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PubMed1981

The distribution of neurons projecting from the retina and visual cortex to the thalamus and tectum opticum of the barn owl, Tyto alba, and the burrowing owl, Speotyto cunicularia.

Using the HRP retrograde transport technique in two different genera of owls (Speotyto and Tyto), we have studied the distribution of neurons projecting to the optic tectum and the visual thalamus. Small injections of HRP were made into these structures from the pial surface after they had been visualized directly by dissection of the overlying bone. In contrast to the findings in mammals, retinal ganglion cells were labeled only in the eye contralateral to the injection site, whether this was in the thalamus or tectum, and the labeled ganglion cells were found on both nasal and temporal sides of the vertical retinal meridian through the fovea. After thalamic injections, labeling was prominent in temporal retina representing the binocular field, temporal to the optic nerve head. Retinothalamic ganglion cells formed roughly concentric lines of isodensity centered on the fovea (Speotyto) or area centralis (Tyto); labeling from thalamic injections involved both large and medium-sized neurons, but did not involve the smallest nor a conspicuous class of very large neurons. Tectal injections led to prominent labeling along the horizontal streak region, with horizontally elongated isodensity contours in both Tyto and Speotyto; retinotectal ganglion cells were heterogeneous and included a group of very large neurons and anther group of small neurons, neither of which was labeled from the thalamus. In the visual Wulst, labeled neurons were confined to the supragranular layers after both tectal and thalamic injections. Corticotectal neurons were found in both ipsilateral and contralateral visual Wulst. They were characterized by large cell bodies and prominent dendrites. Corticotectal neurons were distributed throughout the mediolateral extent of the ipsilateral Wulst and therefore involved both the monocular and binocular representations of the visual field. Corticothalamic neurons, found only in the ipsilateral Wulst, were characterized by smaller cell bodies and fine dendrites. They were confined to the monocular crescent on the extreme medial edge of the World.

Animals↗
PubMed1982

Studies of retinal representations within the cat's optic tract.

The manner in which each retina can be mapped onto a single cross section of the optic tract of the cat has been defined by neuroanatomical methods. It has been found that the contralateral nasal hemi-retina and both temporal hemi-retinae are represented in each tract by multiple, rough maps which partially overlap one another. All maps show the same general orientation, with area centralis represented dorsomedially, lower retina represented dorsolaterally, and upper retina represented ventromedially. The peripheral part of the horizontal meridian is represented ventrolaterally. Labeling all of the fibers from one eye by axonal degeneration or autoradiographic methods shows that the crossed map is displaced dorsally and medially relative to the uncrossed map, leaving a dorsomedial crescent of pure crossed fibers. Localized retinal lesions or injections of 3H-amino acid show the general orientation of the maps. Lesions within the dorsomedial pure crossed crescent show that fibers in this crescent arise from retinal areas close to the optic disc, near the site of the early fetal fissure. Localized injections of horseradish peroxidase into the optic tract show the relationships of the several maps in terms of the retinal distribution of retrogradely labeled retinal ganglion cells. They show that axons of large and small cells map ventrolaterally in the tract while intermediate sizes map dorsomedially. They confirm that the crossed map is displaced relative to the uncrossed maps. It is suggested that the optic tract develops by fibers taking a position in the tract in accordance with their time of arrival at the chiasm. The several maps are displaced because they develop sequentially and the optic tract can be read as a developmental record, the most dorsomedial axons being the oldest.

Animals↗
PubMed1983

Retinotopic patterns of connections of area 17 with visual areas V-II and MT in macaque monkeys.

Patterns of connections of Area 17 with extrastriate visual areas V-II and MT were studied in macaque monkeys by using 3H-proline and HRP as anatomical tracers. The results support previous conclusions that V-II is a split representation of the visual hemifield with most of the outer border formed by the zero horizontal meridian, and that MT is a topological representation of the visual hemifield. V-II was estimated to be about 10-12 mm wide, 110 mm long along its shared border with V-I, and 920 mm2 in surface area, thereby almost equaling Area 17 in size. MT was judged to be 7 mm wide and 12 mm long, with a surface area of 68 mm2, or roughly 7% of the area of Area 17. In some cases, there was evidence for projections from Area 17 to a third area (or areas) of cortex of about 2 mm wide and of uncertain length on the outer boundary of V-II. Terminations in V-II and MT were concentrated in layer IV and extended into inner layer III. Cells projecting back to Area 17 were largely in layers III and V in V-II and in layers III and VI in MT.

Animals↗
PubMed1983

Structural basis of orientation sensitivity of cat retinal ganglion cells.

We investigated the structural basis of the physiological orientation sensitivity of retinal ganglion cells (Levick and Thibos, '82). The dendritic fields of 840 retinal ganglion cells labeled by injections of horseradish peroxidase into the dorsal lateral geniculate nucleus (LGNd) or optic tracts of normal cats. Siamese cats, and cat deprived of patterned visual experience from birth by monocular lid-suture (MD) were studied. Mathematical techniques designed to analyze direction were used to find the dendritic field orientation of each cell. Statistical techniques designed for angular data were used to determine the relationship between dendritic field orientation and angular position on the retina (polar angle). Our results indicate that 88% of retinal ganglion cells have oriented dendritic fields and that dendritic field orientation is related systematically to retinal position. In all regions of retina more that 0.5 mm from the area centralis the dendritic fields of retinal ganglion cells are oriented radially, i.e., like the spokes of a wheel having the area centralis at its hub. This relationship was present in all animals and cell types studied and was strongest for cells located close to the horizontal meridian (visual streak) of the retina. Retinal ganglion cells appear to be sensitive to stimulus orientation because they have oriented dendritic fields.

Animals↗
PubMed1983

Distribution in area 17 of neurons projecting to the pontine nuclei: a quantitative study in the cat with retrograde transport of HRP-WGA.

The number and distribution of corticopontine neurons within area 17 of the cat were studied quantitatively with the use of retrograde transport of horseradish peroxidase-wheat germ agglutinin. Eight cats received stereotactic injections in the pontine nuclei; in three of these complete staining of the parts of the pontine nuclei receiving fibers from the visual cortex was achieved. Labeled cells were counted in frontal sections through the hemisphere, spaced at 0.5 mm. The borders of area 17 were determined cyto- and myeloarchitectonically and a flat map was produced for each animal. A map of the representation of the visual field in 10 degrees X 10 degrees blocks in the first visual area (Tusa et al., '78, '81) was transferred to our maps of area 17. The density and number of labeled corticopontine cells could then be determined within blocks of the cortex representing 10 degrees X 10 degrees of the visual field. The cell density (number of labeled cells per mm2 cortex) was found in general to be highest in parts of the cortex representing peripheral parts of the visual field. The cell density is low in cortex representing the central visual field, but the lowest density was found in the representation of a para-central region in the upper visual field. Furthermore, cortical regions representing the lower part of the visual field have a higher cell density than those representing the upper part; in four cases, 68-86% of all labeled cells were found in parts of area 17 representing the visual field below the horizontal meridian. Since there is an enlarged cortical representation of central vision, the much lower cell densities in "central" parts of area 17 than in "peripheral" parts may mean that all parts of the visual field are represented with equal numbers of corticopontine neurons ("linear" representation). This is not the case, however, since the number of labeled cells per 10 degrees X 10 degrees is considerably higher in the cortex representing the central 10 degrees and medial parts of the lower visual field than in the rest of area 17. Assuming that the corticopontine cells in the visual cortex transmit spatially relevant information, we conclude that there is an overrepresentation of central vision and the medial parts of the lower visual field in the corticopontine projection from area 17.

Animals↗
PubMed1984

The visual field representation of the rat ventral lateral geniculate nucleus.

The representation of the visual field in the ventral lateral geniculate nucleus (LGNv) was studied in rats anesthetized with urethane by recording the response of single units to visual stimulation. Receptive fields of LGNv units were plotted on a campimeter, 60 cm in diameter, which was placed 30 cm from the contralateral eye. LGNv neurons responded mainly to stimulation of the contralateral eye with on-tonic characteristics. Few neurons responded only to stimulation of the ipsilateral eye and no binocular interaction was observed. Retinotopic organization was clearly seen in the LGNv; the nasal visual fields were represented dorsally, the temporal fields ventrally, and the upper to lower visual fields were in the rostrolateral to caudomedial parts of the LGNv. A given point in the visual field is represented along a line running through the LGNv in a rostrocaudal direction. Almost the entire horizontal extent of the contralateral visual field was represented in the LGNv, whereas vertically the visual field between 40 degrees above and 20 degrees below the distribution axis was represented. The major axis of the strip of the visual field containing all the RF centers, which is referred to as the distribution axis, inclined nasally up and temporally down at an angle of 10.4 degrees to the 0 degree horizontal meridian line. The representation of the distribution axis in the retina was in accordance with the major axis of retinal ganglion cell distribution (Fukuda, '77; Schober and Gruschka, '77).

Animals↗
PubMed1985

Development of the human retina: patterns of cell distribution and redistribution in the ganglion cell layer.

Neurogenesis in the ventricular layer and the development of cell topography in the ganglion cell layer have been studied in whole-mounts of human fetal retinae. At the end of the embryonic period mitotic figures were seen over the entire outer surface of the retina. By about 14 weeks gestation mitosis had ceased in central retina and differentiation of photoreceptor nuclei was evident within a well-defined area which constituted about 2% of total retina area. This area was approximately centered on the site of the putative fovea, identified by the exclusive development of cone nuclei at that location. The area of retina in which mitosis had ceased increased as gestation progressed. By mid-gestation mitosis in the ventricular layer occupied about 77% of the outer surface of the retina and by about 30 weeks gestation mitosis in the ventricular layer had ceased. Cell density distributions in the ganglion cell layer were nonuniform at all stages studied (14-40 weeks). Densities were highest at about 17 weeks gestation, and by mid-gestation the adult pattern of cell topography was present with maps showing elevated cell densities in posterior retina and along the horizontal meridian. Cell densities generally declined throughout the remainder of the gestation period, except in the posterior retina, where densities in the perifoveal ganglion cell layer remained high during the second half of gestation. There is a rapid decline in cell density in the foveal ganglion cell layer toward the end of gestation, and it is suggested that the persistence of high densities in the perifoveal region may be related to migration of cells away from the developing fovea. The total population of cells in the ganglion cell layer was highest (2.2-2.5 million cells) between about weeks 18 and 30 of gestation. After this the cell population declined rapidly to 1.5-1.7 million cells. It is suggested that naturally occurring neuronal death is largely responsible for this decline.

Cell Count↗
PubMed1985

Interhemispheric connections of cortical sensory areas in tree shrews.

Interhemispheric connections were studied in tree shrews (Tupaia belangeri) after multiple injections of horseradish peroxidase or horseradish peroxidase conjugated to wheat germ agglutinin into the cortex of one cerebral hemisphere. After an appropriate survival period, the areal pattern of connections was revealed by flattening the other hemisphere, cutting sections parallel to the cortical surface, and staining with tetramethylbenzidine. Architectonic boundaries were identified by using sections stained for myelinated fibers. Labeled cells and axon terminations formed largely overlapping distributions that covaried in density, although labeled cells appeared to be more evenly distributed than labeled terminations. Connections were concentrated along the border of area 17 (V-I) with area 18 (V-II). However, connections also extended as far as 2 mm into area 17 to include cortex representing parts of the visual field 10 degrees or more from the zero vertical meridian. Clusters of dense connections spanned the width of area 18, where they alternated with regions of fewer connections. These clusters roughly corresponded in location to regions with heavier myelination. In the visually responsive temporal cortex, connections were also unevenly distributed. The organization of most of this cortex is not understood, but one subdivision, the temporal dorsal area (TD), has been identified on the basis of reciprocal connections with area 17. The central part of the TD had few interhemispheric connections, while most of the outer border had dense connections. The auditory cortex had dense and patchy connections throughout. The pattern in the primary somatosensory cortex (S-I) varied according to the representation of body parts, so that the cortex related to the forepaw had sparse connections, while connections were dense but uneven over much of the representation of the face, nose, and mouth. A focus of connections was found at the border of the forepaw and face representations, where the myelination of S-I cortex is interrupted. Dense, uneven connections also characterized the second somatosensory area, S-II. The motor cortex was densely connected, with only slightly fewer terminations rostral to the forepaw region of S-I. Other parts of frontal cortex had dense connections. The distribution of cortical connections varied with depth for at least some areas, so that clusters of cells and terminations were found in supragranular layers in S-I, S-II, and TD, while infragranular labeled cells were more evenly distributed.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1985

A callosal projection of area 17 upon the border region of area MT in the marmoset monkey, Callithrix jacchus.

Injections of the retrograde tracer HRP into the border region of the temporal visual area MT and adjoining cortex in Callithrix labeled pyramidal neurons in area 17 of the contralateral hemisphere. Evidence is presented that this newly discovered heterotopic callosal projection of the monkey striate cortex connects regions of representation of the zero vertical meridian of the visual field in a retinotopic order.

Animals↗
PubMed1985

Topographic organization of the optic radiation of the cat.

Pairs of injections of different neuroanatomical tracers--peroxidase-conjugated wheat-germ agglutinin (WGA) and [3H]proline--were made into the lateral geniculate nucleus (LGN) of the cat, and the course of the labeled fibers in the optic radiation was reconstructed. When the two injections were widely separated in the rostrocaudal dimension of the LGN (i.e., one in the representation of the lower quadrant of the visual field and one in the upper quadrant), the two sets of labeled fibers also remained separated in the long (roughly rostrocaudal) axis of the optic radiation. When the injections were widely separated in the mediolateral dimension of the LGN (i.e., one at the representation of the area centralis and one on the horizontal meridian in the far periphery of the field), the two sets of labeled fibers were separated in the short (mediolateral) dimension of the radiation. Shortly before reaching area 17, however, the medially and laterally placed fibers exchanged positions. This crossing is the basis of the topological inversion in the optic radiation deduced previously by Connolly and Van Essen (J. Comp. Neurol. 226:544-564, '84). The retinotopic organization of fibers in the radiation is less precise (in either dimension) than that of their terminal arborizations in visual cortex, but even injections as close as 1 mm to each other gave rise to spatially distinct fiber distributions. The WGA injections also labeled the corticogeniculate fibers by retrograde transport; these fibers traveled in a separate pathway medial to the optic radiation.

Animals↗
PubMed1986

Retinotopic organization within the cat's posterior suprasylvian sulcus and gyrus.

The organization of extrastriate visual areas of the cat's posterior suprasylvian sulcus and gyrus was studied with electrophysiological mapping methods. Analysis of retinotopic organization confirmed the presence of dorsal lateral and ventral lateral suprasylvian (DLS, VLS) visual areas (Palmer et al., '78, J. Comp. Neurol. 177:237-256) and demonstrated new features of organization. Areas DLS and VLS occupy the upper two-thirds of the posterior suprasylvian sulcus, with DLS wholly confined to the upper bank and VLS straddling the sulcal fundus. Both areas contain a partial representation of the lower quadrant of the visual field. A narrow strip of visually responsive cortex (periauditory belt) was identified adjoining DLS on the posterior ectosylvian gyrus; its organization and extent were not explored in detail. The organization of the posterior suprasylvian areas (PS) (Updyke, '82, Soc. Neurosci. Abst. 8:810) was explored in detail. Area PS lies inferior to areas VLS and 21b in the lower third of the posterior suprasylvian sulcus and gyrus, extending onto the fusiform gyrus. PS contains a partial representation of the lower quadrant of the visual field. It shares a representation of horizontal meridian with area 21b and a representation of central gaze with area VLS. Analysis of the PS/20 border region indicates that the representation of the lower quadrant periphery is common to the borders of PS, 20a, and 20b rather than lying internal to areas 20a and 20b as suggested by Tusa and Palmer ('80, J. Comp. Neurol. 193:147-164).

Animals↗
PubMed1986

Course of retinogeniculate projection fibers in the cat optic nerve.

The fiber courses of the cat optic nerve were studied by using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), which was iontophoretically applied to electrophysiologically defined positions in the lateral geniculate nucleus (LGN). Grossly, there were two distinct bends along the length of the optic nerve. The ventrally flexing anterior bend was located approximately 2 mm from the eyeball, while the dorsally flexing posterior bend was found at some 6 mm distant. The optic nerve fibers showed a tendency to scatter toward the chiasm. At the optic nerve head, the fibers from the different retinal areas maintained the retinal topography in a simplified form according to the trajectory of optic fibers surrounding the optic disc. Between the anterior and posterior bends, the fibers from the pericentral, middle-temporal, and most upper areas of the dorsal retina migrated ventrally and were arranged in the middle of the lateral, middle, and medial parts of the optic nerve, respectively, while fibers from the middle-temporal area of the ventral retina migrated dorsally and scattered into the lateral half. The fibers from the temporal and nasal horizontal meridian areas tended to hold their respective positions in the lateral and medial halves of the optic nerve. As a result, in this level they displayed a complex retinotopy in that the fibers from each part of the retina were mixed. Passing the posterior bend, as the optic nerve proceeded toward the chiasm, the characteristic pattern became less defined. Near the chiasm, the retinotopy became very scattered, showing a partial dorsoventral inversion of the retinal topography with substantial overlapping. It was noted from the present findings that the fibers from ventral retina scattered more quickly than the fibers from the dorsal retina, which tended to hold their grouping until the anterior bend, but the central or pericentral retinal fibers proceeded without significant scatter as far as the posterior bend. The analysis of labelled ganglion cells suggested that even if fibers that arise from one certain "mode" of ganglion cells are selected, it is unlikely that they maintain their initial fiber topography along the entire length of the optic nerve.

Animals↗
PubMed1988

Metabolic activity in striate and extrastriate cortex in the hooded rat: contralateral and ipsilateral eye input.

The extent of changes in glucose metabolism resulting from ipsilateral and contralateral eye activity in the posterior cortex of the hooded rat was demonstrated by means of the C-14 2-deoxyglucose autoradiographic technique. By stimulating one eye with square wave gratings and eliminating efferent activation from the other by means of enucleation or intraocular TTX injection, differences between ipsilaterally and contralaterally based visual activity in the two hemispheres were maximized. Carbon-14 levels in layer IV of autoradiographs of coronal sections were measured and combined across sections to form right and left matrices of posterior cortex metabolic activity. A difference matrix, formed by subtracting the metabolic activity matrix of cortex contralateral to the stimulated eye from the ipsilateral "depressed" matrix, emphasized those parts of the visual cortex that received monocular visual input. The demarcation of striate cortex by means of cholinesterase stain and the examination of autoradiographs from sections cut tangential to the cortical surface aided in the interpretation of the difference matrices. In striate cortex, differences were maximal in the medial monocular portion, and the lateral or binocular portion was shown to be divided metabolically into a far lateral contralaterally dominant strip along the cortical representation of the vertical meridian, and a more medial region of patches of more or less contralaterally dominant binocular input. Lateral peristriate differences were less than those of striate cortex, and regions of greater and lesser monocular input could be distinguished. We did not detect differences between the two hemispheres in either anterior or medial peristriate areas, thus indicating either completely binocular input (which seems unlikely given the retinotopic organization of these regions), or a greater dependence than in the lateral peristriate on inputs that were not affected by the visual manipulations.

Animals↗
PubMed1990

Organization of geniculocortical projections in turtles: isoazimuth lamellae in the visual cortex.

The projection from the dorsal lateral geniculate complex to the visual cortex in Pseudemys and Chrysemys turtles was examined by using the anterograde transport of horseradish peroxidase (HRP) in vitro and the retrograde transport of HRP in vivo. In vitro HRP injections into the lateral forebrain bundle were used to fill geniculocortical axons anterogradely, which were then analyzed in cortical wholemount preparations. Geniculocortical axons gain access to the visual cortex along its entire rostral-caudal extent. They course in slightly curved trajectories for up to 2 mm from the lateral edge of the cortex through both the lateral (or pallial thickening) and medial parts of Desan's cortical area D2. Single axons are of fine caliber. They tend to cross each other and sometimes branch in the pallial thickening, but are generally unbranched in the medial part of D2. They bear small, fusiform varicosities at irregular intervals along their lengths. Although axons show small variations in the number of varicosities per 100 microns segment, no consistent variation in varicosity number as a function of distance could be detected. These results indicate that geniculocortical axons project to the visual cortex in an orderly pattern. The retrograde transport experiments provide some clue as to the significance of this pattern. Small, ionotophoretic injections of HRP in the visual cortex retrogradely labeled neurons in the dorsal lateral geniculate complex. Injections in the rostral visual cortex retrogradely labeled neurons in the caudal pole of the geniculate complex. Injections at progressively more caudal loci within the visual cortex labeled neurons at progressively more rostral loci within the geniculate complex. Thus, there is a representation of the rostral-caudal axis of the geniculate complex along the caudal-rostral axis of the visual cortex. Consistent with the anterograde transport experiments that showed individual geniculocortical axons coursing through both lateral and medial parts of the visual cortex, HRP injections restricted to the medial edge of the visual cortex retrogradely labeled neurons along the entire dorsal-ventral axis of the geniculate complex at the appropriate rostral-caudal position. The neurophysiological studies of Mazurskaya ('72: J. Evol. Biochem. Physiol. 8:550-555; respond to a small, moving stimulus anywhere in visual space, implying a convergence of inputs from all points in visual space somewhere along the retinogeniculocortical pathway. The experiments reported here suggest a convergence in the geniculocortical projections of information along the vertical meridians, or azimuth lines, of visual space onto neurons lying along lateral to medial transects through the visual cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Development redistribution of photoreceptors across the Macaca nemestrina (pigtail macaque) retina.

Redistributions of monkey cones and rods during the first year after birth include a fivefold increase in peak foveal cone density from 43,000 to 210,000 cones/mm2, a decrease in the diameter of the rod-sparse area, and a two- to threefold decrease in peripheral photoreceptor density. Two weeks before birth, higher cone density is already apparent in the future fovea, as are the nasotemporal asymmetry in cone distribution, a higher density "cone streak" along the horizontal meridian, a large rod-sparse central fovea, and a ring of high rod density. Despite the early appearance of these basic patterns, photoreceptor distribution is not mature until 1 to 5 years postnatally. Total cones varied from 4 million at birth to 3.1 million in the average adult. The two oldest eyes had fewer cones, suggesting up to a 25% loss late in development. There were 60 to 70 million rods in the adult macaque retina and little evidence of postnatal changes in number. Neither of these small changes is sufficient to account for the reduction in peripheral photoreceptor density and both are in the wrong direction to explain increasing foveal density, ruling out a major role for either photoreceptor death or generation. Retinal area increased by a factor of 2.4 from 2 weeks before birth to adulthood. In contrast, the posterior pole of the retina was dimensionally stable throughout this period, with the distance between the fovea and optic disc varying nonsystematically from 3.37 to 4.05 mm. Retinal coverage of the globe was also stable at 48-60%. Thus postnatal growth can be ruled out as a factor in the density changes occurring in central retina. Adult retinas have a higher proportion of both cones and rods in midperiphery, whereas young retinas have a higher proportion of photoreceptors in far periphery. It appears that photoreceptors are radially redistributed from peripheral toward central retina during postnatal development, resulting in the marked increase in foveal cone density and the decrease in the eccentricity of the rod ring. Up to 13 weeks postnatally, midperipheral growth of the retina is substantial and increases with eccentricity. At later ages, expansion continues only in the very far periphery. Retinal growth appears sufficient to explain the decreases in peripheral rod and cone density with age. These and previous data strongly suggest that differentiated photoreceptors, with complex cytology and synaptic contacts, migrate toward the foveal center, explaining the increase in foveal photoreceptor density.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Topography of ganglion cells in human retina.

We quantified the spatial distribution of presumed ganglion cells and displaced amacrine cells in unstained whole mounts of six young normal human retinas whose photoreceptor distributions had previously been characterized. Cells with large somata compared to their nuclei were considered ganglion cells; cells with small somata relative to their nuclei were considered displaced amacrine cells. Within the central area, ganglion cell densities reach 32,000-38,000 cells/mm2 in a horizontally oriented elliptical ring 0.4-2.0 mm from the foveal center. In peripheral retina, densities in nasal retina exceed those at corresponding eccentricities in temporal retina by more than 300%; superior exceeds inferior by 60%. Displaced amacrine cells represented 3% of the total cells in central retina and nearly 80% in the far periphery. A twofold range in the total number of ganglion cells (0.7 to 1.5 million) was largely explained by a similar range in ganglion cell density in different eyes. Cone and ganglion cell number were not correlated, and the overall cone:ganglion cell ratio ranged from 2.9 to 7.5 in different eyes. Peripheral cones and ganglion cells have different topographies, thus suggesting meridianal differences in convergence onto individual ganglion cells. Low convergence of foveal cones onto individual ganglion cells is an important mechanism for preserving high resolution at later stages of neural processing. Our improved estimates for the density of central ganglion cells allowed us to ask whether there are enough ganglion cells for each cone at the foveal center to have a direct line to the brain. Our calculations indicate that 1) there are so many ganglion cells relative to cones that a ratio of only one ganglion cell per foveal cone would require fibers of Henle radiating toward rather than away from the foveal center; and 2) like the macaque, the human retina may have enough ganglion cells to transmit the information afforded by closely spaced foveal cones to both ON- and OFF-channels. Comparison of ganglion cell topography with the visual field representation in V1 reveals similarities consistent with the idea that cortical magnification is proportional to ganglion cell density throughout the visual field.

Adult↗
PubMed1991

Visual topography of area TEO in the macaque.

Previous studies have mapped the visuotopic organization of visual areas from V1 through V4 in the occipital cortex and of area TE in the temporal cortex, but the cortex in between, at the occipito-temporal junction, has remained relatively unexplored. To determine the visuotopic organization of this region, receptive fields were mapped at 1,200 visually responsive sites on 370 penetrations in the ventral occipital and temporal cortex of five macaques. We identified a new visual area, roughly corresponding to cytoarchitectonic area TEO, located between the ventral portion of V4 and area TE. Receptive fields in TEO are intermediate in size between those in V4 and TE and have a coarse visuotopic organization. Collectively, receptive fields in TEO appear to cover nearly the entire contralateral visual field. The foveal and parafoveal representation of TEO is located laterally on the convexity of the inferior temporal gyrus, and the peripheral field is represented medially on the ventral surface of the hemisphere, within and medial to the occipitotemporal sulcus. Beyond the medial border of TEO, within cyteoarchitectonic area TF, is another visually responsive region, which we have termed VTF; this region may also have some crude visual topography. Bands of constant eccentricity in TEO appear to be continuous with those in V2, V3v, and V4. The upper field representation in TEO is located adjacent to that in ventral V4, with a representation of the horizontal meridian forming the boundary between the two areas. The lower field representation in TEO is located just anterior to the upper field but is smaller. In contrast to the orderly representation of eccentricity in TEO, we found little consistent representation of polar angle, other than the separation of upper and lower fields. The results of injecting anatomical tracers in two animals suggest that TEO is an important link in the pathway that relays visual information from V1 to the inferior temporal cortex. TEO is thus likely to play an important role in pattern perception.

Animals↗
PubMed1993

Geometry of the representation of the visual field on the superior colliculus of the wallaby (Macropus eugenii). I. Normal projection.

In 13 wallabies (Macropus eugenii, the tammar), microelectrode recordings of the activity of units in the superficial layers of the superior colliculus in response to a flashing light spot were used to make a map of the spatial location of their receptive fields. This article describes the projection of a normal eye to the contralateral colliculus. Ten of the 13 animals had one rotated eye and these projections are analysed in the accompanying paper (James et al., this issue). Units responded briskly to the stimulus at light on and off and had receptive fields about 5 degrees across. The centres of receptive fields from a regular array of recording points on the colliculus were plotted with a perimeter and fitted to a flattened representation of the colliculus according to a spline technique. The visual field of each colliculus extends from 25 degrees ipsilateral to the vertical meridian to 120 degrees temporal contralaterally. The lines of isoazimuth are regularly spaced and parallel and run mediolaterally on the colliculus. The horizon is represented by a line running rostrocaudally and the parallels are more widely spaced near the horizon and become compressed in the superior and inferior fields. The variation of areal magnification factor fits the distribution of density of retinal ganglion cells very well. Anisotropy of the projection means that the increased ganglion cell density of the retinal visual streak is entirely accommodated by magnification in the vertical direction, while the magnification of the azimuthal projection is equal over the whole field. No responses were recorded from the ipsilateral eye even though anatomically there is a direct retinal ipsilateral projection.

Animals↗