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M Sur

Publications and source records attributed to M Sur.

At least 91 records · Page 5Linked to original sources
PubMed1989

Effects of convergent strabismus on the development of physiologically identified retinogeniculate axons in cats.

We have studied the effects of surgically induced convergent strabismus (esotropia) on the morphological development of retinogeniculate X and Y axon arbors in cats. Single axons were recorded in the lateral geniculate nucleus or in the optic tract adjacent to the nucleus, classified physiologically, and injected intracellularly with horseradish peroxidase. The arbors of recovered axons were compared with X and Y axon arbors from normally reared adult cats. Our data demonstrate that while X axon arbors are relatively normal, the arbors of Y axons are profoundly affected by rearing with strabismus. Y axons, whether originating from the deviated or the nondeviated eye, have substantially smaller arbors and fewer boutons in the A-laminae of the lateral geniculate nucleus compared to Y axons in normal cats. The C-lamina terminations of contralaterally projecting Y axons in the strabismic cats are unaffected. These results suggest that the postnatal development of retinogeniculate Y axon arbors in the A-laminae is strongly influenced by abnormalities in postnatal visual experience. Furthermore, the present data suggest that, in addition to intraocular competitive interactions between X and Y axons previously proposed to account for the effects of other rearing conditions, interactions between afferents from the two eyes must also be involved in the development of at least Y axons.

Action Potentials↗
PubMed1989

Terminal arbors of single ON-center and OFF-center X and Y retinal ganglion cell axons within the ferret's lateral geniculate nucleus.

The lateral geniculate nucleus of the ferret contains not only eye-specific layers, but a further subdivision of layers A and A1 into inner and outer sublaminae that contain, respectively, ON-center and OFF-center cells (Stryker and Zahs, '83). To study how the arbors of single retinal ganglion cell axons correlate with these cellular divisions, we have examined the morphology of physiologically classified retinal axons in the ferret's lateral geniculate nucleus. As in cats, we could classify retinal axons as X or Y on the basis of a number of physiological criteria. X and Y axons have distinct patterns of termination in the lateral geniculate nucleus. Contralateral X axons innervate lamina A and ipsilateral axons lamina A1. X axons are further segregated in these laminae so that ON-center axons terminate in the inner sublamina, and OFF-center axons in the outer sublamina. We did not observe any branches of X axons innervating the medial interlaminar nucleus or the midbrain. Y axons have much larger terminal arbors and exhibit greater variation in their terminations. Generally, within layers A and A1, ON-center Y axons innervate the inner sublamina and OFF-center Y axons innervate the outer sublamina. However, they often innervate both sublaminae, and occasionally have a few boutons in the inappropriate lamina as well. Y axons also terminate in the dorsal C laminae, the interlaminar zones, and the media interlaminar nucleus; branches of these axons course toward the midbrain, presumably to innervate the superior colliculus. Thus, whereas the Y pathway in the ferret is one of high divergence, the X pathway appears to be the substrate for segregated ON and OFF channels through the lateral geniculate nucleus.

Animals↗
PubMed1989

The arbors of axons terminating in middle cortical layers of somatosensory area 3b in owl monkeys.

The arbors of single axons terminating predominantly in layer IV of the representation of the hand in area 3b of owl monkeys were reconstructed from serial brain sections after axons beneath the cortex were severed and horseradish peroxidase was injected into the white matter. In addition to dense terminations in layer IV, these labeled axons generally had branches extending into deeper layer III, and a few had very sparse terminations in layer VI. Terminal arbors ranged from 100 to 900 microns in diameter, and fine branches with synaptic boutons were unevenly distributed, typically grouped in a large central cluster and one or more smaller side clusters. The results are consistent with three broad conclusions: (1) Since the arbors are large relative to the details of the somatotopic map in area 3b, all regions within a single arbor may not be equally effective in activating cortical cells. (2) Spatially separate branches of single axons may relate to spatially separate modules of neurons of the same class in a manner that allows them to receive the same inputs. (3) Many of the somatotopic changes that have been reported in the hand representation as a result of nerve manipulations in adults could result from alterations in synaptic effectiveness within the arbors of single axons.

Afferent Pathways↗
PubMed1988

Experimentally induced visual projections into auditory thalamus and cortex.

Retinal cells have been induced to project into the medial geniculate nucleus, the principal auditory thalamic nucleus, in newborn ferrets by reduction of targets of retinal axons in one hemisphere and creation of alternative terminal space for these fibers in the auditory thalamus. Many cells in the medial geniculate nucleus are then visually driven, have large receptive fields, and receive input from retinal ganglion cells with small somata and slow conduction velocities. Visual cells with long conduction latencies and large contralateral receptive fields can also be recorded in primary auditory cortex. Some visual cells in auditory cortex are direction selective or have oriented receptive fields that resemble those of complex cells in primary visual cortex. Thus, functional visual projections can be routed into nonvisual structures in higher mammals, suggesting that the modality of a sensory thalamic nucleus or cortical area may be specified by its inputs during development.

Afferent Pathways↗
PubMed1988

Prenatal disruption of binocular interactions creates novel lamination in the cat's lateral geniculate nucleus.

The elimination of retinogeniculate afferents from one eye on embryonic day 44 (E44) has pronounced effects on the formation of the cellular laminae in the cat lateral geniculate nucleus (LGN). Only two laminae form: a dorsal, "magnocellular" layer, and a ventral, "parvocellular" layer. Soma size measurements and previously reported patterns of termination of retinogeniculate axons suggest that the dorsal lamina is a coalescence of the normal A-laminae and the dorsal, magnocellular division of layer C, while the ventral layer is a composite of the parvocellular sublamina of layer C and the remaining C-laminae. This is a novel pattern of lamination in the LGN that differs from that found in the normal nucleus, not only in that there are now only two cell layers rather than the normal five, but also in that the interlaminar zone occurs in an abnormal location. This result is markedly different from that observed in other species where interlaminar zones present after early monocular enucleation are a subset of the ones which would normally be present. We suggest that, in the absence of ongoing binocular interactions, interactions between functionally distinct retinal ganglion cell classes from the remaining eye may direct the formation of cell laminae in the LGN, even when such interactions are not normally operative.

Animals↗
PubMed1988

Axon arbors of X and Y retinal ganglion cells are differentially affected by prenatal disruption of binocular inputs.

In the mammalian visual system, the terminal arbors of retinal ganglion cell axons from the two eyes are restricted to mutually exclusive territories within their thalamic target, the lateral geniculate nucleus (LGN). Here we have investigated some of the factors that determine the adult morphology of terminal arbors in the cat's retinogeniculate system. Removal of one eye during prenatal life at a time when retinogeniculate axons from the two eyes are extensively intermixed within the LGN perturbs the subsequent morphological development of some but not all axons from the remaining eye. The presence of terminal arbors qualitatively normal in size, shape, and location within the LGN suggests that for some retinal axons, ongoing binocular interactions throughout prenatal life are not needed for the development of normal arbor morphology. However, many of the axons form arbors of abnormal size or location, suggesting that such features of axon morphology are not intrinsically determined for these axons but may be susceptible to external influences. Electrophysiological studies reveal that the abnormal arbors all belong to the functionally distinct Y class of retinal ganglion cells, whereas the normal arbors all belong to X cells. The different responses of X and Y axons to prenatal enucleation demonstrate that during development subsets of a single neuronal population projecting to the same target in the central nervous system can be under different developmental controls for axon arbor differentiation.

Animals↗
PubMed1988

Development and plasticity of retinal X and Y axon terminations in the cat's lateral geniculate nucleus.

The technique of injecting single retinogeniculate fibers with horseradish peroxidase enables the terminal arbors of physiologically identified axons to be fully characterized morphologically. We have used this technique to study the postnatal development of retinal X and Y cell arbors within the cat's lateral geniculate nucleus, and the plasticity of these arbors following a variety of manipulations that perturb normal development. These experiments suggest quite specific sequences and mechanisms for the development of individual X and Y retinogeniculate axons. Retinal X axons appear to innervate the lateral geniculate nucleus before Y axons do, and are probably specified innately to their appropriate target lamina A or A1. By 3-4 weeks postnatally, X axons from each eye develop exuberant terminal arbors within the A laminae that by 12 weeks get pruned to the narrow adult form by later developing Y axon arbors from the same eye. The Y arbors progressively expand to form their characteristic broad terminal zones during this period. The laminar location of Y arbors depends on interactions between axons from the two eyes, and their transverse extent on the presence of normal afferent activity in retinogeniculate fibers.

Animals↗
PubMed1988

Expression of a surface-associated antigen on Y-cells in the cat lateral geniculate nucleus is regulated by visual experience.

The monoclonal antibody Cat-301, generated against cat spinal cord (McKay and Hockfield, 1982), recognizes a surface-associated antigen that, in the cat lateral geniculate nucleus (LGN), is selectively expressed on Y-cells (Hockfield et al., 1983; Hendry et al., 1984; Sur et al., 1984). We now report that the antigen recognized by Cat-301 appears late in development, along a time course similar to that described for the maturation of the physiological properties of Y-cells in the LGN, and that its expression is sharply reduced by monocular lid suture or dark-rearing from birth, 2 visual deprivation procedures that lead to a reduction in the proportion of Y-cells recorded physiologically in the LGN (Sherman et al., 1972; Kratz et al., 1979; reviewed in Sherman and Spear, 1982). Monocular lid suture in the adult has no effect on Cat-301 antigen levels or, as previously reported (Sherman et al., 1972), on the proportion of physiologically recorded Y-cells. In addition, reversing the monocular deprivation in adulthood by opening the neonatally sutured eye and suturing closed the previously normal eye for 6 months restores neither normal levels of Cat-301 labeling nor, as previously reported (Geisert et al., 1982), the proportion of recordable Y-cells. The development of Cat-301 immunoreactivity thus parallels the development of LGN Y-cell physiology. The relative reduction in levels of immunoreactivity consequent to neonatal, but not adult, visual deprivation shows that Cat-301 antigen expression does not simply reflect the level of visually evoked electrical activity in the LGN, but rather reflects a process that depends on the nature of visual experience early in life.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

The morphology of retinogeniculate X- and Y-cell axonal arbors in dark-reared cats.

The cat's retinogeniculate pathway, immature at birth, develops physiologically and anatomically over the first three postnatal months. Visual deprivation from birth interferes with this maturation. Thus, monocular eye lid suture from birth leads to pronounced abnormalities in the morphology of retinogeniculate terminations and geniculate neurons, and to a reduction in the proportions of Y-cells recorded physiologically in the lateral geniculate nucleus (e.g. see Sherman and Spear 1982). This "loss" of geniculate Y-cells could possibly be due to reduced retinogeniculate Y-cell terminations and expanded X-cell terminations in the A-laminae (Sur et al. 1982), so that many geniculate cells that normally receive retinal Y-cell input accept and retain retinal X-cell input (Friedlander et al. 1982). Dark-rearing from birth also leads to a reduction in the proportions of Y-cells recorded in the lateral geniculate nucleus (Kratz et al. 1979). Such a loss might also be due to abnormalities in retinogeniculate X- and Y-cell terminations. To test this possibility, we injected horseradish peroxidase into physiologically identified retinogeniculate axons of dark-reared cats. Surprisingly, we found that our sample of retinogeniculate X- and Y-cell axons in dark-reared cats had normal morphology. If our sample is representative of the entire population of retinogeniculate X- and Y-cell axons, retinogeniculate axon morphology in dark-reared cats differs from that in monocularly sutured cats. Yet, using extracellular recording, we replicated the observation that physiologically identified geniculate Y-cells are encountered less often in dark-reared cats than in normal cats. Given the apparent normality of the retinogeniculate axons in these cats, the "loss" of geniculate Y-cells in dark-reared cats could then conceivably be due to conduction block in retinogeniculate afferents, tonic inhibition on Y-cells, or deficits in non-retinal influences that may importantly affect Y-cell development.

Animals↗
PubMed1987

Morphology of physiologically identified retinogeniculate X- and Y-axons in the cat.

1. We studied the morphology of individual, physiologically identified retinogeniculate axons in normal adult cats. The axons were recorded in the lateral geniculate nucleus or in the subjacent optic tract, characterized as X or Y by physiological criteria, penetrated, and injected with horseradish peroxidase. With subsequent application of appropriate histochemistry, the enzyme provides a complete label of the terminal arbors and parent trunks for morphological analysis. We have recovered for such analysis 26 X- and 25 Y-axons; of these, 14 X- and 12 Y-axons were studied in detail. 2. Within the optic tract, the parent trunk of every X-axon is located closer to the lateral geniculate nucleus and thus further from the pial surface than that of every Y-axon. This probably reflects the earlier development of X- than of Y-axons. Furthermore, the parent axon trunks of the X-axons are noticeably thinner than are those of the Y-axons. Every retinogeniculate X- and Y-axon in our sample branches within the optic tract. One of these branches heads dorsally to innervate the lateral geniculate nucleus and one heads medially and rostrally toward the midbrain, although none of these labeled axons were traced to a terminal arbor beyond the lateral geniculate nucleus. For Y-axons, all branches are of comparable diameter, but for X-axons, the branch heading toward the lateral geniculate nucleus is always noticeably thicker than is the branch directed toward the midbrain. 3. Every retinogeniculate X- and Y-axon produces the greatest portion of its terminal arbor in lamina A (if from the contralateral retina) or A1 (if from the ipsilateral retina). These arbors typically extend across most of the lamina along a projection line. Not a single terminal bouton from any axon was found in the inappropriate lamina A or A1 (i.e., in lamina A for ipsilaterally projecting axons or in lamina A1 for contralaterally projecting ones). Occasionally, an X-axon also innervates the medial interlaminar nucleus, and even more rarely does an X-axon innervate the C-laminae. In contrast, nearly all Y-axons from the contralateral retina branch to innervate part of the C-laminae (probably lamina C), and most from either retina also innervate the medial interlaminar nucleus. Although these details imply considerable variation in the overall pattern of retinogeniculate innervation for both X- and Y-axons, we found no physiological properties to correlate with this variation.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Callosal and ipsilateral cortical connections of the body surface representations in SI and SII of tree shrews.

Injections of horseradish peroxidase (HRP) were used to study the connections of the first and second somatosensory areas (SI and SII) in tree shrews. The locations of callosally projecting neurons in SI were determined by placing large injections of HRP in the SI region of one cerebral hemisphere and determining the organization of SI of the other cerebral hemisphere with microelectrode mapping. Many callosally projecting neurons were revealed in lateral SI representing the face, especially the glabrous nose. A sparse scattering of callosally projecting neurons were located more centrally in SI in portions representing the forepaw; these neurons tended to be in cortex devoted to the dorsal hand and pads of the palm rather than the digits. Part of medial SI, representing the forelimb and trunk, had a moderately dense distribution of callosally projecting neurons. More restricted injections in SI indicated that callosally projecting neurons were largely within comparable portions of contralateral SI, although a few neurons projecting callosally to SI were located in SII and cortex caudal and rostral to SI. Large injections of HRP in SII labeled neurons throughout contralateral SII, including representations of the forepaw and hindpaw. More restricted injections in SII labeled neurons in somatotopically comparable parts of the contralateral SII. A few labeled neurons were also seen in somatotopically matched parts of contralateral SI. The results also demonstrated strong somatotopically organized connections between SI and SII of the same hemisphere, and connections of SI and SII with adjoining subdivisions of parietal and frontal cortex. The major thalamic projections to both SI and SII originated in the ventroposterior nucleus.

Afferent Pathways↗
PubMed1986

Morphology of retinogeniculate X and Y axon arbors in monocularly enucleated cats.

We examined the terminal arbors of single, physiologically identified retinogeniculate X and Y axons from the remaining retinas of adult cats raised from birth with monocular enucleation. These were compared with arbors of X and Y axons in normally reared cats. We used intra-axonal injections of horseradish peroxidase to label each axon after recording its response properties. While the axons in monocularly enucleated cats exhibited normal response properties, both X and Y axons in these cats had abnormally large terminal arbors. Each of the hypertrophied X arbors appeared to be completely confined to the single geniculate lamina A or A1 appropriate to its eye of origin (i.e., lamina A for the contralateral retina and lamina A1 for the ipsilateral retina). In contrast, in addition to their normal terminations, most of the Y arbors seemed to extend well into laminae normally innervated only by the retina that was removed. Thus most or all of the translaminar sprouting previously reported for monocularly enucleated cats appears to reflect extensions of Y axon arbors. These data, in addition to earlier, analogous data from young kittens and cats reared with monocular lid suture, suggest the following sequelae during postnatal development: the retinogeniculate X arbors mature first and develop exuberant arbors that are later competitively pruned as the Y axons expand their innervation of the lateral geniculate nucleus; monocular lid suture prevents the Y axons from succeeding in this competition, so they fail to establish normal arbors and cannot reduce the exuberant X arbors; monocular enucleation offers a less resistant path in the denervated laminae for the rapidly growing Y arbors from the remaining eye, and the expansion of these arbors there reduces the competitive pressure on the exuberant X arbors. Thus, in monocularly enucleated cats, sprouting is limited to Y axons, either because only they possess the capacity to sprout or because they are in the midst of a period of relatively rapid growth at the time of the neonatal enucleation. The X axon arbors are also abnormally large within their appropriate laminae. This occurs presumably because they are able to maintain their immature exuberance, although we cannot rule out the possibility that they are pruned and later regrow to the final size seen in our experiments.

Animals↗
PubMed1986

Role of competitive interactions in the postnatal development of X and Y retinogeniculate axons.

The cat's retinogeniculate pathway is largely composed of X and Y axons, which represent two distinct neuronal streams organized in parallel. Our earlier data, summarized in the previous paper, suggest that the postnatal development of retinogeniculate axon arbors is characterized by competitive interactions between the X and Y axons. Thus, during development, X arbors in lamina A or A1 are initially broad or exuberant before the Y arbors begin to develop adultlike arbors; the X arbors then shrink to their adult form as the Y arbors grow and establish their mature complement of connections; monocular lid suture prevents the rapid growth of Y arbors, which in turn prevents the pruning of X arbors; and monocular enucleation at birth allows X arbors from the remaining eye to retain their exuberance although completely confined to their appropriate lamina A or A1, whereas the Y arbors develop aberrant extensions into adjacent, previously denervated laminae. We now provide additional evidence for the role of competition between retinogeniculate X and Y axons during development. The addition of visual deprivation by lid suture of the remaining eye to monocular enucleation at birth causes no apparent change in the morphology of X arbors in laminae A and A1. In contrast, the Y arbors of such cats continue to form extensive translaminar sprouts in the previously denervated laminae despite severely reduced terminations in the lamina A or A1 normally innervated by the remaining eye. We interpret these new data, in conjunction with our earlier data, as follows. If retinogeniculate X and Y arbors complete for synaptic space during postnatal development, terminations of Y axons can be affected by lid suture only in geniculate laminae where terminations of X axons are also present. Thus, Y axon arbors are severely reduced in deprived lamina A or A1 following lid suture whether or not the other eye is removed. Where X arbors are not present, such as in lamina C or the laminae inappropriate for the remaining eye after removal of the other, the lid suture has no obvious effect on development of the Y arbors.

Animals↗
PubMed1986

Functional reorganization in somatosensory cortical areas 3b and 1 of adult monkeys after median nerve repair: possible relationships to sensory recovery in humans.

Previous studies have shown that the primary somatosensory cortex of adult mammals undergoes somatotopic reorganization in response to peripheral nerve transection. The present study assesses how cortical organization is affected when a transected nerve subsequently regenerates. The median nerve to one hand of adult owl monkeys was transected and repaired. Following nerve regeneration, the representations of the hand in cortical areas 3b and 1 were studied with neurophysiological mapping methods. The major results were as follows: Peripherally, median nerve transection, repair, and regeneration resulted in reinnervation of the median nerve skin territory. Centrally, both the initial loss and subsequent regeneration of median nerve inputs caused reorganizational changes in cortex. Reorganizational changes were specifically restricted to regions of the hand cortex where inputs from the median nerve were normally represented. The functional features of cortical regions that recovered tactile responsiveness from reinnervated skin regions were abnormal in several respects. Most notably, these regions contained recording sites with abnormally located or multiple cutaneous receptive fields, and contained major topographical changes, such as reestablishment of palmar pad or digit representations in small, discontinuous patches of cortex. Normal organizational features were reestablished to a more limited extent. These features included recovery of delimited, discrete receptive fields and reestablishment of topographic representations for localized skin areas. Different transformations in topographical organization were seen in areas 3b and 1 of the same monkey. These results suggest that nerve regeneration reestablishes the cortical capacity to process tactile information from reinnervated skin via a prolonged reorganizational process that appears dependent on peripheral and central factors. Cortical recovery mechanisms clearly appear to have limitations, since preinjury patterns of cortical organization are not widely recovered even almost 1 year after repair. We suggest possible relationships between cortical reorganizational changes in these primates, and postrepair sensory changes in humans.

Animals↗
PubMed1985

Somatosensory cortex in macaque monkeys: laminar differences in receptive field size in areas 3b and 1.

We have examined receptive field sizes of neurons in granular, supragranular and infragranular layers within somatosensory cortical areas 3b and 1 in macaque monkeys. Receptive fields of neurons in layer 4 are smaller than receptive fields of neurons above or below layer 4. In addition, neurons in area 1 have larger receptive fields than neurons in corresponding layers of area 3b.

Animals↗
PubMed1985

Projection patterns of individual X- and Y-cell axons from the lateral geniculate nucleus to cortical area 17 in the cat.

Horseradish peroxidase was injected intracellularly into single, physiologically-identified X- and Y-cell geniculocortical axons projecting to area 17 of the cat. This injection anterogradely labeled the axon terminal fields in cortex and retrogradely labeled the somata of these same axons in laminae A and A1 of the lateral geniculate nucleus (LGN). The laminar projections of 21 X- and 15 Y-cell axons were analyzed. For these, the laminar terminations of ten X- and seven Y-cell axons were also related to their cells' positions in the A-laminae. The terminal fields of X- and Y-cell axons overlapped substantially in layers IV and VI of area 17. Some X-cells terminated mainly in IVb, others mainly in IVa, and still others throughout IVa and IVb. The latter two groups also projected up to 100 micron into lower layer III. Y-cells terminated primarily in layer IVa and projected up to 200 microns into lower layer III. Some also arborized throughout the depth of layer IVb. Both X- and Y-cell axons terminated throughout the depth of layer VI, although more so in the upper half. We found no relationship between the diameter of the parent axon and its sublaminar projection within layer IV. Within layer IV, X-cell axons generally terminated within a single, continuous clump and had surface areas of 0.6 to 0.9 mm2. Axons of Y-cells often terminated in two to three separate clumps, separated by terminal free gaps 400 to 600 micron wide. Their total surface areas, including gaps, were 1.0 to 1.8 mm2, roughly 1.6 times the surface areas of X-cell axons. Despite considerable overlap, Y-cell arbors contained significantly more boutons than did X-cell arbors. The sublaminar projections of the X- and Y-cell axons within layer IV reflected the locations of the cells' somata within the depth of the A-laminae. X-cells located in the dorsal or ventral thirds of the depths of the laminae projected mainly to layer IVa or throughout layer IV in cortex. Those located in the central thirds projected mainly to layer IVb. Y-cells showed a similar positional relationship, but they appeared to follow different rules. Y-cells in the outer thirds of the A-laminae projected mainly to layer IVa; those in the central thirds, in addition, expanded their projections to include layer IVb. In general, larger sized somata in the LGN gave rise to more widely spreading terminal arbors and greater numbers of boutons in cortex than did smaller somata.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

Termination patterns of individual X- and Y-cell axons in the visual cortex of the cat: projections to area 18, to the 17/18 border region, and to both areas 17 and 18.

Horseradish peroxidase was injected intracellularly into single, physiologically identified X- and Y-cell geniculocortical axons that projected to area 18, to the 17/18 border region, or to both areas 17 and 18 via branching axons. The axon terminal fields in cortex were labeled anterogradely, and the cell bodies of the axons in the A-laminae, lamina C, and the medial interlaminar nucleus (MIN) of the dorsal lateral geniculate nucleus (LGN) were labeled retrogradely. The laminar projections in area 18 of eight Y-cells and one geniculate, non-Y-cell were analyzed. Most of the cells arborized densely within layer IVa and the lower 200 to 400 microns of layer III. Most provided little or no input to layer IVb or layer VI. Thus, the laminar projections of Y-cells to layer IV of area 18 were similar to those of their area 17 counterparts, although the input to layer III was greater and rose much higher in area 18 than in area 17. The terminal arbors in area 18 were two to three times larger in lateral extent than those in area 17. They spread over 2.0 to 2.8 mm2 of layer IV and occupied proportionately much greater regions of area 18 than the Y-cell arbors in area 17. This may partially account for the large receptive fields of cortical cells in area 18, and it indicates that a small region of area 18 may receive converging inputs from a relatively wide retinotopic region of the LGN. The terminal arbors were also highly asymmetric, generally being two to four times longer anteroposteriorly than mediolaterally. These asymmetric arbors may form the structural basis for the anisotropic organization of the retinotopic map in area 18. We recovered three cells (two Y, one X) whose axons arborized in the border zone between areas 17 and 18. One Y-cell axon had a receptive field located in the ipsilateral visual hemifield and it arborized in a small region restricted almost exclusively to the border zone. The other two cells had receptive fields on or adjacent to the vertical meridian, and they terminated on either side of the 17/18 border region as well as within it. Thus, geniculate afferents representing the ipsilateral hemifield or the vertical meridian appear to have different patterns of termination on and adjacent to the 17/18 border zone. Also, some X-cell input may invade area 18 in the region immediately adjacent to the border zone.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗