The postnatal development of clustered intrinsic connections in area 18 of the visual cortex in kittens

Developmental Brain Research, 24 (1986) 31- 38 Elsevier

31

BRD 50306

The Postnatal Development of Clustered Intrinsic Connections in Area 18 of the Visual Cortex in Kittens DAVID J. PRICE

University Laboratory of Physiology, Parks Road, Oxford OX1 3PT (U. K.) (Accepted June 4th, 1985)

Key words: area 18 - - kitten - - intrinsic connection - - retrogradely transported neuronal tracer

Retrogradely transported neuronal markers were injected into area 18 of the visual cortex in normal kittens of various ages and in animals that had been binocularly deprived of patterned visual stimulation by eyelid suture. In normal kittens aged 20 days or more, distinct clusters of retrogradely labelled cells were identified in area 18 surrounding the injection sites; these cells lay mainly in cortical layers II, III, and the upper part of IV, but with some in layers V and VI. In kittens younger than 10 days, labelled cells were observed around injection sites, although they were not organized into clusters. The results suggest that the typical clustered pattern of neurones forming intrinsic connections in area 18 emerges during the second week postnatally from an immature non-patchy distribution. Binocular deprivation did not prevent the appearance of these patches of cells. INTRODUCTION Clustering of the intrinsic connections of the visual cortex has been observed in cats it and primates24,25; discrete, periodically spaced patches of cells possessing intrinsic projections to any particular point in area 18 of the cat have been described 15-17. The aim of this study was to use neuroanatomical methods to follow the postnatal development of the clustering of cells forming intrinsic connections within area 18, and to discover whether the maturation of these patchy patterns, which occurs around the time when kittens' eyelids open naturally, is dependent on the onset of patterned visual stimulation. During prenatal and early postnatal development, the cells of origin of certain neural pathways in the mammalian nervous system form more widespread connections than in the adult, some of which are subsequently eliminated e. In the visual system, the selective loss of 'exuberant' connections is, in some cases, influenced by the animal's experience6.13,14,28. However, for the association projection from area 17 to area 18, which initially arises from an aberrant pattern of cells in area 17 and gradually matures over the

second and third postnatal weeks, binocular deprivation (BD) has little or no effect on the elimination of exuberant connectionse0,23. It seems possible that similar regressive events occur in the intrinsic pathways within a particular cortical area, and that they play an important role in determining the mature adult form of these local cortical circuits. Since full physiological maturation of visual cortical cells in kittens requires patterned visual stimulation (reviewed recently by Fregnac and Imbert8), deprivation might be expected to interfere with the development of intrinsic cortical connections. MATERIALS AND METHODS

Animals Successful experiments were performed on 8 kittens of various ages. The conjunctivae and eyelids of two animals were closed under ketamine anaesthesia at 6 days of age, before the time of natural eye-opening (which is at about 10 days postnatally), using the procedure described by Blakemore and Van Sluyters 4, and these kittens were allowed to survive until they were 4 weeks old.

Correspondence: D.J. Price, University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, U.K. 0165-3806/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)

"1 A.

Injection of tracers

two BD kittens (BD1 and BD2), both of which were

Each animal was anaesthetized with halothane and

aged 28 days. A pressure injection of 500 nl of a 2cv~

placed in a stereotaxic holder 7. A small craniotomy

solution of diamidino yellow (DY) was made into

was made over the lateral gyrus a few millimetres an-

area 18 in a 30-day-old normal animal (N6). All ani-

terior to the coronal plane passing through the rudi-

mals were perfused after 24 h except for the BD kit-

mentary external auditory meatuses (AP0) on one

tens, which were allowed to survive for only 8 h;

side of the brain (over the presumed position of area 18) and a single deposit of a tracer was made in the cortex.

Histological preparation The animals were deeply anaesthetized with pen-

Area 18 was injected iontophoretically (3/zA cur-

tobarbitone sodium before perfusion. Kittens in-

rent pulses for 2 h) with a 10% solution of horserad-

jected with W G A - H R P were perfused first with a bo-

ish peroxidase conjugated with wheat germ aggluti-

lus of normal saline, followed by a solution o f l ~

nin ( W G A - H R P ) dissolved in Tris (hydroxymethyl)-

paraformaldehyde and 1.25% glutaraldehyde, and

methylamine at pH 7.8. This was done in normal kit-

the fixative was flushed out with ~1. l M phosphate

tens, on postnatal days 2 (animal n u m b e r N1), 4 (two animals: N2 and N3), 10 (N4) and 20 (N5), and in the

buffer containing 10% sucrose l~. The animal that had received DY was perfused with a solution containing

8

18

Fig. 1, Coronal sections through the visual cortex of a 20-day normal kitte n (N5) injected with WGA-HRP in area 18. a: tracings of a series of 3 representative sections showing the injection site (fflied areas show dense centre of injection site and not diffuse halo, from which uptake probably does not occur Ls) and clusters of labelled cells lying mainly in layers II, III and upper IVi with some in layers V and VI, but very few in lower layer IV. The borders of area 18, indicated with arrowheads, were defined by studying adjacent Nissland cytochrome oxidase-stained sections, b: a dark-field photomicrograph of the upper layers of area 18 showing two patches of]abelled cells - this section is close to the edge of the dense core of the injection site, which accounts for the background 0f granules of WGAHRP spread throughout the cortex, c: a lower power dark-field photomicrograph showing a single cluster of labelled ceils clearlYlseParated from the edge of the injection site. Most labelled neurones are seen in superficial layers; although a few are present in deep layers. In b and c laminae were defined by reference to adjacent cytochrome oxidase-stained sections. Scale bars: l ram.

33 4% p a r a f o r m a l d e h y d e alone. Sections through the cortex and the lateral geniculate nucleus ( L G N ) were cut (at 50/~m) and m o u n t e d on gelatinized slides; cortex was always sectioned coronally, while the L G N was cut either in a coronal or a parasagittal plane. Sections of cortex and L G N from all the animals injected with W G A - H R P were incubated using the m e t h o d of Mesulam is with tetramethylbenzidine (TMB) as the chromogen and were counterstained with neutral red. A t regular intervals throughout the brains of all 8 animals, sections of cortex and L G N were stained with cresyl violet to allow identification of cortical laminae and the outline of the L G N . A n other series of cortical sections from each animal were used for the histochemical demonstration of cytochrome oxidase activity 29 which helps define the area 17/18 and 18/19 borders, even in young kittens 21.

Analysis Sections were e x a m i n e d using bright- and darka

field or fluorescence microscopy as appropriate. The extent of spread of tracer from the point of injection was measured in each case. Drawings were made of the positions of labelled cells in area 18 around the injection site, as well as of the boundaries of cortical laminae (see below), using a camera lucida. In each kitten, the L G N on the injected side of the brain was examined. The positions of any labelled cells found within the L G N were m a r k e d on lowpower drawings m a d e using a camera lucida. The sizes of cells labelled with W G A - H R P and intermingled unlabelled cells were measured: a graticule marked with a 100 ~tm x 100/~m square was used to demarcate a series of regions within the area of labelled cells, and within each of these regions the outline of every cell in which the nucleus and nucleolus could clearly be seen was drawn at higher power. A sample of several hundred labelled and unlabelled cells was obtained from each animal. Cross-sectional areas of cells were calculated from the drawings using a

18

19

7'

"-

IV

f

I mm I

IOOum Fig. 2. Coronal sections through the visual cortex of a 28-day BD kitten (BD1) injected with WGA-HRP in area 18. a: tracings of a series of representative 'sections (dense core of injection site filled) through area 18 showing clusters of labelled cells; the overall pattern is similar to that seen in normal animals of similar age (see Fig. 1). b: high-power dark-field photomicrograph of one such cluster of labelled cells (a large artefact consisting of crystals of TMB is seen in lower layer III and layer IV).

34 graphics tablet linked to a computer.

The definition of cortical laminae In this study cortical laminae in area 18 were defined in cytochrome oxidase-stained sections. As I have previously described 21. a band of high cytochrome oxidase activity is found in area 18 in cats and kittens overlying layer IV and also extending into lower layer IlI {laminae defined conventionally in Nissl-stained sectionsl; this dense band of staining covering layers IV and lower Ill may demarcate a region receiving direct geniculate inputs 21. Since the cytochrome oxidase method was used to define cortical layers, in this study the term 'layer IV' covers layers IV and lower Ill defined conventionally, and 'layer III', which could not be distinguished from layer II, is equivalent to upper layer Ill defined in Nisslstained sections. Otherwise. cortical laminae defined in cytochrome oxidase-stained material correspond with those seen following a cresyt violet stain. RESULTS

Injection sites The position of each injection site within the visual cortex in relation to the area 17/18 and 18/19 borders was estimated by using cytoarchitectonic criteria 9, by examining the pattern of cvtochrome oxidase activity2~ in cortex around the deposit of tracer, and by studying the uptake of the marker by cells of the LGN. I considered that area 18 had been successfully and exclusively injected if a restricted group of labelled cells was found lying in the centre of the L G N in the coronal plane, and these neurones had a considerably larger mean cross-sectional area than those of the total sample of cells from that region of the LGN12, 23. In all normal animals described in this report, this pattern of label was indeed observed in the LGN. In the 28-day BD animals no L G N cells were clearly labelled, probably because 8 h survival time was insufficient to allow W G A - H R P to accumulate in the nucleus Is However. the injection sites in these animals were similar in size and position to those in the normal animals, and lay within the boundaries of area 18, judged by cytoarchitectonic and cytochrome oxidase patterns 21. as well as by the position of the group of labelled association cells in area 17 (ref. 23). The injection sites varied between 1 and 2 mm in

width, but all involved the full depth of the grey matter The smallest injection site {about 1 mm wide~ was in the 30-day-old kitten injected with DY. Bundies of axons filled with tracer were observed running from the centre of injection sites tthe area of most effective uptaketSl towards the thalamus in all animals.

Labelling around injection stws Older normal and BD kittens. In the two normal animals aged 20 and 30 days, and in the two BD animals aged 28 days. clusters of retrogradely labelled cells surrounded the injection sites (Figs. I and 2). These cells lay mainly in upper layers (ll. llI and the upper part of IVL some were seen in layers V and VI, but very few were located in the lower portion of laver IV. Patches of labelled cells in these 4 avfimals were particularly evident where they were separated from the deposit of tracer bv a region ot cortex that contained no labelled cells. In some sections from these 4 kittens, groups of labelled cells coukt be seen merging with the edge of an injection site Isee Fig. 2a for example), and where these cells were distributed specifically in layers 1I. 11I. upper IV. and partly in layers V and V1 li.e the laminar pattern observed m patches clearl~ separated from the edge of a deposit of tracer), they probably also represented clusters of intrlnsm projection cells labelled retrogradely rather than neurones stained by direct spread. Reconstructions of these clusters of cells forming intrmsm connections within area l x were made from serial sections from these animals to give surface wews of the patches. This was done by demarcating 100 um wide radial strips of cortex m area 18 in a number of serial sections l a l-in-3 series of 5t/ um thick secnonsl and counting the total number of labelled cells in all layers in each stop; Fig. 3 shows such surface views of area 18 in a 20-day normal and a 28-day BD animal that had been injected with W G A HRP. The centres of the patches of labelled cells around the injection site were between 500 um and 1 mm apart, and there was no consistent tendency for these patches to be elongated in a surface-parallel direction. The only abnormality identified in BD animals was that the density of labelled cells within the patches of cells of origin of the intrinsic projections appeared to be lower than in normal kittens of similar age (Fig, 3L However. further studies are required to rule out

35 17

a

is

Rostral

-0 18 19

more t h a n 8 x 1 0 4 cells mm -3

17

4x10 4 - 8x10 4

18

cells mm -3 1 - 4 x 1 0 4 c e l l s mm -3

lOOum

b

18

19

Fig. 3. Surface viewsof injections of WGA-HRP and patches of labelled cells in area 18, each reconstructed from a series of coronal sections. Average densities throughout the cortical depth are indicated across the cortical surface, a: 20-day normal kitten (N5). b: 28-day BD animal (BD2).

the possibility of this being due to differences in experimental technique between animals (including the shorter survival of the BD kittens). The important point is that the laminar pattern of intrinsically labelled cells and the separations of the clusters were qualitatively very similar to those seen in normal animals of similar age. Neonatal normal kittens. In normal animals aged 2 or 4 days, labelled cells were scattered with a roughly uniform density over all layers throughout the cortex immediately surrounding the deposit of traeer, and there were no clear patches of labelled cells separated from the injection site by unlabelled regions of cortex (Fig. 4). The question arises as to whether these cells were labelled by retrograde transport along intrinsic axons or not. While retrograde transport of tracer almost certainly accounts for the labelling of cells lying in discrete clusters around an injec-

tion site in older kittens, a diffuse scatter of labelled cells around injection sites, like that seen in very young kittens, might be produced by direct spread of tracer from the point of injection, particularly since tracers injected into the cortex of immature kittens tend to diffuse more widely than they do in older animals. The fact that distinctly labelled cells, with granular reaction product typical of retrogradely labelled cells, were seen up to 1 mm from the distinct edge of the halo of extracellular tracer at the injection site, suggests that at least some of the neurones had intrinsic connections into the core of the injection site. Cells with intrinsic projections probably do exist in area 18 in very young kittens, but there is no evidence that they are distributed in a patchy fashion. The absence of patches in young kittens could not be explained by there being a difference in the sizes of injection sites relative to the width of area 18 from the area 17/18 border to the 18/19 border at different ages; the proportion of the width of area 18 (estimated from the labelling of cells of the LGN as well as from demarcation of the boundaries in cytochrome oxidase and Nissl-stained sections) that was occupied by an injection of W G A - H R P in a 20-day-old kitten was the same as that involved in an injection that had a smaller absolute size in a 2-day kitten, yet in the former animal clusters of retrogradely labelled cells were found, while in the latter case they were not. In a 10-day-old kitten that was injected with W G A - H R P , some evidence for an uneven distribution of retrogradely labelled cells around the injection site was seen in some sections of area 18. These cells were distributed with a higher density in superficial layers (II, III and upper IV) and deep layers (V and VI), although they were never separated from the edge of the injection site by a region of cortex that contained no labelled cells. This pattern of uptake may represent an intermediate state of organization that is not yet clearly patchy, as in older kittens. In kittens that had received injections of WGAHRP, although the dense core of the injection was confined to grey matter, white matter underlying the injection sites was diffusely stained to a depth of several hundred micrometres. However, W G A - H R P is probably not taken up by intact axonsS, and, in any case, since W G A - H R P infiltrated white matter equally at all ages, the difference in the distributions of labelled cells seen around injection sites at various

36

a 18

/ J

jfJ

J

J

!

_

!

Fig, 4. Coronal sections through the visual cortex of kittens during the first postnatal week. following inlccnons ot WGA-HRP into area 18. a: tracings of a series of 3 representative sections from a 4-day kitten (N2), showing the dcnsc corc of file mjection sitc filled area) and. surrounding it. labelled cells in all cortical laminae, b: a dark-field photomicrograph of a section rostral to the m ~ection sitc showing labelled cells in area 18 in this 4-day kitten (N2). c: photomicrograph of the edge of the injecuon site m ,~ 2-day animal (N 1~ showing the distribution of labelled cells around the deposit of tracer. No clustering of cells containing WGA-HRP was sccn and although labelled neurones were identified surrounding the injection sites, whether they were labelled by direct spread or by retrograde transport is not entirely clear. Scale bars: 1 mm

ages is unlikely to be e x p l a i n e d by v a r i a t i o n in u p t a k e

that the cells f o r m i n g t h e m are not a r r a n g e d in clus-

f r o m fibres-of-passage. In a d d i t i o n , p a t c h e s of cells

ters. but are u n i f o r m l y distributed a r o u n d their tar-

m a k i n g intrinsic c o n n e c t i o n s w e r e seen f o l l o w i n g the i n j e c t i o n of D Y . which did not i n v o l v e w h i t e m a t t e r

get in a r e a 18. In adult cats. within all clusters o f cells that f o r m

at all.

intrinsic c o n n e c t i o n s to o n e point m a r e a 18. all neu-

DISCUSSION

lusl5-17: It a p p e a r s that local c o r n e a l i n t e r c o n n e c t l o n s

rones r e s p o n d to a similarly o r i e n t e d visual stimuare m a d e b e t w e e n cell p o p u l a t i o n s w h o s e prefe rredT h e s e results s h o w that the clustered distribution of

o r i e n t a t i o n s are o r t h o g o n a ] to each o t h e r w, Thus_

cells f o r m i n g intrinsic c o n n e c t i o n s in a r e a 18 of the

M a t s u b a r a and h e r co-workers~5-~v have p r o p o s e d

visual c o r t e x is p r e s e n t in kittens of 20 days or o l d e r .

that ' o r i e n t a t i o n selectivity plays a d o m i n a n t role in

and t h e y suggest that this p a t c h y p a t t e r n first a p p e a r s life. T h e s e

local cortical c o n n e c t i o n s ' , bul have s u g g e s t e d that the spatial a r r a n g e m e n t s of local c o n n e c t i o n s do not

patches of cells w e r e n o t d e t e c t a b l e d u r i n g the first

b e a r any r e l a t i o n s h i p to the spalial o r g a n i z a t i o n oi

w e e k p o s t n a t a l l y , a l t h o u g h it is p r o b a b l e that intrin-

ocular d o m i n a n c e p r o p e r t i e s 15-~: W h a t little inform a t i o n is available on the d e v e l o p m e n t of o r i e n t a t i o n

d u r i n g the s e c o n d w e e k of p o s t n a t a l

sic c o n n e c t i o n s are p r e s e n t at these early ages and

37 columns in area 18 suggests that the spatial organization of the orientation domain seen in adult cats is not well developed until the beginning of the third postnatal week, although there may be some order in the arrangement of those neurones that are orientationselective before this age2,3,22. It appears, then, that the development of the clustered distribution of cells producing intrinsic connections in area 18 may occur at a similar age to that at which orientation columns become readily detectable, during the third week postnatally. There may also be a relationship between the development of intrinsic connections of the visual cortex and the maturation of the cortico-cortical pathway from area 17 to area 18; both of these projections arise in adult cats and older kittens from cells lying in clusters in layers II, III, upper IV, and, to a lesser extent, V and VI 1,1°,2°,23, and the clusters of cells of origin of both pathways appear during the third postnatal week20, 23. However, the nature of any possible relationship between cortico-cortical and intrinsic connections is unclear. Axonal elimination has been shown to occur during the postnatal maturation of many neural pathways in a number of mammalian species (reviewed by Cowan et al.6), and it is probably a major developmental mechanism for eliminating the many inappropriate projections that form prenatally. Both axonal elimination and cell death appear to play parts in the development of the clusters of cells in area 17 that form cortico-cortical projections to area 18 (Price and Blakemore, in preparation), and it is possible that the emergence of the patches of cells producing intrinsic connections is caused by a similar combination of processes. By the age at which the first evidence is seen of a patchy distribution of ceils projecting internally within area 18, the eyes have opened, but patterned visu-

REFERENCES 1 Albus, K. and Meyer, G., Spiny stellates as cells of origin of association fibres from area 17 to area 18 in the cat's neocortex, Brain Res., 210 (1981) 335-341. 2 Albus, K. and Wolf, W., Early postnatal development of neuronal function in the kitten's visual cortex: a laminar analysis, J. Physiol. (London), 348 (1984) 153-185. 3 Blakemore, C. and Price, D.J., Organization and development of orientation columns in area 18 of the cat's visual cortex, J. Physiol. (London), 358 (1985) 16P,

al stimulation is not required for the appearance of these patches, since they were just as obvious in 28day BD animals as in normal kittens of similar age. Singer and Tretter 26 studied the effects of BD by eyelid-suture on the physiological properties of neurones in area 18 in cats and concluded that, while in deprived animals the responses of individual cells were generally weaker and the receptive-field properties were poorly specified, the afferent, efferent and intrinsic connections of area 18 were intact. The results of this present study show that the anatomical organization of the intrinsic connections of area 18 appears qualitatively normal in one-month-old BD kittens, although it is possible that the density of these projections is reduced. Indeed, it has been shown that the number of callosally projecting neurones in areas 17 and 18 is lower in the BD cat than in normal animals 13. It is also possible that a prolonged period of BD, perhaps into adulthood, would produce a deterioration in the clustered arrangement of intrinsically projecting neurones in area 18. Finally, it would be interesting to know whether intrinsic connections also have a normal arrangement in dark-reared kittens, since in lid-sutured animals activation of cells in the visual cortex by stimulation through the closed eyelids may still be possible 27, particularly in area 18 where cells generally respond to low spatial frequencies ~9. ACKNOWLEDGEMENTS 1 thank Colin Blakemore for his help throughout the study, Karen Wenk-Salamone for technical assistance, and the Medical Research Council for financial support. The author is an MRC Training Fellow and the experiments were funded from MRC Programme Grants (G979/49 and G7900491) to Colin Blakemore.

4 Blakemore, C. and Van Sluyters, R.C., Innate and environmental factors in the development of the kitten's visual cortex, J. Physiol. (London), 248 (1975) 663-716. 5 Brodal, P., Dietrichs, E., Bjaalie, J.G., Nordby, T. and Walberg, F., Is lectin-coupled horseradish peroxidase taken up and transported by undamaged as well as by damaged fibres in the central nervous system, Brain Res., 278 (1983) I-9. 6 Cowan, W.M., Fawcett, J.W., O'Leary, D.D.M. and Stanfield, B.B., Regressive events in neurogenesis, Science, 225 (1984) 1258-1265.

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7 Eldridge, J.L., H e n d e r s o n , Z. and Wilkins, V.C., A stereotaxic m e t h o d for neonatal kitten brain, J. Physiol. (London), 320 (19811 12P. 8 Fregnac, Y. and Imbert, M , Development of neuronal selectivity in primary visual cortex of cat, Physiol. Rev,. 64 (19841 325-434, 9 Garey, L.J., A light and electron microscopic study of the visual cortex of the cat and m o n k e y , Proc. R. Soc. London Ser. B.. 179 (19711 21-40. 10 Gilbert, C.D. and Kelly, J.P., The projections of cells in the different layers of the cat's visual cortex, J. Cornp. Neurol., 163 (1975) 81-106. 11 Gilbert, C.D. and Wiesel, T . N . . Clustered intrinsic connections in cat visual cortex, J. Neurosci., 3 (1983) 1116-1133. 12 Henderson, Z., A n anatomical investigation of projections from lateral geniculate nucleus to visual cortical areas 17 and 18 in newborn kitten, Exp. Brain Res.. 46 (19821 177-185. 13 lnnocenti, G.M. and Frost, D . O . , The postnatal development of visual callosal connections in the absence of visual experience or of the eyes, Exp. Brain Res., 39 (1980) 365-375. 14 Lund, R . D . , Mitchell, D.E. and Henry, G . H . , Squint-induced modification of callosal connections in cats, Brain Res., t44 (19781 169-172. 15 Matsubara, J. and Cynader, M., Clustered intracortical connections in cat visual cortex: physiological identification of injection site and projection areas, A R V O Abstr.. 24 (1983) 1(/5. t6 Matsubara. J. and Cynader, M., The role of orientation tuning in the specificity of local intracortical connections in the cat visual cortex: an anatomical and physiological study, Soc. Neurosei. Abstr., 9 (19831 475. 17 Matsubara, J., Cynader, M., Swindale, N.V. and Stryker, M.P., Intrinsic projections within visual cortex: evidence for orientation-specific local connections, Proc. Natl. Acad. 5ci. U.S.A., 82 (1985) 935-939. 18 Mesulam, M.-M., Principles ot horseradish peroxidase neurohistochemistry and their applications for tracing neural pathways - - axonal transport, enzyme histochemistry

and light microscopic analysis. In M.-M. M e s u l a m (Ed.l Tracing Neural Connections with Horseradish Peroxidase. John Wiley. New York, 1982, pp. 1-15 I. it,) Movshon. J.A.. T h o m p s o n , I.D. and Tothurst. D.J., Spatial and temporal contrast sensttiv~ty of neurones in areas 17 and 18 of the cat's visual cortex. J. Phv,,i~d. (LondonJ. 283 { 19781 l / ) l - 12(/. 2(I Price. D.J., The postnatal development <)~ the assocmtlon projection from visual cortical area 17 to area t8 in the cat, J. Physiol, (London), 346 ( 19841 31P. 21 Price. D.J.. Patterns of cytochromc oxidasc acnvity m areas 17. 18 and 19 of the visual cortex ol cats and kittens, Exp. Brain Res. 58 11985) 125-133. 22 Price. D.J. and Blakemore. C . The postnatal development of orientation-selectivity in area 18 of the cat's visual cortex. Soc. Neurosci. Abstr.. 10 1984147~) 23 Price. D.J. and Blakemore. C.. The p(,stnatal development of the association projection from visual cortical area 17 to area 18 in the cat J. Neurosci.. in press 24 Rockland. K S . and Lund. J.S.. Widespread periodic intrinsic connections in the tree shrew visual cortex Science. 215 ~19821 1532-1534, 25 Rockland, K.S. and Lund. J.S.. Intrinsic laminar latticc connections m primate visual cortex, J. (ornp Neurol.. 216 19831 3(/3-318. 26 Singer, W and Tretter. F.. Receptive-field propcrues and neuronal connecliwty m striate and parastriate cortex of contour deprived cats. J. Neur(z~hvsiol.. 39 {1976~ ¢~1 3 - 6 3 0 . 27 Spear, P.D., l o n g , L. and Langsetm(), A.. Striate cortex neurones of binocularly deprived kittens respond to vtsual stimuli through the closed eyelids, Brain Res'. 155 119781 1 4 t - 146 28 Swindale, N.V.. Absence nl ocular dominance patches m dark-reared cats, Nature t LondonL 2~1l 19811 3 3 2 - 3 3 3 29 Wong-Riley, M.. Changes in the visual system of monocutarlv sutured or enucleated cats demonstrabIe with cytochrome oxidase histochemistry. Brain Res. 171 ~19791 " 1--28.