Geniculocortical Afferents Research Articles

Morphology of Single Geniculocortical Afferents and Functional Recovery of the Visual Cortex after Reverse Monocular Deprivation in the Kitten

To investigate the possible anatomical basis for the functional recovery of visual cortical responses after reverse monocular deprivation, we have studied the morphology of single geniculocortical afferents to area 17. In kittens reverse-sutured for 10 d after an initial week of monocular deprivation, single-unit and intrinsic signal optical recordings confirmed that the effects of the initial deprivation were largely reversed. Responses through the originally nondeprived (OND) eye were drastically diminished, but remained much more selective for orientation than after an initial monocular deprivation (Crair et al., 1997). Responses through the originally deprived (OD) eye recovered completely. Geniculocortical afferent arbors in layer IV of area 17 were filled by iontophoresis of Phaseolus lectin into lamina A of the lateral geniculate nucleus (LGN) and were serially reconstructed. Arbors serving both the OD and the OND eye were analyzed. The plastic changes of both OD and OND arbors were evaluated by comparison with arbors reconstructed in normal animals and in animals studied after an equivalent initial period of deprivation (Antonini and Stryker, 1996). These analyses demonstrate that closure of the OND eye caused a substantial shrinkage of the arbors serving that eye. Moreover, reopening the OD eye induced regrowth only in some arbors, whereas others appeared to be largely unaffected and continued to have the characteristics of deprived arbors. Quantitatively, the initial and the second deprivation caused similar proportional changes in total arbor length and numbers of branches, whereas several other features were more severely affected by the initial deprivation.

Competition for Neurotrophic Factors: Ocular Dominance Columns

Activity-dependent competition between afferents in the primary visual cortex of many mammals is a quintessential feature of neuronal development. From both experimental and theoretical perspectives, understanding the mechanisms underlying competition is a significant challenge. Recent experimental work suggests that geniculocortical afferents might compete for retrograde neurotrophic factors. We show that a mathematically well-characterized model of retrograde neurotrophic interactions, in which the afferent uptake of neurotrophic factors is activity-dependent and in which the average level of uptake determines the complexity of the axonal arbors of afferents, permits the anatomical segregation of geniculocortical afferents into ocular dominance columns. The model induces segregation provided that the levels of neurotrophic factors available either by activity-independent release from cortical cells or by exogenous cortical infusion are not too high; otherwise segregation breaks down. We show that the model exhibits changes in ocular dominance column periodicity in response to changes in interocular image correlations and that the model predicts that changes in intraocular image correlations should also affect columnar periodicity.

The layout of orientation and ocular dominance domains in area 17 of strabismic cats.

In the primary visual cortex of strabismic cats, the elimination of correlated activity between the two eyes enhances the segregation of the geniculocortical afferents into alternating ocular dominance domains. In addition, both tangential intracortical fibres and neuronal synchronization are severely reduced between neurons activated by different eyes. Consequently, ocular dominance columns belonging to different eyes are functionally rather independent. We wondered whether this would also affect the organization of orientation preference maps. To this end, we visualized the functional architecture of area 17 of strabismic cats with both optical imaging based on intrinsic signals and double labelling of orientation and ocular dominance columns with [14C]2-deoxyglucose and [3H]proline. As expected, monocular iso-orientation domains had a patchy appearance and differed for the two eyes, leading to a clear segregation of the ocular dominance domains. Comparison of 'angle maps' revealed that orientation domains exhibit a pinwheel organization as in normally reared cats. Interestingly, the map of orientation preferences did not show any breaks at the borders between ocular dominance columns: iso-orientation domains were continuous across these borders. In addition, iso-orientation contours tended to cross the borders of adjacent ocular dominance columns at right angles. These data suggest that the basic relations between the layout of orientation maps and ocular dominance columns are not disturbed by artificial decorrelation of binocular input. Therefore in cat area 17, the orientation map does not seem to be modified by experience-dependent changes of thalamic input connections. This suggests the possibility that use-dependent rearrangement of geniculocortical afferents into ocular dominance columns is due to Hebbian modifications whereby postsynaptic responsivity is constrained by the scaffold of the orientation map.

Monocular Core Zones and Binocular Border Strips in Primate Striate Cortex Revealed by the Contrasting Effects of Enucleation, Eyelid Suture, and Retinal Laser Lesions on Cytochrome Oxidase Activity

In primate striate cortex, geniculocortical afferents in layer IVc terminate in parallel stripes called ocular dominance columns. We propose that this segregation of ocular inputs generates a related but distinct columnar system of monocular core zones alternating with binocular border strips. Evidence for this functional parcellation was obtained by comparing the effects of enucleation, eyelid suture, and retinal laser lesions on cytochrome oxidase (CO) activity in eight macaques. Enucleation produced a high-contrast pattern of dark and light columns in layer IVc, corresponding precisely to the ocular dominance columns, whereas eyelid suture produced a low-contrast pattern of thin dark columns alternating with wide pale columns. [3H]Proline eye injection showed that the thin dark columns corresponded to the core zones of the open eye's ocular dominance columns. The wide pale columns resulted from loss of CO activity in the sutured eye's core zones and within both eyes' border strips. Loss of CO activity within both eyes' border strips suggested that these regions are binocular. To confirm our findings, we compared different CO patterns in the same cortex by making retinal laser lesions in four animals. They produced a CO pattern tantamount to "focal" enucleation, although contrast was low when laser damage was confined to the outer retina. CO levels in cortical scotomas remained severely depressed for months after retinal lesions, even when the other eye was enucleated. This observation provided little anatomical support for the notion of topographic plasticity after visual deafferentation. In a single human subject with macular degeneration, CO revealed a low-contrast pattern of ocular dominance columns, resembling the pattern in monkeys with laser-induced photoreceptor damage.

Developmental and sensory-dependent changes of phosphoinositide-linked metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) can modulate synaptic transmission, and there is evidence that phosphoinositide (PI)-linked mGluRs may be involved in sensory-dependent plasticity during the development of cat visual cortex. Consequently, we asked the questions: Where are the PI-linked mGluRs (mGluR1alpha and mGluR5) in the visual cortex? Does the quantity and distribution of these receptors change in the cat visual cortex during postnatal development, and are these features sensory-dependent? We found that the quantity of mGluR1alpha decreases with age, whereas the laminar distribution of mGluR1alpha remains the same. Quantity of mGluR5 also decreases, but the laminar distribution of mGluR5 changes during development. The pattern and timing of the mGluR5 change in distribution follow the development of geniculocortical afferents. Immunostaining indicates that reduction of receptor occurs mainly in layers V-VI for mGluR1alpha and outside layer IV for mGluR5. Dark-rearing postpones the laminar change of mGluR5 and produces an increased level of mGluR5 between postnatal 1.5-6 weeks of age but has no significant effect on the mGluR1alpha distribution or the mGluR1alpha quantity. These results suggest that mGluR1alpha and mGluR5 are involved in different aspects of cortical development. The mGluR5 is more likely to be involved in sensory-dependent events than mGluR1alpha. The lack of developmental correlation between mGluR quantities and the critical period for ocular dominance plasticity also suggests that other factors besides mGluR quantities are important for ocular dominance plasticity.

Geniculo-cortical afferents form synaptic contacts with vasoactive intestinal polypeptide (VIP) immunoreactive neurons of the rat visual cortex

The lateral geniculate nucleus of the rat was injected with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) to see if geniculo-cortical axons terminate on vasoactive intestinal polypeptide immunoreactive (VIP-IR) neurons of the primary visual cortex. PHA-L-labelled boutons attached to VIP-IR perikarya and dendrites were identified as presynaptic parts of asymmetrical synapses. This geniculo-cortical projection to VIP-IR cells in the visual cortex and comparable findings in the somatosensory cortex suggest that sensory input from specific thalamic nuclei may influence local circuit inhibition and the metabolic state within the cortical domain via VIP-IR neurons.

Cortical direction selectivity without directional experience.

How neurones in the visual cortex acquire their response properties during postnatal development is an important question with far-reaching implications. In the present study, we demonstrate the developmental specification of geniculo-cortical afferents using our previously proposed model for the activity-dependent self-organization of neural networks. Our results indicate, in contrast to common beliefs, that both orientation and direction selectivity can be achieved in the primary visual cortex even if the retinae were never exposed to oriented and/or moving objects during development.

Development of Orientation Preference Maps in Ferret Primary Visual Cortex

The development of orientation preference maps was studied in ferret primary visual cortex using chronic optical imaging of intrinsic signals. The emergence and maturation of the maps were examined over time in single animals. The earliest age at which cortical domains selectively responsive to particular stimulus orientations were observed varied considerably between individuals, from postnatal day 31 to 36. In all cases, the earliest maps seen were low-contrast, with regions of orientation-specific activity that were difficult to distinguish from noise. These early maps matured over a period of several days into the high-contrast, patchy maps typical of adult animals. The structure of the orientation maps was remarkably constant over time. The indistinct features in the earliest maps were always patches of the same sizes and shapes and at the same locations as in the maps obtained in subsequent recording sessions. Details of the more mature maps, including the relative intensities of individual iso-orientation domains, were also constant from one recording session to another over periods of several weeks. The patterning of iso-orientation domains in ferret primary visual cortex thus is established early in development and remains stable over time, unaffected by either normal visual experience or the anatomical rearrangements of geniculocortical afferents into eye-specific domains.

Plasticity of geniculocortical afferents following brief or prolonged monocular occlusion in the cat

During a critical period in early life, physiological studies reveal that either prolonged or brief periods of monocular occlusion induce similar plastic changes in the primary visual cortex (area 17) of the cat, leading to a nearly complete loss of visual responses from the deprived eye (Hubel and Wiesel [1970], J. Physiol. (London) 160: 106-154). However, the recovery of function is markedly different in the two conditions, being complete and thorough only after a brief period of monocular occlusion (Movshon [1976] J. Physiol. (London) 261: 125-174). In search for anatomical correlates that distinguish between these two experimental conditions, we compared the geniculocortical connectivity in normal kittens with that following brief (4 days and 6-7 days) or prolonged (> 5 weeks) periods of occlusion of vision in one eye. Transneuronal labeling of the geniculocortical pathway revealed changes in both cases, and single afferent arbors reconstructed in serial sections were similarly shrunken after short or long periods of deprivation. However, a significant expansion of the geniculocortical afferents serving the nondeprived eye was evident mainly after prolonged deprivation. These findings provide further evidence for rapid, activity-dependent remodeling of afferents during development.

Plasticity of geniculocortical afferents following brief or prolonged monocular occlusion in the cat

During a critical period in early life, physiological studies reveal that either prolonged or brief periods of monocular occlusion induce similar plastic changes in the primary visual cortex (area 17) of the cat, leading to a nearly complete loss of visual responses from the deprived eye (Hubel and Wiesel [1970], J. Physiol. (London) 160: 106–154). However, the recovery of function is markedly different in the two conditions, being complete and thorough only after a brief period of monocular occlusion (Movshon [1976] J. Physiol. (London) 261: 125–174). In search for anatomical correlates that distinguish between these two experimental conditions, we compared the geniculocortical connectivity in normal kittens with that following brief (4 days and 6–7 days) or prolonged (>5 weeks) periods of occlusion of vision in one eye. Transneuronal labeling of the geniculocortical pathway revealed changes in both cases, and single afferent arbors reconstructed in serial sections were similarly shrunken after short or long periods of deprivation. However, a significant expansion of the geniculocortical afferents serving the nondeprived eye was evident mainly after prolonged deprivation. These findings provide further evidence for rapid, activity-dependent remodeling of afferents during development. © 1996 Wiley-Liss, Inc.

An adult-like pattern of ocular dominance columns in striate cortex of newborn monkeys prior to visual experience

In macaque monkeys, the geniculocortical afferents serving each eye segregate in layer IVc of striate cortex during early life into a pattern of alternating inputs called ocular dominance columns. It has been disputed whether visual experience is necessary for the formation of ocular dominance columns. To settle this issue, fetal monkeys were delivered prematurely by Caesarean section at embryonic day 157 (E157), 8 d before the end of normal gestation. To avoid light exposure, the Caesarean section and all subsequent feedings and procedures were done in absolute darkness, using infrared night-vision goggles. Tritiated proline was injected into the right eye 1 d after delivery (E158). One week later at postnatal age 0 (P0), the equivalent of a full-term pregnancy (E165/P0), alternate sections of unfolded and flattened visual cortex were prepared for autoradiography or cytochrome oxidase (CO). All three newborns studied at E165/P0 had well segregated ocular dominance columns organized into the characteristic mosaic present in adults. In the upper layers, a mature pattern of CO patches (also known as blobs or puffs) was visible, aligned with the ocular dominance columns in layer IVc. Every other row of patches in layers II, III was labeled by [3H]proline. In V2, a distinct system of alternating thick-pale-thin-pale CO stripes was present. These findings indicate that stimulation of the retina by light is not necessary for the development of columnar systems in the visual cortex. Ocular dominance columns, patches, and V2 stripes all are well formed before visual experience. Even the thalamic input to the patches in the upper layers of striate cortex is segregated by eye in newborns.

Functional architecture of area 17 in normal and monocularly deprived marmosets (Callithrix jacchus).

The organization of the primary visual cortex (VI) of the common marmoset (Callithrix jacchus) was studied both physiologically and by means of transneuronal labelling of geniculocortical afferents. We addressed the question whether monocular deprivation (MD) could stabilize segregation into ocular dominance (OD) columns, which are not seen in normal adult marmosets but are present in juvenile animals (Spatz, 1979, 1989). Properties of neurons in normal marmosets closely resembled those of other New-World and Old-World monkeys and orderly tangential progressions of preferred orientation were observed. However, in contrast to species that display well-defined OD columns, neurons of layer 4 in V1 of normal adult marmosets received balanced inputs from the two eyes. Early MD (even though followed by prolonged binocular experience into adulthood) resulted in a reduction of cell size in laminae of the lateral geniculate nucleus with input from the deprived eye and a dramatic overall shift in ocular dominance towards the nondeprived eye in the cortex. However, isolated clusters of cells dominated by the deprived eye were found in both layers 4 and 6. Injection of lectin-conjugated horseradish peroxidase (WGA-HRP) into the deprived eye revealed elongated patches of terminal label, about 350 microns wide, in flat-mounted sections through layer 4. Afferent segregation was sharper and more regular in the region of V1 representing parafoveal visual space than in that representing the fovea. Our findings support the notion that all Old-World and New-World monkeys possess the capacity for segregation of geniculocortical afferents into OD columns.

Relationships between dendritic fields and functional architecture in striate cortex of normal and visually deprived cats

We examined relationships between the pattern of geniculocortical innervation and the dendritic fields of cells in layer 4 of in cat primary visual cortex. Experiments were performed on normal animals and on cats in which the geniculocortical projection was altered by monocular deprivation or by the induction of divergent squint during the critical period. Thalamic afferents providing the input from the contralateral eye were anterogradely labeled by injecting the fluorescent tracer Dil into lamina A of the lateral geniculate nucleus. Intracellular staining with Lucifer yellow in slice preparations allowed simultaneous visualization of the morphology of individual cells and the thalamic afferents. Our results demonstrate that spiny stellate cells close to the upper and lower margin of the geniculocortical input have highly asymmetric dendritic fields, and thereby confine their dendrites to the termination zone of these afferents. This effect was specific for the cell class; it was not observed in pyramidal neurons. These dendritic asymmetries perpendicular to the laminar borders of spiny stellate cells were not altered by monocular deprivation or strabismus. In contrast, visual deprivation strongly influenced the dendritic arbors of spiny stellate cells near the borders between adjacent ocular dominance columns. In normal animals, the dendrites of cells near columnar borders remained preferentially within one column. These dendritic asymmetries became much more pronounced in strabismic animals. Monocular deprivation weakened the influence of the columnar borders on dendritic fields. Spiny stellate cells within the columns of the open eye exhibited a slight tendency to confine their dendrites to these columns. Cells in the columns of the deprived eye showed the opposite effect; they extended their dendrites preferentially into the adjacent columns of the open eye. These results demonstrate that the segregation of geniculocortical afferents into ocular dominance columns and its perturbation by manipulation of the visual input plays an important role in defining the morphology of cortical target cells. Thus, activity-dependent structural changes not only occur at the level of the presynaptic terminals, but also at the level of the postsynaptic target cells, and thereby contribute to build up the functional architecture of the cortex.

A developmentally regulated nerve growth factor-induced gene, VGF, is expressed in geniculocortical afferents during synaptogenesis.

The expression of the nerve growth factor-inducible gene VGF has been examined by in situ hybridization. Western blot and immunohistochemical studies in the developing and adult rat central nervous system, with particular emphasis on the visual system. Both the messenger RNA and the protein are particularly abundant in the developing dorsal lateral geniculate nucleus, appearing, respectively, at embryonal day 16 and 18. After its onset at E16, VGF messenger RNA expression increases progressively in the dorsal lateral geniculate nucleus and remains high during the first two post-natal weeks; afterwards, it gradually decreases and, at the offset of the plasticity period, it reaches very low levels maintained in adulthood. A similar time course has been observed for VGF protein in the dorsal lateral geniculate nucleus area, by semi-quantitative Western blots. In addition to the presence of the protein in the geniculate neurons, a strong, transient immunoreactivity has been found at the embryonic cortical subplate at E18, reflecting the presence of the antigen in axonal terminals originating from thalamic neurons. Interestingly, we found that the blockade of afferent electrical activity by intraocular injection of tetrodotoxin strongly reduces the level of VGF messenger RNA in the dorsal lateral geniculate nucleus. Although the function of the VGF protein is not known, it had been previously proposed that VGF could be a precursor for neuropeptide/s. The spatiotemporal expression of VGF, together with the observation of a regulation by electrical activity, suggest that this protein may be relevant in the process of synaptogenesis and/or synaptic stabilization in the developing geniculocortical connections.

Segregation of geniculocortical afferents during the critical period: a role for subplate neurons

To investigate the cellular interactions within the mammalian visual cortex that are important in ocular dominance column formation, we have examined the role of subplate neurons in this process. LGN axons segregate in layer 4 of the cat's visual cortex between the third and sixth postnatal weeks to give rise to the adult pattern of ocular dominance columns. Subplate neurons are a transient population of neurons that sit in the white matter but have extensive projections into the overlying cortex, particularly layer 4, during neonatal life. Many subplate neurons are present at birth, but most are gone by the end of the period of LGN axon segregation. To examine whether these neurons are required for the segregation of LGN axons, we deleted them by intracortical injections of kainic acid either just after LGN axons had grown into layer 4 (first postnatal week) or later, just before the onset of segregation (third postnatal week). The consequences for the patterning of geniculocortical terminals were evaluated by transneuronal transport of 3H-proline injected into one eye at times when segregation would normally be complete. Following deletion of subplate neurons at either age, LGN axons failed to segregate into ocular dominance columns. Following the late deletions only, geniculocortical axons lost their laminar restriction to layer 4 and projected to layers 2 and 3 as well. Deletion of subplate neurons also resulted in long-term changes in the cytoarchitecture of layer 4. These observations suggest that the interactions that mediate segregation of LGN axons within layer 4 of visual cortex are susceptible to influences from subplate neurons. Although the mechanisms by which subplate neurons exert their effect are not yet clear, these experiments strongly suggest that interactions between LGN axons and layer 4 neurons are not sufficient for column formation, and that subplate neurons most likely play a critical role in interactions leading to ocular segregation.

Axonal sprouting accompanies functional reorganization in adult cat striate cortex.

Removal of sensory input from a focal region of adult neocortex can lead to a large reorganization of cortical topography within the deprived area during subsequent months. Although this form of functional recovery is now well documented across several sensory systems, the underlying cellular mechanisms remain elusive. Weeks after binocular retinal lesions silence a corresponding portion of striate cortex in the adult cat, this cortex again becomes responsive, this time to retinal loci immediately outside the scotoma. Earlier findings showed a lack of reorganization in the lateral geniculate nucleus and an inadequate spread of geniculocortical afferents to account for the cortical reorganization, suggesting the involvement of intrinsic cortical connections. We investigated the possibility that intracortical axonal sprouting mediates long-term reorganization of cortical functional architecture. The anterograde label biocytin was used to compare the density of lateral projections into reorganized and non-deprived cortex. We report here that structural changes in the form of axonal sprouting of long-range laterally projecting neurons accompany topographic remodelling of the visual cortex.

Development of gemculocortical projections to visual cortex in rat: Evidence for early lngrowth and synaptogenesis

Anterograde movement of DiI and transneuronal transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were used to study the temporal and laminar patterns of ingrowth of the geniculocortical projection to visual cortex in fetal and postnatal rats. The development of this projection was compared to patterns of migration and settling of [3H]-thymidine-labeled neurons destined for cortical layer IV, and to geniculocortical synapse formation. DiI-labeled geniculocortical axons were found in the intermediate zone beneath the lateral cerebral mantle at embryonic day (E)17 and in the subplate layer underlying visual cortex by E18. On E19 they appeared to accumulate and grow radially into an expanding subplate layer and into the deep part of developing cortical layer VI. By postnatal day (P)0, DiI or WGA-HRP-labeled geniculocortical axons were found in developing cortical layers VI and V. By P1, they invaded the deep portion of the cell-dense cortical plate, where they were in position to make initial contact with neurons that would later form layer IV. A few axons traversed the cortical plate to reach the marginal zone. Layer IV became an identifiable layer on P2, and a clear projection to layer IV was evident by P3. These results suggest that geniculocortical afferents grow continuously from the intermediate zone, initially into an expanding subplate layer and then sequentially into each of the developing cortical layers without evidence of "waiting." Electron microscopic data suggest that geniculocortical axons begin to form immature synapses with dendrites and neuronal perikarya as they first encounter cortical neurons, first in the subplate layer and then in developing layers VI, V and marginal zone, in addition to the primary target layer IV. The precise targeting and overall temporal and laminar patterns of ingrowth and synaptogenesis suggest that geniculocortical axons are directed to the visual cortex by guidance cues within the internal capsule and subplate. Further, they reach the occipital pole early enough to influence the specification and histogenesis of cortical area 17, perhaps by exerting an influence on the deep-to-superficial "wave" of neuronal differentiation in sequentially developing subplate and cortical layers VI, V and IV.

Plasticity of an aberrant geniculocortical pathway in neonatally lesioned cats

Geniculocortical afferents to the lateral suprasylvian (LS) visual area come only from the C layers in normal adult cats. After neonatal lesions of areas 17 and 18, inputs to the LS area also arise from the A layers. We studied the plasticity of this aberrant pathway. We made lesions in areas 17 and 18 of newborn kittens, some of which were then monocularly deprived. In lesioned undeprived cats, retrograde tracing confirmed projections to the LS area from both the A and A1 as well as the C geniculate layers. In lesioned deprived cats, geniculate afferents from the A layers to the LS area came mainly from the layer receiving a normal visual input. We conclude that the development of the abnormal pathway from the A layers to the LS area is influenced by patterned visual activity.

Numerical relationships between geniculocortical afferents and pyramidal cell modules in cat primary visual cortex.

An analysis has been made of the quantitative data available on the number of pyramidal cell modules of layer IV neurons, and of geniculocortical axons and their synapses in cat striate cortex. It is found that the convergence of geniculocortical afferents upon any one pyramidal cell module is enormous, since in any one location there is overlap between 360-540 X-axons and 300-540 Y-axons. In total, the X- and Y-axonal arbors provide some 1640 x 10(6) synapses to area 17, which is equivalent to a ratio of 160-200 synapses per layer IV neuron. These values assume that geniculocortical terminals synapse only with the spiny stellate cells of layer IV. The values are reduced to 100-125 per spiny stellate cell when account is taken of the synapses that involve the dendrites that enter layer IV from neurons with cell bodies in other layers. Since each layer IV neuron receives some 2500 asymmetric synapses, this means that only 5% of the total excitatory input to a layer IV neuron seems to be provided by the geniculocortical afferents. Further, if the boutons in the geniculocortical axonal arbors are distributed homogeneously across layer IV, each axon could only provide one synapse to about one in four of the layer IV neurons encompassed by its plexus. It may be, however, that instead of being spread evenly, boutons in individual arbors converge upon individual neurons to supply a number of synapses to them. But even so, it seems unlikely that any individual geniculate axon could dominate the activity of a particular cortical neuron.

Receptive field dynamics in adult primary visual cortex.

The adult brain has a remarkable ability to adjust to changes in sensory input. Removal of afferent input to the somatosensory, auditory, motor or visual cortex results in a marked change of cortical topography. Changes in sensory activity can, over a period of months, alter receptive field size and cortical topography. Here we remove visual input by focal binocular retinal lesions and record from the same cortical sites before and within minutes after making the lesion and find immediate striking increases in receptive field size for cortical cells with receptive fields near the edge of the retinal scotoma. After a few months even the cortical areas that were initially silenced by the lesion recover visual activity, representing retinotopic loci surrounding the lesion. At the level of the lateral geniculate nucleus, which provides the visual input to the striate cortex, a large silent region remains. Furthermore, anatomical studies show that the spread of geniculocortical afferents is insufficient to account for the cortical recovery. The results indicate that the topographic reorganization within the cortex was largely due to synaptic changes intrinsic to the cortex, perhaps through the plexus of long-range horizontal connections.