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Behavioral tests of compression of retinotectal projection after partial tectal ablation in goldfish
liSl’liKl~II~N’~AJ.
NKIJHULOGY
Behavioral
54, 579-590
Tests of Compression of Retinotectal Projection after Partial Tectal Ablation in Goldfish MARGARET
Division
(1977)
of Biology,
California
Iwtitute Rcceivrd
YOSHIKO of October
SCOTT
Techrcology,
f Pasadrna,
California
91125
4,1976
The course of functional recovery of vision during the compression of the retinotectal projection that follows hemitectal ablations was mapped by behavioral methods in goldfish (Carassius aikratus) Differential suppression of respiration to red or green stimuli was used as a behavioral measure. Results show that vision was restored throughout the temporal half-field, originally blinded by tectal lesion, within 84 to 112 days of surgery. The scotoma ‘diminished in an orderly fashion, starting from the central edge and progressing caudalward. Color discriminability was concomitant with visual recovery. The return of color vision indicates prespecification of retinal and tectal cells for color and selective reconnection of neurons according to their specificities. Functional topographic reorganization of the retinotectal projection implies that locus-specific aflinities between retinal and tectal neurons, which may play a prominent role in the direction of nerve growth and formation of synapses under more normal circumstances, are not permanently fixed even in mature goldfish and may yield to compensatory developmental pressures created by the size disparity.
INTRODUCTION Recent studies indicate an exceptional plasticity in the retinotectal projection in goldfish (3, 7, 14, 1.5, 21, 22). After removal of the caudal halftectum, the normal topographic projection from the retina, which is regularly restored on the tectum after transection and regeneration of the optic nerve (2, 4, 8, 9, 16, 17), was found to be reinstated in complete but compressed form on the residual rostra1 half-tectum. This orderly compression of the retinal projection onto the half-tectum in goldfish has since been 1 I am indebted to R. W. Sperry for many helpful suggestions and also thank C. R. Hamilton and R. L. Meyer for a free exchange of ideas and useful discussions. The technical assistance of P. Jonkhoff, E. Ogawa, and E. Iwaki are gratefully acknowledged. This work was supported by U.S. Public Health Service Grant 00086, National Institute of Mental Health Grant MH 03372 to R. W. Sperry; and the Evelyn Sharp Graduate Student Scholarship in Behavioral Biology to the author.
Copyright
0 1977 by Academic Press, Inc.
All
of reproduction
rights
in any
form
reserved.
ISSN
0014-4886
580
MARGARET
YOSHIKO
SCOTT
confirmed by many studies and with a number of surgical variations. To achieve the compressed map, considerable readjustment of original connections seemsnecessary. The underlying mechanism by which the plastic remapping occurs and the extent to which the projection may be functional through the tectum and the efferent system remains to be determined. Evidence for the compression comes primarily from electrophysiological studies in which elaborate arguments have been presented that the mapped potentials are from optic axon terminals rather than from tectal cells or axons coursing across the tectum (7). It has been assumedalmost without question that synapsis occurs although possible postsynaptic recordings have been reported by only one investigator (23). Despite the absence of direct physiological evidence for synapse formation, evoked tectal potentials, in the context of retinotectal plasticity, have generally come to be translated into “presynaptic terminals” (8). The aim of the present investigation was to determine whether or not the compressedprojection is behaviorally functional. A visually mediated conditioned respiratory response was utilized as an assay for function at various intervals after removal of the caudal half-tectum, with the optic nerve left intact. The results show a gradual behavioral recovery of the conditioned response over the entire visual field. A preliminary report has been published ( 13) . MATERIALS
AND METHODS
Surgery. Goldfish (Carassius auratus) 6 to 7.5 cm in body length were housed in glass aquaria under a 12-h light : 12-h dark schedule. For surgery, fish were anesthetized in a 0.05% solution of tricain methanesulfonate (Finquel, Ayerst) and the tectum exposed through a reflected cranial bone flap. The caudal half-tectum was separated from the rostra1 half by an incision, down to the ventricle, made with an electrolytically sharpened tungsten knife. The caudal half was then lifted out by aspiration and teased away from the brain and the cranial flap wedged back into position. The optic nerve was left intact in all cases. Operated fish were quickly revived by infusion of water over the gills, and they recovered without apparent motor deficits. Fish were anesthetized and the extraocular muscles torn away from the globe with jewelers’ forceps prior to mapping sessions. Apparatus. Fish were lightly anesthetized, then immobilized, by securing an elasticized strap wrapped around the body, to a clay block mounted on a mechanical maniplator in a 20 X 40-cm water-filled tank adjacent to a 46cm water-filled hemispheric dome (10, 12). In an effort to reduce reflective images, the interior surface of the hemisphere was sandblasted except for a 3-cm circular region at the apex which was left clear. The interior of the tank was lined with flat-black closed-cell neoprene, except for a 1.2-cm
PLASTICITY
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round peephole left uncovered, to expose the test eye. The transilluminated optic disc was visualized through the peephole and aligned along the central axis of a perimeter circumventin g the hemisphere at the equator. Differential recording electrodes were positioned behind the gills contralateral to the test eye, and just in front of the mouth, with the ground embedded in the clay block behind the fish. Respiration was visually displayed on a Beckman Dynograph recorder with simultaneous audiomonitoring. Training and Testing Procedure. A visual discrimination task was selected in these mapping experiments to determine whether or not the compressed projection, if functional, was transmitting useful information. A conditioned respiratory response technique \vas developed as a behavioral measure becausethe procedure readily allows sequential mappings from the sameanimal, without undue damage, and at the sametime enablessufficient immobilization of the fish to map the visual field accurately. Differential suppression of respiration to the S-s presentation of a red or green flashing light was accomplished by pairing one of the colors with an electroshock to the tail. The shock, delivered through stainless-steel electrodes positioned on either side of the tail, was adjusted to an effective value for each fish individually, and ranged from 1.5 to 50 mA for 200 to 500 ms. In order that the stimuli be presented to the same point in space, a Y-shape fiber optics light pipe was used. The stem of the optical conduit was terminated at a mount on the perimeter, forming a light source of 1.5” visual angle. The mount then could be slid through 1SO” of the perimeter. The lights were equated for brightness according to Yager (20). Fish were habituated in preconditioning sessions by presenting the stimuli (20 trials each) without shock. Most fish habituated within the first five presentations and some did not respond to either light. It was observed that fish not giving an initial startle response were more difficult to train later. In conditioning sessions,stimuli were presented at 40” in the anterior field. After a shock-reinforced trial, a 120-s recovery period was allowed before the next presentation, and at least 10 min of quiet after each series of five reinforced trials. After the first block of five continuously reinforced trials, shock was administered intermittently until 20 consecutive responses to the positive stimulus and less than 20% to the neutral stimulus were obtained. As fish generalized throughout the field even when reinforced at only one position, it was not necessary to train at different angles, Some fish initially gave weak responsesat extreme posterior positions but responded vigorously to later presentations at those angles when weak responseswere immediately followed by a light-shock trial at the 40” training position. A few early fish were reinforced at several positions in the anterior field during training.
582
MARGARET
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SCOTT
In general, it was more difficult to obtain differential responding when the green light was paired with shock. Although readily reacting to the green light, it appeared difficult to inhibit generalization of the “fear” response to red light neutral stimulus presentation, which may indicate an innate aversion to the color red. Six fish were discarded because adequate differential responding could not be obtained. During mapping sessions, discriminanda were presented pseudorandomly at variable intervals ranging from 20 to 90 s. The visual field was mapped along the equator by alternating the test angle between anterior and posterior fields. All angles were tested at least twice with three to five positive stimuli each time to give a total of 10 or more trials per angle tested. Test angles ranged from 60” in the anterior field (+60”) to 80” in the posterior field (-SO’) with the optic disc falling on 0”. As seen in Fig. 1, a dramatic suppression of respiration to the onset and sometimes the duration of the positive stimulus makes the conditioned response clearly differentiable from normal prestimulus respiration, whereas respiration to the neutral stimulus remains unchanged. Responses to reinforced and neutral stimuli were tallied for each angle. Trials were deleted if prestimulus noise interfered with unambiguous scoring of responses. Check for Hue vs Brightness Discri+&ation. After the final training session, two fish (one green-positive and one red-positive stimulus) were given additional trials under varying stimulus brightness conditions. The intensity of the positive stimulus was decremented in steps of one-half by inserting one or more Kodak neutral density filters (N.D. 0.3) into the
,
FIG. 1. Respiratory potentials 5-s red and green lights in two and duration of the two colors.
showing goldfish.
differential Bars above
suppression to presentation and below baseline denote
”
I..
of onset
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light path. Both fish continued responding normally to the positive stimulus until the intensity was decreased to one-eighth the original value, where responding ceased altogether. Neither fish switched response preference to the neutral stimulus at any of the decreased positive stimulus intensities. Responding resumed when the positive stimulus was increased by one-half so that light intensity was brought up to one-fourth the original value. Varying the brightness of the neutral stimulus had no effect on response pattern. RESULTS Individual visual field maps of five fish before and 1 week after excision of the caudal half-tectum are shown in Fig. 2A, B. Differential responding obtained at all angles prior to tectal surgery, shown by separation of the positive and neutral stimulus curves in Fig. 2A, indicates good discrimina-
ECCENTRICITY
(DEGREES)
FIG. 2. Visual field maps of the left eye obtained behaviorally. The upper set of curves in each graph show responses to the reinforced (positive) stimulus and the lower show responses to the neutral stimulus. Positive degrees = anterior visual field; negative degrees= posterior visual field. A-Normal fish. B-Same fish as in A 1 week after lesion of the caudal half of the contralateral optic tectum, showing scotoma in the posterior half-field. C-A different group of fish showing complete restoration of the visual field 8 to 18 months after removal of contralateral caudal half-tectum.
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tion throughout the field. After tectal ablation, responding was normal in the anterior field but decreased to both stimuli at the vertical meridian, indicating a scotoma in the posterior half-field (Fig. 2B). Recovery after Tectal Ablation. Mapping 8 to 18 months after surgery showed recovery of vision in the blind field created by tectal lesion in all eight fish in this series. Recovery maps from six fish (four red- and two green-positive stimuli) are plotted in Fig. 2C. Differential responding is evident throughout, and the curves show remarkable similarity to the visual field maps of normal fish in Fig. 2A. Red-green discrimination does not appear to be impaired after tectal lesion and reorganization of the retinotectal projection onto the remaining half-tectum. Two fish, not included in Fig. 2C, are treated in Fig. 3, Fish 3A (Fig. 3A) (red-positive stimulus) did not show full recovery on the first testing at 532 days with a scotoma remaining in the posterior 40” of the visual field. Retesting the following day, however, showed full visual field recovery. Responses to the neutral stimulus were significantly depressed in the first
+60 +40 +20 ECCENTRICITY
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-80
GEYxEi~
FIG. 3. A-Response curves from a fish showing partial (solid line at 532 days postoperatively) and full recovery of the visual field (dashed line at 533 days postoperatively). Red-positive stimulus. B-Response maps from another fish, 312 (solid) and 320 (dashed) days after surgery showing incomplete recovery of the visual field in both testings. Green-positive stimulus. C-Same fish as in B after reversal training to red as the positive stimulus, showing a fully recovered field. See text for discussion.
PLASTICITY
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58.5
compared to the second testing and compared to those obtained from fish in Fig. 2C. The diminished responses to the neutral stimulus on the first day may have reflected an attention/motivational factor which differentially affected performance in the recovered posterior field, suggesting a possible threshold or other difference in the reconnection btween temporal and nasal fibers in the reorganized projection. Fish 3B (green-positive stimulus) showed deficits in the posterior visual field when tested on two occasions, 8 days apart (Fig. 3B). Vision appeared to be marginal, at best, beyond -2O”, with complete disappearance of differential responding. The unusual increase in responding to the neutral stimulus in the posterior field was suspicious, and the fish was subsequently trained on a reverse discrimination task, which it readily acqured. The resultant map, after reversal, is shown in Fig. 3C and shows full-field recovery, at least for red light. Whether or not this fish could see green light in the posterior field is unclear, but the results of the two other greenpositive fish, included in Fig. 2C, indicate that recovery of vision for green light does occur. Reversal training was attempted on other fish originally conditioned to red to see if a similar scotoma could be produced in the posterior field. These attempts were unsuccessful, however, as differential responding could not be obtained after commencement of shocks to green light. Time Course of Recozlery. In this series, four fish were trained and mapped prior to surgery. Seven days after ablation of the caudal half-tecturn, tests established the extent of the blind field and served as a reference for comparison with subsequent intermittent mappings from the same fish. All fish were found to be blind posterior to the vertical meridian in the initial postoperative test. After 3 weeks, response maps showed no change in the blind field, but during the fifth week, a noticable expansion of vision over 20 to 30” had occurred. From the fifth week, variability of recovery between fish increased with restoration of vision across the entire field ranging from 84 to 112 days. Time sequences of recovery for the four fish are shown in Fig. 4. Responses to the neutral stimulus have been omitted for clarity. Fish 4A (Fig. 4A), with an initial scotoma in the entire posterior field, gradually recovered vision in the blind field in an orderly anterior-posteror fashion as seen in the sequential mappings at the indicated postsurgical intervals. Response maps from the other three fish (Fig. 4B through D) again show an orderly reestablishment of connections from retina to tectum, with the central edge of the scotoma estending gradually into the temporal field. These curves clearly illustrate the regularity of recovery, with expansion always proceeding from the center to the periphery ; no indication of vision was found in the posterior field before restoration in the central regions.
586
MARGARET
YOSHIKO
SCOTT
Another observation from these data was that color vision returned concomitantly with the first indication of vision. In Fig. 5, the difference in percentage of responses to the positive and neutral stimuli, averaged for four fish, are plotted for each indicated recovery interval. These curves are unambiguous in showing immediate discrimination between red and green lights at all angIes where vision was first demonstrable. A comparison of Fig. 4 and 5 shows striking similarities in recovery curves determined by percentage of responses to the positive stimulus alone and by percentage of differential responses to the positive and neutral stimuli. DISCUSSION Behavioral mapping of the visual field, using a conditioned respiratory response, indicates recovery of vision over the entire formerly blind field created by excision of the caudal half-tectum. Differential suppression of respiration to red and green lights throughout indicates, furthermore, the restoration of color vision to the recovered field. The present results demonstrate that the compressed retinotectal projection mapped by electrical methods is behaviorally functional through the tectum and the efferent system. Sequential recovery maps show a gradual orderly expansion of the visual field, caudalward from the central region that extends to the posterior pole and reaches a plateau within 84 to 112 days after surgery. The observed behavioral recovery is consistent with the time for compression determined by electrophysiology with the optic nerve left intact (21). The expansion, starting from the central edge of the scotoma, extended 20 to 30” every several weeks. Biweekly mappings sometimes failed to show a noticeable change in the size of the field from a previous testing; for example, at 3 weeks, the field map was essentially unchanged from the l-week-postoperative reference map. When field expansion did occur, it was always found to be orderly and continuous with the previously recovered field, with no islands of recovery restored sporadically within the scotoma. Although differential responding was always obtained, the conditioned responses at newly recovered points were sometimes weaker (shorter in duration) than elsewhere. The differences in response characteristics disappeared by the time the full field was recovered, however, perhaps as the result of maturation of the new synaptic connections, or the more effective driving of tectal cells through concentration of synaptic contacts by addition of new synapses, or spatial localization of existing terminals. There was some indication that the reestablished representation of the posterior visual field was not always equivalent to a normal projection, even after long recovery intervals, but these cases were exceptional, and recovered fish on
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FIG. 4. Sequential response maps from four fish (A through D) showing visual field expansion in an orderly center-periphery fashion. Recovery interval in days. Responses to the neutral stimulus have been omitted for clarity.
the whole performed in a manner indistinguishable from normal fish in their ability to detect and differentiate the 1.5” stimuli. Functional rejoining of fibers from the whole eye to the remnant halftectum, like the results of electrical mapping of compression, may appear at first to violate the principles of the neurospecificity model of Sperry which explains the formation of orderly connections in terms of cytochemical differentiation and specific labeling of individual neurons in the retinal and tectal fields with selective matching of presynaptic and postsynaptic elements according to the prescribed specificities (16-1s). However, the orderly graded progression of recovery in the foregoing does suggest rather
selective
reconnection
in the sense that the nasal fibers,
deprived
of
588
MARGARET
YOSHIKO
SCOTT
A%
-20’
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+40
+20 0 ECCENTRICITY
-20 -40 (DEGREES)
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FIG. 5. Differential response curves (percentage of response to the positive stimulus minus percentage of response to the neutral stimulus) showing sequential recovery of color vision at the indicated recovery intervals. Each point is the average of the four fish in Fig. 4.
their terminal connections after excision of the caudal tectum, formed synapsesin the order of their original proximity to the remaining tectum. Without cell-unique specification for position and positional preferences of some kind, a random recovery throughout the posterior field would be expected because all axons coursing beyond the rostrocaudal midline of the tectum were interrupted concurrently and presumably had an equal opportunity to make connections on a purely mechanical nonselective basis. Respecification of synaptic affinities has been proposed (11, 19, 21-23) as one means by which the half-tectum could accommodate the influx of foreign axon terminals. Other investigators (5, 6) deny the necessity for affinities between retinal and tectal neurons for the formation of ordered maps and suggest that retinal fibers merely align themselves according to nearest neighbors on the available tectal space. The pattern of visual recovery found here does not distinguish the above two divergent positions. Another aspect of the results, however, is suggestive of the existence of predetermined affinities. The majority of operated fish after recovery performed throughout as well as normal fish on the red-green discrimination task, indicating a return of color vision to the restored temporal field. Differential responding in the time-course study was immediately apparent with the first clear indication of vision in the expanding field. Great precision and selectivity in matching up of color cells of the retina and tectum would seem a logical necessity. These findings concur with those of Arora and Sperry ( 1) in supporting the notion of cytospecific differentiations of neurons in the retinal and tectal fields along the color dimension (17). Observations from compression studies in goldfish in which the optic nerve was divided and allowed to regenerate (3, 14, 23) indicate that only appropriate retinotectal connections are formed initially. Reorganization appears to occur secondarily with time, suggesting that the influence of the position-dependent affinities may be diminished in light of the mismatch
situation.
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The present results show that the reorganized retinotectal projection induced by extensive damage to the optic tectum in goldfish is behaviorally functional. The full functional capacity of the compressed projection is yet to be determined. Visual acuity tests, for example, were not administered, but acuity is presumably diminished by the greater convergence of optic axons required in reconnecting the whole eye to a half-tectum. Perceptual distortions may also be introduced by the compression, and the accuracy of visual localization remains to be determined. Another unknown factor concerns the extent to which the plastic changes extend to deeper centers of visuomotor coordination. REFERENCES H. L., AND R. W. SPERRY. 1963. Color discrimination after optic nerve regeneration in the fish Astronotus ocellatus. Develop. Biol. 7: 234-243. ATTARDI, D. G., AND R. W. SPERRY. 1963. Preferential selection of central pathways by regenerating optic fibers. Exp. Neurol. 7: 4M4. COOK, J. E., AND T. J. HORDER. 1974. Interactions between optic fibres in their regeneration to specific sites in the goldfish tectum. /. Physiol. (Lond.) 241: 89-9OP. GAZE, R. M. 1970. The Formation of Ncrzje Connectiow. Academic Press, London. GAZE, R. M., AND R. A. HOPE. 1976. The formation of continuously ordered mappings. Prog. Brain Res. 45: 327.-357. GAZE, R. M., AND M. J. KEATING. 1972. The visual system and “neuronal specificity.” Nature (London) 231: 375-378. GAZE, R. M., AND S. C. SHARMA. 1970. Axial differences in the reinnervation of the goldfish optic tectum by regenerating optic nerve fibres. Exp. BY& Rcs. 10: 171-181. HUNT, R. Ii., AND M. JACOBSON. 1974. Neuronal specificity revisited. Czdrr. Top.
1. ARORA, 2.
3.
4. 5.
6. 7.
8.
Develop. 9. JACOBSON,
10. 11.
12.
13. 14. 15. 16.
Biol.
8 : 203-259.
M. 1970. Developwaental Neurobiology. Holt, Rinehart and Winston, New York. MEYER, R. L. 1974. Factors Affecting Regeneration of the Retirzofecfal Projection, Doctoral Dissertation. California Institute of Technology, Pasadena, California. MEYER, R. L., AND R. W. SPERRY. 1976. Retinotectal specificity: chemoaffinity theory. Pages 111-149 i~t G. GOTTLIEB, Ed., Studies on the Development of Behavior axd fke Nervous System, Vol. 3. Academic Press, New York. SCHWASSMANN, H. O., AND L. KRUGER. 1965. Organization of the visual projection upon the optic tectum of some freshwater fish. I. Camp. Neztrol. 124: 113126. SCOTT, M. Y. 1975. Functional capacity of compressed retinotectal projection in goldfish. Attot. Rec. 181: 474. SIIARMA, S. C. 1972. Redistribution of visual projections in altered optic tecta of adult goldfish. Proc. Nat. Acad. Sci. USA 69: 2637-2639. SHARMA, S. C. 1972. Reformation of Tetinotectal projections after various tectal ablations in adult goldfish. E.zp. Neurol. 24: 171-182. SPERRY, R. W. 1944 Optic nerve regeneration with return of vision in anurans. J. Neuropkysiol.
7:
57-69.
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R. W. 1951. Mechanism of neural maturation. Pages 236280 in S. S. STEVENS, Ed., Handbook of Experimental Psychology. Wiley, New York. 18. SPERRY, R. W. 1965. Embryogenesis of behavioral nerve nets. Pages 161-186 in R. L. DEHAAN AND H. URSPRUNG, Eds., Organogcnesis. Holt, New York. 19. SPERRY, R. W. 1975. Models, new and old, for growth of retinotectal connections. Pages 191-215 in M. MAROIS, Ed.., From Theoretical Physics to Biology. American Elsevier, New York. 20. YAGER, D. 1967. Behavioral measures and theoretical analysis of spectral sensitivity and spectral saturation in the goldfi’sh, Carassius awatus. Vision Res. 7 : 707-727. 21. YOON, M. 1971. Reorganization of retinotectal projection following surgical operations on the optic tectum in goldfish. Exp. Neural. 33: 395411. 22. YOON, M. 1972. Reversibility of the reorganization of retinotectal projection in goldfish. Exg. Neural. 35: 565-577. 23. YOON, M. 1975. Effects of post-operative visual environment on reorganization of retinotectal projection in goldfish. J. Physiol. (Lond.) 246: 673-694. 17. SPERRY,
NKIJHULOGY
Behavioral
54, 579-590
Tests of Compression of Retinotectal Projection after Partial Tectal Ablation in Goldfish MARGARET
Division
(1977)
of Biology,
California
Iwtitute Rcceivrd
YOSHIKO of October
SCOTT
Techrcology,
f Pasadrna,
California
91125
4,1976
The course of functional recovery of vision during the compression of the retinotectal projection that follows hemitectal ablations was mapped by behavioral methods in goldfish (Carassius aikratus) Differential suppression of respiration to red or green stimuli was used as a behavioral measure. Results show that vision was restored throughout the temporal half-field, originally blinded by tectal lesion, within 84 to 112 days of surgery. The scotoma ‘diminished in an orderly fashion, starting from the central edge and progressing caudalward. Color discriminability was concomitant with visual recovery. The return of color vision indicates prespecification of retinal and tectal cells for color and selective reconnection of neurons according to their specificities. Functional topographic reorganization of the retinotectal projection implies that locus-specific aflinities between retinal and tectal neurons, which may play a prominent role in the direction of nerve growth and formation of synapses under more normal circumstances, are not permanently fixed even in mature goldfish and may yield to compensatory developmental pressures created by the size disparity.
INTRODUCTION Recent studies indicate an exceptional plasticity in the retinotectal projection in goldfish (3, 7, 14, 1.5, 21, 22). After removal of the caudal halftectum, the normal topographic projection from the retina, which is regularly restored on the tectum after transection and regeneration of the optic nerve (2, 4, 8, 9, 16, 17), was found to be reinstated in complete but compressed form on the residual rostra1 half-tectum. This orderly compression of the retinal projection onto the half-tectum in goldfish has since been 1 I am indebted to R. W. Sperry for many helpful suggestions and also thank C. R. Hamilton and R. L. Meyer for a free exchange of ideas and useful discussions. The technical assistance of P. Jonkhoff, E. Ogawa, and E. Iwaki are gratefully acknowledged. This work was supported by U.S. Public Health Service Grant 00086, National Institute of Mental Health Grant MH 03372 to R. W. Sperry; and the Evelyn Sharp Graduate Student Scholarship in Behavioral Biology to the author.
Copyright
0 1977 by Academic Press, Inc.
All
of reproduction
rights
in any
form
reserved.
ISSN
0014-4886
580
MARGARET
YOSHIKO
SCOTT
confirmed by many studies and with a number of surgical variations. To achieve the compressed map, considerable readjustment of original connections seemsnecessary. The underlying mechanism by which the plastic remapping occurs and the extent to which the projection may be functional through the tectum and the efferent system remains to be determined. Evidence for the compression comes primarily from electrophysiological studies in which elaborate arguments have been presented that the mapped potentials are from optic axon terminals rather than from tectal cells or axons coursing across the tectum (7). It has been assumedalmost without question that synapsis occurs although possible postsynaptic recordings have been reported by only one investigator (23). Despite the absence of direct physiological evidence for synapse formation, evoked tectal potentials, in the context of retinotectal plasticity, have generally come to be translated into “presynaptic terminals” (8). The aim of the present investigation was to determine whether or not the compressedprojection is behaviorally functional. A visually mediated conditioned respiratory response was utilized as an assay for function at various intervals after removal of the caudal half-tectum, with the optic nerve left intact. The results show a gradual behavioral recovery of the conditioned response over the entire visual field. A preliminary report has been published ( 13) . MATERIALS
AND METHODS
Surgery. Goldfish (Carassius auratus) 6 to 7.5 cm in body length were housed in glass aquaria under a 12-h light : 12-h dark schedule. For surgery, fish were anesthetized in a 0.05% solution of tricain methanesulfonate (Finquel, Ayerst) and the tectum exposed through a reflected cranial bone flap. The caudal half-tectum was separated from the rostra1 half by an incision, down to the ventricle, made with an electrolytically sharpened tungsten knife. The caudal half was then lifted out by aspiration and teased away from the brain and the cranial flap wedged back into position. The optic nerve was left intact in all cases. Operated fish were quickly revived by infusion of water over the gills, and they recovered without apparent motor deficits. Fish were anesthetized and the extraocular muscles torn away from the globe with jewelers’ forceps prior to mapping sessions. Apparatus. Fish were lightly anesthetized, then immobilized, by securing an elasticized strap wrapped around the body, to a clay block mounted on a mechanical maniplator in a 20 X 40-cm water-filled tank adjacent to a 46cm water-filled hemispheric dome (10, 12). In an effort to reduce reflective images, the interior surface of the hemisphere was sandblasted except for a 3-cm circular region at the apex which was left clear. The interior of the tank was lined with flat-black closed-cell neoprene, except for a 1.2-cm
PLASTICITY
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round peephole left uncovered, to expose the test eye. The transilluminated optic disc was visualized through the peephole and aligned along the central axis of a perimeter circumventin g the hemisphere at the equator. Differential recording electrodes were positioned behind the gills contralateral to the test eye, and just in front of the mouth, with the ground embedded in the clay block behind the fish. Respiration was visually displayed on a Beckman Dynograph recorder with simultaneous audiomonitoring. Training and Testing Procedure. A visual discrimination task was selected in these mapping experiments to determine whether or not the compressed projection, if functional, was transmitting useful information. A conditioned respiratory response technique \vas developed as a behavioral measure becausethe procedure readily allows sequential mappings from the sameanimal, without undue damage, and at the sametime enablessufficient immobilization of the fish to map the visual field accurately. Differential suppression of respiration to the S-s presentation of a red or green flashing light was accomplished by pairing one of the colors with an electroshock to the tail. The shock, delivered through stainless-steel electrodes positioned on either side of the tail, was adjusted to an effective value for each fish individually, and ranged from 1.5 to 50 mA for 200 to 500 ms. In order that the stimuli be presented to the same point in space, a Y-shape fiber optics light pipe was used. The stem of the optical conduit was terminated at a mount on the perimeter, forming a light source of 1.5” visual angle. The mount then could be slid through 1SO” of the perimeter. The lights were equated for brightness according to Yager (20). Fish were habituated in preconditioning sessions by presenting the stimuli (20 trials each) without shock. Most fish habituated within the first five presentations and some did not respond to either light. It was observed that fish not giving an initial startle response were more difficult to train later. In conditioning sessions,stimuli were presented at 40” in the anterior field. After a shock-reinforced trial, a 120-s recovery period was allowed before the next presentation, and at least 10 min of quiet after each series of five reinforced trials. After the first block of five continuously reinforced trials, shock was administered intermittently until 20 consecutive responses to the positive stimulus and less than 20% to the neutral stimulus were obtained. As fish generalized throughout the field even when reinforced at only one position, it was not necessary to train at different angles, Some fish initially gave weak responsesat extreme posterior positions but responded vigorously to later presentations at those angles when weak responseswere immediately followed by a light-shock trial at the 40” training position. A few early fish were reinforced at several positions in the anterior field during training.
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In general, it was more difficult to obtain differential responding when the green light was paired with shock. Although readily reacting to the green light, it appeared difficult to inhibit generalization of the “fear” response to red light neutral stimulus presentation, which may indicate an innate aversion to the color red. Six fish were discarded because adequate differential responding could not be obtained. During mapping sessions, discriminanda were presented pseudorandomly at variable intervals ranging from 20 to 90 s. The visual field was mapped along the equator by alternating the test angle between anterior and posterior fields. All angles were tested at least twice with three to five positive stimuli each time to give a total of 10 or more trials per angle tested. Test angles ranged from 60” in the anterior field (+60”) to 80” in the posterior field (-SO’) with the optic disc falling on 0”. As seen in Fig. 1, a dramatic suppression of respiration to the onset and sometimes the duration of the positive stimulus makes the conditioned response clearly differentiable from normal prestimulus respiration, whereas respiration to the neutral stimulus remains unchanged. Responses to reinforced and neutral stimuli were tallied for each angle. Trials were deleted if prestimulus noise interfered with unambiguous scoring of responses. Check for Hue vs Brightness Discri+&ation. After the final training session, two fish (one green-positive and one red-positive stimulus) were given additional trials under varying stimulus brightness conditions. The intensity of the positive stimulus was decremented in steps of one-half by inserting one or more Kodak neutral density filters (N.D. 0.3) into the
,
FIG. 1. Respiratory potentials 5-s red and green lights in two and duration of the two colors.
showing goldfish.
differential Bars above
suppression to presentation and below baseline denote
”
I..
of onset
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light path. Both fish continued responding normally to the positive stimulus until the intensity was decreased to one-eighth the original value, where responding ceased altogether. Neither fish switched response preference to the neutral stimulus at any of the decreased positive stimulus intensities. Responding resumed when the positive stimulus was increased by one-half so that light intensity was brought up to one-fourth the original value. Varying the brightness of the neutral stimulus had no effect on response pattern. RESULTS Individual visual field maps of five fish before and 1 week after excision of the caudal half-tectum are shown in Fig. 2A, B. Differential responding obtained at all angles prior to tectal surgery, shown by separation of the positive and neutral stimulus curves in Fig. 2A, indicates good discrimina-
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FIG. 2. Visual field maps of the left eye obtained behaviorally. The upper set of curves in each graph show responses to the reinforced (positive) stimulus and the lower show responses to the neutral stimulus. Positive degrees = anterior visual field; negative degrees= posterior visual field. A-Normal fish. B-Same fish as in A 1 week after lesion of the caudal half of the contralateral optic tectum, showing scotoma in the posterior half-field. C-A different group of fish showing complete restoration of the visual field 8 to 18 months after removal of contralateral caudal half-tectum.
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tion throughout the field. After tectal ablation, responding was normal in the anterior field but decreased to both stimuli at the vertical meridian, indicating a scotoma in the posterior half-field (Fig. 2B). Recovery after Tectal Ablation. Mapping 8 to 18 months after surgery showed recovery of vision in the blind field created by tectal lesion in all eight fish in this series. Recovery maps from six fish (four red- and two green-positive stimuli) are plotted in Fig. 2C. Differential responding is evident throughout, and the curves show remarkable similarity to the visual field maps of normal fish in Fig. 2A. Red-green discrimination does not appear to be impaired after tectal lesion and reorganization of the retinotectal projection onto the remaining half-tectum. Two fish, not included in Fig. 2C, are treated in Fig. 3, Fish 3A (Fig. 3A) (red-positive stimulus) did not show full recovery on the first testing at 532 days with a scotoma remaining in the posterior 40” of the visual field. Retesting the following day, however, showed full visual field recovery. Responses to the neutral stimulus were significantly depressed in the first
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FIG. 3. A-Response curves from a fish showing partial (solid line at 532 days postoperatively) and full recovery of the visual field (dashed line at 533 days postoperatively). Red-positive stimulus. B-Response maps from another fish, 312 (solid) and 320 (dashed) days after surgery showing incomplete recovery of the visual field in both testings. Green-positive stimulus. C-Same fish as in B after reversal training to red as the positive stimulus, showing a fully recovered field. See text for discussion.
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compared to the second testing and compared to those obtained from fish in Fig. 2C. The diminished responses to the neutral stimulus on the first day may have reflected an attention/motivational factor which differentially affected performance in the recovered posterior field, suggesting a possible threshold or other difference in the reconnection btween temporal and nasal fibers in the reorganized projection. Fish 3B (green-positive stimulus) showed deficits in the posterior visual field when tested on two occasions, 8 days apart (Fig. 3B). Vision appeared to be marginal, at best, beyond -2O”, with complete disappearance of differential responding. The unusual increase in responding to the neutral stimulus in the posterior field was suspicious, and the fish was subsequently trained on a reverse discrimination task, which it readily acqured. The resultant map, after reversal, is shown in Fig. 3C and shows full-field recovery, at least for red light. Whether or not this fish could see green light in the posterior field is unclear, but the results of the two other greenpositive fish, included in Fig. 2C, indicate that recovery of vision for green light does occur. Reversal training was attempted on other fish originally conditioned to red to see if a similar scotoma could be produced in the posterior field. These attempts were unsuccessful, however, as differential responding could not be obtained after commencement of shocks to green light. Time Course of Recozlery. In this series, four fish were trained and mapped prior to surgery. Seven days after ablation of the caudal half-tecturn, tests established the extent of the blind field and served as a reference for comparison with subsequent intermittent mappings from the same fish. All fish were found to be blind posterior to the vertical meridian in the initial postoperative test. After 3 weeks, response maps showed no change in the blind field, but during the fifth week, a noticable expansion of vision over 20 to 30” had occurred. From the fifth week, variability of recovery between fish increased with restoration of vision across the entire field ranging from 84 to 112 days. Time sequences of recovery for the four fish are shown in Fig. 4. Responses to the neutral stimulus have been omitted for clarity. Fish 4A (Fig. 4A), with an initial scotoma in the entire posterior field, gradually recovered vision in the blind field in an orderly anterior-posteror fashion as seen in the sequential mappings at the indicated postsurgical intervals. Response maps from the other three fish (Fig. 4B through D) again show an orderly reestablishment of connections from retina to tectum, with the central edge of the scotoma estending gradually into the temporal field. These curves clearly illustrate the regularity of recovery, with expansion always proceeding from the center to the periphery ; no indication of vision was found in the posterior field before restoration in the central regions.
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Another observation from these data was that color vision returned concomitantly with the first indication of vision. In Fig. 5, the difference in percentage of responses to the positive and neutral stimuli, averaged for four fish, are plotted for each indicated recovery interval. These curves are unambiguous in showing immediate discrimination between red and green lights at all angIes where vision was first demonstrable. A comparison of Fig. 4 and 5 shows striking similarities in recovery curves determined by percentage of responses to the positive stimulus alone and by percentage of differential responses to the positive and neutral stimuli. DISCUSSION Behavioral mapping of the visual field, using a conditioned respiratory response, indicates recovery of vision over the entire formerly blind field created by excision of the caudal half-tectum. Differential suppression of respiration to red and green lights throughout indicates, furthermore, the restoration of color vision to the recovered field. The present results demonstrate that the compressed retinotectal projection mapped by electrical methods is behaviorally functional through the tectum and the efferent system. Sequential recovery maps show a gradual orderly expansion of the visual field, caudalward from the central region that extends to the posterior pole and reaches a plateau within 84 to 112 days after surgery. The observed behavioral recovery is consistent with the time for compression determined by electrophysiology with the optic nerve left intact (21). The expansion, starting from the central edge of the scotoma, extended 20 to 30” every several weeks. Biweekly mappings sometimes failed to show a noticeable change in the size of the field from a previous testing; for example, at 3 weeks, the field map was essentially unchanged from the l-week-postoperative reference map. When field expansion did occur, it was always found to be orderly and continuous with the previously recovered field, with no islands of recovery restored sporadically within the scotoma. Although differential responding was always obtained, the conditioned responses at newly recovered points were sometimes weaker (shorter in duration) than elsewhere. The differences in response characteristics disappeared by the time the full field was recovered, however, perhaps as the result of maturation of the new synaptic connections, or the more effective driving of tectal cells through concentration of synaptic contacts by addition of new synapses, or spatial localization of existing terminals. There was some indication that the reestablished representation of the posterior visual field was not always equivalent to a normal projection, even after long recovery intervals, but these cases were exceptional, and recovered fish on
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FIG. 4. Sequential response maps from four fish (A through D) showing visual field expansion in an orderly center-periphery fashion. Recovery interval in days. Responses to the neutral stimulus have been omitted for clarity.
the whole performed in a manner indistinguishable from normal fish in their ability to detect and differentiate the 1.5” stimuli. Functional rejoining of fibers from the whole eye to the remnant halftectum, like the results of electrical mapping of compression, may appear at first to violate the principles of the neurospecificity model of Sperry which explains the formation of orderly connections in terms of cytochemical differentiation and specific labeling of individual neurons in the retinal and tectal fields with selective matching of presynaptic and postsynaptic elements according to the prescribed specificities (16-1s). However, the orderly graded progression of recovery in the foregoing does suggest rather
selective
reconnection
in the sense that the nasal fibers,
deprived
of
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FIG. 5. Differential response curves (percentage of response to the positive stimulus minus percentage of response to the neutral stimulus) showing sequential recovery of color vision at the indicated recovery intervals. Each point is the average of the four fish in Fig. 4.
their terminal connections after excision of the caudal tectum, formed synapsesin the order of their original proximity to the remaining tectum. Without cell-unique specification for position and positional preferences of some kind, a random recovery throughout the posterior field would be expected because all axons coursing beyond the rostrocaudal midline of the tectum were interrupted concurrently and presumably had an equal opportunity to make connections on a purely mechanical nonselective basis. Respecification of synaptic affinities has been proposed (11, 19, 21-23) as one means by which the half-tectum could accommodate the influx of foreign axon terminals. Other investigators (5, 6) deny the necessity for affinities between retinal and tectal neurons for the formation of ordered maps and suggest that retinal fibers merely align themselves according to nearest neighbors on the available tectal space. The pattern of visual recovery found here does not distinguish the above two divergent positions. Another aspect of the results, however, is suggestive of the existence of predetermined affinities. The majority of operated fish after recovery performed throughout as well as normal fish on the red-green discrimination task, indicating a return of color vision to the restored temporal field. Differential responding in the time-course study was immediately apparent with the first clear indication of vision in the expanding field. Great precision and selectivity in matching up of color cells of the retina and tectum would seem a logical necessity. These findings concur with those of Arora and Sperry ( 1) in supporting the notion of cytospecific differentiations of neurons in the retinal and tectal fields along the color dimension (17). Observations from compression studies in goldfish in which the optic nerve was divided and allowed to regenerate (3, 14, 23) indicate that only appropriate retinotectal connections are formed initially. Reorganization appears to occur secondarily with time, suggesting that the influence of the position-dependent affinities may be diminished in light of the mismatch
situation.
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The present results show that the reorganized retinotectal projection induced by extensive damage to the optic tectum in goldfish is behaviorally functional. The full functional capacity of the compressed projection is yet to be determined. Visual acuity tests, for example, were not administered, but acuity is presumably diminished by the greater convergence of optic axons required in reconnecting the whole eye to a half-tectum. Perceptual distortions may also be introduced by the compression, and the accuracy of visual localization remains to be determined. Another unknown factor concerns the extent to which the plastic changes extend to deeper centers of visuomotor coordination. REFERENCES H. L., AND R. W. SPERRY. 1963. Color discrimination after optic nerve regeneration in the fish Astronotus ocellatus. Develop. Biol. 7: 234-243. ATTARDI, D. G., AND R. W. SPERRY. 1963. Preferential selection of central pathways by regenerating optic fibers. Exp. Neurol. 7: 4M4. COOK, J. E., AND T. J. HORDER. 1974. Interactions between optic fibres in their regeneration to specific sites in the goldfish tectum. /. Physiol. (Lond.) 241: 89-9OP. GAZE, R. M. 1970. The Formation of Ncrzje Connectiow. Academic Press, London. GAZE, R. M., AND R. A. HOPE. 1976. The formation of continuously ordered mappings. Prog. Brain Res. 45: 327.-357. GAZE, R. M., AND M. J. KEATING. 1972. The visual system and “neuronal specificity.” Nature (London) 231: 375-378. GAZE, R. M., AND S. C. SHARMA. 1970. Axial differences in the reinnervation of the goldfish optic tectum by regenerating optic nerve fibres. Exp. BY& Rcs. 10: 171-181. HUNT, R. Ii., AND M. JACOBSON. 1974. Neuronal specificity revisited. Czdrr. Top.
1. ARORA, 2.
3.
4. 5.
6. 7.
8.
Develop. 9. JACOBSON,
10. 11.
12.
13. 14. 15. 16.
Biol.
8 : 203-259.
M. 1970. Developwaental Neurobiology. Holt, Rinehart and Winston, New York. MEYER, R. L. 1974. Factors Affecting Regeneration of the Retirzofecfal Projection, Doctoral Dissertation. California Institute of Technology, Pasadena, California. MEYER, R. L., AND R. W. SPERRY. 1976. Retinotectal specificity: chemoaffinity theory. Pages 111-149 i~t G. GOTTLIEB, Ed., Studies on the Development of Behavior axd fke Nervous System, Vol. 3. Academic Press, New York. SCHWASSMANN, H. O., AND L. KRUGER. 1965. Organization of the visual projection upon the optic tectum of some freshwater fish. I. Camp. Neztrol. 124: 113126. SCOTT, M. Y. 1975. Functional capacity of compressed retinotectal projection in goldfish. Attot. Rec. 181: 474. SIIARMA, S. C. 1972. Redistribution of visual projections in altered optic tecta of adult goldfish. Proc. Nat. Acad. Sci. USA 69: 2637-2639. SHARMA, S. C. 1972. Reformation of Tetinotectal projections after various tectal ablations in adult goldfish. E.zp. Neurol. 24: 171-182. SPERRY, R. W. 1944 Optic nerve regeneration with return of vision in anurans. J. Neuropkysiol.
7:
57-69.
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R. W. 1951. Mechanism of neural maturation. Pages 236280 in S. S. STEVENS, Ed., Handbook of Experimental Psychology. Wiley, New York. 18. SPERRY, R. W. 1965. Embryogenesis of behavioral nerve nets. Pages 161-186 in R. L. DEHAAN AND H. URSPRUNG, Eds., Organogcnesis. Holt, New York. 19. SPERRY, R. W. 1975. Models, new and old, for growth of retinotectal connections. Pages 191-215 in M. MAROIS, Ed.., From Theoretical Physics to Biology. American Elsevier, New York. 20. YAGER, D. 1967. Behavioral measures and theoretical analysis of spectral sensitivity and spectral saturation in the goldfi’sh, Carassius awatus. Vision Res. 7 : 707-727. 21. YOON, M. 1971. Reorganization of retinotectal projection following surgical operations on the optic tectum in goldfish. Exp. Neural. 33: 395411. 22. YOON, M. 1972. Reversibility of the reorganization of retinotectal projection in goldfish. Exg. Neural. 35: 565-577. 23. YOON, M. 1975. Effects of post-operative visual environment on reorganization of retinotectal projection in goldfish. J. Physiol. (Lond.) 246: 673-694. 17. SPERRY,