Roger Sperry and his chemoaffinity hypothesis

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Pergamon

PII] S9917Ð2821"87#99941Ð8

Neuropsycholo`ia\ Vol[ 25\ No[ 09\ pp[ 846Ð879\ 0887 Þ 0887 Elsevier Science Ltd[ All rights reserved Printed in Great Britain 9917Ð2821:87 ,08[99¦9[99

Roger Sperry and his chemoaf_nity hypothesis RONALD L[ MEYER Developmental and Cell Biology\ University of California\ Irvine\ CA 81586!1164\ U[S[A[ "Received 08 November 0885^ accepted 3 February 0887#

Abstract*In the early 0839s\ Roger Sperry performed a series of insightful experiments on the visual system of lower vertebrates that led him to draw two important conclusions] When optic _bers were severed\ the regenerating _bers grew back to their original loci in the midbrain tectum to re!establish a topographical set of connections^ and the re!establishment of these orderly connections underlay the orderly behavior of the animal[ From these conclusions\ he inferred that each optic _ber and each tectal neuron possessed cytochemical labels that uniquely denoted their neuronal type and position and that optic _bers could utilize these labels to selectively navigate to their matching target cell[ This inference was subsequently formulated into a general explanation of how neurons form ordered interconnections during development and became known as the chemoa.nity hypothesis[ The origins of this hypothesis\ the controversies that surrounded it for several decades and its eventual acceptance\ are discussed in this article[ Þ 0887 Elsevier Science Ltd[ All rights reserved Key Words] retina^ tectum^ speci_city^ plasticity^ axon growth[

and provided critical evidence that connectivity was fundamentally important to function[ In the process of so doing\ however\ he became Roger Sperry\ the devel! opmental neurobiologist\ by asking the corollary ques! tion of how these speci_c connections came to be formed[ His answer was the chemoa.nity hypothesis which states that neuronal di}erentiation determines connectivity[ Ironically\ he may be better remembered for this chemoa.nity hypothesis which addressed a secondary and arguably less important issue than he is for his con! tribution to the fundamental issue of connectivity and function[ Perhaps it is that we have long accepted the importance of connectivity for function while the chemoa.nity hypothesis has\ at times\ been controversial and the developmental issues it raised continue to be a current topic of research[ This article will focus on Sper! ry|s chemoa.nity hypothesis\ from its origins in the reti! notectal system to its present status[ I will argue that its most basic and important tenets have become accepted fact in contemporary developmental neurobiology although there are certain aspects and details of his pro! posals that are unproven or incorrect[ To understand the origin of the chemoa.nity hypoth! esis we have to go back to the milieu of!the 0829|s when Sperry began his work[ The stage had been set by the turn of the century by the great pioneering neuro! anatomists\ most notably Cajal "see Finger\ ð21Ł#[ They made spectacular use of the newly developed silver stain! ing methods\ especially the method of Golgi ð34Ł\ to reveal

Background and precedents to chemoaf_nity The modern view of the structure and function of the nervous system rests on a number of premises that have been accepted for so long that their controversial origins and hard won acceptance have been forgotten[ They have become our intellectual re~exes\ integral parts of our thinking but not of our conscious thought[ One of these is the idea that the function of the nervous system depends in a fundamental way on the pattern of connections between neurons[ We take for granted that speci_c con! nections dictate speci_c functions[ We know that the language of individual neurons is a common one\ con! sisting of action potentials and synaptic transmission and that it is who these neurons talk to that imparts mean! ingful language[ The hackneyed analogy is that of the computer] The transistors are all the same and com! municate with the same electrons^ it is how they are wired together that imbues them with useful function[ Although it seems astounding by current standards\ as late as the 0829|s and 0839|s prominent neuroscientists were espous! ing the diametrically opposed conclusion that speci_c connections were not necessary for function[ Roger Sperry\ the neuropsychologist\ set out to test this idea  To whom all correspondence should be addressed] Ronald L[ Meyer\ Developmental and Cell Biology\ University of California\ Bio Sci 1\ ZOT 1164^ Irvine\ CA 81586!1164\ U[S[A[[ Fax] "838# 713!5848^ E!mail] rlmeyerÝuci[edu

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a nervous system that was cellularly complex but yet highly ordered[ They described in exhaustive detail that perhaps only a neuroanatomist could love\ but that every! one could appreciate\ the morphology of neurons and their interconnecting tracts in many of the nuclei and lamina of the brain and spinal cord[ As a result of this impressive e}ort\ the existence of orderly nerve con! nections was well known[ The developmental question as to how these orderly connections came to be formed had also been raised before Sperry|s time[ Cajal had observed growth cones at the end of growing axons and had sug! gested that connections were formed by the orderly out! growth of axonal extensions of the neuron ð092\ 093Ł[ This may seem obvious in today|s perspective\ but there was an ongoing debate about whether axons were actu! ally processes that extended from individual neurons ð21Ł[ Ross Harrison laid this issue to rest in 0809 with his invention of tissue culture ð49Ł[ He observed process out! growth from individual embryonic frog neurons in vitro[ Cajal further suggested that growth cones might be attracted to their appropriate targets by detecting some chemical cue exuded by the target that is\ by chemotaxis ð86Ł[ Although this suggestion is sometimes cited as hav! ing presaged Sperry|s chemoa.nity hypothesis\ Cajal did no experiments to demonstrate this and had also sug! gested a number of other mechanisms such as mechanical guidance and the elimination of incorrect growth[ Sub! sequent e}orts by Harrison who used his tissue culture technique to look for chemotaxis failed to provide any evidence that neurons were chemotactically directed to their targets as supposed by Cajal "see below#[ One person who probably deserves recognition for pre! saging Sperry is J[ N[ Langley who was contemporaneous with Cajal[ Langley ð69\ 60Ł cut preganglionic _bers to the superior cervical ganglion and allowed them to regen! erate[ By selectively stimulating various spinal trunk nerves\ he was able to show that speci_c autonomic re~exes associated with the di}erent trunk nerves were re!established by the regenerated _bers[ He concluded that the regenerated _bers must have formed connections onto the speci_c class of ganglionic neurons that innerv! ate di}erent peripheral targets and suggested\ apparently independently of Cajal\ that this selective re!innervation was probably mediated by a process like chemotaxis[ However\ Langley|s work\ which was limited to the auto! nomic system\ appears to have made little impact on the thinking of the 0829|s\ much of which was concerned with complex CNS function and behavior[ It was for! gotten and overshadowed by the contrary _ndings and thinking of Weiss and Harrison as described below[

Functionalism\ an anti!speci_city view The thinking of the 0829|s\ especially in America\ had come to be strongly in~uenced by functionalism\ the idea that learning\ not precise neuronal connectivity\ gen! erated animal behavior "see reviews by Sperry ð014\ 015Ł^

Hunt and Cowan ð50Ł^ Finger ð21Ł#[ Behavior was thought far too complex to be explained by Connectivity which at best might account for primitive re~exes[ Behavioral function was postulated to arise through learning by a selection of the appropriate responses and elimination of inappropriate ones[ Coghill ð06\ 07Ł had described the development of organized behavior in modeles as emerg! ing from a crude thrashing behavior\ as if the animal were actively selecting the correct responses and eliminating the incorrect ones[ In a word\ the functionality of a par! ticular neuronal activity pattern and its behavioral e}ect were postulated to determine whether the activity pattern was eliminated or whether it survived and was enhanced[ The cellular substrate\ that is\ the pattern of nerve con! nections was not critical for this activity[ Rather\ activity was thought of in holistic or Gestalt!like terms which were being championed in England by Head ð41Ł and in America by Franz ð22Ł\ Lashley ð61Ł and Goldstein ð33Ł[ These patterns of nerve activity were supposed to exist independently of speci_c nerve connections or precise brain locations[ Di}erent spatiotemporal sequences of neuronal activity generated di}erent behaviors[ Although neuronal activity depended on interconnected neurons\ these were simply the substrate over which the patterns of neuronal activity traveled[ Neurons were the supporting media\ not the determinants of the pattern of activity ð042\ 044Ł[ The analogy was that of sound waves traveling through matter[ Neurons were the matter which allowed the sound waves to propagate\ but they imparted very little to the actual pattern of the waves of activity[ There was\ of course\ plenty of evidence cited as sup! porting the functionalist view[ A large body of clinical evidence showed that there was extensive recovery fol! lowing injury to peripheral or central nervous system ð22\ 33\ 41\ 61Ł[ This recovery was interpreted to mean that behavior was achieved through learning and did not depend on the speci_cs of circuitry[ As long as the mini! mal neuronal substrate was present\ learning could com! pensate for the damaged neurons[ Similar lesion studies in animals were similarly interpreted[ More important\ however\ were experiments in which nerve connections were directly altered[ Most of these were done on the neuromuscular system where it was relatively simple to rearrange nerve connections[ When nerve _bers in the limb were transposed so as to connect to foreign muscle groups or conversely when di}erent muscles were trans! posed to foreign positions\ the animal would recover apparently normal behavior[ Perhaps the most remark! able reports of this period were the ectopic limb experi! ments of Paul Weiss ð049Ł[ He transplanted an extra limb onto the ~ank of a salamander and seemingly found that these ectopic legs became innervated by nerves that did not normally innervate limb muscles[ In spite of this foreign innervation\ these legs moved in synchrony with the neighboring normal leg[ His explanation\ resonance theory ð040Ł\ postulated that the pattern of muscle move! ment was controlled by patterns of nerve activity that originated in the central nervous system and ~owed out

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along all nerves in the vicinity of the limb[ Speci_c limb muscles contracted at speci_c times because di}erent muscles were tuned to\ {{resonated|| to\ di}erent fre! quency patterns[ Since all nerves could carry the same pattern\ it did not matter what muscle they connected to[ A corollary of this functionist view was that there was no necessity for neurons to be selective about what other neurons or muscles they connected to[ Indeed\ diverse surgical segments in the neuromuscular system showed nerves to be very promiscuous about what muscles they innervated "reviewed by Sperry ð011Ł#[ Virtually any nerve could connect to any muscle[ This made sense in that if speci_c connections did not have functional relevance\ they would not need to form speci_c connections[ There would be no evolutionary pressure to do so[ The problem\ the ~y in the ointment\ was that neurons did\ in fact\ normally form ordered connections[ The elegant and vol! uminous work of the early anatomists showed this quite clearly[ If ordered connections were not important\ why then were they normally so organized< The functionalist answer was that this order was an epiphenomena of devel! opment[ Organization was an incidental outcome of other developmental processes rather than something that was itself directly regulated[ The available evidence was thought to support their view[ E}orts to show the contrary\ that growing nerve _bers expressed inherent preferences as to where they grew and with what they connected\ met with failure[ A major player in this was Ross Harrison who utilized his newly invented method of tissue culture to test for directed growth of neurites ð49Ł[ He reasoned that if\ as Cajal had suggested\ axons were detecting a substance exuded by their proper target cells\ then one should be able to show directional growth in culture by putting targets together with the a}erents ð40Ł[ His several e}orts came up empty[ He found no evidence that the direction of axonal growth could be a}ected by the presence of targets[ Similar tissue culture studies by Weiss ð041Ł were likewise negative[ Weiss instead found evidence of mech! anical e}ects on nerve _bers[ In collagen gels\ he noticed that neurites tended to follow along oriented collagen _brils[ He was able to create gels with di}erently oriented _brils in di}erent regions and showed that when a grow! ing nerve _bers encountered a new orientation\ they turned accordingly[ All nerve _bers regardless of their type and origin behaved similarly[ He concluded that nerve _bers were guided solely by the mechanical features of their environment such as cells\ extracellular material and other nerve _bers[ Except for their size\ which might predispose large and small axons to respond di}erentially to various sized features in their environment all neurons were intrinsically identical[ The organization of the ner! vous system was hypothesized to arise from the time and position at which neurons were born and di}erentiated and where and when they extended axons ð045Ł[ These extending axons then reacted non!speci_cally to the mechanical features in their environment[ These ideas\

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which diminished the individuality of neurons\ _t well with the functionism of the time ð044Ł[

The birth of Sperry|s chemoaf_nity Sperry joined Paul Weiss at the University of Chicago as a doctoral graduate student in biology after having obtained a masters degree in psychology at Oberlin College[ His dual interest in psychology and biology\ which he retained throughout his life\ led to him to address the structureÐfunction question[ Sperry\ in hall! mark fashion\ identi_ed this as a major contemporary issue and embarked on a series of critical experiments to resolve it ð005Ð007Ł[ Working with adult rats\ he trans! posed motor nerves to force them to re!innervate the incorrect muscle groups as had been done previously but then proceeded to assay behavior much more carefully than had been done before[ His conclusions were quite di}erent from the then current view[ Although he also saw some recovery of function\ it was incomplete and resulted in behavior that was always abnormal[ The rat could learn to walk again but in a much cruder fashion than before[ The subtle control over motor movement was permanently impaired and the basic re~exes remained abnormal in correspondence to the abnormal connections[ Sperry|s interpretation was that the rat was intelligent enough to learn to use the dysfunctional nerve connections in the leg in an adaptive manner much like someone might learn to walk with a leg in a cast[ Since normal function was never regained when nerves were miswired\ it must mean that connections were important[ In subsequent studies\ he transposed sensory nerves and reached similar conclusion[ Sperry later critically evalu! ated the literature in a review ð011Ł that was impressive in detail and scope and took the functionalist view to task[ Sperry|s conclusions meshed with the earlier elec! trophysiological experiments of Wiersma ð046Ł[ Wiersma had shown that an impulse in a motor nerve led reliably to a muscle contraction\ contradicting Weiss|s notion that muscle selectively responded to particular patterns of impulse activity[ These results of Sperry and Wiersma\ however\ did not spell the end of functionalism[ The theory adapted to the new evidence[ Weiss ð042\ 045Ł conceded that motorneurons were hardwired to speci_c muscles but postulated that the motor neurons in spinal cord were not similarly driven by neurons in the CNS[ It was the motor neurons\ instead of the muscles\ that preferentially responded to the critical patterns of impulse activity that radiated around in the central ner! vous system[ In e}ect the resonator for motor control was moved centrally from the muscle into the spinal cord[ This explanation required the motorneuron to know what!muscle it was connected to[ To explain this\ it was postulated that di}erent muscles have speci_c charac! teristics or labels that they imbued to the motorneurons that happen to connect to them[ A motorneuron could

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connect to any muscle and then the muscle would tell the motorneuron which muscle it was connected to[ To account for the limited recovery of function following nerve crossing in mammals\ these muscle labels were hypothesized to be strong during development but weak in adults[ In salamanders\ which are neotenous animals\ these muscle labels were postulated to persist beyond initial development and could thereby generate complete recovery of function such as Weiss had seen in ectopic limbs ð049\ 043\ 044Ł[ The functionist view survived for the CNS[ Around this time\ it had been reported by several groups ð5\ 66Ł\ most notably\ Stone ð024\ 025Ł at Yale\ that lower vertebrates could recover vision following transection of the optic nerve[ With time\ frogs and sala! manders regained the ability to respond to visual stimuli such as by feeding on small food objects[ Histologically\ this recovery correlated with the regeneration of the optic nerve from the surviving retina or from the regenerated retina in cases where the retinal blood supply was inter! rupted and retina itself degenerated[ In some animals\ it\ proved possible to remove the entire eye from its socket put it back in place and obtain visual recovery[ In one of Stone|s experiments\ the eye had become rotated in its socket and visual recovery was not only observed but was reportedly normal[ The signi_cance of these experiments for the structure!function issue was not lost on Sperry[ At the time\ these results could and were interpreted as supporting the functionalist view[ Histological obser! vations on the nerve had shown that the optic nerve _bers were very disorganized at the site of injury indicating that the regenerated _bers did not restore their normal order following regeneration[ The recovery of normal vision from disorderly connections _t well with the functionalist theory[ Stone|s observation of visual recovery from a rotated eye further bolstered the case for functionalism[

Sperry recognized not only the importance of these results but also the value of the lower vertebrate visual system\ the retinotectal system for addressing the struc! ture!function issue[ It had several attractive assets that were missing from the rat leg model he had been working with[ It was a CNS system[ Not only did optic _bers terminate in the brain but the retina itself was embryo! logically part of the CNS[ The system could therefore provide a critical test of the resonance theory which had now retreated into the CNS[ Equally important the sys! tem lent itself to critical testing of function[ Frogs and salamanders had good visual localization[ They readily identi_ed potential food objects\ oriented to them and ate them[ They were also dumb[ Compared to rats\ their responses were much more stereotypic so there was little learning to confuse the interpretation of the behavior[ And Sperry|s interest was undoubtedly piqued by Stone|s incidental observation that a rotated eye could mediate normal behavior[ Sperry was very suspicious of this result[ After all\ it would mean that Sperry was wrong about speci_c nerve connections mediating speci_c func! tion[ Sperry set upon a series of now famous experiments on the retinotectal system of the newt and frog in which he combined his considerable surgical skill with careful behavioral testing "Figs 0Ð3#[ The two that are perhaps best remembered are the eye rotation ð008Ł and optic nerve uncrossing experiments ð012Ł[ In the newt\ he rotated the eye by 079> and used prey localization to test vision ð008Ł[ If the nerve was kept intact\ the newt behaved as if its visual world were correspondingly rotated[ When food was presented in front of the animal\ it turned rear! ward[ When food was present above the animal\ it twisted its head to look down[ If the optic nerve was crushed and the nerve allowed to grow back from the rotated eye\ the same kind of mislocalization was observed[ Of course\ if

Fig[ 0[ Schematic drawing of the retinotectal system of the gold_sh showing the back of the eyes from which the optic nerves originate\ their crossing at the optic chiasm at the base of the forebrain and the course of _bers from the right eye along the optic tract and into left tectum[ The visual projection of amphibia is similar[ Taken from Fig[ 0 of Attardi and Sperry\ 0852 ð2Ł[

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Fig[ 1[ Schematic drawing of the amphibian visual system showing the nerve scar where optic _bers were severed and through which _bers had regenerated back to the optic tectum "optic lobe#[ The disruption of the _ber order in the scar is illustrated by the tangled mass of _bers[ Taken from Fig[ 0 of Sperry\ 0840 ð015Ł[

the nerve was simply cut without rotating the eye\ the newt behaved normally after regeneration as previously reported ð019Ł[ In the frog\ he uncrossed both optic nerves so that optic _bers regenerated to the ipsilateral side instead of the contralateral side ð012Ł[ Following regen! eration\ prey localization behavior was left!right reversed[ When food was presented to the left visual _eld\ the frog orientated to the mirror image position in the

right visual _eld[ In all cases\ the maladaptive behavior was permanent[ If the animals weren|t manually fed\ they would have starved to death[ Sperry ð008Ð010\ 012Ł drew the obvious conclusion from the persistent mislocalization that learning could not account for the behavior[ Since behavior was not mediated by learning\ then the only alternative was that it was mediated by orderly connections[ The behavior

Fig[ 2[ Illustration of the behavior of a frog that had its eye rotated by 079> and the nerve severed and allowed to regenerate[ When a ~y was presented in the upper visual _eld behind the frog\ the frog struck downward and frontward as indicated by the thick arrow[ Taken from Scienti_c American article by Sperry in 0845 ð016Ł[

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Fig[ 3[ Illustration of the optic nerve uncrossing surgery in frog and resulting behavior[ In A\ the normal optic chiasm is shown on the left wherein optic nerves cross to the contralateral side of the brain and the surgically uncrossed nerves are shown at the right wherein each optic nerve was de~ected so as to regenerate to the ipsilateral side of the brain[ In B\ the mislocalization of ~ies by the frog with uncrossed optic nerves is shown by the position of the ~ies and the corresponding response indicated by the thick arrows[ Taken from Scienti_c American article by Sperry in 0845 ð016Ł[

was organized so the connections had to be organized[ In the absence of learning\ one could not get orderly behavior from disorderly connections[ He further rea! soned that since the localization behavior was organized retinotopically\ that is\ was a function of the topographic position of the retina at which visual stimuli fell\ the regenerated optic _bers must have reformed reti! notopically organized connections with the brain[ There was previous evidence for topography in the normal vis! ual system based on anatomical observations and local lesion studies ð032Ł[ Localized lesions to the tectum had been shown to produce discrete blind spots or scotomas in the visual _eld[ Sperry ð010Ł subsequently took advan! tage of this work on scotomas to obtain more direct evidence that the regenerated optic _bers had reformed a retinotopic projection onto tectum[ As had others\ he found that in a normal frog\ local tectal lesions produced a scotoma in the corresponding part of visual _eld[ When he then repeated this in frogs which had had their optic nerve previously severed\ he found that tectal lesions produced scotomas of comparable size and position[ This

result further bolstered his initial inference about the reformation of retinotopic connections[ In hallmark fashion\ Sperry immediately appreciated the importance of his conclusions for the developmental question of how nerve _bers form patterned connections[ He was led to reject mechanical guidance for several reasons[ If mechanical guidance were the explanation\ regenerating _bers should be found to follow orderly paths through the nerve since it postulates a preservation of initial order[ He ð019\ 012Ł examined silver stained sections of the optic nerve and contrary to the prediction of mechanical guidance\ found _bers to be highly dis! ordered at the site where they had been sectioned[ He described the _bers as looking like a bird|s nest[ In some experiments\ Sperry ð019Ł deliberately scrambled the cut ends of the optic _bers to destroy initial order and yet visual localization was restored[ In several of his experi! ments where the orientation of the nerve was altered with respect to tectum\ the results predicted from mechanical guidance were not obtained[ With 079> eye rotation ð008Ł\ for example\ mechanical guidance would predict that the

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_bers should preserve their new orientation to form a projection onto tectum in accord with their new position\ not with their original position[ This should produce normal prey localization behavior regardless of rotation[ Similar argument could be made for experiments where the optic nerve was uncrossed or the eye transposed from left to right[ There was also the related possibility of di}erential timing in the outgrowth of _bers\ but as far as anyone could tell\ the severed _bers all grew back at about the same time[ To Sperry|s way of thinking\ there were only two gen! eric possibilities[ Either optic _bers were intrinsically identical and passively followed mechanical tracts back to the tectum as proposed in the mechanical guidance theory\ or optic _bers di}ered according to their retinal origin and actively followed cues based on their intrinsic di}erences[ Since he could rule out the former\ this meant _bers had to be intrinsically di}erent[ He further recog! nized the necessity of having intrinsic regional di}erences in the brain for the _bers to recognize[ If there were no regional di}erences in the brain itself\ then it would be di.cult to account for the di}erential growth of _bers[ Fibers had to recognize what part of the brain to grow to\ that is\ to the tectum and within the tectum\ what region to terminate in[ These reasons led Sperry ð008Ð 010\ 012\ 014\ 015Ł to postulate that both _bers and tectum exhibited position dependent di}erences and that _bers di}erentially grew in response to these intrinsic di}erences so as to reconnect with their original location[ These postulated intrinsic neuronal characteristics raised the issue of how they came about[ Sperry|s explanation\ which was heavily in~uenced by the embryological thought of the time\ was that these characteristics were the product of di}erentiation during development[ The implication\ of course\ was that these developmentally acquired characteristics were what produced the initial organization of the optic projection during development\ not just during regeneration[ These early inferences con! stituted the basics of the chemoa.nity hypothesis[ He had postulated] "0# axons had di}erential markers\ "1# target cells had corresponding markers\ "2# markers were the product of cellular di}erentiation and "3# axonal growth was actively directed by markers to establish spec! i_c connections[ I will refer to this as the {{General Chemoa.nity Hypothesis||[ Sperry quickly embarked on a series of similar studies on other systems in amphibia[ He transected and scram! bled VIII nerve and examined vestibular re~exes ð013Ł\ redirected dorsal roots to the wrong side of the spinal cord and looked at wiping responses and proprioception ð014Ł\ scrambled the trigeminal nerve root and tested for recovery of function ð022Ł\ performed cross union between opthalmic and mandibular branches of the tri! geminal nerve and looked for head withdrawal re~exes ð014Ł\ transplanted and rotated pieces of skin and tested for wiping re~exes ð014Ł and created supernumerary limbs and tested withdrawal re~exes ð85\ 014Ł[ There were three classes of results[ The _rst were those cases where the

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behavior was inappropriate and this malfunction clearly correlated with surgically induced miswiring[ For exam! ple\ when dorsal roots were made to innervate the wrong side of spinal cord in frog\ noxious stimulation of one leg produced withrawal of the leg on the other side ð014Ł[ The frog never learned otherwise[ This reinforced his previous conclusions that it was the wiring that deter! mined behavior[ Learning alone could not explain it[ The second class of results were those in which appropriate behavior emerged following disruption and regrowth of the connections[ A notable example of this was his study ð013Ł on the VIII nerve wherein he cut the nerves\ teased them apart and then rearranged their relative positions[ Normal responses to tilt and acceleration were observed[ Learning was ruled unlikely because these responses could also be observed in a decerebrate animal[ He inter! preted these _ndings to mean that _bers had regenerated back to their original locations in the brain based on their intrinsic chemical markers denoting what part of the vestibular apparatus they had originated from[ He ruled out mechanical guidance because of the surgical scrambling and because the histology showed extensive disarray at the transaction site[ The third class of results were more problematical for chemoa.nity[ These were cases in which the behavior was inappropriate while the connections were seemingly appropriate[ The most notable were the skin rotation and supernumerary leg experiments done with Miner ð85\ 014Ł[ They rotated a large piece of ~ank skin in a frog tadpole so that back skin was positioned on the belly and vice versa[ With metamorphosis\ the skin di}erentiated according to its origin so that dorsal pigmentation was now observed on the belly in the rotated graft[ When the back position of the graft skin was stimulated\ the frog wiped its belly with its foreleg[ When the belly skin was stimulated\ the frog instead used its hindleg to wipe the back[ This was behavior appropriate for the origin of the skin[ Equivalent results were obtained with super! numerary legs transplanted to the frog|s back[ Based on anatomical inspection\ they believed that the skin ~ap and ectopic leg had become innervated by nerves that were appropriate for the position and were consequently inappropriate for the transplanted skin and leg[ Sperry|s ð014Ł explanation was that the skin of the ~ank or leg had a {{local sign|| that denoted its origin and that this labeled the sensory nerves which then selected its central con! nections in accord with this peripherally acquired label[ He invoked this peripherally induced labeling for motorneurons as well to account for Weiss|s results[ The explanation for Weiss|s synchronous movement of super! numerary limb was that the local sign of muscles similarly imparted chemical di}erentiation to the motorneurons which then caused it to receive appropriate connections[ Basically\ he made the general suggestion for the per! ipheral nervous system that when the neuronal cell bodies were located centrally\ they grew out relatively non!selec! tively and then relied on retrograde labeling from their targets to acquire the speci_c labels that regulated the

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formation of central connections[ This _t the then current observations that motorneurons could innervate any muscle without preference[ This last suggestion did not hold up[ Sperry himself subsequently obtained contrary data[ Working with Arora nearly 19 years later ð020Ł\ they found good evidence for selective re!innervation of eye muscles[ Sperry stated that his earlier conclusion for no selectivity in the periphery {{may have been overly hasty|| and needed to be re!evaluated ð020Ł[ The correct explanation for Weiss|s synchronously moving ectoptic limbs came only later with the work of Grimm in 0860 ð36Ł[ Using electrophysiological methods\ Grimm showed that the ectopic limbs had become surreptitiously innerv! ated by motorneurons corresponding to the nearby nor! mal limb[ The dissection method used in the early studies to trace nerve _bers was simply not sensitive enough to detect these _bers[ Sperry|s quali_er to chemospeci_city for the peripheral nervous system was no longer needed[

The maturation of chemoaf_nity The major impact of all this work on chemoa.nity was to bolster it and to show that it applied generally[ The next intellectual advance in chemoa.nity didn|t really come until the early 0859|s[ Prior to that\ Sperry|s methods had been behavioral[ Consequently he was infer! ring that appropriate connections were being made but didn|t have any direct evidence of it[ Although it is a tribute to his keen insight that he was able to see so clearly the implications of his behavioral work\ other interpretations could be argued[ He set out to obtain direct anatomical evidence for speci_city of re!inner! vation[ Before he could do so\ however\ Lettvin and Maturana and colleagues obtained electrophysiological evidence for selective innervation in the frog retinotectal system ð67Ł[ They developed microelectrode methods for recording from optic _bers and found that they could record a retinotopic map of retina on tectum ð62Ł[ Although their interest was in visual physiology\ they realized they could use their mapping method to deter! mine whether severed optic _bers indeed regenerated back to their original tectal positions as proposed by Sperry[ They found ð67Ł that not only did a retinotopic map reform but so did the laminar speci_c innervation of di}erent classes of optic _bers[ This result provided strong con_rmation of Sperry|s behavioral work\ but there was arguably room for doubt[ The interpretation of the electrophysiological map rested on the inference that the terminal arbors of optic _bers were being selec! tively recorded[ Although Lettvin and colleagues pro! vided some good arguments for this ð62Ł\ they did not have direct proof[ Sperry\ who was a bit distrustful of electro! physiological methods\ thought that it was still important to obtain direct anatomical evidence for selective re! innervation[ Working with Attardi ð2Ł\ he developed a modi_cation of a Bodian silver stain that preferentially

stained regenerating optic _bers in the gold_sh[ In order to visualize what _bers from di}erent retinal regions were doing\ they removed half of the retina\ crushed the optic nerve and analysed the\ ingrowth of _bers from the remaining retinal half "Fig[ 4#[ They found and published in 0852 ð2Ł that optic _bers from superior\ inferior\ nasal or temporal half retinas selectively re!innervated their appropriate lateral\ dorsal\ posterior or anterior half tec! tum respectively[ They further noticed that _bers had selected the appropriate part of the optic tract in route to tectum[ Fibers from superior retina grew into the lateral brachial division of the tract in route to lateral tectum and inferior _bers grew into the medial brachia[ These _ndings provided the sought after direct anatomical con! _rmation of selective innervation but also went well beyond this in two ways[ The _rst was in showing that axons were able to selectively navigate along the pathway to their target choosing the correct path over the incorrect one[ Previously\ Sperry|s evidence had been limited to preferential choice within the target region and\ if anything\ indicated there to be no selectivity within the pathway itself[ The second way was in showing that _bers were capable of growing past incorrect regions that were left uninnervated in favor of the correct regions[ This was a much stronger preference than had been demonstrated in previous studies where the entire complement of axons was present and consequently all target regions were innervated[ In the Attardi and Sperry study\ half of tec! tum remained without innervation[ They created the _rst size disparity experiment\ many more of which were to come[ Using these new results as a springboard\ Sperry for! mulated his chemoa.nity theory in its most detailed form with speci_c reference to the retinotectal system in two publications ð017\ 018Ł[ He hypothesized that during neuronal di}erentiation retinal ganglion cells and tectal cells acquire at least two categories of cytochemical labels "Fig[ 5#[ The _rst was class labels denoting the neurons as being retinal ganglion or tectal cells and as belonging to functional subclasses such as color speci_city[ The other category of labels were positional labels that denoted the topographic position of the retinal ganglion cells in retina and the tectal position of the corresponding retinoreceptive cells in tectum[ Sperry thought that it was unlikely that each positional label was entirely unique since this would require an enormous number of di}erent molecules[ He was inspired by earlier ideas from embry! ology that tissue might be organized as gradients and _elds and was led to suggest that the positional properties could be distributed as a concentration gradient of mol! ecules across retina and tectum[ In order to explain the topography\ he thought it necessary to have at least two gradients[ For the retinotectal system of lower vertebrates\ he suggested that in retina one might exist along the nasal temporal axis and the other across the inferior superior axis[ He also suggested that these or similar cytochemical gradients must be distributed across the optic pathway to explain appropriate choice of bra!

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854

Fig[ 4[ Illustration of four of the retinal surgeries and resulting innervation pattern of the optic tectum of regenerating _bers in gold_sh described by Attardi and Sperry[ Half of retina was removed and _bers from the remaining half of retina as indicated by the radial lines were allowed to regenerate after crushing the optic nerve[ The selective choice of _bers through the optic tract and re!innervation of the appropriate half of tectum is illustrated for the corresponding eye surgery[ Taken from Figs 2 and 4 of Attardi and Sperry\ 0852 ð2Ł[

Fig[ 5[ Illustration of Sperry|s conception of how retinal cells might have acquired several position dependent properties that uniquely denote their locus[ Three postulated gradients of retinal labels are shown from left to right] an anterior to posterior gradient\ a dorsal to ventral gradient and a central to peripheral gradient[ Taken from Fig[ 2 of Sperry\ 0840 ð015Ł[

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chia in the optic tract[ As before\ he postulated that there were additional labels denoting classes of optic _bers such as those di}erentially responsive to color[ For the mammalian visual system he was led to suggest additional radial gradients in order to explain the convergence of the visual _eld from both eyes into dorsal lateral genicu! late[ The idea of a radial gradient had actually been suggested to Sperry by one of his students\ Charles Hamilton[ Sperry\ as others had been\ was intrigued by the dynamic extension and retraction of _lopodia from growth cones "Fig[ 6# and interpreted this as exploratory activity for guidance cues ð017\ 018Ł[ He suggested that these _lopodia were the chemosensory detectors for chemoa.nity[ The _lopodia expressed the same labels that were on the parent cell bodies and di}erentially responded to cytochemical cues in their environment by orienting their growth toward their matching labels[ In other words\ the growth cone could detect local gradients of chemoa.nity molecules and could orient their growth accordingly[ In this way\ growing optic axons selected the appropriate part of optic tract and within tectum grew toward their appropriate locus and terminated on the matching labels at the correct part of tectum[ Based on the results from the half retinal experiments in which _bers bypassed denervated incorrect regions in favor of correct regions\ he suggested that these chemoa.nity preferences were so strong that they appeared to preclude incorrect termination[ I will refer to this last postulate as the {{strong chemoa.nity hypothesis||[ This detailed formulation of chemoa.nity\ the strong chemoa.nity postulate and Attardi and Sperry|s spec! tacular experiments were arguably the pinnacle of Sperry|s chemoa.nity hypothesis[ The publications from this period ð2\ 017\ 018Ł have been and continue to be among Sperry|s most cited work[ Only the earlier eye rotation and optic nerve uncrossing studies have had as much impact[ Unfortunately\ this experimental work of 0852 may be his most questionable and the speci_c ver! sion of chemoa.nity that emerged from these studies became controversial[ Ironically\ the source of the controversy arose mainly from variations of the size dis!

Fig[ 6[ Illustration of the tip of a growing nerve _ber at various time points along its path showing possible pathway choice points indicated by A\ B and C through which _bers were physically capable of growing[ Chemical cues detected by the _lopodia protruding from the growth cone were postulated to determine the selection of the pathway[ Taken from Sperry\ 0852 ð016Ł[

parity experiments that he had _rst used on the reti! notectal system[

Challenge to chemoaf_nity One of the _rst contrary studies to appear was the compound eye experiments from Mike Gaze|s lab in 0852 and 0854 ð27\ 28Ł[ They removed half an eye in Xenopus embryos and replaced it with the mirror image half from another embryo to make double nasal or double temporal eyes[ When the embryos grew into frogs\ they recorded an electrophysiological map and found that these eyes projected across the entire tectum\ not just to half of the tectum as might be predicted from chemoa.nity[ They were further able to show that functional connections were being made in the {{wrong|| part of tectum[ It was argued that this {{expansion|| of the projection was a problem for chemoa.nity hypothesis because _bers were not projecting to their matching chemoa.nity locus[ However\ there was a loophole in these experiments which Sperry pointed out ð018\ 021Ł[ One was that the tectum may have undergone positional hyperplasia and hypoplasia during development in response to its partial innervation so that the denervated half of tectum was diminished and the innervated half was increased ð018\ 021Ł[ In other words\ the tectum that eventually formed in the adult was really an enlarged half tectum cor! responding to the correct chemoa.nity labels[ This explanation was subsequently disproved by Gaze and associates ð32\ 026Ł[ They produced frog embryos with one compound eye and then after metamorphosis uncrossed optic _bers at the chiasm so that now the compound eye projected to the normal tectum and the normal eye projected to the {{compound eye tectum|| previously innervated by the compound eye ð030Ł[ The result was that the compound eye projected across the entire normal tectum and the normal eye projected across the entire compound eye tectum contrary to Sperry|s expectation[ Sperry ð82\ 83\ 029Ł subsequently suggested that the development of the eyes rather than tectum might have been altered[ It was possible that the eyes underwent positional regulation during development so that each half of these compound eyes became complete retinas each containing the full range of positional values[ This would be similar to the regulation observed following removal of half a limb bud wherein the remaining half limb bud eventually forms a normal limb with all digits[ In e}ect\ the compound eyes were really two normal eyes in one globe[ Subsequently\ there were also size disparity experi! ments done on mature gold_sh that proved more di.cult to accommodate in terms of embryonic regulation[ The most notable were the experiments of Gaze and Sharma ð31Ł in which they removed the posterior half of the tec! tum in gold_sh and severed the optic nerve[ Following optic nerve regeneration\ the electrophysiological map showed that the entire retina was represented on the

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remaining anterior half of the tectum thereby forming a {{compressed|| projection "Fig[ 7#[ In this case\ the lesioned tectum did not grow signi_cantly and so the missing tissue could not have been replaced[ This result was again taken as directly contradicting the chemoa.! nity hypothesis which\ it was thought\ predicted that only the correct half of retina would be represented on the half tectum[ Chemoa.nity predicted {{speci_city|| but the _nding was {{plasticity|| "Fig[ 7#[ At this point the criti! cism became sharp[ Gaze published a book and argu! ments against chemoa.nity were spelled out in great detail[ A controversy was on[ Sperry greatly disliked these expansion and com! pression results[ They were at variance with his own size disparity experiments in gold_sh showing that growing _bers had strong preferences for their correct targets[ He was initially led to question their validity and encouraged a newly arrived postdoctoral electrophysiologist in the lab\ Myong Yoon\ to re!examine the half tectum experi! ment in gold_sh[ Unfortunately for Sperry\ Yoon ð059\ 053Ł con_rmed compression[ Sperry still had reser! vations[ He questioned the reliability of electro! physiological mapping for determining the distribution of optic _bers which he viewed as less direct and reliable than the anatomical tracing he had used[ However\ elec! trophysiological mapping was generally accepted in the _eld as a valid measure of the termination pattern of optic _bers and those of us in the lab who were doing electrophysiology at the time argued in favor of it[ He reluctantly accepted these results[ On the other hand\ there were also electrophysiology _ndings that strongly supported chemoa.nity[ The earl!

856

ier results of Lettvin et al[ ð62\ 67Ł showing that regen! erating optic _bers reformed a highly ordered projection were con_rmed many times in both frog and gold_sh ð39\ 55Ł[ More dramatic con_rmation came from the tectal rotation experiments in adult frog by Levine and Jacobson ð64Ł and in gold_sh by Yoon ð051\ 052Ł while he was in Sperry|s lab[ In these studies a piece of the tectum was removed and re!implanted with a 89> or 079> rotation and in some cases the optic nerve was also cut[ After allowing time for _bers to re!grow back onto the implant the projection was mapped electrophysio! logically[ The projection surrounding the implant was found to be normal but that onto the implant was rotated in accord with the intrinsic polarity of the tectal fragment[ This was an important con_rmation of chemoa.nity because it strongly argued against mechanical guidance[ If one argued that _bers used mechanical guidance to reform the retinotopic projection onto the intact tectum\ then these _bers should have preserved this order when invading the implant to form an unrotated projection[ Instead\ _bers must have had to drastically change course to terminate at their original position[ The obvious interpretation was that _bers were able to read some sort of local cues on tectum as postulated by chemoa.nity[ At this point\ the retinotectal _eld appeared to split into two opposed camps[ One was Sperry|s chemoa.nity group that continued to argue for strong chemoa.nity[ The other group argued against the chemoa.nity hypothesis[ The chemoa.nity camp argued that plasticity\ in particular\ compression onto a half tectum did not violate chemoa.nity because there was size regu! lation analogous to that proposed to have taken place

Fig[ 7[ Schematic illustration of the possible retinotectal projections onto anterior half tecta following removal of the posterior half tectum[ The letters {{a||\ {{b|| and {{c|| indicate retinal positions along the nasal!temporal axis with corresponding locations in visual _eld and the position of corresponding optic terminals in tectum[ Normally {{a||\ {{b|| and {{c|| would be distributed across the entire extent of the whole tectum[ If the posterior half of tectum is removed\ the strong chemoa.nity or {{original model|| would predict that only the appropriate half of retina would project to anterior tectum as illustrated for the left tectum in the lower half of the _gure[ The actual result observed in gold_sh is that _bers form a compressed projection onto the anterior half as predicted from the {{sliding scale model|| of Gaze shown at the upper half of the _gure[ Taken from Fig[ 2 of Sperry\ 0864 ð018Ł[

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in the compound eye experiments ð82\ 83\ 053Ł[ It was suggested that gold_sh retained the embryonic ability to regulate because the tectum was still growing[ Therefore\ when the posterior half of tectum was removed\ the remaining anterior half could size regulate its position dependent labels so as to become a complete tectum[ The compressed projection\ therefore\ actually represented an exact chemoa.nity match between optic _bers and tectum[ Time course studies of compression from Sperry|s lab were taken as supporting regulation ð053Ł[ When optic _bers _rst invade an anterior half tectum in gold_sh\ they form an uncompressed projection[ The compressed projection forms gradually thereafter as might be expected from a time dependent regulation of positional labels[ Half tectal experiments in post! metamorphic tree frogs ð82Ł in which tectal growth was not occurring provided evidence consistent with regu! lation[ In these animals\ no compression was detected[ Around this time\ plasticity was also observed following removal of half of retina in gold_sh ð096\ 097Ł[ However\ like compression\ this was a time dependent change con! sistent with regulation of labels ð096Ł[ The initial pro! jection from a temporal half retina was to the correct anterior half of tectum in agreement with Attardi and Sperry|s original observation[ With time\ a retinotopic expansion of the projection from half a retina across the entire tectum was observed[ If the nerve was then severed a second time\ the projection that initially formed was already expanded ð097Ł[ This expansion could\ therefore\ represent a time dependent regulation of the half retina so that it acquired labels equivalent to a whole retina[ The anti!chemoa.nity camp took the plasticity results to mean that chemoa.nity was partly or entirely incor! rect[ At one end of the spectrum were a relatively small group of developmental anatomists who resurrected mechanical guidance ð02\ 48\ 000Ł[ Using newer neuro! anatomical tracing methods\ they were able to dem! onstrate that there was considerable spatial and temporal organization in the formation of the retinotectal pro! jection during development[ Retinal ganglion cells were born in an orderly central to peripheral sequence and extended axons along the optic pathway in a way that preserved their retinotopic and chronotopic organ! ization[ They were led to suggest that this pathway order could generate the retinotopic projection with little or no need for retinal or tectal chemoa.nity labels[ There were two problems with this neomechanical guidance theory[ It could not account for much of the experimental data such as Sperry|s original eye rotation studies or the more recent tectal rotation studies[ The second was that further tracing studies on pathway organization showed that optic _bers underwent complex rearrangement and adjustments\ especially at the chiasm\ that could not be readily explained by passive mechanical guidance and that _ber paths were not very precisely organized in some species even though these species had precise retinotopy in their projection ð037Ł[ Although neomechanical guid! ance was not a serious contender as a comprehensive

explanation\ it was nevertheless invoked by others to explain selected results and to minimize the need for chemoa.nity[ The major {{anti!chemoa.nity|| group\ most notably Gaze and associates ð05\ 26\ 30\ 31Ł conceded that retinal ganglion cells had to have some positional labels in order to account for the experimental data but questioned whether tectum had any labels[ Invoking the plasticity results\ they instead suggested that _bers were able to arrange themselves in relative position with each other on the tectum based on their own positional labels with! out the need for local tectal labels[ In this way\ _bers could form an expanded or compressed projection[ In order to explain Sperry|s original eye rotation result\ they acknowledge that some central cues were required to actively order _bers but that these were quite minimal[ Prior to the tectal rotation studies\ it was suggested that local cues in the optic pathway could provide orientation which could then be simply preserved by passive mech! anical guidance into tectum[ The tectum itself need not have any such cues[ The subsequent tectal rotation results required there be some information in tectum but this was postulated to be minimal[ Some sort of tectal cue providing polarity information was thought to be su.cient ð47Ł[

Evidence against strong chemoaf_nity This controversy spawned an incredible variety of cut and paste experiments in which various pieces of retina were removed and tectum variously ablated or trans! planted ð17\ 038Ł[ Although basically many of these con! stituted various examples of expansion and compression\ there were several that were critical to the controversy[ A major problem for the strong chemoa.nity hypothesis came from experiments aimed at testing whether position dependent labels were undergoing regulation during expansion and compression[ Some of the _rst evidence actually came from Sperry|s lab[ It had been shown for gold_sh that when both the posterior half of the tectum and the temporal half of retina were removed at the same time\ the nasal half of retina would form a retinotopic projection across its inappropriate anterior half of tectum ð050Ł[ This was compatible with regulation since the half retina and half tectum could have both regulated[ However\ when the projection was examined ana! tomically using autoradiography ð70Ł\ it was found that the projection was strongly biased toward the posterior end[ Many _bers were piled up at the cut edge[ In contrast\ when only a temporal half retina was left\ the regenerated innervation was uniformly distributed across anterior tectum[ It was also shown that expansion could occur in gold_sh under conditions that precluded regu! lation[ When a subset of optic _bers was surgically diverted across the tectal midline into the opposite tectum\ they grew to their appropriate quadrant if host _bers were present ð73Ł[ If the host tectum were dener!

R[ L[ Meyer:Chemoa.nity

vated of its normal optic innervation\ however\ the de~ected _bers expanded ð71Ł[ Since retina was intact there was no opportunity for it to undergo size regulation of its positional labels[ Furthermore careful analysis of the projection that was formed when de~ected _bers and host _bers coinnervated one tectum showed that the pres! ence of the extra _bers caused a compression of the host _bers ð73Ł[ Since tectum was intact\ regulation was not possible[ And\ expansion of a half retina across an intact tectum was prevented by coinnervation from an intact eye ð096\ 001Ł[ Further studies in Xenopus also argued against regu! lation[ When double ventral compound eyes were made in frog\ _bers were shown to expand across tectum as had been observed with double nasal and double tem! poral eyes[ However\ optic _bers from these eyes selec! tively traversed the medial branch of the optic tract as appropriate for ventral _bers\ leaving the lateral branch unoccupied ð29\ 039Ł[ This indicated they had not regu! lated into whole retinas[ Additional evidence came from studies in which _bers from compound eyes were made to coinnervate a tectum with a normal eye[ Instead of forming an expanded projection\ optic _bers from the compound eye occupied only the appropriate half of tectum indicating they had not regulated ð031Ł[ The other major class of problematical results showed that _bers could make mistakes[ Anatomical tracing methods which were more selective and sensitive than those available to Sperry showed that when the optic nerve of adult lower vertebrates was severed some _bers traversed through the wrong part of the optic path and entered the incorrect region of tectum ð12\ 25\ 74\ 036Ł[ The magnitude of these errors could be quite large[ Fibers that normally terminated in the medial half of tectum\ for example\ could be seen to grow through the lateral branch of the optic tract and enter the lateral half of tectum[ This misbehavior ran counter to the inference from Attardi and Sperry|s work that _bers were strongly guided by chemoa.nity cues to strictly follow along their original path and trajectory to their original tectal targets[ On the other hand\ these errant _bers invariably ter! minated at their appropriate tectal position in spite of their highly abnormal entry route ð12\ 25\ 74\ 036Ł[ Some _bers that had incorrectly invaded the lateral half of tectum could be shown to turn medially to terminate in their correct medial position[ So although this result was not in agreement with the assertion of Attardi and Sperry that regenerating _bers followed their original pathways\ it did argue against mechanical guidance and thereby further supported chemoa.nity[

Evidence for chemoaf_nity and against the no tectal label hypothesis The proposition presented by Gaze that tectum had no position markers and had only minimal polarity markers quickly ran into trouble[ A critical test was done in gold!

858

_sh wherein pieces of tectum were transposed from anterior to posterior tectum ð47\ 054Ł[ Following regen! eration\ optic _bers were found to have tracked down the translocated pieces of tectum so as to again project onto their original piece of tectum[ Gaze ð47Ł was among the _rst to perform these experiments and quickly concluded that there were indeed positional labels in the tectum of adult gold_sh[ However\ he argued that these results from regeneration did not necessarily generalize to devel! opment[ It was possible\ he suggested\ that during devel! opment optic _bers form a retinotopic projection onto tectum without the bene_t of positional labels using mechanical guidance\ pathway cues and polarity indi! cators in tectum[ Once the projection had formed\ these _bers then somehow imparted their positional markers to the tectal cells[ When the optic nerve was cut in the adult animal\ _bers then used these developmentally derived tectal markers to locate their original tectal cells[ There were some immediate problems with the idea that tectal positional labels were absent in development[ To begin with\ this notion lacked a certain plausibility in that the tectal cells supposedly lacked labels when they were most needed\ during the initial formation of the projection and instead possessed these labels when optic _bers no longer required them\ after the projection had formed[ The function of the acquired labels was unclear[ The other di.culty was with the experimental data cited in support of this idea[ There was only one major devel! opmental study that o}ered any support and it was open to alternative interpretation[ The study in question was the often cited tectal rotation experiment done in develop! ing frog ð05Ł[ When entire midbrain was rotated in early frog embryos before the ingrowth of optic _bers\ the projection that eventually formed was found to be rotated in some animal and unrotated in others[ The _nding that it was sometimes unrotated was interpreted to mean that the orientation of the _bers as they invaded tectum deter! mined the polarity of the entire map[ As pointed out by Sperry|s group ð75\ 81Ł\ however\ there were no good markers to independently monitor tectal polarity or the survival of the transplanted tissue[ Therefore the possi! bility of embryonic regulation in which the tectal markers sometimes developed in normal orientation following these early surgeries could not be ruled out[ The other studies cited in support of the idea were far less direct in that they were regeneration studies\ not developmental studies and the _ndings were contra! dictory[ These supporting studies ð096\ 097Ł\ mostly in gold_sh\ were thought to show that optic _bers could relabel the tectum[ The main observation was that when half a retina was removed in a normal gold_sh and the optic nerve severed\ the initial projection was appro! priate\ that is\ not expanded[ If\ however\ the nerve was severed in a _sh in which expansion had already occurred\ the initial projection was expanded[ The interpretation was that _bers had relabeled the tectal cells[ Subsequent studies failed to con_rm these observations[ An expanded projection could initially form when a normal projection

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had previously existed ð80Ł[ Also\ contrary to the expec! tation that optic denervation would lead to a loss of tectal labels\ a gold_sh tectum could be deprived of optic _bers for 07 months and yet could be shown to retain position dependent labels ð89Ł[ Several developmental studies also provided clear evi! dence against the no tectal label hypothesis[ One of the earliest was a study of the developing retinotectal system of chick ð14Ł[ In this study\ various sectors of retina were removed and the innervation examined much as Attardi and Sperry had done in gold_sh[ However\ in the chick\ the retinal lesions were done in embryos before the ingrowth of optic _bers so there was no opportunity for _bers to label tectum[ Using autoradiography to trace optic _bers\ _bers were found to innervate their appro! priate part of tectum even bypassing inappropriate regions to do so[ These observations provided strong evidence that tectal labels existed when optic _bers _rst invaded tectum[ These experiments actually predated the hypothesis that no tectal labels existed in development and were basically ignored[ Similar experiments were later done in newborn hamsters when optic _bers were beginning to invade superior colliculus ð24Ł[ When large retinal lesions were made and the fate of the remaining optic _bers traced\ selective innervation of the appro! priate part of colliculus was observed[ The retinotopically incorrect regions were left uninnervated[ Again this poin! ted to pre!existing tectal labels and again this data was essentially ignored[ Equivalent experiments in developing frog were attempted but proved to be technically di.cult[ When retina was lesioned\ new retinal tissue quickly formed so as to make the experiment impossible[ Attempts to raise a frog that had no optic innervation of tectum by remov! ing one eye "{{virgin tectum||# were also frustrated by the tendency of _bers from the remaining eye to invade the denervated tectum[ In the few cases where virgin tecta were thought to be successfully made\ _bers from the remaining eye were then made to be regenerated into this virgin tectum and a retinotopically ordered projection was observed ð028Ł[ A safer alternative was discovered by Hunt who found that when he repeatedly rotated the eye during development\ the projection in the adult was scrambled\ lacking any retinotopic order "reported in a review\ ð23Ł#[ If he then removed the eye and allowed _bers from the opposite eye to regenerate into this tectum\ the projection was retinotopically ordered[ In some cases\ pieces of tectum were translocated to ectopic positions before diverting _bers into them and _bers were found to track down their appropriate part of tectum[ Later Bonhoe}er|s group ð8\ 09Ł developed in vitro assays in which chick optic _bers could distinguish between anterior and posterior tectal cells as predicted by tectal markers[ These assays worked equally well when the eye was removed prior to the ingrowth of optic _bers thereby depriving tectal cells of any labels derived from optic _bers[ Finally\ developmental studies aimed at examining the role of spatiotemporal factors in the formation of the

projection failed to support the proposition that orderly ingrowth of optic _bers was important for generating retinotopography[ Working in urodeles and Xenopus\ Harris and Holt ð38\ 45\ 46Ł and coworkers transplanted eyes into various ectopic locations forcing _bers to approach tectum through highly abnormal routes[ Or eyes or fragments of eyes were transplanted between di}erent aged animals to produce temporal mismatches[ In spite of these extreme disruptions of spatial and tem! poral factors\ the resulting projections were retin! otopically ordered as expected from tectal markers[

Chemoaf_nity vs plasticity\ Sperry|s response The upshot of all this work was something of a di! chotomy for chemoa.nity[ On the one hand\ chemoa.nity received further support and survived the threat of alternative hypotheses[ On the other hand\ plas! ticity became _rmly established and resisted the attempt ð029Ł to explain it away as a regulative alteration of chemoa.nity labels[ Sperry was disappointed that regu! lation did not turn out to be the explanation for plasticity[ This meant that the formation of ordered connections could not be entirely explained in terms of the exact chemoa.nity matching he had proposed in his strong version of the chemoa.nity hypothesis ð2\ 017\ 018Ł[ This had two adverse implications for the chemoa.nity theory[ One was that under some conditions there\ could be a violation of strict matching between chemoa.nity labels[ The other was that some processes besides chemoa.nity must play a role in the patterning of the projection[ These corollaries arguably diminished the importance of chemoa.nity in the sense that other factors beside chemoa.nity had to be postulated[ However\ Sperry maintained that chemoa.nity was still the primary explanation for the formation of orderly nerve connections including that of the retinotectal pro! jection[ He put forth his ideas in a short symposium report ð029Ł and in a co!authored review chapter ð84Ł[ At that time\ Sperry|s general view was that chemoa.nity determined most features of the reti! notectal projection such as pathway organization\ lami! nation\ class speci_c termination ðe[g[ color codingŁ and topography[ The various experimentally produced plas! ticities and projection mistakes were not incompatible with the existence of chemoa.nity labels[ Plasticity and pathway errors simply meant that chemoa.nity guidance and matching were not rigid and that other factors besides chemoa.nity were at work[ This was not a new suggestion but a rea.rmation of his original proposal[ It was mainly Sperry writing ð84Ł\ {{It has never been claimed that these cytochemical a.nities are exclusive or rigid or are the only factors regulating nerve growth and con! nections and it has repeatedly been emphasized that the exact way in which they are expressed can be expected to vary in detail in di}erent systems\ di}erent species\ growth stages and under di}erent pathological and exper!

R[ L[ Meyer:Chemoa.nity

imental conditions||[ He favored the view that the expan! sion and compression that were produced in gold_sh were a kind of pathology that the gold_sh was especially susceptible to since similar plasticities had not been seen in chick and tree frog[ The initial pathway errors made by some _bers during regeneration could also be explai! ned by the surgical disruption of tissue caused by severing the optic nerve since similar errors were not described for development[ {{In sum\ the plastic reorganizations that follow tectal lesions are not only compatible with chemoa.nity concepts\ but some of the observed e}ects such as the [ [ [ asymmetry of re!innervation following non!corresponding retinal and tectal lesions\ directly sup! port the idea of matching retinotectal preferences[ These remapping phenomena\ however\ do bring to light the presence of additional competitive!type factors that importantly determine _ber growth and connectivity|| ð84Ł[ The competition by _bers to _ll available target space could account for the observation that _bers from an intact eye squeezed into a half tectum or _bers from a half retina expanded across tectum but it was still necessary to explain how the projection maintained its retinotopic organization in a manner consistent with chemoa.nity[ Two suggestions for this were o}ered[ The one favored by Sperry himself was the {{nearest chemoa.nity match|| idea ð84\ 029Ł[ According to this\ _bers were initially directed to their appropriate tectal position by chemoa.nity and then under pressure from competition secondarily adjusted their position to the next best mat! ching position[ In the case of an anterior half tectum\ _bers normally terminating in the posterior half would be directed to the most posterior part of the half tectum[ Fibers normally terminating just posterior to the cut edge would be a close chemoa.nity match to those at the edge and so could easily terminate there displacing the existing _bers anteriorly[ This would be repeated until all _bers were accommodated[ Sperry further suggested that as compression occurred\ _bers would alter the chemoa.! nity labels on the tectal neurons so as to conform to that of the impinging _bers[ In this manner\ the _bers would progressively relabel tectal cells so that _bers would be actually terminating on their correct chemoa.nity match[ A similar suggestion was made for expansion[ "It should be noted that Sperry|s relabeling was a transient modulation of tectal labels acquired during development\ not the de novo acquisition of labels postulated by Gaze and others as described above[# This idea of relabeling was really Sperry|s "the co!author being far less enthusi! astic#[ He especially liked the idea that it nearly preserved strict chemoa.nity matching while accommodating plas! ticity[ In principle\ the projection from compound eyes could have been similarly explained although this was not expressly hypothesized because morphollactic regu! lation of labels "respeci_cation of positional labels so that each half eye was essentially a whole eye in terms of positional labels# had not yet been disproved for com! pound eyes[

860

The other explanation ð84Ł which was a reiteration of one I had made previously ð70Ł was that there were two chemoa.nity interactions at work[ There was the optic _ber to tectum a.nity as originally proposed[ And there was a selective _ber to _ber chemoa.nity such that _bers from the same retinal locus preferentially terminated with each other[ The _ber to tectum a.nity was responsible for generating general topographic order but these a.n! ities were not so strong as to preclude mismatches that were driven by a competition for synaptic sites following retinal or tectal lesions[ The _ber to _ber a.nity pre! served local order during the expansion and compression thereby producing a projection that was always reti! notopically ordered[ Although this was not Sperry|s _rst choice as an explanation since it somewhat reduced the role of _ber to target matching\ he did view it as a variant of chemoa.nity[ This was signi_cant in that it recognized the possibility that the matching between axons and tar! gets could vary by a considerable extent in response to other factors and that there could be instructive factors besides the _ber to target chemoa.nity matching[ At the time of that review ð84Ł\ all of the reported plasticities involved surgical interventions such as ablations\ transplantations and tissue trauma[ It was argued ð84Ł\ therefore\ that expansion and compression were surgically induced abnormalities since analogous plasticities were not observed during normal develop! ment[ Similarly\ the pathway errors observed by some regenerating _bers following section of the nerve could be explained as the pathological result of tissue damage to the nerve which could have destroyed chemoa.nity cues[ Again similar errors had not been seen in normal development[ Some reports of induced abnormal pro! jections in mammals had also appeared ð037Ł[ Most notable were the ectopic projections formed by optic _bers following lesions of colliculus in neonatal hamster[ From the perspective of chemoa.nity\ it was argued that these abnormal innervations were secondary choices forced upon _bers because their _rst target choice had been eliminated[ In summary\ the argument was made that these plasticities were pathological and served only to indicate the relative strengths of chemoa.nity pref! erences[ These _ndings could be viewed as being perfectly compatible with the proposal that normal growth and innervation was guided by chemoa.nity preferences[

The retinotectal work during Sperry|s philosophical years Sperry|s interest in the development of nerve con! nections had progressively decreased since the early 0859|s[ He had become much more interested in the split brain work and had increasingly turned to the philo! sophical questions concerning the nature of con! sciousness and basis of values[ He did not author any signi_cant publications on chemoa.nity since the above mentioned review chapters ð84\ 029Ł[ However\ work on

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the retinotectal system continued in his lab until the end of 0868 when the last of us "the author# left[ Sperry was an exceptionally generous supervisor in allowing students and postdocs who were supported by his grant to author articles without his name[ Although they did not bear his name\ Sperry was\ nevertheless\ very much involved with these articles[ He edited these manuscripts with such detail and scrutiny that producing a _nal version often meant seemingly endless revisions until he was satis_ed with every word[ Part of this arose from Sperry|s love of the English language which had been sparked in his undergraduate years of studying English literature[ But he also wanted anything to come out of his lab to be _rst quality\ to meet his standards and to uphold the reputation of the lab[ So although he did not himself publish much on chemoa.nity during this time\ the art! icles that did appear met with his approval[ He did help write many of them and agreed with their content[ These articles\ then\ as well as personal communications can be taken to re~ect his late thinking on chemoa.nity[ The major {{problem|| for strong chemoa.nity during this time was the emergence of evidence that plasticity could be part of normal development[ This meant that plasticity could not be dismissed as experimental path! ology[ The most dramatic data in the retinotectal system came from thymidine labeling studies on frog and gold! _sh[ These demonstrated that new neurons were added over an extended period of post embryonic growth[ Gaze|s lab showed that new cells were added to the mar! gins of retina and tectum of larval frog ð026\ 027Ł and Sperry|s ð72Ł and Easter|s labs ð56Ł similarly showed anal! ogous cell addition occurred in the gold_sh retinotectal system throughout their life[ The problem for chemoa.nity was that the pattern of cell addition was mismatched[ Retina added cells around its entire cir! cumference while tectum cells added neurons medially\ laterally and posteriorly but not at the anterior end[ This meant that new ganglion cells in temporal retina had no new tectal cells at the anterior end of tectum to terminate on[ The implication was that older optic _bers that were terminating at the anterior end had to move posteriorly to make room for the new temporal _bers so that the retinotopic organization of the projection would be pre! served[ These posteriorly shifted _bers would\ in turn\ displace more posterior _bers in cascade fashion so that the entire projection progressively shifted in the posterior direction[ Later studies in which the path of _bers was traced using HRP con_rmed this shift by showing that _bers from older retinal ganglion cells exhibited a pos! teriorly directed component to their path through tectum ð13\ 16Ł[ Other anatomical and electrophysiological stud! ies at various stages of growth also implicated a pos! teriorly directed shift ð30\ 72\ 094Ł[ The upshot of these {{shifting connections|| as they became known was that some retinotopic plasticity was a normal part of devel! opment[ At about this same time\ a number of developmental studies in mammals demonstrated axonal remodeling

during postnatal development[ There had been earlier studies demonstrating transient projections during devel! opment that were eliminated with maturation but this elimination was either known to be due to cell death or cell death could not be excluded ð53\ 038Ł[ These earlier studies\ therefore\ did not clearly contradict chemoa.! nity[ It was possible to argue that these transient pro! jections were developmental mistakes in which axons became misdirected along their pathway and as a result of the chemoa.nity mismatch in the target\ axons could not form connections and the cell died[ Or it could be argued that these transient projections were actually cor! rectly guided by chemoa.nity and that there was an independently controlled program for cell death that per! haps evolved to regulate cell numbers[ The advent of long!lived vital dyes\ however\ demonstrated that some transient projections were eliminated even though the neuron survived[ The most spectacular _ndings were obtained in rodent cortex where it was shown that some pyramidal cells extended axonal collaterals into con! tralateral cortex or into the spinal cord at early postnatal times and then subsequently eliminated these {{incorrect|| projections while retaining their mature projections ð4\ 52\ 54\ 88Ł[ Similar _ndings were later obtained from the mammalian visual system where optic _bers initially formed large arbors that extended into the wrong lamina of lateral geniculate and then retracted the incorrectly projecting part of the arbor ð002Ł[ These and other exam! ples of transient projections clearly meant that _bers were not always rigidly guided to their _nal destination and that considerable rearrangements could take place as part of normal development[ Fiber rearrangement could not be dismissed as just pathology[ These and similar discoveries prompted some to ques! tion chemoa.nity as a general model for the development of the nervous system and to conclude that the rearrange! ment of connections was a general feature of develop! ment[ Axonal rearrangement was proposed as an alter! native to chemoa.nity to explain the formation of orderly connections[ This proposal was\ in a fundamental sense\ not really an alternative to chemoa.nity[ Whereas chemoa.nity provided a mechanistic explanation for the formation of connections at the cellular and molecular levels\ _ber rearrangement did not[ The {{rearrangement hypothesis|| was really more of an account of what the axons did rather than an explanatory model for the mech! anism by which they did it[ Nevertheless\ it was often viewed as an alternative explanation for the formation of connections[ There were two other aspects of the {{rearrangement hypothesis|| that limited its viability as a general expla! nation for the formation of connections and as being an alternative mechanism to chemoa.nity[ First\ it did not address the many examples of the initial selectivity of growing _bers[ In the development of the visual system\ for example\ growing _bers showed a strong preference for optic pathways leading to the visual centers and within the visual centers showed a strong preference for

R[ L[ Meyer:Chemoa.nity

the appropriate visual lamina or nuclei ð10\ 002\ 038Ł[ There were few examples of optic _bers straying beyond the con_nes of the visual pathways and target regions[ In fact\ most systems showed considerable initial order[ The extensive rearrangements seen in projecting neurons in cortex were not typical of the large majority of systems[ Second\ in the cases where _ber rearrangements occurred\ this rearrangement took place within an ordered frame! work[ The initial projections were far from random[ Cortical neurons that sent axons to contralateral cortex and then withdrew them were a case in point ð4\ 52\ 54 58Ł[ These axons grew into callosum and on into the homonymous position and appropriate layer of con! tralateral cortex[ Their growth was evidently precisely guided as predicted from chemoa.nity[ The bottom line was that rearrangement could be considered a phenom! enon that occurred in addition to chemoa.nity rather than as a replacement for chemoa.nity[ During the early 0879|s convincing evidence emerged for another hypothesis that was more of an alternative to chemoa.nity in that it entailed a cellular mechanism] activity mediated ordering of connections[ The idea that neural activity could pattern connections was hardly new[ It had been discussed at the time Sperry did his seminal work in the 0839|s and was acknowledged to be a con! tributing factor ð014Ł[ There had also been a substantial history of evidence for it[ Most notable was the work of Hubel and Weisel who showed the response properties of neurons in the visual cortex of cat and monkey could be altered by visual experience ð59\ 047\ 048Ł[ Occluding one eye of a kitten\ for example\ led to a reduction in the number of cells that could be driven by the deprived eye ð047Ł[ Many similar studies ensued[ It was not clear from these earlier studies\ however\ that nerve connections were actually being patterned by activity[ Many of the observations could be interpreted to mean that synaptic e}ectiveness was being altered but not the anatomical pattern of connectivity[ A _nding in support of synaptic e}ectiveness was that in monocularly reared cats\ the deprived eye could be made to suddenly drive cortical neurons within minutes of removing the good eye ð57Ł[ Similar interpretations could be made of studies in which cats were reared viewing stripes of one orientation and were subsequently found to have a predominance of cort! ical cells that preferentially responded to the same orien! tation ð6\ 44Ł[ These results could be viewed as a selective deprivation of one orientation so that cells of that orien! tation were no longer being e}ectively driven[ The fact that orientation selective cells could be recorded during the early development of cats deprived of all visual experi! ence ð7\ 01Ł argued that they did not require any instruc! tive role of visual experience and supported the interpretation of selective deprivation[ There were some anatomical correlates of experience!dependent changes but their interpretation was not entirely clear[ In the case of unilateral monocular deprivation in cats\ the cortical columns corresponding to the deprived eye were smaller than those from the good eye ð003Ł[ It could be argued

862

that activity was simply needed to develop healthy neurons so that neurons deprived of activity were abnormal[ The critical issue was whether activity was truly instructive for the patterning of connectivity[ A number of studies in both mammals and lower vertebrates added convincing evidence that activity could be instructive[ In lower vertebrates\ each eye normally innervates only the contralateral tectum[ When two eyes were surgically induced to coinnervate the same tectum in frog or gold! _sh\ optic _bers from each eye formed non!overlapping ocular dominance columns ð11\ 65\ 73Ł[ The only obvious way to explain this in terms of chemoa.nity would be to suppose that there are eye!speci_c labels for each eye[ However\ this unlikely idea was ruled out by the _nding that {{compound|| eyes in which each half of retina pro! jects across the entire tectum form segregated columns between each half of retina ð20\ 51Ł[ Positional and timing cues were also ruled out[ In the case of gold_sh\ it is possible to make _bers from both eyes regenerate into one tectum simultaneously ð77Ł[ These _bers were observed to overlap prior to segregating thus indicating that seg! regation was an active process[ Similar observations were obtained for the mammalian visual pathway[ In striate cortex\ geniculocortical _bers corresponding to each eye initially formed overlapping projections prior to forming dominance columns ð63\ 091Ł[ The question became if the formation of ocular domi! nance columns could not be explained in terms of chemoa.nity\ di}erential timing or positional factors\ what was the instructive cue that then could distinguish _bers from each eye so that they would {{know|| to seg! regate< The only plausible mechanism appeared to be di}erential impulse activity[ The case for activity was strongly supported by experiments in which retinal activity was eliminated with TTX[ In this case\ the for! mation of columns was completely inhibited in lower vertebrates ð00\ 76\ 094Ł and in mammals ð033Ł for as long as impulse blockade was maintained[ For lower vertebrates\ this _nding begged the question of what the normal role of activity is[ Optic _bers normally only projected to one tectum and consequently did not nor! mally form ocular dominance columns[ To answer this\ activity was blocked during optic nerve regeneration from one eye into one tectum in gold_sh ð77\ 098Ł[ Fibers grew into tectum and formed a rough retinotopic pro! jection but failed to form _ne retinotopy for as long as activity was blocked[ This result suggested that while chemoa.nity might be responsible for guiding _bers to the general retinotopically correct position of tectum\ their precise positioning was mediated by impulse activity[ The kind of activity needed was suggested by much earlier electrophysiological studies on retinal gan! glion cells[ These showed that neighboring retinal gan! glion cells with overlapping receptive _elds had correlated spontaneous activity while retinal ganglion cells that were further apart did not ð0\ 1Ł[ This led to the proposition that _bers that _red together\ stayed together\ thereby

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generating _ne retinotopography from the cruder top! ography generated by chemoa.nity[ The hypothesis that correlated activity can be an instructive determinant for the patterning of nerve con! nections is widely accepted as the explanation for the formation of _ne retinotopy in the retinotectal projection of frog and gold_sh\ the segregation of optic _bers into eye!speci_c lamina of geniculate and the generation of ocular dominance columns in mammalian cortex and in dually innervated tectum of lower vertebrates ð81\ 009\ 002\ 038Ł[ The activity dependent ordering\ however\ was not a general alternative to chemoa.nity[ In those sys! tems where activity was found to play a role\ it was that of {{_ne tuning|| in the later stages of the formation of connections[ Much of the patterning still required chemoa.nity[ In the retinotectal system\ for example\ chemoa.nity was presumably required to direct _bers to the optic pathway\ to the correct layer of tectum and to the roughly retinotopically appropriate position in tec! tum since all of this can occur in the absence of activity[ Likewise\ in visual cortex\ chemoa.nity would appear to be required to explain how geniculocortical _bers follow a stereotypic path to cortex\ selectively form synapses in layer IV and form a retinotopic projection[ In some lower vertebrate species such as axolotl ð38Ł\ no e}ect of impulse blockade on the precision of the projection could be detected[ Likewise for mammals\ some species do not exhibit ocular dominance columns[ Outside of the visual system\ the evidence that activity plays a major role in the anatomical patterning of connections is quite limited and in many cases contrary[ The available data indicates that activity is not responsible for generating som! atosensory connections such as whisker barrels or con! nections in the auditory system ð04\ 43Ł[

Chemoaf_nity] current status During the last decade\ a curious thing has happened to the chemoa.nity hypothesis] the argument over it has essentially disappeared[ The essential features of Sperry|s {{General Chemoa.nity Hypothesis|| are now broadly accepted[ To reiterate these are] "0# axons have di}er! ential markers\ "1# target cells "and pathways# have cor! responding markers\ "2# markers are the product of cellular di}erentiation\ "3# axonal growth is actively directed by markers to establish speci_c connections[ The question has changed from whether such labels might exist to explain directed growth and the formation of selective connections to what these labels actually are in molecular terms and how these labels are transduced into directed growth[ Although the word {{chemoa.nity|| is infrequently used to denote this explanation and Sperry is sometimes no longer cited\ this generally accepted idea is\ in essence\ Sperry|s chemoa.nity hypothesis[ There are a number of reasons for this broad accept! ance of the essential postulates of the chemoa.nity hypothesis[ One is that the circumstantial evidence for

label directed growth has become so strong as to become virtually irrefutable[ This evidence has come from both in vivo and in vitro studies[ The in vivo analysis of growing _bers has moved to ever greater levels of detail[ In part\ this has been made possible through technological advances in nerve tracing such as by the use of DiI\ more favorable preparations such as transparent zebra_sh larva and improved imaging techniques such as confocal microscope[ It has also been the consequence of pressing forward with ever more re_ned studies[ These studies have revealed many clear examples of actively directed growth strongly implicating that it is guided by axonal labels and molecular cues in their vicinity[ In the develop! ing zebra_sh\ as one example\ the outgrowth from ident! i_able early spinal neurons was analysed at the single cell level ð099Ł[ These neurons were seen to be located immediately adjacent to each other and to initiate process outgrowth at about the same time[ Yet the di}erent neu! rons invariably innervated speci_c regions of axial muscle[ Ablation of individual neurons prior to process extension had no e}ect on the pattern of innervation indicating that each neuron was di}erentially responding to local cues[ In vitro studies were no less persuasive[ The most notable of these were those of Bonhoe}er and colleagues on the chick retinotectal system ð8\ 09\ 023\ 038Ł[ They devised a number of methods to test for directed growth of optic _bers in response to tectal cues in culture[ They were able to demonstrate that membrane vesicles from anterior or posterior tectum were capable of directing the growth of nasal vs temporal _bers as predicted by chemoa.nity[ Perhaps a more important explanation for the move toward chemoa.nity has been the mounting molecular and biochemical evidence for labels[ This more direct evidence has come from a number of systems using sev! eral di}erent approaches[ Using a molecular genetic approach in Drosophila\ a number of molecules such as the fasciclins and semaphorins have been identi_ed that are expressed in subsets of neurons and have been shown by de_ciency analysis and other genetic methods to be involved in selective fasciculation and other aspects of axonal navigation "reviewed in Goodman\ cf[ ð35Ł#[ Anal! ogous genetic studies have been conducted for nematode with similar results ð37\ 42\ 68Ł[ A large e}ort is underway in zebra_sh to identify guidance molecules for vertebrates ð86\ 004Ł and already a number of mutations have been found that have speci_c e}ects on the growth and guid! ance of optic and other axons ð58Ł[ In vertebrate spinal cord\ the monoclonal antibody approach has been used to identify a molecule that is expressed in commissural neurons just when they grow through ~oor plate and subsequent biochemical studies have identi_ed a mol! ecule made by ~oor plate that has chemotropic guidance activity for commissural neurons ð08Ł[ In the retinotectal system\ several monoclonal anti! bodies have been produced that identify molecular gradi! ents across retina and tectum along two axes as postulated by chemoa.nity ð19\ 79\ 090\ 034\ 035Ł[ In

R[ L[ Meyer:Chemoa.nity

chick tectum\ a molecule has been isolated that is dis! tributed along the anteriorÐposterior dimension which exhibits guidance activity for retinal axons in culture ð3Ł[ Recently\ two transcriptional regulators have been identi_ed that are distributed across the nasal temporal axis in a mutually exclusive fashion ð055Ł[ Misexpression of these regulators during development using retroviral vectors causes a disruption of the topographic dis! tribution of optic _bers in tectum[ This result represents a functional demonstration that the pattern of gene expression\ that is\ the position dependent di}erentiation of retinal cells\ determines the distribution of optic _bers as predicted by chemoa.nity[ Also recently\ a tyrosine kinase receptor of the Eph family\ has been found to be distributed in a gradient fashion in retinal ganglion cells and its cognate ligand has been shown to have a matching gradient distribution in tectum ð03\ 15Ł[ One of the tectal ligands\ ELF!0\ is most concentrated in posterior tectum[ Topographic misexpression of the ligand in chick tectum during development selectively disrupts the projection of optic _bers from temporal retina ð87Ł providing strong functional con_rmation that these cell surface molecules regulate targeting of optic _bers[ These Eph family mem! bers are the strongest current candidate for being one of Sperry|s chemoa.nity molecules he had postulated for the retinotectal system[ It is highly likely that the next decade will bring to light a host of other guidance mol! ecules that will bring further molecular de_nition to the chemoa.nity hypothesis[ There is an excellent chance that we will soon see the de_nitive identi_cation of the chemoa.nity molecules in the retinotectal system that Sperry postulated over 49 years ago[ In retrospect\ much of the argument over chemoa.! nity\ especially that in the retinotectal system\ was over details that did not greatly a}ect the general validity of the chemoa.nity hypothesis[ Sperry recognized this and it was one of the reasons for his declining interest in the _eld over time[ Sperry was really interested in the major issues\ {{the big picture||\ as he liked to refer to it[ He had the insight to crystallize the critical question and brilliance of mind to devise incisive experiments to answer the key question[ To him\ details were not so important[ This came home to me in a visit to his o.ce shortly before leaving his lab[ I was describing some of the experiments on the retinotectal system I was planning on doing in my new position when I recognized his characteristic glazed look that he got when he was being bored[ I stopped abruptly[ After a pregnant pause\ Sperry asked {{Why are you still working on this< I solved this problem years ago||[ He was right[

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References 0[ Arnett\ D[\ Statistical dependence between neigh! boring retinal ganglion cells in gold_sh[ Exp[ Brain Res[\ 0867\ 21\ 38Ð42[ 1[ Arnett\ D[ and Spraker\ T[\ Cross correlation analy!

08[ 19[

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sis of the maintained discharge of rabbit retinal[ ganglion cells[ J[ Physiol[ Lond[\ 0870\ 206\ 18Ð36[ Attardi\ D[ G[ and Sperry\ R[ W[\ Preferential selec! tion of central pathways by regenerating optic _bers[ Exp[ Neurol[\ 0852\ 6\ 35Ð53[ Baier\ H[ and Bonhoe}er\ F[\ Axon guidance by gradients of a target!derived component[ Science\ 0881\ 144\ 361Ð364[ Bates\ C[ and Killackey\ H[\ The emergence of a discretely distributed pattern of coticospinal pro! jection neurons[ Dev[ Brain Res[\ 0872\ 02\ 154Ð162[ Beers\ D[ Return of vision and other observations in transplanted amphibian eyes[ Proc[ Soc[ Exp[ Biol[\ N[Y[\ 0818\ 15\ 366Ð368[ Blakemore\ C[ and Cooper\ G[\ Development of the brain depends on visual environment[ Nature\ Lond[\ 0869\ 117\ 366Ð367[ Blakemore\ C[ and Van Sluyters\ R[\ Innate and environmental factors in the developmental of the kitten|s visual cortex[ J[ Physiol[\ 0864\ 137\ 552Ð 605[ Bonhoe}er\ F[ and Huf\ J[\ In vitro experiments on axon guidance demonstrating an anterior!posterior gradient on the tectum[ EMBO\ 0871\ 0\ 316Ð320[ Bonhoe}er\ F[ and Huf\ J[\ Position!dependent properties of retinal axons and their growth cones[ Nature\ 0874\ 204\ 398Ð309[ Boss\ V[ C[ and Schmidt J[ T[\ Activity and the formation of ocular dominance patches in dually innervated tectum of gold_sh[ J[ Neurosci[\ 0873\ 3\ 1780Ð1894[ Buisseret\ P[ and Imbert M[\ Visual cortical cells] Their developmental properties in normal and dark! reared kittens[ J[ Physiol[ "Lond[#\ 0865\ 144\ 400Ð 414[ Bunt\ S[ M[\ Retinotopic and temporal organization of the optic nerve and tracts in the adult gold_sh[ J[ Comp[ Neurol[\ 0871\ 195\ 198Ð115[ Cheng\ H[!J[\ Nakamoto\ M[\ Bergeman\ A[ and Flanagan\ J[\ Complementary gradients in expression and binding of ELF!0 and Mek3 in development of the topographic retinotectal pro! jection map[ Cell\ 0884\ 71\ 260Ð270[ Chiaia\ N[\ Fish\ S[\ Bauer\ W[\ Figley\ B[\ Bennett! Clarke\ C[ and Rhoades\ R[\ E}ects of postnatal blockade of cortical activity with tetrodotoxin upon lesion!induced reorganization of vibrissae!related patterns in the somatosensory cortex of rat[ Dev[ Brain Res[\ 0883\ 06\ 290Ð295[ Chung\ S[ H[ and Cooke\ J[\ Observations on the formation of the brain and of nerve connections following embryonic manipulations of the amphib! ian neural tube[ Proc[ R[ Soc[ Lond[ B[\ 0867\ 190\ 224Ð262[ Coghill\ G[\ Anatomy and the Problem of behavior[ Cambridge University Press\ Cambridge\ 0818[ Coghill\ G[\ Individuation vs integration in the development of behavior[ J[ `en[ Psychol[\ 0829\ 2\ 320Ð324[ Colamarino\ S[ and Tessier!Lavigne\ M[\ The role of the ~oor plate in axon guidance[ Ann[ Rev[ Neuro! sci[\ 0884\ 07\ 386Ð418[ Constantine!Paton\ M[\ Blum\ A[\ Mendez!Oero\

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R[ and Barnstable\ C[\ A cell surface molecule dis! tributed in dorsalventral gradient in the perinatal rat retina[ Nature\ 0875\ 213\ 348Ð351[ Constantine!Paton\ M[\ Cline\ H[ and Debski\ E[\ Patterned activity\ synaptic convergence and the NMDA receptor in developing visual pathways[ Annu[ Rev[ Neurosci[\ 0889\ 02\ 018Ð043[ Constantine!Paton\ M[ and Law\ M[ I[\ Eye!speci_c termination bands in tecta of three!eyed frogs[ Science\ 0867\ 191\ 528Ð530[ Cook\ J[ E[\ Errant optic axons in the normal gold! _sh retina reach retinotopic tectal sites[ Brain Res[\ 0871\ 149\ 043[ Cook\ J[ E[\ Rankin\ E[ C[ and Stevens\ H[ P[\ A pattern of optic axons in the normal gold_sh tectum consistent with the caudal migration of optic ter! minals during development[ Exp[ Brain Res[\ 0872\ 41\ 036Ð040[ Crossland\ W[ J[\ Cowan\ W[ M[\ Rogers\ L[ A[ and Kelly\ J[ P[\ The speci_cation of the retino!tectal projection in the chick[ J[ Comp[ Neurol[\ 0863\ 044\ 016Ð053[ Drescher\ U[\ Kernoser\ C[\ Handwerker\ C[\ Los! chinger\ J[\ Noda\ M[ and Bonhoe}er\ F[\ In vitro guidance of retinal ganglion cell axons by RAGS\ a 14 kDa tectal protein related to ligands for Eph receptor tyrosine kinases[ Cell\ 0884\ 71\ 248Ð269[ Easter\ S[ and Stuermer\ C[\ An evaluation of the hypothesis of shifting terminals in the gold_sh optic tectum[ J[ Neurosci[\ 0873\ 3\ 0941Ð0952[ Edds\ M[\ Gaze\ P\\ Schneider\ G[ and Irwin\ L[\ Speci_city and plasticity of retinotectal connec! tions[ Neuosciences Research Pro`ram Bulletin\ 0868\ 06\ 132Ð264[ Fawcett\ J[ W[\ Retinotopic maps\ cell death and electrical activity in the retinotectal projections[ In The Makin` of the Nervous System\\ eds[ J[ Parna! velas\ C[ Stern and R[ Stirling[ Oxford University Press\ Oxford\ 0877\ pp[ 284Ð305[ Fawcett\ J[ W[ and Gaze\ R[ M[\ The retinotectal _bre pathways from normal and compound eyes in Xenopus[ J[ Embryol[ Exp[ Morph[\ 0871\ 61\ 08Ð 26[ Fawcett\ J[ W[ and Willshaw\ D[ J[\ Compound eyes project stripes on the optic tectum in Xenopus[ Nature\ 0871\ 185\ 249Ð240[ Finger\ S[\ Ori`ins of neuroscience[ Oxford Uni! versity Press\ New York\ 0883[ Franz\ S[ Conceptions of cerebral functions[ Psycholo`ical Rev[\ 0812\ 29\ 327Ð335[ Fraser\ S[ E[ and Hunt\ R[ K[\ Retinotectal speci! _city] models and experiments in search of a map! ping function[ Ann[ Rev[ Neurosci[\ 0879\ 2\ 208Ð 241[ Frost\ D[ O[ and Schneider\ G[ E[\ Plasticity of retinofugal projections after partial lesions of the retina in newborn syrian hamster[ J[ Comp[ Neurol[\ 0868\ 074\ 406Ð457[ Fujisawa\ H[\ Retinotopic analysis of _ber path! ways in the regenerating retinotectal system of the adult newt Cynops pyrrhogaster[ Brain Res[\ 0870\ 195\ 16Ð26[ Gaze\ R[ M[\ The Formation of Nerve Connections[ Academic Press\ London\ 0869[

27[ Gaze\ R[ M[\ Jacobson\ M[ and Szekely\ G[\ The retinotectal projection in Xenopus with compound eyes[ J[ Physiol[\ 0852\ 054\ 373Ð388[ 28[ Gaze\ R[ M[\ Jacobson\ M[ and Szekely\ G[\ On the formation of connexions by compound eyes in Xenopus[ J[ Physiol[\ 0854\ 065\ 398Ð306[ 39[ Gaze\ R[ M[ and Keating\ M[ J[\ Further studies on the restoration of the contralateral retinotectal projection following regeneration of the optic nerve in the frog[ Brain Res[\ 0869\ 10\ 072Ð084[ 30[ Gaze\ R[ M[\ Keating\ M[ J[ and Chung\ S[ H[\ The evolution of the retinotectal map during develop! ment in Xenopus[ Proc[ R[ Soc[ Lond[ B[\ 0863\ 074\ 290Ð229[ 31[ Gaze\ R[ M[ and Shama\ S[ C[\ Axial di}erences in the re!innervation of the gold_sh optic tectum by regenerating optic nerve _bers[ Exp[ Brain Res[\ 0869\ 09\ 060Ð070[ 32[ Gaze\ R[ M[ and Straznicky\ K[\ Regeneration of optic nerve _bres from a compound eye to both tecta in Xenopus] Evidence relating to the state of speci_cation of the eye and tectum[ J[ Embryol[ Exp[ Morphol[\ 0879\ 59\ 014Ð039[ 33[ Goldstein\ K[\ The Or`anism[ American Book Com! pany\ New York\ 0828[ 34[ Golgi\ C[\ Sulla struttara della grigia del cervello[ Gazetta Medica Italiani Lombardi\ 0762\ 5\ 133Ð135[ 35[ Goodman\ C[\ Mechanisms and molecules that con! trol growth cone guidance[ Ann[ Rev[ Neurosci[\ 0885\ 08\ 230Ð266[ 36[ Grimm\ L[\ An evaluation of myotypic respeci! _cation in axolotls[ J[ Exp[ Neurol[\ 0860\ 067\ 368Ð 385[ 37[ Hamelin\ M[\ Zhou\ Y[\ Su\ M[\ Scott\ I[ and Culotti\ J[\ Expression of the UNC!4 guidance receptor in the touch neurons of C[ ele`ans\ steers their axons dorsally[ Nature\ 0882\ 253\ 216Ð229[ 38[ Harris\ W[ A[\ Axonal path_nding in the absence of normal pathways and impulse activity[ J Neurosci[\ 0873\ 3\ 0042Ð0051[ 49[ Harrison\ R[\ The outgrowth of the nerve _ber as a mode of protoplasmic movement[ J[ Exp[ Zool[\ 0809\ 8\ 676Ð735[ 40[ Harrison\ R[\ On the origin and development of the nervous system studied by the methods of exper! imental embryology[ Proc[ Roy[ Soc[ London\ ser B\ 0824\ 007\ 044Ð085[ 41[ Head\ H[\ Studies in Neurolo`y[ Hodder and Stroughton\ London\ 0819[ 42[ Hedgecock\ E[\ Culotti\ J[ and Hall\ D[\ The unc! 4\ unc!5 and unc!39\ genes guide circumferential migrations of pioneer axons and mesodermal cells in the epidermis in C[ ele`ans[ Neuron\ 0889[ 1\ 50Ð 74[ 43[ Henderson\ T[\ Woolsey\ T[ and Jacquin\ M[\ Intraorbital nerve blockade from birth does not disrupt central trigeminal pattern formation in the rat[ Brain Res[\ 0881\ 55\ 035Ð041[ 44[ Hirsch\ H[ and Spinelli\ D[\ Visual experience modi! _es distribution of horizontally and vertically ori! ented receptive _elds in cats[ Science\ 0869\ 057\ 758Ð760[ 45[ Holt\ C[ E[\ Does timing of axon outgrowth in~u!

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ence initial retinotectal topography in Xenopus< J[ Neurosci[\ 0873\ 3\ 0029Ð0041[ Holt\ C[ E[ and Harris\ W[ A[\ Order in the initial retinotectal map in Xenopus] a new technique for labelling growing nerve _bers[ Nature\ 0872\ 290\ 049Ð041[ Hope\ R[ A[\ Hammond\ B[ J[ and Gaze\ R[ M[\ The arrow model] retinotectal speci_city and map formation in the gold_sh visual system[ Proc[ R[ Soc[ Lond[ B\ 0865\ 083\ 336Ð355[ Horder\ T[ J[ and Martin\ K[ A[ C[\ Morphogenctics as an alternative to chemospeci_city in the for! mation of nerve connections[ In Cell!cell Reco`! nition[ Soc[ for Exp[ Biol[ Symp[\ Vol 21\ ed[ A[ S[ G[ Curtis[ Cambridge University Press\ Cambridge\ 0867\ pp[ 164Ð247[ Hubel\ D[ and Wiesel\ T[\ Binocular interaction in striate cortex of kittens reared with arti_cial squint[ J[ Neurophysiol[\ 0854\ 17\ 0930Ð0948[ Hunt\ R[ and Cowan\ W[\ The chemoa.nity hypothesis] an appreciation of Roger W[ Sperry|s contributions to developmental biology[ In Brain Circuits and Functions of the Mind[ Essays in Honor of R[ W[ Sperry\\ ed[ C[ Trevarthen[ Cambridge University Press\ Cambridge\ 0889\ pp[ 08Ð63[ Ide\ C[ F[\ Fraser\ S[ E[ and Meyer\ R[ L[\ Eye dominance columns from an isogenic double!nasal frog eye[ Science\ 0872\ 110\ 182Ð184[ Innocenti\ G[\ Growth and reshaping of axons in the establishment of visual callosal connections[ Science\ 0870\ 101\ 713Ð716[ Innocenti\ G[ Loss of axonal projections in the development of the mammalian brain[ In The Mak! in` of the Nervous System\ eds[ J[ Parnavelas\ C[ Stern and R[ Stirling[ Oxford University Press\ Oxford\ 0877\ pp[ 208Ð228[ Ivy\ G[ O[ and Killackey\ H[ P[\ Ontogenic changes in the projections of neocortical neurons[ J[ Neuro! sci[\ 0871\ 5\ 624Ð632[ Jacobson\ M[ and Gaze\ P[ M[\ Selection of appro! priate tectal connections by regenerating optic nerve _bers in adult gold_sh[ Exp[ Neurol[\ 0854\ 02\ 307Ð 329[ Johns\ P[ R[\ Growth of the adult gold_sh eye III[ Source of the new cell[ J[ Comp[ Neurol[\ 0867\ 065\ 232Ð247[ Kratz\ K[ E[\ Spear\ P[ D[ and Spear\ D[ C[\ Post critical!period reversal of e}ects of monocular deprivation on striate cortex cells in the cat[ J Neu! rophysiol[\ 0865\ 28\ 490Ð400[ Kuwada\ J[\ Development of the zebra_sh nervous system] genetic analysis and manipulation[ Curr[ Opin[ Neurobiol[\ 0884\ 4\ 49Ð43[ Langley\ J[\ Note on regeneration of pre!ganglionic _bres of the sympathetic[ J[ Physiol[\ 0784\ 07\ 179Ð 173[ Langley\ J[\ On the regeneration of pre!ganglionic and post!ganglionic visceral nerve _bers[ J[ Physiol[\ 0786\ 11\ 104Ð129[ Lashley\ K[\ Brain Mechanisms and Intelli`ence[\ University of Chicago Press\ Chicago\ 0818[ Lettvin\ J[\ Maturana\ H[\ McCulloch\ W[ and Pitts\ W[\ What the frog|s eye tells the frog|s brain[ Proc[ I[ R[ E[\ 0848\ 36\ 0839Ð0840[

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63[ LeVay\ S[\ Stryker\ M[ P[ and Shatz\ C[ J[\ Ocular dominance columns and their development in layer IV of the cat|s visual cortex[ J[ Comp[ Neurol[\ 0867\ 068\ 112Ð133[ 64[ Levine\ R[ L[ and Jacobson\ M[\ Deployment of optic nerve _bers is determined by positional mar! kers in the frog|s tectum[ Exp[ Neurol[\ 0863\ 416Ð 427[ 65[ Levine\ R[ L[ and Jacobson\ M[\ Discontinuous mapping of retina onto tectum innervated by both eyes[ Brain Res[\ 0864\ 87\ 061Ð065[ 66[ Matthey\ R[\ La gre} de l|oeil[ Etude experimentale de la gre}e de l|oeil chez le triton[ Arch[ f Entw[ Mech[ d[ Or`an\ 0815\ 098\ 215Ð230[ 67[ Maturana\ H[ R[\ Lettvin\ J[ Y[\ McCulloch\ W[ S[ and Pitts\ W[ H[\ Physiological evidence the cut optic nerve _bers in a frog regenerate to their proper places in the tectum[ Science\ 0848\ 029\ 0698Ð0609[ 68[ McIntire\ S[\ Garriga\ G[\ White\ J[\ Jacobson\ D[ and Horvitz\ H[\ Genes necessary for directed axonal elongation or fasciculation in C[ ele`ans[ Neuron\ 0881\ 7\ 296Ð211[ 79[ McLoon\ S[\ A monoclonal antibody that dis! tinguishes between temporal and nasal retinal axons[ J[ Neurosci[\ 0880\ 00\ 0369Ð0366[ 70[ Meyer\ R[ L[\ Tests for _eld regulation in the reti! notectal system of gold_sh[ In Developmental Biolo`y\ Pattern Formation\ Gene Re`ulation\ ed[ D[ McMahon[ W[ A[ Benjamin\ Menlo Park\ 0864\ pp[ 146Ð164[ 71 Meyer\ R[ L[\ De~ection of selected optic _bers into a denervated tectum in gold_sh[ Brain Research\ 0867\ 044\ 102Ð116[ 72 Meyer\ R[ L[\ Evidence from thymidine labelling for continuing growth of retina and tectum in juvenile gold_sh[ Exp[ Neurol[\ 0867\ 48\ 88Ð000[ 73[ Meyer\ R[ L[\ {{Extra|| optic _bers exclude normal _bers from tectal regions in gold_sh[ J[ Comp[ Neurol[\ 0868\ 072\ 772Ð891[ 74[ Meyer\ R[ L[\ Mapping the normal and regenerating retinotectal projection of gold_sh with auto! radiographic methods[ J[ Comp[ Neurol[\ 0879\ 078\ 162Ð178[ 75[ Meyer\ R[ L[\ Ordering of retinotectal connections] a multivariate operational analysis[ Curr[ Top[ Dev[ Biol[\ 0871a\ 06\ 090Ð034[ 76[ Meyer\ R[ L[\ Tetrodotoxin blocks the formation of ocular dominance columns in gold_sh[ Science\ 0871b\ 107\ 478Ð480[ 77[ Meyer\ R[ L[\ Tetrodotoxin inhibits the formation of re_ned retinotopography in gold_sh[ Brain Res[\ 0872\ 5\ 182Ð187[ 78[ Meyer\ R[ L[\ The growth and formation of ocular dominance columns by de~ected optic _bers in gold_sh[ Dev[ Brain Res[\ 0872\ 5\ 168Ð180[ 89[ Meyer\ R[ L[\ Target selection by surgically mis! directed optic _bers in the tectum of gold_sh[ J[ Neurosci[\ 0873 3\ 123Ð149[ 80[ Meyer\ R[ L[\ Tests for relabelling the gold_sh tec! tum by optic _bers[ Dev[ Brain Res[\ 0876\ 20\ 201Ð 207[ 81[ Meyer\ R[ L[\ The case for chemoa.nity in the retinotectal system] recent studies[ In Brain Circuits

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and Functions of the Mind[ Essays in Honor of R[ W[ Sperry\ ed[ C[ Trevarthen[ Cambridge University Press\ Cambridge\ 0889\ pp[ 090Ð012[ Meyer\ R[ L[ and Sperry\ R[ W[\ Tests for neu! roplasticity in the anuran retinotectal system[ Exp[ Neurol[\ 0862\ 39\ 414Ð428[ Meyer\ R[ L[ and Sperry\ R[ W[\ Explanatory mod! els for neuroplasticity in retinotectal connections[ In Plasticity and Recovery of Function in the Central Nervous System\ eds[ D[ G[ Stein\ J[ J[ Rosen and N[ Butters[ Academic Press\ New York\ 0863\ pp[ 34Ð52[ Meyer\ R[ L[ and Sperry\ R[ W[\ Retinotectal speci! _city] Chemoa.nity theory[ In Studies on the Devel! opment of Behavior and the Nervous System[ Vol[ 2] Neural and Behavioral Speci_city\ ed[ G[ Gottlieb[ Academic Press\ New York\ 0865\ pp[ 000Ð038[ Miner\ N[\ Cutaneous localization following 079 degree rotation of skin grafts[ Anat[ Rec[\ 0840\ 098\ 215Ð216[ Mullins\ M[\ Hammerschmidt\ M[\ Ha/er\ P[ and Nusselin!Volhard\ C[\ Large!scale mutagenesis in the zebra_sh] In search of genes controlling devel! opment in a vertebrate[ Curr[ Opin[ Biol[\ 0883\ 3\ 078Ð191[ Nakamoto\ M[\ Cheng\ H[!J[\ Friedman\ G[ C[\ McLaughlin\ T[\ Hansen\ M[ J[\ Yoon\ C[ H[\ O|Leary\ D[ D[ M[ and Flanagan\ J[ G[\ Topo! graphically speci_ed e}ects of ELF!0 on retinal axon guidance in vitro and retinal axon mapping in vivo[ Cell\ 0885\ 75\ 644Ð655[ O|Leary\ D[\ Stan_eld\ B[ and Cowan\ W[\ Evidence that the early postnatal restriction of the cells of origin of the callosal projection is due to the elim! ination of axonal collaterals rather than to the death of neurons[ Dev[ Brain Res[\ 0870\ 0\ 596Ð506[ Pike\ S[ H[ and Eisen\ J[ S[\ Identi_ed primary motoneurons in embryonic zebra_sh select appro! priate pathways in the absence of other primary motoneurons[ J[ Neurosci[\ 0889\ 09\ 33Ð38[ Rabacchi\ S[\ Neve\ R[ and Drager\ U[\ A positional marker for the dorsal embryonic retina is hom! ologous to the high!a.nity laminin receptor[ Devel! opment\ 0889\ 098\ 410Ð420[ Rakic\ P[\ Prenatal genesis of connections sub! serving ocular dominance in the rhesus monkey[ Nature\ 0865\ 150\ 356Ð360[ Ramon y Cajal\ S[\ Sur l|origine et les rami_cations des _bres nerveuses de la moelle embryonaire[ Anat[ Anz[\ 0789\ 4\ 000Ð008[ Ramon y Cajal\ S[\ Die Retina der Wirbelthiere[ Thomas 0861\ Spring_eld\ 0783[ Reh\ T[ and Constantine!Paton\ M[ Retinal ganglion cell terminals change their projection sites during larval development of Rana pipiens[ J[ Neu! rosci[\ 0873\ 3\ 331Ð046[ Reh\ T[ and Constantine!Paton\ M[\ Eye!speci_c segregation requires neural activity in three!eyed Rana pipiens[ J[ Neurosci[\ 0874\ 4\ 0021Ð0032[ Schmidt\ J[ T[\ Retinal _bers alter tectal positional markers during the expansion of the retinal pro! jection in gold_sh[ J[ Comp[ Neurol[\ 0867\ 066\ 168Ð299[

097[ Schmidt\ J[ T[\ Cicerone\ C[ M[ and Easter\ S[ S[\ Expansion of the half retinal projection to the tec! tum in gold_sh] an electrophysiological and ana! tomical study[ J[ Comp[ Neurol[\ 0867\ 066\ 146Ð 167[ 098[ Schmidt\ J[ T[ and Edwards\ D[ L[\ Activity sharpens the map during the regeneration of the retinotectal projection in gold_sh[ Brain Res[\ 0872\ 158\ 18Ð28[ 009[ Schmidt\ J[ T[ and Tieman\ S[ B[\ Eye!speci_c seg! regation of optic a}erents in mammals\ _sh and frog] The role of activity[ Cell[ Mol[ Neurobiol[\ 0874\ 4\ 4Ð23[ 000[ Scholes\ J[ Nerve _ber topography in the retinal projection to the tectum[ Nature\ 0868\ 167\ 519Ð 513[ 001[ Sharma\ S[ C[ and Tung\ Y[ L[\ Interactions between nasal and temporal hemiretinal _bers in adult gold_sh tectum[ Neuroscience\ 0868\ 3\ 002Ð 008[ 002[ Shatz\ C[ J[ Impulse activity and the patterning of connections during CNS development[ Neuron\ 4\ 634Ð645[ 003[ Shatz\ C[ J[ and Stryker\ M[ P[\ Ocular dominance in layer IV of the cat|s visual cortex and the e}ects of monocular deprivation[ J[ Physiol[\ 0867\ 170\ 156Ð172[ 004[ Solnica!Krezel\ L[\ Schier\ A[ and Driever\ W[\ E.cient recovery of ENU!induced mutations from the zebra_sh germline[ Genetics\ 0883\ 025\ 0390Ð 0319[ 005[ Sperry\ R[ W[\ The functional results of muscle transposition in the hind limb of the rat[ J[ Comp[ Neurol[\ 0839\ 62\ 268Ð393[ 006[ Sperry\ R[ W[\ The e}ect of crossing nerves to antagonistic muscles in the hind limb of the rat[ J[ Comp[ Neurol[\ 0830\ 64\ 0Ð08[ 007[ Sperry\ R[ W[\ Transplantation of motor nerve and muscles in the forelimb of the rat[ J[ Comp[ Neurol[\ 0831\ 65\ 172Ð210[ 008[ Sperry\ R[ W[\ E}ect of 079 degree rotation of the retinal _eld on visuomotor coordination[ J[ Expl[ Zool[\ 0832a\ 81\ 152Ð168[ 019[ Sperry\ R[ W[\ Visuomotor coordination in the newt "Triturus viridescens#\ after regeneration of the opitc nerve[ J[ Comp[ Neurol[\ 0832b\ 68\ 22Ð44[ 010[ Sperry\ R[ W[\ Optic nerve regeneration with return of vision in anurans[ J[ Neurophysiol[\ 0833\ 6\ 46Ð 58[ 011[ Sperry\ R[ W[\ The problem of central nervous re! organization after nerve regeneration and muscle transposition\ Quart[ Rev[ Biol[\ 0834a\ 19\ 200Ð258[ 012[ Sperry\ R[ W[\ Restoration of vision after crossing of optic nerves and after contralateral transposition of the eye[ J[ Neurophysiol[\ 0834b\ 7\ 04Ð17[ 013[ Sperry\ R[ W[\ Centripetal regeneration of the 7th cranial nerve root with systematic restoration of vestibular re~exes[ Am[ J[ Physiol[\ 0834c\ 033\ 624Ð 630[ 014[ Sperry\ R[ W[\ Mechanisms of neural maturation[ In Handbook of Experimental Psycholo`y\ ed[ S[ S[ Stevens[ Wiley\ New York\ 0840a\ pp[ 125Ð179[ 015[ Sperry\ R[ W[\ Regulative factors in the orderly

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growth of neural circuits[ Growth "Symposium#\ 0840b\ 09\ 52Ð76[ Sperry\ R[ W[\ The eye and the brain[ Sci[ Am[\ 0845\ 083\ 692Ð609[ Sperry\ R[ W[ Chemoa.nity in the orderly growth of nerve _ber patterns and connections[ Proc[ Nat[ Acad[ Sci[ U[S[A[\ 0852\ 49\ 692Ð609[ Sperry\ R[ W[\ Embryogenesis of behavioral nerve nets[ In Or`ano`enesis\ eds[ R[ L[ Dehaan and H[ Ursprung[ Holt Rinehart and Winston\ New York\ 0854\ pp[ 050Ð075[ Sperry\ R[ W[\ Models\ new and old\ for growth of retino!tectal connections[ In From Theoretical Physics to Biolo`y\ ed[ M[ Marois[ Elsevier\ New York\ 0864[ Sperry\ R[ W[ and Arora\ H[ L[\ Selectivity in regen! eration of the oculomotor nerve in the cichlid _sh\ Astronotus ocellatus[ J[ Embryol[ Exp[ Morph[\ 0854\ 03\ 296Ð206[ Sperry\ R[ W[ and Hibbard\ E[\ Regulative factors in the orderly growth of retino!tectal connexions[ In Ciba Foundation Symposium on Growth of the Nervous System\ eds[ G[ Wolstenholme and M[ O|Connor[ J[ A[ Churchill\ London\ 0857[ Sperry\ R[ W[ and Miner\ N[\ Formation within sensory nucleus V of synaptic associations media! ting cutaneous localization[ J[ Comp[ Neurol[\ 0838\ 89\ 392Ð312[ Stahl\ B[\ Muller\ B[\ Boxberg\ Y[ V[\ Cox\ E[ C[ and Bonhoe}er\ F[\ Biochemical characterization of a putative axonal guidance molecule of the chick visual system[ Neuron\ 0889\ 0889\ 624Ð632[ Stone\ L[ S[\ Functional polarization in retinal development and its re!establishment in regen! erating retinae of rotated grafted eyes[ Proc[ Soc[ Exp[ Biol[ Med[\ 0833\ 46\ 02Ð03[ Stone\ L[ S[ and Ussher\ N[\ Return of vision and other observations in replanted amphibian eyes[ Proc[ Soc[ Exp[ Biol[ Med[\ 0816\ 14\ 102Ð104[ Straznicky\ K[ and Gaze\ R[ M[\ The growth of the retina in Xenopus laevis]\ an autoradiographic study[ J[ Embryol[ Exp[ Morph[\ 0860\ 15\ 56Ð68[ Straznicky\ K[ and Gaze\ R[ M[\ The development of the tectum in Xenopus laevis] An auto! radiographic study[ J[ Embryol[ Exp[ Morph[\ 0861\ 17\ 76Ð004[ Straznicky\ K[ and Gaze\ R[ M[\ The innervation of a virgin tectum by a double!temporal or a double! nasal eye in Xenopus[ J[ Embry[ and Exper[ Morph[\ 0871\ 57\ 8Ð10[ Straznicky\ K[\ Gaze\ R[ M[ and Horder\ T[ J[\ Selection of appropriate medial branch of the optic tract by _bres of ventral retinal origin during devel! opment and in regeneration] An autoradiographic study in Xenopus[ J[ Embryol[ Exp[ Morph[\ 0868\ 49\ 142Ð156[ Straznicky\ K[\ Gaze\ R[ M[ and Keating\ M[ J[\ The retinotectal projections after uncrossing the optic chiasma in Xenopus with one compound eye[ J[ Embryol[ Exp[ Morph[\ 0860\ 15\ 412Ð431[ Straznicky\ K[ and Tay\ C[\ Retinotectal map for! mation in dually innervated tecta] A regeneration study in Xenopus with one compound eye following

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bilateral optic nerve section[ J[ Comp[ Neurol[\ 0871\ 15\ 008Ð029[ Stroer\ W[\ Zur vergleichenden Anatomie des pri! maren optischen Systems bei Wirbeltieren[ Z Anat[ Entuckin`s`esch\ 0828\ 009\ 290Ð210[ Stryker\ M[ P[ and Harris\ W[ A[\ Binocular impulse blockade prevents the formation of ocular domi! nance columns in cat visual cortex[ J[ Neurosci[\ 0875\ 5\ 1006Ð1022[ Trisler\ D[ and Collins\ F[\ Corresponding spatial gradients of TOP molecules in the developing retina and optic tectum[ Science\ 0876\ 126\ 0197Ð0198[ Trisler\ D[\ Schneider\ M[ D[ and Nirenberg\ M[\ A topographic gradient of molecules in retina can be used to identify neuron position[ Proc[ Nat[ Acad[ Sci[\ 0870\ 67\ 1034Ð1038[ Udin\ S[ B[\ Permanent disorganization of the regenerating optic tract in the frog[ Exp[ Neurol[\ 0867\ 47\ 344Ð369[ Udin\ S[ B[ and Fawcett\ J[ W[\ Formation of topo! graphic maps[ Annu[ Rev[ Neurosci[\ 0877\ 00\ 178Ð 216[ Walter\ J[\ Henke!Fahle\ S[ and Bonhoe}er\ F[\ Avoidance of posterior tectal membranes by tem! poral retinal axons[ Development\ 0876\ 090\ 898Ð 802[ Weiss\ P[\ Die Funktion transplantierfer Amphi! bienextremitaten[ Aufstellung einer Resonanz! theorie der motorischen Nerventatigkeit auf Grund abgestimmter Endorgane[ Roux| Archiv[\ 0813\ 091\ 524Ð561[ Weiss\ P[\ Das Resonanzprinzip der Nerv! entatigkeit[ Wein[ Klin[ Wochenschr[\ 0820\ 28\ 0Ð 06[ Weiss\ P[\ In vitro experiments on the factors deter! mining the course of the outgrowing nerve _ber[ J[ Exp[ Zool[\ 0823\ 57\ 282Ð337[ Weiss\ P[\ Selectivity controlling the central!per! ipheral relations in the nervous system[ Biol[ Rev[\ 0825\ 00\ 383Ð420[ Weiss\ P[\ Further experimental investigations on the phenomenon of homologous response in trans! planted amphibian limbs[ I[ Functional obser! vations[ J[ Comp[ Neurol[\ 0826\ 55\ 070Ð198[ Weiss\ P[\ Self!di}erentiation of the basic patterns of coordination[ Comp[ Psych[ Mono`raphs\ 0830a\ 06\ 0Ð85[ Weiss\ P[\ Nerve patterns] The mechanics of nerve growth[ Growth Suppl[\ 0830b\ 4\ 052Ð192[ Wiersma\ C[\ An experiment on the {{resonance theory||[ Arch[ Neurol[\ 0820\ 05\ 226Ð234[ Wiesel\ T[ and Hubel\ D[\ Single!cell responses in striate cortex of kittens deprived of vision in one eye[ J[ Neurophysiol[\ 0852\ 15\ 0992Ð0906[ Wiesel\ T[ N[ and Hubel\ D[ H[\ Comparison of the e}ects of unilateral and bilateral closure on cortical unit responses in kittens[ J[ Neurophysiol[\ 0854\ 17\ 0918Ð0939[ Yoon\ M[ G[\ Reorganization of retinotectal pro! jection following surgical operations on the optic tectum in gold_sh[ Exp[ Neurol[\ 0860\ 22\ 284Ð300[ Yoon\ M[ G[\ Transposition of the visual projection from the nasal hemiretina onto the foreign rostral

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zone of the optic tectum in gold_sh[ Exp[ Neurol[\ 0861\ 26\ 340Ð351[ 051[ Yoon\ M[ G[\ Retention of the original topographic polarity by the 079 degrees rotated tectal reimplant in young adult gold_sh[ J[ Physiol[\ 0862\ 122\ 464Ð 477[ 052[ Yoon\ M[ G[\ Readjustment of retinotectal pro! jection following reimplantation of a rotated or inverted tectal tissue in adult gold_sh[ J[ Physiol[\ 0864\ 141\ 026Ð047[

053[ Yoon\ M[ G[\ Progress of topographic regulation of the visual projection in the halved optic tectum of adult gold_sh[ J[ Physiol[\ 0865\ 146\ 510Ð532[ 054[ Yoon\ M[ G[\ Retention of topographic addresses by reciprocally translocated tectal re!implants in adult gold_sh[ J[ Physiol[\ 0879\ 297\ 086Ð104[ 055[ Yuasa\ J[\ Hirano\ S[\ Yamagata\ M[ and Noda\ M[\ Visual projection map speci_ed by topographic expression of transcription factors in the retina[ Nature\ 0885\ 271\ 521Ð524[