Retinal ganglion cells that project to the dorsal lateral geniculate nucleus in the macaque monkey

Parasol and midget ganglion cells of the primate retina

We have intracellularly filled the dendritic arbors of 996 midget and parasol ganglion cells with horseradish peroxidase (HRP) in macaque and baboon retinas. Only minor differences in the properties of these cell groups were found between species. Ninety of these cells were cut from their retinas, embedded in methacrylate, and transversely sectioned. According to their depth of stratification, there are two types of parasol cells (termed a-parasol and b-parasol), and two types of midget ganglion cells (a-midget and b-midget). Each of these four types stratifies at a different level within the IPL. The dendritic fields of midget ganglion cells lie either near the border of the ganglion cell layer (GCL) or near the border of the amacrine cell layer (ACL). The dendrites of the two types of parasol cells stratify closer to the center of the IPL, where they divide it into three approximately equal parts. There was no vertical overlap in the dendritic fields of a-parasols and b-parasols; they were always separated by at least 1 micron. The border between the a- and b-sublaminae of the IPL, defined in terms of this narrow gap between the stratification of the two parasol cell types, lies approximately at the center of the IPL. The dendritic-field thickness for each of these types, on average, is no greater than 30% of the IPL thickness. At a similar location, there is no significant difference between the dendritic-field diameters of the two parasol types or between those of the two midget types. As previously reported (Perry et al.: Neuroscience 12:1101-1123, '84) the dendritic fields of both parasol and midget ganglion cells are smaller in the nasal retina than at a position in the temporal retina equidistant from the fovea. Because dendritic-field diameters prove to depend upon local ganglion-cell density, the scatter in these diameters as a function of retinal eccentricity is due in part to the asymmetric distribution of ganglion cells. We have devised a measure, termed equivalent eccentricity, that allows data points of cells from regions having the same local ganglion-cell density to be plotted at the same position on this scale. The use of this measure, rather than eccentricity per se, significantly reduces the scatter of dendritic-field diameters. The dendritic-field diameters of parasol cells within the nasal quadrant of the retina are not fully brought into line with those of cells lying elsewhere in the retina.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective degeneration of the parvocellular-projecting retinal ganglion cells in a New World monkey, Saimiri sciureus

Selective degeneration of retinal ganglion cells projecting to parvocellular layers of the dorsal lateral geniculate nucleus (LGN) was observed in squirrel monkeys (Saimiri sciureus) exposed to a range of doses of acrylamide monomer. Similar acrylamide-induced neuronal loss has previously been reported in parvocellular-projecting ganglion cells of macaques, but no such selective degeneration has been found in acrylamide-dosed rats, squirrels, rabbits or cats. The extent of ganglion cell loss observed in the present study suggests that in the squirrel monkey, as in the macaque, a majority of ganglion cells project to parvocellular layers of the LGN. The locus of optic tract degeneration suggests that the squirrel monkey parvocellular pathway passes in dorsolateral optic tract, as does that of the macaque. Patterns of decreases in cytochrome oxidase activity confirm that, in both of these primates, geniculocortical pathways driven by these vulnerable neurons project to cortical layers 4A and 4C beta. These results suggest close parallels in the neuroanatomical projections and toxic vulnerability of the parvocellular-projecting pathway in New and Old World monkeys. They indicate that acrylamide intoxication can be used to selectively damage this pathway in order to study the functional roles of parallel visual pathways in both New and Old World monkeys.

Photoreceptor topography of the retina in the adult pigtail macaque (Macaca nemestrina)

In spite of the crucial role retinal photoreceptors play in mapping optical images into a pattern of neural excitation, there are no complete studies of photoreceptor topography in any primate retina. We have measured the spatial density and inner segment areas of cones and rods across the whole mounted retinas of three adult pigtail macaques (Macaca nemestrina) and constructed maps of photoreceptor density and inner segment diameter. These retinas contain an average of 3.1 million cones (2.8-3.3 million), with an average peak foveal cone density of 210,000 cones/mm2 (190,000-260,000 cones/mm2). Cone density falls steeply with increasing eccentricity, to 100,000 cones/mm2 at 200 microns from the fovea, and to 50,000 cones/mm2 at 750 microns. Imposed on this gradient is a "streak" of higher cone density along the horizontal meridian. At equivalent eccentricities, cone density is higher in nasal and inferior retina. Cone inner segments increase in diameter from 2.3 microns at the foveal center to 11 microns in far temporal retina and 10 microns in far nasal retina. These retinas contain an average of 60.1 million rods (44.9-75.3 million). Rod density is zero within 20 microns of the foveal center, increases to the crest of a "rod ring" at the eccentricity of the optic disk, and then declines. Central rod topography is asymmetric, with higher densities in superior retina. Density along the crest of the rod ring peaks in superior retina at 177,000 rods/mm2, dips as low as 120,000 rods/mm2 along the horizontal meridian, and increases to about 150,000 rods/mm2 in inferior retina. Far peripheral rod topography is relatively symmetric around the fovea. Rod inner segment diameter ranged from 1.5 microns in the fovea to 4 microns at the temporal edge and 3.4 microns at the nasal edge of the retina. At eccentricities exceeding 6 mm, rod inner segment diameter was greater temporally than nasally. Cone inner segments cover 85-90% of the central fovea, with extrareceptor space accounting for the remainder. Cone coverage declines with increasing eccentricity to 20% at the temporal edge and 35% at the nasal edge of the retina. In contrast, rod coverage increases from zero at the foveal center to a maximum of 65% in temporal retina and 50% in nasal retina. The photoreceptor topography of the pigtail macaque is qualitatively similar to that of other macaques and to humans. Photoreceptor topography is formed by a complex interaction between regional changes in cone and rod density and inner segment diameter.

Parameters affecting the loss of ganglion cells of the retina following ablations of striate cortex in primates

Partial lesions of striate cortex were made in newborn and adolescent or young adult macaque monkeys, one newborn squirrel monkey, and adult squirrel and owl monkeys. After survival times ranging from 3 1/2 weeks to 8 years, the retinas were examined for transneuronal retrograde ganglion cell loss and retinal projections to the dorsal lateral geniculate nucleus, and other targets were examined for changes. After lesions in infant macaque monkeys and long postoperative survivals, nearly 80% of the ganglion cells were lost in the altered portions of the retinas. The degeneration appeared to be exclusively of ganglion cells projecting to the parvocellular layers of the lateral geniculate nucleus, and the loss of this class of cell appeared to be complete or nearly complete for the affected portions of the retina. Cases with shorter survivals showed that nine-tenths of the potential loss occurred within 6 months, and about half of the potential loss took place within one month. In cases where lesions were placed in adolescent and young adult macaque monkeys, the loss also was of ganglion cells projecting to the parvocellular layers. However, the rate of cell loss was slower so that little or no cell loss was apparent after six months, and only one-third to three-fourths of the potential loss occurred within 12-14 months. A cell loss of 22% was measured in the altered portions of the retina of a squirrel monkey lesioned as an infant and surviving for 6 months, but no regions of ganglion cell loss were apparent in the retinas of owl and squirrel monkeys lesioned as adults and surviving as long as two or more years. We conclude that nearly 80% of the ganglion cells project to the parvocellular layers in macaque monkeys, and that the ultimate survival of these ganglion cells depends on the presence of target neurons in the parvocellular layers. Age is important in that the loss of ganglion cells proceeds rapidly in infant macaque monkeys, but slowly in older animals. Infant New World monkeys, judging from one squirrel monkey, are also susceptible to ganglion cell loss, although apparently at a rate comparable to older macaque monkeys. Finally, adult New World monkeys do not appear to be susceptible to ganglion cell loss. These age and species differences in rates of loss and susceptibility to loss challenge a "sustaining collateral" hypothesis proposed earlier (Weller et al., 1979), and suggest alternatives and modifications.

Extrinsic determinants of retinal ganglion cell development in primates

As in all mammals studied to date, primate retina contains morphologically distinct classes of retinal ganglion cells (Polyak: The Retina. Chicago: University of Chicago Press, '41; Boycott and Dowling: Philos. Trans. R. Soc. Lond. [Biol.] 225:109-184, '69; Leventhal et al.: Science 213:1139-1142, '81; Perry et al.: Neuroscience 12:1101-1123, '84; Rodieck et al.: J. Comp. Neurol. 233:115-132, '85; Rodieck: In H.D. Steklis and J. Erwin (eds): Comparative Primate Biology, Volume 4: Neurosciences. New York: Alan R. Liss, Inc., pp. 203-278, '88). We have now studied the morphologies, central projections, and retinal distributions of the major morphological classes of ganglion cells in the normal adult monkey, the newborn monkey, and the adult monkey in which restricted regions of retina were depleted of ganglion cells at birth as a result of small lesions made around the perimeter of the optic disc. Both old-world (Macaca fascicularis) and new-world (Saimiri sciureus) monkeys were studied. Our results indicate that, at birth, the major morphological classes of monkey retinal ganglion cells are recognizable; cells in central regions are close to adult size whereas cells in peripheral regions are much smaller than in the adult. As in the adult (Stone et al.: J. Comp. Neurol. 150:333-348, '73), in newborn monkeys there is a very sharp division between ipsilaterally and contralaterally projecting retinal ganglion cells (nasotemporal division). Consistent with earlier work (Hendrickson and Kupfer: Invest. Ophthalmol. 15:746-756, '76) we find that the foveal pit in the neonate is immature and contains many more ganglion cells than in the adult. In the adult monkey in which the density of retinal ganglion cells in the central retina was reduced experimentally at birth, the fovea appeared immature, and an abnormally large number of retinal ganglion cells were distributed throughout the foveal pit. The cell bodies and dendritic fields of ganglion cells that developed within cell-poor regions of the central retina were nearly ten times larger than normal. In peripheral regions the effects were smaller. The dendrites of the abnormally toward the foveal pit. They did not extend preferentially into the cell-poor region as do the abnormally large cells on the borders of experimentally induced cell-poor regions of cat central retina (Leventhal et al.: J. Neurosci. 8:1485-1499, '88) or, as we found here, in paracentral regions of primate retina.(ABSTRACT TRUNCATED AT 400 WORDS)

Sensitivity of macaque retinal ganglion cells to chromatic and luminance flicker

1. We have studied the sensitivity of macaque retinal ganglion cells to sinusoidal flicker. Contrast thresholds were compared for stimuli which alternated only in luminance ('luminance flicker') or chromaticity ('chromatic flicker'), or which modulated only the middle- or long-wavelength-sensitive cones ('silent substitution'). 2. For luminance flicker, the lowest thresholds were those of phasic, non-opponent ganglion cells. Sensitivity was maximal near 10 Hz. 3. Tonic, cone-opponent ganglion cells were relatively insensitive to luminance flicker, especially at low temporal frequencies, but were sensitive to chromatic flicker, thresholds changing little from 1 to 20 Hz. Those with antagonistic input from middle- and long-wavelength-sensitive (M- and L-) cones had a low threshold to chromatic flicker between red and green lights. Those with input from short-wavelength-sensitive (S-) cones had a low threshold to chromatic flicker between blue and green. Expressed in terms of cone contrast, the S-cone inputs to blue on-centre cells had higher thresholds than M- and L-cone inputs to other cell types. 4. Phasic, non-opponent cells responded to high-contrast red-green chromatic flicker at twice the flicker frequency. This frequency-doubled response is due to a non-linearity of summation of M- and L-cone mechanisms. It was only apparent at cone contrasts which were above threshold for most tonic cells. 5. M- or L-cones were stimulated selectively using silent substitution. Thresholds of M- and L-cone inputs to both red and green on-centre cells were similar. This implies that these cells' sensitivity to chromatic flicker is derived in equal measure from centre and surround. Thresholds of the isolated cone inputs could be used to predict sensitivity to chromatic flicker. The high threshold of these cells to achromatic contrast is thus, at least in part, due to mutual cancellation by opponent inputs rather than intrinsically low sensitivity. 6. Thresholds of M- and L-cone inputs to phasic cells were similar at 10 Hz, and were comparable to those of tonic cells, suggesting that at 1400 td cone inputs to both cell groups are of similar strength. 7. The modulation transfer function of phasic cells to luminance flicker was similar to the detection sensitivity curve of human observers who viewed the same stimulus. For chromatic flicker, at low temporal frequencies thresholds of tonic cells (red or green on-centre cells in the case of red-green flicker or blue on-centre cells in the case of blue-green flicker) approached that of human observers. We propose the different cell types are the substrate of different channels which have been postulated on the basis of psychophysical experiments. 8. At frequencies of chromatic flicker above 2 Hz, human sensitivity falls off steeply whereas tonic cell sensitivity remained the same or increased. This implies that high-frequency signals in the chromatic, tonic cell pathway are not available to the central pathway respons

Starburst amacrine cells of the primate retina

A group of readily recognized amacrine cells were observed in Golgi-impregnated and flat-mounted macaque, baboon, and human retinas. These cells had roughly-circular or oval dendritic fields that were narrowly stratified within the inner plexiform layer (IPL). Most of these cells stratified in the inner half (sublamina b) of the IPL, and they had their somata in the ganglion-cell layer; a few stratified in the outer half (sublamina a) of the IPL and had their somata in the amacrine-cell layer. Typically, a single dendrite issued from the soma, and, after passing for 10 microns or so, gave rise to five or more radiate processes. As these processes neared the edge of the dendritic field they branched, turned, and became varicose. Most showed no evidence of an axon, although a few had a short process extending inward, toward the optic-fiber layer. Dendritic-field diameters were about 100 microns near the fovea and increased to about 350 microns in the peripheral retina. Mean somal diameter also increased slightly from near the fovea (7.8 microns) to the periphery (8.7 microns). Although the primate cells are smaller, and there are some minor differences in the form of the dendritic fields, these cells appear to be morphologically equivalent to the starburst amacrines of the rabbit retina, whose counterparts have also been observed in the retinas of rats and cats. Presuming that these cells correspond to the choline acetyltransferase immunoreactive primate cells described by Mariani and Hersh (J. Comp. Neurol. 267:269-280, '87), their overlap factor is about ten for the type whose somata lay in the ganglion-cell layer and about 0.25 for those whose somata lay in the amacrine-cell layer.

Retinogeniculate projection fibers in the monkey optic nerve: a demonstration of the fiber pathways by retrograde axonal transport of WGA-HRP

Seven Japanese monkeys (Macaca fuscata) were used to investigate the fiber pathways of the optic nerve. Optic nerve fibers and retinal ganglion cells were retrogradely labeled by iontophoretic injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into electrophysiologically defined positions of the lateral geniculate nucleus (LGN). By gross anatomical observation, the optic nerve usually had one distinct bend, which flexed dorsally 3-4 mm from the eyeball, and occasionally another ventrally directed bend was found just behind the eyeball. In the optic nerve head, fibers from the various retinal areas were arranged in a wedge according to the fiber trajectory on the retinal surface. For about a 3 mm distance from the disc, fibers rapidly spread out radially. Subsequently, rather than scattering dorsoventrally, they progressed to the chiasm with a gradual increase in the degree of mediolateral (nasotemporal) scatter. The degree of the scatter was different depending on the retinal site from which the axons originated. Fibers from the peripheral retina spread out widely for a few millimeters behind the eyeball. Thereafter the scatter was rather limited until the chiasm. On the other hand, the scatter of fibers from the foveal and parafoveal areas progressed gradually through the nerve. The present study also suggests that the difference in scatter depends on the types of cells of origin. Fibers from large ganglion cells displayed more extensive scatter than fibers from medium-sized cells. In spite of the extensive scatter of fibers, two clear segregations were found; one was a dorsoventral segregation, which was displayed by both central and peripheral retinal fibers, and the other was a center-peripheral segregation in which the fibers from the nasal central (papillomacular) retina were located almost exclusively in the central part of the optic nerve surrounded by peripheral retinal fibers. However, the temporal central retinal fibers were located in the lateral periphery of the nerve, and they overlapped significantly with fibers from the temporal peripheral retina. Furthermore, a broad intermingling was found between nasal and temporal peripheral retinal fibers owing to their mediolateral scatter. Thus, the present findings based on more precise anatomical techniques indicate that the classical notion of the retinal quadrant topography in the monkey optic nerve probably is suspect. In addition, the "rotation" of the fiber arrangement was not demonstrated.

Immunohistochemical localization of GABAA receptors in the retina of the new world primate Saimiri sciureus

A large population of amacrine cells in the retina are thought to use GABA as an inhibitory neurotransmitter in their synaptic interactions within the inner plexiform layer. However, little is known about their synaptic targets; the neurons that express the receptors for GABA have not been clearly identified. Recently, the GABAA receptor has been isolated and antibodies have been raised against it. These antibodies have proven useful for the immunocytochemical localization of the receptor, and two brief reports describing the distribution of GABAA receptor immunoreactivity in the retina have appeared (Richards et al., 1987; Mariani et al., 1987). We used a monoclonal antibody (62-3G1) against the GABAA receptor to study the retina of the New World primate Saimiri sciureus. Labeled somata were found in the inner nuclear layer (INL) and ganglion cell layer (GCL). The staining was confined to what appeared to be the cell's plasmalemma and small cytoplasmic granules. Most of the labeled neurons in the INL had small somata (5-7 microns in diameter) located at the vitreal edge of the layer. They arborized in two laminae (approximately 2 and 4) of inner plexiform layer (IPL). Ventral to the optic disc (2.5 mm) they comprised 29% of the cells present. A few of the labeled neurons appeared to be interplexiform cells or flat bipolar cells, with labeled processes that extended into both the IPL and the inner half of the outer plexiform layer. In the GCL, the labeled somata were among the largest present (13-20 microns in diameter), and 2.5 mm ventral to the optic disc they made up 15% of the cells present. Experiments in which immunoreactive somata were retrogradely labeled following the injection of fluorescent tracers into the optic tract provided a conclusive demonstration that some of the immunoreactive somata were ganglion cells. The antibody often labeled their axons in the optic fiber layer. This suggests that the GABAA receptors are transported anterogradely to the retinal terminal fields. The dendrites of the immunoreactive ganglion cells extended into the 2 laminae of labeled processes in the IPL, and their primary dendritic arbors were, at any given eccentricity, quite similar in appearance. This homogeneity suggests that they comprise a particular subset of the ganglion cells. Sections simultaneously labeled with the monoclonal antibody against the GABAA receptor and antisera against either L-glutamic acid decarboxylase (GAD) or GABA revealed that the GAD/GABA was distributed much more widely in the IPL than the GABAA receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinal ganglion cell atrophy correlated with automated perimetry in human eyes with glaucoma

We measured the number and size of retinal ganglion cells from six human eyes with glaucoma. In each, the histologic findings were correlated with visual field results. Five age-matched normal eyes were studied for comparison. In general, there were fewer remaining large ganglion cells in retinal areas with atrophy. In the perifoveal area, however, no consistent pattern of cell loss by size was found. Our estimates suggest that visual field sensitivity in automated testing begins to decline soon after the initial loss of ganglion cells. Throughout the central 30 degrees of the retina, 20% of the normal number of cells were gone in locations with a 5-dB sensitivity loss, and 40% cell loss corresponded to a 10-dB decrease. There were some remaining ganglion cells in areas that had 0-dB sensitivity in the field test.

Selective loss of pattern discrimination in early glaucoma

A new perimetric pattern discrimination test was compared with conventional automated perimetry (Humphrey program 30-2 or Octopus program 32) in glaucoma patients, glaucoma suspects, and control subjects. The new test is based on the rationale that a greater percentage of retinal ganglion cells should be needed to detect a stimulus by its shape, or pattern, than by its brightness. The pattern discrimination stimulus was apatch of nonrandom dots embedded in a surrounding random dot field of the same average density. Pattern discrimination thresholds were measured by changing the degree of regularity, or coherence, of the stimulus dots. The fully coherent target was a static, 1-s duration, 20 x 20-dot checkerboard. Using a criterion-free relative operating characteristic analysis, we estimated the ability of both the pattern discrimination and conventional tests to distinguish the normal data distribution from the suspect and glaucoma distributions. The pattern discrimination test appeared to produce separations greater than conventional perimetry for glaucoma suspects and separations equivalent to conventional perimetry for glaucoma patients.

Morphology of retinogeniculate axons in the macaque

The size, pattern of terminal arborizations, and laminar specificity of individual retinogeniculate axons were studied in the macaque following injections of HRP into the optic tract. Axons that terminated in the magnocellular layers had significantly larger fiber diameters and wider terminal fields than those that terminated in the parvocellular layers. Terminal fields of magnocellular fibers spanned most of the width of their target layer, whereas those of parvocellular fibers were restricted to approximately one-half the width of their target layers; almost all terminal fields were oriented along lines of projection. All of the optic tract fibers that we examined terminated in only one layer of the lateral geniculate nucleus (GL), including a population of fine caliber fibers that project to the intercalated layers, and none had collateral projections outside the GL. The results suggest that each layer--magnocellular, parvocellular, and intercalated--receives projections from a morphologically distinct population of optic tract fibers.

Timing and distribution of flash-evoked activity in the lateral geniculate nucleus of the alert monkey

Simultaneous recording of activity from multiple cortical laminae in alert monkeys, using multichannel electrodes, has been used to identify the intracranial generators of surface-recorded, visually evoked potentials (VEP) to stroboscopic flash. Beyond their clinical implications, these results offer an unique view of the timing and sequence of cortical visual processing in the alert monkey, including the somewhat surprising findings of an extremely short-latency response in lamina IVA, a contra- over ipsilateral latency advantage throughout lamina IV, and the lack of a consistent flash-evoked response in the major cortical recipient of the magnocellular system, lamina IVCa. The present study used similar techniques to examine flash-evoked activity in LGN and in optic tract, both to elucidate the role of the subcortical pathways in establishing this pattern, and to provide a parallel, detailed view of the timing of visual activity in LGN and optic tract in the alert monkey. Flash-evoked responses are robust in both parvo- and magnocellular laminae, but these responses differ along several dimensions: (1) parvocellular multiunit activity (MUA) is 1/4 to 1/2 the amplitude of magnocellular MUA; (2) oscillatory activity is higher in frequency and shorter in duration in parvo- than in magnocellular responses; (3) inhibitory processes appear less prominent and diverse in parvo- than in magnocellular activity; (4) mean onset latencies of MUA are longer in parvo- than in magnocellular laminae, but there is extensive overlap in these distributions. Latencies encountered in ipsilateral lamina 3, and at laminar borders dorsal to 3, group more clearly with those of the magnocellular laminae than with those of the other parvocellular laminae. As a result, in the parvocellular division as a whole, the average latency to ipsilateral stimulation is shorter than that to contralateral stimulation. The optic tract exhibits a dorsal-to-ventral progression of onset latency and oscillation frequency consistent with a dorsal/ventral segregation of the inputs to parvo- and magnocellular layers. Comparison of optic tract and LGN data reveals that while many LGN response characteristics are initiated in the retina, significant modification of retinal output occurs at LGN. The techniques used here permit a particularly sensitive and reliable assessment of the timing and distribution of visual responses in the optic tract and LGN of alert monkeys. Our data support the view that in the alert monkey, the surface-VEP to passive, binocular flash primarily reflects activation of parvocellular thalamorecipient laminae of Area 17.(ABSTRACT TRUNCATED AT 400 WORDS)

Transneuronal retrograde degeneration of retinal ganglion cells after damage to striate cortex in macaque monkeys: selective loss of P beta cells

We examined the retinae of two monkeys whose left striate cortex had been removed eight years previously and compared the transneuronally degenerated hemiretina of each eye with the normal hemiretina, and with the retinae of normal monkeys. All retinae were prepared as whole mounts. One from each pair was stained with Cresyl Violet; the other was reacted for horseradish peroxidase two days after placing pellets of the enzyme in the optic nerve. Measurements of ganglion cell density in the Nissl-stained retina of the contralateral right eye showed that approximately 80% of retinal ganglion cells were missing in the central 30 degrees of the degenerated hemiretinae. More peripherally the percentage loss was less extensive. Measurements of cell soma size and dendritic field size of peroxidase-labelled classified surviving cells in the degenerated temporal hemiretina of the ipsilateral eye showed them to be morphologically normal. In comparison with the normal hemiretina, however, the mean soma size at three selected eccentricities was larger than normal, suggesting selective loss of smaller ganglion cells. Classification of peroxidase-labelled ganglion cells in the normal and degenerated hemiretinae revealed that the population of P beta cells was reduced by as much as 85% in the degenerated region. There was comparable change in the density of P alpha or P gamma cells. The degeneration of the great majority of P beta cells, which are believed to be the morphological substrate of ganglion cells with small and colour-opponent receptive fields, must set limits on the visual sensitivity and discrimination that survive damage to striate cortex.

Retinal ganglion cell distribution in the cebus monkey: a comparison with the cortical magnification factors

The distribution of ganglion cells was determined in whole-mounted Cebus monkey retinae. Ganglion cell density along the horizontal meridian was asymmetric, being 1.2-4.3 higher in the nasal retinal region when compared to temporal retina at the same eccentricities. The total number of ganglion cells varied from 1.34 to 1.4 million. Ganglion cell density peaked at 49,000/mm2 about 0.5 mm nasal to the fovea. Comparison between ganglion cell density and areal cortical magnification factors for V1 and V2 reveals that the relative representation of the fovea increases in the visual cortex. This effect seems to be a general feature of the visual system of primates.

Quantitative immunogold analysis reveals high glutamate levels in retinal and cortical synaptic terminals in the lateral geniculate nucleus of the macaque

An immunogold procedure has been used on ultrathin sections of the parvo- and magnocellular layers of the dorsal lateral geniculate of the rhesus monkey to estimate quantitatively at the electron microscopic level the intensity of immunoreactivity to an antibody against glutamate over profiles of retinal, cortical, GABAergic synaptic terminals and glial cells. GABAergic terminals were identified directly by immunogold reactivity to a GABA antibody or by ultrastructural features. The results showed that in both of the main subdivisions of the geniculate the densities of immunogold particles over cortical and retinal terminals were about two- to three-fold higher than those over GABAergic terminals or glial profiles. In addition, cortical and retinal terminals showed higher positive correlations of glutamate immunogold particle densities to synaptic vesicle densities than did GABAergic terminals. These differences suggest higher and lower concentrations of glutamate corresponding to transmitter and metabolic pools of this amino acid in axon terminals of retinal and cortical origins versus GABAergic terminals, respectively, in the dorsal lateral geniculate nucleus of the macaque.

Differences between fovea and parafovea in visual search processes

Visual objects that differ from the surroundings for some simple feature, e.g. colour or line orientation, or for some shape parameters ("textons", Julez, 1986) are believed to be detected in parallel from different locations in the visual field without requiring a serial search process. Tachistoscopic presentations of textures were used to compare the time course of search processes in the fovea and parafovea. Detection of targets differing for a simple feature (line orientation or line crossings) from the surrounding elements was found to have a time course typical of parallel processing for coarse textures extending into the parafovea. For fine textures confined into the fovea the time course was suggestive of a serial search process even for these textons. These findings are consistent with the hypothesis that parallel processing of lines or crossings is subserved by a coarse network of detectors with relatively large receptive field and low resolution. For the counting of coloured spots in a background of a different colour the parafovea has the same time requirements as the fovea.

GABA-immunoreactive synaptic plexus in the nerve fiber layer of primate retina

Synaptic contacts onto fibers and somata in the nerve fiber layer (NFL) and ganglion cell layer (GCL) of macaque and human retina were demonstrated at the electron microscopical (EM) level. Many presynaptic processes in monkey NFL are gamma aminobutyric acid (GABA) immunoreactive, using anti-GABA antiserum with an EM immunogold procedure. Immunocytochemistry at the light microscopic level revealed that many GABA-reactive cells in the GCL send branching processes into the NFL, forming a sparse synaptic plexus. The presence of long, unbranched GABA-reactive fibers running horizontally in the NFL and entering the optic nerve suggests that some ganglion cells may be GABAergic. GABA-reactive cells contributing to the plexus appear to be a new class of displaced amacrines that arborize in the NFL.

Development of the optic nerve of the opossum (Didelphis virginiana)

The development of the optic nerve of a marsupial, the North American opossum, was examined in 24 animals from postnatal days 5 to 78 (P5-P78): gestation is 13 days. The estimated number of axons increased from 24,000 at P5, to 267,000 at P27, approximately 2.7 times the mean number in the adult. Following P27, axon numbers decreased rapidly to 140,000 at P40, then decreased more slowly, attaining adult values between P50 and P59. Thus, the opossum is similar to placental mammals examined in evidencing an overproduction and later attenuation to adult values in the number of axons in the optic nerve during development. Monocular enucleation of 3 animals at P17, 10 days before peak axon counts, resulted in a mean population increase of 24,000 (range 19,000-30,000) above the normal adult mean. Additionally, a 4th animal monocularly enucleated on P7, 3 days prior to the arrival of migrating fibers to central target sites, had a similar value of 26,500 supernumerary axons. Our findings in the opposum, when coupled with previous reports in other mammals, suggest that binocular interactions during development account only for optic nerve axon loss approximately equal in magnitude to the ipsilateral projection from one eye.

Axon diameter distributions across the monkey's optic nerve

The distribution of axons according to diameter has been examined in the optic nerve of old world monkeys. Axon diameters were measured from electron micrographs, and histograms were constructed at regular intervals across a section through the optic nerve to reveal the local axon diameter distribution. The total axon diameter distribution was also estimated. Fine-calibre optic axons (less than 2.0 micron in diameter) are found at all locations across the optic nerve. They are most frequent centrotemporally, where very few coarse optic axons can be found, but also make up the majority at the optic nerve's periphery. Coarse optic axons (greater than 2.0 microns in diameter) are increasingly common at progressively peripheral positions in the nerve. Around the nerve's circumference, these coarse optic axons are least numerous temporally, and most common dorsonasally. The axon diameter distribution peaks around 1.25 microns at most locations across the optic nerve, but there are more, slightly larger (1.5-2.0 microns), optic axons dorsally than ventrally. The estimated total axon diameter distribution is unimodal, peaking at 1.0-1.25 microns, with an extended tail towards larger diameters. This centroperipheral gradient of increasing axon diameters across the optic nerve is not substantial enough to account for the partial segregation of axons by size in the monkey's optic tract: there, coarse optic axons form a conspicuously greater proportion of the local axon diameter distribution along the tract's superficial (sub-pial) border, and fine optic axons are the only axons present near the tract's deep border. Hence, the fibre distribution in the optic tract cannot be formed by a simple combination of the fibre distributions of the two respective half-nerves, as described in the classic neuro-ophthalmologic literature. Rather, the present results, in conjunction with previous results from the optic tract, demonstrate that there must be a reorganization of axons by size in or near the optic chiasm.

The physiological basis of heterochromatic flicker photometry demonstrated in the ganglion cells of the macaque retina

1. Heterochromatic flicker photometry is a way of measuring the spectral sensitivity of the human eye. Two lights of different colour are sinusoidally alternated at, typically, 10-20 Hz, and their relative intensities adjusted by the observer until the sensation of flicker is minimized. This technique has been used to define the human photopic luminosity, or V lambda, function on which photometry is based. 2. We have studied the responses of macaque retinal ganglion cells using this stimulus paradigm. The responses of the phasic ganglion cells go through a minimum at relative radiances very similar to that predicted from the V lambda function. At this point, defined as equal luminance, an abrupt change in response phase was observed. A small residual response at twice the flicker frequency was apparent under some conditions. 3. The spectral sensitivity of parafoveal phasic cells measured in this way corresponded very closely to that of human observers minimizing flicker on the same apparatus. 4. Minima in phasic cell activity were independent of flicker frequency, as is the case in the psychophysical task. 5. The response minima of phasic cells obey the laws of additivity and transitivity which are important characteristics of heterochromatic flicker photometry. 6. As the relative intensities of the lights were altered responses of tonic, spectrally opponent cells usually underwent a gradual phase change with vigorous responses at equal luminance. The responses of tonic cells treated individually or as a population could not be related to the V lambda function in any meaningful way. 7. We conclude that the phasic, magnocellular cell system of the primate visual pathway underlies performance in the psychophysical task of heterochromatic flicker photometry. It is likely that other tasks in which spectral sensitivity conforms to the V lambda function also rely on this cell system.

Intraretinal axons of ganglion cells in the Japanese monkey (Macaca fuscata): conduction velocity and diameter distribution

In anesthetized and immobilized Japanese monkeys (Macaca fuscata), intraretinal conduction velocities of the ganglion cell axons were measured. The field potentials elicited by optic chiasm shocks consisted of fast and slow components with estimated conduction velocities of 1.19 and 0.72 m/s in recordings from the optic nerve fiber layer, and 1.65 and 1.00 m/s in recordings from the ganglion cell layer. Single cell recordings verified that the time course of the fast component corresponded to the antidromic spike latencies of Y-like cells, whereas that of the slow component covered the latency range of both X-like and W-like cells. In an electron microscopic study of the cross-sections of the intraretinal optic nerve fiber bundles, the axon diameter histograms of large samples (n = 3000-6000) all showed a unimodal distribution with a sharp peak at 0.3-0.6 micron and a long tail extending to 2-3 micron. The mean diameter was largest in the ventral and nasal bundles, smallest in the papillomacular bundle and intermediate in the dorsal, upper arcuate and lower arcuate bundles. However, diameter histograms of a small number of regional axons (n = 255-300) showed a broad tail distinct from the peak at 0.3-0.6 micron, enabling us to segregate a group of larger axons from the medium-sized to small axons. From such regional axon diameter histograms we estimated the mean relative occurrences of the larger axons (7.1-11.3%) and their mean diameters (0.9-1.3 micron). We further applied this relative frequency to the unimodal distribution of the histograms with larger samples in the upper and lower arcuate bundles and estimated the mean axon diameter of the large axons (1.1 micron) and that of the medium-sized to small axons (slightly below 0.5 micron). Finally, in studying the relation between axon diameter and conduction velocity in the two arcuate fiber bundles, we found it to be somewhat different from that previously reported for the cat retina.

The functional logic of cortical connections

Patterns of anatomical connections in the visual cortex form the structural basis for segregating features of the visual image into separate cortical areas and for communication between these areas at all levels to produce a coherent percept. Such multi-stage integration may be a common strategy throughout the cortex for producing complex behaviour.

Retinal afferent arborization patterns, dendritic field orientations, and the segregation of function in the lateral geniculate nucleus of the monkey

Optic tract fibers and cell bodies in the lateral geniculate nucleus of the monkey were studied intracellularly with micropipette electrodes containing the marker enzyme horseradish peroxidase. Single optic-tract fibers always projected to only one of the six geniculate layers. The majority of the axons innervating the four parvocellular laminae were red/green opponent color units; their terminations formed cylindrical columns that were perpendicular to the layers. In similar fashion, the geniculate cells in the parvocellular layers were mostly red/green units with narrow, bipolar dendritic fields oriented normal to the laminar borders. The majority of the retinal axons ending in parvocellular layers 6 and 5 were on-center units; nearly all geniculate cells in these two laminae were also on-center neurons. In layers 4 and 3 most terminating optic-tract fibers, as well as the geniculate cells themselves, were off-center units. All axons projecting to the magnocellular layers were broad-band units with spherical terminal arborizations. The magnocellular geniculate neurons, which were also broad band, had extensive spherical dendritic fields that often crossed laminar borders. Thus, the terminal patterns of each class of retinogeniculate axon closely resembled the dendritic orientations of the functionally related geniculate target cells.

Check-size specific changes of pattern electroretinogram in patients with early open-angle glaucoma

The pattern electroretinogram was recorded in patients with initial stages of visual field defects due to open-angle glaucoma and in age-matched normal subjects. Both normal subjects and glaucoma patients had a visual acuity above 0.8. Counterphasing checkerboard patterns were used as visual stimuli with a range of check sizes from 0.8 degree to 15 degrees at 7.8 reversals/s. Whereas the amplitude in glaucoma patients was nearly normal for large check sizes, it was significantly reduced for small check sizes (p = 0.003). Possibly two separate mechanisms that generate the pattern electroretinogram for small and large checks are differentially affected; they may be related to the magnocellular and parvocellular systems. The difference between normals and glaucoma patients was even more significant when the ratios of the amplitudes at small and large check sizes were compared (p less than 0.0002). When this ratios is used, the amplitude variability can be partly overcome and the pattern electroretinogram can be a sensitive indicator of ganglion cell function.

Morphology of ganglion cells which project to the dorsal lateral geniculate and superior colliculus in the ground squirrel

We wished to determine whether retinal ganglion cells that have axons terminating in the dorsal lateral geniculate and/or the superior colliculus have specific sizes of somata, comprising only part of the entire size range of ganglion cell somata. If so, then perhaps the specific functional types described by Michael might be associated with morphological types based on soma size. HRP was injected into either the superior colliculus (SC) or dorsal lateral geniculate nucleus (LGd) of thirteen-lined ground squirrels. Soma diameter of labeled ganglion cells was measured and the relation between cell size and frequency determined. After SC injections HRP-filled cells were mostly small and medium-sized. They ranged in diameter from 3 to 14 microns and the mean diameter of labeled neurons was 7.35 microns. Cells labeled after SC injections were often distributed as doublets or triplets in the retina. After LGD injections the majority of labeled cells were medium and large-sized. They ranged from 4 to 18 microns in diameter with a mean of 9.1 microns and were more regularly spaced within the retinal region of labeled cells. Thus, the present results provide reason to believe that functional classes of ganglion cells in ground squirrels may be correlated with particular morphological types.

Elimination of neurons from the rhesus monkey's lateral geniculate nucleus during development

The timing, magnitude, and spatial distribution of neuron elimination was studied in the dorsal lateral geniculate nucleus of 57 rhesus monkeys (Macaca mulatta) ranging in age from the 48th day of gestation to maturity. Normal and degenerating cells were counted in Nissl-stained sections by using video-enhanced differential interference contrast optics and video-overlay microscopy. Before embryonic day 60 (E60), the geniculate nucleus contains 2,200,000 +/- 100,000 neurons. Roughly 800,000 of these neurons are eliminated over a 40- to 50-day period spanning the middle third of gestation. Neurons are lost at an average rate of 300 an hour between E48 and E60, and at an average rate of 800 an hour between E60 and E100. Very few neurons are lost after E100, and as early as E103 the population has fallen to the adult average of 1,400,000 +/- 90,000. Degenerating neurons are far more common in the magnocellular part of the nucleus than in the parvicellular part. In 20 of 29 cases, the number of neurons is greater on the right than on the left side. The right-left asymmetry averages about 8.5% and the difference is statistically significant (phi 2 = 38, p less than .001). The period of cell death occurs before the emergence of cell layers in the geniculate nucleus, before the establishment of geniculocortical connections, and before the formation of ocular dominance columns (Rakic, '76). Most important, the depletion of neurons in the geniculate nucleus begins long before the depletion of retinal axons. The number of geniculate neurons is probably a key factor controlling the number of the retinal cells that survive to maturity.

The axon initial segment as a possible determinant of retinal ganglion cell dendritic geometry

In wholemounted retinae of cat, rat and monkey, in which ganglion cells were retrogradely labelled with horseradish peroxidase, a quantitative analysis of the direction of the axon initial segment with respect to the optic disc and of the relationship between the axon initial segment and the direction and distribution of primary dendrites was performed on the class of largest ganglion cells. The results show the following. (1) In all 3 species, the majority of primary dendrites of ganglion cells are directed away from the axon initial segment. (2) Primary dendrites arise with a greater frequency from the region of the cell body opposite to the axon initial segment than close to it. (3) In cat the direction of the axon initial segments show less variance in their initial direction with respect to the optic disc than in rat or monkey. In adult cats the nucleus of alpha-ganglion cells occupies a central position. In the kitten the position of the nucleus is eccentric and lies in a part of the cell body opposite to the axon initial segment. The nucleus moves to a central position over the next 3 weeks. The position of the axon initial segment is discussed as a possible determinant of ganglion cell dendritic geometry.

The lengths of the fibres of Henle in the retina of macaque monkeys: implications for vision

In Golgi preparations of retinae from macaque monkeys the lengths of the fibres of Henle from photoreceptors, and Müller's fibres were measured. It was shown that the lengths of Müller's fibres provide a good estimate of the lengths of adjacent fibres of Henle of photoreceptors. The fibres form a radiate pattern with respect to the fovea. They are longest at the fovea and their length decreases in a systematic way with distance from the fovea. The implications of the fibre length are considered with respect to the relationship between the ganglion cell distribution and central magnification factors. We show that even when the functional offset introduced by the fibres of Henle and by bipolar and ganglion cells is taken into account there is not a constant proportional relationship between ganglion cells and central magnification factors. The representation of the central few degrees of the visual field on the striate cortex is greater than would be predicted on the basis of the ganglion cell density for the central retina.

'Hidden lamination' in the dorsal lateral geniculate nucleus: the functional organization of this thalamic region in the rat

The cyto-and myeloarchitecture of the rat's dorsal lateral geniculate nucleus (dLGN) display none of the laminar features characteristic of this thalamic region in carnivores and primates. Despite this, the rodent's nucleus contains a segregation of functionally and ocularly distinct afferents--organizational properties manifested in the prominent lamination of these other mammalian forms. The rat's dLGN can be divided into two main regions: an inner core and an outer shell. The inner core contains two ocular laminae receiving direct retinotopic projections from the contralateral nasal and ipsilateral temporal retinae, mapping the contralateral visual hemifield. The outer shell receives a retinotopic projection from the complete contralateral retina only, the representation of the ipsilateral hemifield being extremely compressed at the medial edge of this lamina. The retinotopic maps in these three ocular laminae (contra, ipsi, contra) are in conjugate register, so that lines of projection course rostro-ventro-medially from the optic tract at the thalamic surface through these laminae. Three morphologically distinct retinal ganglion cell types project to the dLGN, and the axons of these ganglion cells are partially segregated within the optic tract in anticipation of their segregation within the nucleus, where they terminate at distinct locations along the lines of projection. Type I and III cells terminate in the inner core of the nucleus, while type II and III cells terminate in the outer shell. The outer shell also receives a direct projection from the superior colliculus. These characteristics of the afferent termination within the rat's dLGN support the view of a general mammalian plan for the organization of this thalamic region, and provide a basis for further experimentation to test speculations about potentially homologous subdivisions of this nucleus. Conclusions regarding functionally analogous pathways are proposed with less confidence, due to the paucity of definitive evidence for physiologically distinct cell classes. The type I cells in the rat's retina are the likely homologues of the cat's alpha-cell. Geniculocortical relay cells driven by them have properties similar to the cat's Y-cell. The inner core of the nucleus then may transmit information of a Y-like nature onto striate cortex. The outer shell of the rat's nucleus, a portion of which receives collicular as well as retinal innervation, may convey W-like information onto striate cortex. The rat's retinogeniculate projection appears to be lacking a beta-cell-like pathway that may subserve X-cell function altogether.

Functional lamination in the ganglion cell layer of the macaque's retina

Close to the fovea of the primate retina the ganglion cell layer is at its maximal thickness and several layers of cells deep. In whole-mount preparations in which the ganglion cells had been retrogradely labelled to reveal the dendritic trees we have studied the distribution of the different ganglion cell types across the depth of the ganglion cell layer. The ganglion cells which project to the parvocellular layers (P ganglion cells) are found more vitread than those which project to the magnocellular layers (M ganglion cells). The cells which project to the midbrain lie in the outer part of the ganglion cell layer among the M cells and adjacent to the inner plexiform layer. Within the P and M classes of ganglion cell the On-centre cells lie more vitread than the Off-centre cells. These results are discussed with relation to the proportions of different cell types sampled with intraocular recordings from ganglion cells and the possible significance for the development of different types of ganglion cell.

The nasotemporal division in primate retina: the neural bases of macular sparing and splitting

In primates, each hemisphere contains a representation of the contralateral visual hemifield; unilateral damage to the visual pathways results in loss of vision in half of the visual field. Apparently similar severe, unilateral lesions to the central visual pathways can result in two qualitatively different central visual field defects termed macular sparing and macular splitting. In macular sparing a 2 degrees to 3 degrees region around the fovea is spared from the effects of unilateral damage to the visual pathways. In macular splitting there is no such spared region and the scotoma produced by unilateral brain damage bisects the fovea. The patterns of decussation of the different classes of retinal ganglion cells in both New World (Saimiri sciureus) and Old World (Macaca fascicularis) monkeys have been determined by horseradish peroxidase injection. In both species the distributions of ipsilaterally and contralaterally projecting ganglion cells in the central retina are different from those in other mammals and suggest neural bases for macular sparing and splitting, respectively.

Anatomy of macaque fovea and spatial densities of neurons in foveal representation

Fine visual sampling in the macaque depends on the high density of cone outer and inner segments in the fovea. Cone pedicles, at the opposite, presynaptic end of the cone, are absent from the center of the fovea. Both ends of the cones, inner segments and pedicles, are closely packed within their respective monolayers, but the spatial density of foveal pedicles is lower because foveal pedicles are wider than inner segments. Because there is one pedicle for every inner segment, and because pedicles are wider than inner segments, increase in eccentricity finds increasing lateral displacement of the cone's pedicle from its inner segment. Further increase of eccentricity finds inner segment density falling below pedicle density, and so this lateral displacement declines. By 2-3 mm from the center, inner segments catch up with pedicles. Additional lateral displacements, of bipolar cells from pedicles and ganglion from bipolar cells, are largest for central-most elements and fall steeply with eccentricity. By taking into account all of these lateral displacements, the eccentricity of the cone inner segment(s) associated with a ganglion cell was determined, as was the area of inner segments represented by a unit area in the ganglion cell layer. Then raw ganglion cell densities were transformed to densities comparable to densities of inner segments and of cells in dorsal lateral geniculate nucleus. On average there appears to be close to 2 ganglion cells for each cone in the central fovea out to about 2.5 degrees. Thus, the density of foveal ganglion cells is sufficient to allow each red and each green cone to connect to 2 midget ganglion cells, and each blue cone to connect to 1 ganglion cell. Furthermore, there appears to be a single dorsal lateral geniculate cell for each ganglion cell.

Transient and sustained responses in four extrastriate visual areas of the owl monkey

Single neuron responses to stationary flashed bars were recorded from four extrastriate visual areas in the owl monkey: the middle temporal area (MT), the dorsal lateral area (DL), the dorsal medial area (DM), and the medial area (M). Data were collected at the optimum bar size and orientation for each cell. Each post-stimulus histogram was normalized to its maximum bin height. A cumulative histogram was produced for each area by adding together all the corresponding cell histograms. The cumulative histograms reveal a short latency, transient component and a longer latency, sustained component to the response for each of the areas. In all four areas there was a strong response, but the sustained component was much larger in DL and DM than in MT or M. The transient response in DL had a much longer latency than in the other areas. The dichotomy between areas which are slow-sustained responding and areas which are fast-transient responding is similar to the differences found between the magnocellular and parvocellular pathways.

Visual resolution of macaque retinal ganglion cells

1. The visual resolving ability of different types of macaque retinal ganglion cells was estimated at different retinal eccentricities, by measuring the amplitude of modulated responses to black-white gratings of spatial frequencies near the resolution limit for each cell. 2. The resolving ability of tonic, spectrally opponent ganglion cells was usually similar to that of phasic, non-opponent ganglion cells at similar eccentricities, except that at eccentricities greater than 10 deg some tonic ganglion cells with remarkably high resolution (up to ca. 15 cycles/deg) were found. Our cell sample was limited within the central 2 deg of the visual field, however. 3. Only a small proportion of phasic ganglion cells showed an increase of mean firing level to gratings near the resolution limit. The maintained firing of tonic ganglion cells was higher than that of phasic ganglion cells. 4. With red-black or green-black gratings, the resolution of phasic ganglion cells was unaffected. For red or green on-centre ganglion cells, a marked deterioration of resolving ability occurred when the grating was of a colour to which a cell responded poorly (green-black gratings for red on-centre cells, and red-black gratings for green on-centre cells). A slight improvement in resolving ability occurred when the grating was of an excitatory colour. 5. For a sub-sample of cells, we compared resolution limit with centre size as determined from area-threshold curves. For both phasic and tonic ganglion cells, resolution limit (the period length just resolved) was about half the centre diameter, as is the case for cat ganglion cells. This implies that the centre sizes of phasic and tonic monkey ganglion cells are similar at most eccentricities. 6. We attempt to relate these results to primate retinal anatomy and visual resolution, determined behaviourally.

Segregation of functionally distinct axons in the monkey's optic tract

The classical neuro-ophthalmologic literature describes the organization of the primate's optic tract as containing a single topographic representation of the complete contralateral visual hemifield. In contrast, cats have separate visual field representations for the optic axons of the functionally distinct retinal ganglion cell classes. As the line of decussation for each ganglion cell class in the cat occupies a different location on the retinal surface, whereas in primates they are all superimposed, such a species difference might be expected. We report that implants of horseradish peroxidase placed in either the deep or superficial extremes of the monkey's optic tract produce retrograde labelling of distinct retinal ganglion cell classes, and produce anterograde labelling confined to distinct laminae of the lateral geniculate nucleus. Hence, the optic tract of the primate cannot contain a single representation of the contralateral visual hemifield; rather, independent visual field representations for the functionally distinct optic axons must exist. Their anatomical segregation may account for the clinical observation of selective impairments of distinct visual abilities following partial interruption of the optic tract in man.

Thresholds to chromatic spots of cells in the macaque geniculate nucleus as compared to detection sensitivity in man

1. The relation between wavelength and psychophysical threshold for chromatic spots on a white background provides evidence for the existence of chromatic channels in the primate visual system. To find the physiological substrate of this task, we compared increment thresholds of different cell types in the macaque lateral geniculate nucleus with human psychophysical thresholds to the same stimuli, using two spot sizes, 4 and 0.4 deg. 2. At different wavelengths, different opponent cell classes in the parvocellular layers of the nucleus were most sensitive, so that at long wavelengths (greater than 600 nm) red on-centre cells were most sensitive, while at short wavelengths (less than 500 nm) S-cone, blue on-centre cells were most sensitive, from 500 to about 550 nm green on-centre cells being most sensitive. A rare cell type with inhibition from S-cones was most sensitive at about 570 nm, although its maximum contrast increment sensitivity was poor compared with that of other cell types. Variation in strength of cone opponency caused a considerable range in threshold in each of the opponent cell classes of the parvocellular layers. 3. On- and off-centre cells from the magnocellular layers were more sensitive than opponent cells to white and yellow spots (as is the case with achromatic gratings). 4. With different wavelengths and spot sizes, the most sensitive cells found approached (to within 0.1-0.3 log units) human psychophysical sensitivity, suggesting that the most sensitive cells available may underlie detection. 5. Measurements of psychophysical chromatic discrimination thresholds, both with nearly monochromatic spots and with spots of differing saturation (purity), support this hypothesis. When magnocellular cell sensitivity corresponded to psychophysical threshold, a suprathreshold stimulus, capable of activating opponent cells, was required for chromatic discrimination.

Stratified distribution of synapses in the inner plexiform layer of primate retina

Distributions of bipolar (B) and amacrine (A) synapses and postsynaptic ganglion cell (G) dendritic profiles in the inner plexiform layer (IPL) were analyzed in EM montages of monkey central and human foveal and peripheral retinae. Synapses and profiles were counted and plotted for each 5% interval of IPL, with 0% at the inner edge of the inner nuclear layer and 100% at the outer edge of the ganglion cell layer. In monkey and human retinae, both A and B synapses occur throughout the IPL, but the ratio of A to B synapses varies from 2:1 to more than 6:1. In the monkey central retina, four bands of A conventional synapses are concentrated at 15, 35, 60, and 80% depth. In the human foveal slope, there are two main A bands at 45 and 85%, whereas in the human periphery, there are five bands at 15, 35, 60, 75, and 90%. In both species, A processes containing large dense-core vesicles are concentrated in three bands at 10-20, 50, and 80-90% depth, corresponding to previously described levels of peptides, dopamine, and GABA. B ribbon synapses are distributed fairly evenly throughout the IPL, with a suggestion of four broadly overlapping bands. Most B ribbons are presynaptic to one A and one G (B----A/G). In the human, there are significantly more B dyads with postsynaptic G's (B----A/G, B----G/G) in the fovea (91%) than in the periphery (66%), implying greater A cell processing peripherally. Also in the human, B terminals containing glycogenlike granules are concentrated in the outer half of the IPL, with agranular terminals in the inner half. Our results demonstrate multiple strata containing different types of synaptic contacts in primate IPL.

Alpha ganglion cells in mammalian retinae

Retinae from species of six orders of mammals (table 1) were processed by an on-the-slide neurofibrillar staining method to establish whether alpha-type ganglion cells are generally present in placental mammals. Alpha cells of the domestic cat, where they were first defined as a type, are used as a standard of reference. Alpha cells were found in all the twenty species examined; characteristically they have the largest somata and large dendritic fields with a typical branching pattern. In keeping with the common morphology there are inner and outer stratifying subpopulations and therefore a presumptive 'on-centre' and 'off-centre' responsiveness to light. Depending on the species, alpha cells form between 1 and 4% of the ganglion-cell population and their dendritic fields cover the retina three to four times. The morphology of alpha ganglion cells, and many of their quantitative features, are conserved in mammals coming from different habitats and having a wide variety of behaviours. Because it is known different habitats and having a wide variety of behaviours. Because it is known from the cat that alpha ganglion cells have brisk-transient or Y receptive fields it is possible that all placental mammals possess this physiological system.

Regional variation of contrast sensitivity across the retina of the achromat: sensitivity of human rod vision

1. Detection thresholds for two-dimensional Gabor functions of varying spatial and temporal frequency were used to investigate the post-receptoral sensitivity across the retina of the typical and complete achromat. 2. Under photopic conditions there is no evidence for post-receptoral cone function at any retinal eccentricity investigated. Sensitivity saturates in a way consistent with known psychophysical and electrophysiological measures of rod saturation. This occurs in a unitary fashion across the retina. 3. Under scotopic conditions the regional fall-off in spatio-temporal sensitivity is similar for the achromat and duplex retina. This suggests that the rods in the achromat make normal neural connections. 4. Taken together this supports the contention that the typical and complete achromat is a functional rod monochromat and hence can be used to explore the sensitivity of the isolated rod post-receptoral mechanism under mesopic conditions where its sensitivity is optimal. This is where its contribution is most difficult to isolate in the duplex retina. 5. For the human rod mechanism, mesopic post-receptoral sensitivity for all spatio-temporal stimuli is optimal in the central region of the retina and falls off as a function of eccentricity. 6. For localized stimuli, peripheral spatial sensitivity is reduced evenly at all spatial frequencies compared with that of the central retina. A similar displacement of the spatial sensitivity function of the rod mechanism occurs as illuminance is reduced. 7. For localized stimuli, temporal acuity of the rod mechanism is around 20-25 Hz irrespective of retinal position. As the illuminance is further lowered dynamics of the rod pathway are reduced irrespective of retinal position and the sensitivity function maintains a bandpass shape. 8. The regional distribution sensitivity of the rod mechanism changes as illuminance is reduced from mesopic to scotopic levels.

Distribution of axons according to diameter in the monkey's optic tract

The distribution of axonal diameters in the optic tract of Old World monkeys was examined by light and electron microscopy. Axon diameters were measured in samples of 100 axons taken from several locations in a cross section of the tract about 5 mm behind the optic chiasm. Fine-caliber axons (less than 1.75 micron in diameter) were found in all parts of the tract. Dorsally no coarse axons were present. Further ventrally, coarse axons gradually appeared and increased steadily in proportion. The largest optic axons (greater than 2.5 micron) were found in the most ventral parts of the tract, near the pial surface. This pattern of segregation of axons of differing diameters in the optic tract is a rearrangement of the distribution of axon diameters seen in the nerve rather than a continuation of the same pattern. Examination of axon diameters in the optic nerve has shown that there is a preponderance of fine axons centrally, while coarser axons are found in the periphery, near the pial surface; however, histograms from central parts of the nerve contain a greater proportion of coarse axons than the dorsal parts of the optic tract, while histograms from the periphery of the optic nerve contain a conspicuously greater proportion of fine axons than do histograms from the most ventral parts of the tract. This relatively greater segregation of axons according to diameter in the optic tract demonstrates that the distribution of axons in the tract cannot be formed by the simple combination of two hemiretinal maps contained in each optic nerve, as suggested in classic descriptions.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of image degradation on retinal illuminance and pattern responses to checkerboard stimuli

The contrast of a retinal image is less than that of the external stimulus owing to a process of optical degradation. Theoretical studies have shows that this affects the pattern and illuminance detectors of the retina differently and provides a new insight into the nature of contrast stimulation and the mechanisms responsible for the pattern electroretinogram. Consensus data on the optical transfer function of the eye are applied to the Fourier transform of the pattern stimulus and the retinal illuminance distribution is determined. The checkerboard image is shown to undergo substantial degradation for those check sizes used in experimental and clinical observations. Current concepts of contrast and modes of stimulation are examined and methods are described for quantifying the effects on stimulation of illuminance and pattern detectors. The findings are applied to experimental data on spatial tuning functions of electroretinograms elicited by checkerboard pattern stimulation. It is concluded that the signal predominantly originates from local illuminance, but that this can account for only part of the response with small check sizes. The remainder must be a highly selective response to spatial frequency. If the predicted degradation is accepted, a similar conclusion must be reached for many previously reported tuning functions of the pattern electroretinogram as well as to the present experiment on eight normal eyes.

The retinal ganglion cell mosaic defines orientation columns in striate cortex

A computer simulation was used to demonstrate that the tangential organization of orientation columns is a natural consequence of the orderly projection of the mosaic of retinal ganglion cells onto the visual cortex. Parameters of the simulation were taken from published anatomical and electrophysiological data, and the resulting columnar organization of the simulated visual cortex shows many similarities with observations from animals. The model is able to account for a variety of experimental observations, including the presence of orientation columns in visually inexperienced animals.

Ganglion cell dendritic structure and retinal topography in the rat

The dendritic field size, the distribution of the dendrites relative to the cell body, and the overall shape of the dendritic field of type I ganglion cells in the rat retina were analyzed. These features of neuronal structure were related to the topography of the rat retina. As in the cat, the cell bodies of type I ganglion cells are arranged in a nonrandom mosaic. Previous work has demonstrated that the density of type I cells in the rat retina does not covary with the density of all ganglion cells. Type I dendritic field size varies over the retina; the increase in dendritic field size is accounted for better by the decrease in type I density than by the decrease in overall ganglion cell density. The center of the dendritic field of most type I cells is displaced in the plane of the retina from the cell body. Unlike in carnivore retina (Schall and Leventhal: J. Comp. Neurol. 257:149-159, '87), the dendritic fields in the rat are not displaced down the ganglion cell density gradient. Rather, there is a tendency for the dendritic trees, especially in temporal retina, to be displaced toward dorsal retina. Most of the dendritic fields are elongated, but the degree of elongation is less than that observed in carnivore or primate retina. Unlike in carnivore and primate retina (Leventhal and Schall: J. Comp. Neurol. 220:465-475, '83; Schall et al.: Brain Res. 368:18-23, '86), there is no relationship between dendritic tree orientation and position relative to any point on the retina in the rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Intensity coding and luxotonic activity in the ground squirrel lateral geniculate nucleus

The responses of contrast-sensitive cells in the ground squirrel LGN were studied. In most cells the response to an on-off stimulus was comprised of two components: a sustained on and a transient on-off. The sustained component amplitude was a power function of the light intensity and disappeared altogether at low temperature while the transient component was insensitive to the light intensity and to a drop in temperature. These cells were also luxotonic; they increased their average firing rate when the intensity of a steady stimulus was increased. The possible relation of the luxotonic activity to the diurnal nature of the ground squirrel is discussed.

Representation of edges of variable blur by neuronal responses in the lateral geniculate body and the visual cortex of cats: limits of linear prediction

We have measured the responses of cells in the cats lateral geniculate body and the visual cortex to edges which were blurred to various degrees (cosinusoidal blur). For the same cells also the responses were determined to sinusoidal gratings of various fundamental frequency and to slits of various blur and width. All stimuli were moved across the receptive fields at various speeds. The responses of most cells increased with increasing edge sharpness, but usually reached a maximum at a blur corresponding to a high frequency cutoff at 0.6-1.2 c/deg. The responses to the sharpest edges were usually smaller than those to a blurred edge (up to -50% in individual cells and -15% in the average). After normalization, the responses predicted from the spatial frequency tuning curves and the Fourier transform of the edge stimuli corresponded well to the measured blur functions up to the maximum of the edge response which varied considerably between cells, however. At edge sharpness beyond that maximum, the predicted curves rose up to edge sharpness with high frequency cutoff 1.6-1.8 times above that which produced the experimental neuronal response maximum. On the other hand, responses could increase with edge sharpening in spatial frequency regions, in which no or only small responses were seen with sinusoidal gratings (e.g. at lower spatial frequencies in "band pass neurons"). Geniculate X- and cortical simple cells as well as those geniculate Y-cells which showed phase locked grating responses behaved similarly in all respects. We concluded that edge sharpness is not represented by response amplitude of individual neurons but by the spatial distribution of excitatory peaks across the representation of the retinotopic cortical map. Our findings further indicate that spatial models of receptive fields assuming linear signal summation have only a limited value for predicting edge sharpness.