We investigated the representation of color in cortical area V2 of macaque monkeys, and the association of color with other stimulus attributes. We measured the selectivity of individual V2 neurons for color, motion, and form. Most neurons in V2 were orientation selective, about half of them were selective for color, and a minority of cells (about 20%) were selective for size or direction. We correlated these physiological measurements with the anatomical location of the cells with respect to the cytochrome oxidase (CO) compartments of area V2. There was a tendency for color-selective cells to be found more frequently in the thin stripes, but color-selective cells also occurred frequently in thick stripes and inter-stripes. We found no difference in the degree of color selectivity between the different CO compartments. Furthermore, there was no negative correlation between color selectivity and selectivity for other stimulus attributes. We found many cells capable of encoding information along more than one stimulus dimension, regardless of their location with respect to the CO compartments. We suggest that area V2 plays an important role in integrating information about color, motion, and form. By this integration of stimulus attributes a cue-invariant representation of the visual world might be achieved.

Vertical Slices of postnatal day 6 (P6) rat retina were cut and cultured using the roller-tube technique. The organotypic differentiation during a culture period of up to 30 days has been described in a previous study (Feigenspan et al., 1993a). Here we concentrated on the synaptic organization in the retinal slice culture. Electron microscopy revealed the presence of ribbon synapses in the outer plexiform layer and conventional and ribbon syanpses in the inner plexiform layer. Immunofluroscence with antibodies that recognize specific subunits of GABAA or glycine receptors revealed a punctuate distribution of the receptors. They were aggregated in “hot spots” that correspond to a concentration of receptors at postsynaptic sites. Different isoforms of GABAA and glycine receptors occured in the slice cultures. The experiments show that there is a differentiation of synapses and a diversity of transmitter receptors in the slice cultures that is comparable to the in vivo retina.

The purpose of the present study was to compare visual orienting behavior in the adult cat during (1) unilateral and bilateral cooling deactivation of posterior-middle suprasylvian (pMS) sulcai cortex, and (2) unilateral and bilateral deactivation of the superior colliculus. As expected, unilateral cooling deactivation of either pMS cortex or the superior colliculus resulted in a profound visual neglect of the contracooled hemifield. The addition of cooling the homotopic region in the opposite hemisphere largely reversed this deficit and restored visual orienting into the previously neglected hemifield. These results show that (1) pMS cortex and the superior colliculus are essential for normal detection and orienting to visual targets, and (2) unilateral visual neglect results from an imbalance of activities in the two hemispheres induced at either cortical or subcortical levels. These conclusions have implications for understanding neural bases of visual hemineglect following unilateral lesions in humans.

Gq-type GTP-binding protein (Gq) plays an important role in invertebrate visual phototransduction. The subcellular localization of the alpha subunit of visual Gq in crayfish photoreceptor was investigated immunocytochemically and biochemically to demonstrate the details of the rhodopsin-Gq interaction. The localization of Gq(alpha) changed depending on the light condition. In the dark, Gq(alpha) was localized in the whole rhabdoms as the membrane-bound form. In the light, half of the Gq(alpha) was localized in the cytoplasm as the soluble form. The translocation of Gq(alpha) was reversible. The light-modulated translocation possibly controls the amount of Gq that can be activated by rhodopsin. In vitro hydroxylamine treatment of rhabdomeric membranes suggested that the translocation was regulated by the fatty-acid modification of Gq(alpha).

Modulation by two temporal frequencies differentiates visual processing at the fundamentals (1Fs), second harmonics (2Fs), and second-order intermodulation components (IMCs), the latter created neurally as the sum or difference of the two modulation frequencies. Steady-state VEPs were recorded while stereo-normal adults viewed luminance or grating stimuli modulated by up to three temporal frequencies under dichoptic, monocular, or ordinary (binocular) viewing conditions arranged using liquid crystal light shutters. In Experiment 1, modulation of luminance by a single temporal frequency produced strong 1F and 2F VEP components, but modulation of gratings produced only 2Fs. Modulation by two temporal frequencies resulted in IMCs, often in the absence of evoked activity in the EEC at the 1Fs. IMCs were generally larger during pattern as compared to luminance modulation. Amplitudes of 1Fs and IMCs were smaller, but 2Fs were larger, during dichoptic as compared to ordinary viewing. Although the 2F to a single modulation presented to one eye was not reduced when a second frequency was added to the opposite eye, monocular IMCs were diminished when a frequency was added to the opposite eye. We conclude that 2Fs and IMCs are associated with different neural substrates. Results are consistent with a two pathway model with one pathway having a nonlinear filter prior to binocular combination, the other pathway having a nonlinearity following binocular linear summation. Implications of these data for binocular function are discussed.

Using a combination of retrograde horseradish peroxidase (HRP) labelling, silver staining, and electron microscopy, we have assessed the relationship between retinal ganglion cell soma size and axon diameter in the adult ferret (Mustela putorius furo). Retinal ganglion cells were labelled following injections of HRP into the lateral geniculate nucleus (LGN), superior colliculus (SC), or LGN+SC. The soma size distributions following LGN, SC, or LGN+SC injections were all unimodal showing considerable overlap between different cell classes. This was confirmed for alpha cells, identified on the basis of dendritic filling or from neurofibrillar-stained retinae. Analysis of the soma size and axon diameters of a population of heavily labelled retinal ganglion cells showed a significant correlation between the two. However, the overall distribution of intraretinal axon diameter was bimodal with an extended tail. Analysis of the ganglion cell distributions in the adult ferret indicates that beta cells comprise about 50.5–55%, gamma 42.5–47%, and alpha 2.5% of the ganglion cell population. This implies that the proportion of gamma, beta, alpha cells in both cat and ferret retina is highly conserved despite differences in visual specialization in the two species.

We studied the anatomical substrates of spatial vision in a New World monkey, the marmoset Callithrix jacchus. This species has good visual acuity and a foveal specialization which is qualitatively similar to that of humans and other Old World primates.We measured the spatial density of retinal ganglion cells and photoreceptors, and calculated the relative numbers of these cell populations. We find that ganglion cells outnumber photoreceptors by between 2.4:1 and 4.2:1 in the fovea. The peak sampling density of ganglion cells is close to 550,000 cells/mm2. This value falls by almost 1000-fold between the fovea and peripheral retina; a value which approaches recent estimates of the centroperipheral ganglion cell gradient for human and macaque monkey retina and primary visual cortex. The marmoset shows a sex-linked polymorphism of color vision: all male and some female marmosets are dichromats. Six of the retinas used in the present study came from animals whose chromatic phenotype was identified in electrophysiological experiments and confirmed by polymerase chain reaction (PCR) amplification of cone opsin encoding genes. One animal was a trichromat and the others were dichromats. Antibodies against short wavelength-sensitive (SWS) cones labeled close to 8% of all cones near the fovea of onedichromat animal, consistent with electrophysiological evidence that the SWS system is present inall marmosets. The topography and spatial density of cone photoreceptors and ganglion cells was similar to that reported for macaque retina, and we found no obvious difference between dichromatic and trichromatic marmoset retinas. These results reinforce the view that the main determinate of primate foveal topography is the requirement for maximal spatial resolution.

Two types of spiking neuron were identified among acutely dissociated neurons from the Xenopus retina by their responses to a depolarizing current step: single spikers and multiple spikers. In culture, multiple spikers had perikaryal diameters >15 μm, whereas single spikers had smaller somata, 5—10 μm in diameter. Using a conventional whole-cell patch-clamp technique, both T- and L-type calcium currents were identified in multiply spiking cells whereas only an L-type current was present in singly spiking cells. The metabotropic glutamate receptor (mGluR) agonist trans-(1S-3R)-1-amino-1,3-cyclopentane-dicarboxylic acid (trans-ACPD) significantly decreased the L-type calcium current by 46 ± 3% (mean ± S.E.M.) in both types of cell but had only a minor effect on the T-type current in multiply spiking neurons. In the presence of 50 μm 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 100 μM quisqualate (a potent mGluR1/5 agonist) decreased the L-type calcium current by 47 ± 9% but had no effect on the T-type current. The selective mGluR4/6/7 agonist (±) 2-amino-4-phosphonobutyric acid (L-AP4, 100 μM), and the mGluR2/3 agonist (2S,3S,4S)-α-(carboxycyclopropyl)glycine (L-CCG1, 100μM) decreased the L-type calcium current by 12 ± 3% and 14 ± 2%, respectively. The inhibition of calcium current by trans-ACPD was reduced when the patch pipette contained the G-protein inhibitor, GDPβS. The presence of the G-protein activator GTPγS in the patch pipette irreversibly reduced the L-type calcium current, but was without effect on the T-type current. Heparin applied intracellularly significantly reduced the inhibitory effect of quisqualate, indicating an involvement of the inositol triphosphate (IP3) pathway in the mGluR-induced reduction of calcium current. Replacement of internal EGTA with BAPTA significantly reduced the inhibitory effect of quisqualate. In contrast, internal application of cAMP did not prevent an inhibition of calcium current by quisqualate. Thus, the mechanism by which calcium current is inhibited by mGluR seems not to involve an intracellular cAMP cascade. Our findings indicate that activation of mGluR1/5 results in the inhibition of a high-threshold calcium current. This process is mediated by the activation of a G-protein and is consistent with inhibition occurring by an lPrstimulated release of internal calcium.

Two synapse-rich layers are well recognized in the mammalian retina, the inner and outer plexiform layers. However, synapses occur also in other layers, particularly in the innermost nerve fiber layer. These synapses form a tenuous layer at times referred to as the superficial plexiform layer. We have found that staining for GABAA receptors in whole-mounted rabbit retina demonstrates this layer. It is most well developed in the region of the visual streak 2–4 mm below the center of the myelinated streak and is very sparse in other parts. Most or all of the processes in the plexus originate from cells in the ganglion cell layer.

Difference-of-Gaussians (DOG) models for the receptive fields of retinal ganglion cells accurately predict linear responses to both periodic stimuli (typically moving sinusoidal gratings) and aperiodic stimuli (typically circular fields presented as square-wave pulses). While the relation of spatial organization to retinal anatomy has received considerable attention, temporal characteristics have been only loosely connected to retinal physiology. Here we integrate realistic photoreceptor response waveforms into the DOG model to clarify how far a single set of physiological parameters predict temporal aspects of linear responses to both periodic and aperiodic stimuli. Traditional filter-cascade models provide a useful first-order approximation of the single-photon response in photoreceptors. The absolute time scale of these, plus a time for retinal transmission, here construed as a fixed delay, are obtained from flash/step data. Using these values, we find that the DOG model predicts the main features of both the amplitude and phase response of linear cat ganglion cells to sinusoidal flicker. Where the simplest model formulation fails, it serves to reveal additional mechanisms. Unforeseen facts are the attenuation of low temporal frequencies even in pure center-type responses, and the phase advance of the response relative to the stimulus at low frequencies. Neither can be explained by any experimentally documented cone response waveform, but both would be explained by signal differentiation, e.g. in the retinal transmission pathway, as demonstrated at least in turtle retina.

Blocking GABAA-receptor-mediated inhibition reduces the selectivity of striate cortical neurons for the orientation of a light bar primarily by reducing the selectivity of their onset transient (initial 200 ms) response. Blocking GABAB-receptor-mediated inhibition with phaclofen, however, is not reported to reduce the orientation selectivity of these neurons when it is measured with a light bar. We hypothesized that blocking GABAB-receptor-mediated inhibition would instead affect the orientation selectivity of cortical neurons by reducing the selectivity of their sustained response to a prolonged stimulus. To test this hypothesis, we stimulated 21 striate cortical neurons with drifting sine-wave gratings and measured their orientation selectivity before, during, and after iontophoretic injection of 2-hydroxy-saclofen (2-OH-S), a selective GABAB-receptor antagonist. 2-OH-S reduced the orientation selectivity of six of eight simple cells by an average of 28.8 (± 13.2)% and reduced the orientation selectivity of eight of 13 complex cells by an average of 32.3 (± 27.4)%. As predicted, 2-OH-S reduced the orientation selectivity of the neurons' sustained response, but did not reduce the orientation selectivity of their onset transient response. 2-OH-S also increased the length of spike “bursts” (two or more spikes with interspike intervals ≤ 8 ms) and eliminated the orientation selectivity of these bursts for six cells. These results are the first demonstration of a functional role for GABAB receptors in visual cortex and support the hypothesis that two GABA-mediated inhibitory mechanisms, one fast and the other slow, operate within the striate cortex to shape the response properties of individual neurons.

A series of experiments investigated perceived direction of motion and depth segregation in motion transparency displays consisting of two planes of dots moving in different directions. Direction and depth judgments were obtained from human observers viewing these "bi-directional" animation sequences with and without explicit stereoscopic depth information. We found that (1) misperception of motion direction ("direction repulsion") occurs when two spatially intermingled directions of motion are within 60 deg of each other; (2) direction repulsion is minimal at cardinal directions; (3) perception of two directions of motion always results in separate motion planes segregated in depth; and (4) stereoscopic depth information has no effect on the magnitude of direction repulsion, but it does disambiguate the depth relations between motion directions. These results are developed within the context of a two-stage model of motion transparency. On this model, motion directions are registered within units subject to inhibitory interactions that cause direction repulsion, with the outputs of these units pooled within units selective for direction and disparity.

Previous experiments in animals have shown that early unilateral eyelid suture, a model of amblyopia induced by cataract, causes shrinkage of ocular dominance columns serving the deprived eye in the striate cortex. It is unknown whether the ocular dominance columns are affected in amblyopia produced by strabismus. We examined specimens of striate cortex obtained postmortem from a 79-year-old woman with a history of amblyopia in her left eye (20/800) since age 2 from accommodative esotropia. Four years prior to her death, she suffered an ischemic infarct of the left optic disc. This injury to the left optic disc made it possible to label the ocular dominance columns using cytochrome oxidase histochemistry. The pattern of ocular dominance columns was reconstructed throughout most of the right striate cortex. No shrinkage of columns was found. In the left cortex only half the column mosaic was labelled, because the patient had some residual vision in the temporal retina of her left eye. The columns within the labelled portion of the overall mosaic appeared normal. These findings indicate that shrinkage of ocular dominance columns does not occur in humans with amblyopia caused by accommodative esotropia. The ocular dominance columns are probably no longer susceptible to shrinkage at the age when most children with this condition begin to develop amblyopia.

Two distinct opsin-encoding cDNAs, designated MANOP1 and MANOP2, were isolated as 3′ fragments from the sphingid moth Manduca sexta. They were obtained by reverse transcription of retinal RNA and amplification with the polymerase chain reaction (PCR) using a degenerate primer designed to an amino-acid sequence conserved in arthropod opsins. The cDNA fragments labelled bands at approximately 1.8 kb on Northern blots of retinal RNA extracts. Levels of opsin message were compared in retinas from normal moths, whose diets were fortified with carotenoid precursors of the Manduca rhodopsin chromophore, 3-hydroxyretinal, and those reared on carotenoid/retinoid (vitamin A) deficient diets. The chromophore-depleted retinas contained more opsin mRNA; this was particularly true for MANOP2. Thus, the chromophore is not required for opsin gene transcription in Manduca.

To establish the topography of the lateral division of the lateral posterior region (LP1) projections to area 18, up to five different anatomical tracers were injected in separate rostrocaudal locations in area 18 of four adult cats, and patterns of retrogradely labeled LP1 cells were identified. LP1 inputs to area 18 arose from both caudal and rostral nuclei and were topological, organized in patterns that indicate that lower visual space is represented anteroventrally, and more central and upper visual space is represented caudodorsally. In the caudal LP1 nucleus, patches of labeled cells formed bands that ran parallel to the medial and lateral LPI borders and encompassed medial portions of the nucleus. In rostral LP1, the patches of labeled cells formed clusters giving the connections with area 18 a more modular appearance, and were nearer the lateral LP1 border. Injections made nearest area centralis representations in area 18 labeled more neurons than injections in cortex representing more peripheral visual space. Also, neighboring injections in area 18 labeled overlapping patches of cells, but no double-labeled cells were observed. These findings are consistent with previous conclusions based on electrophysiological mapping studies, that two retinotopically organized nuclei constitute LP1.

Using a reduced-silver neurofibrillar method, we stained a population of bipolar cells in the capybara retina. These cells are distributed throughout the retina following the same topography of other retinal cell classes as the A-type horizontal cells and ganglion cells. The level of axonal stratification, mosaic regularity, and dendritic coverage factor suggest that these neurofibrillar bipolar cells comprise a population of sublamina a cone bipolar cells.

Visual motion, i.e. the pattern of changes on the retinae caused by the motion of objects or the observer through the environment, contains important cues for the accurate perception of the three-dimensional layout of the visual scene. In this study, we investigate if neurons in the visual system, specifically in area MT of the macaque monkey, are able to differentiate between various velocity gradients. Our stimuli were random dot patterns designed to eliminate stimulus variables other than the orientation of a velocity gradient. We develop a stimulus space (“deformation space”) that allows us to easily parameterize our stimuli. We demonstrate that a substantial proportion of MT cells show tuned responses to our various velocity gradients, often exceeding the response evoked by an optimized flat velocity profile. This suggests that MT cells are able to represent complex aspects of the visual environment and that their properties make them well suited as building blocks for the complex receptive-field properties encountered in higher areas, such as area MST to which many cells in area MT project.

Studies of the numbers of retinal ganglion cells and lateral geniculate nucleus (LGN) neurons in primates suggest that the numbers of both types of neurons may vary over a two-fold range from one individual to another. This raises the question of whether the numbers of ganglion cells and LGN neurons are related or vary independently from individual to individual. We used stereological procedures to obtain unbiased estimates of the numbers of both cell types in seven rhesus monkeys. We found no significant correlation (rs. = −0.21) between the numbers of retinal and LGN cells in the same animals. In agreement with previous studies, the average ratio of the number of retinal ganglion cells that project to the LGN and the number of LGN cells was approximately 1:1. However, this ratio varied over a two-fold range, from 0.78:1 to 1.64:1, in individual animals. These results have important implications for understanding the mechanisms of retino-geniculate development and for understanding the connectional wiring between the retina and LGN.

The early development of the optic tract in hamsters was studied by labeling retinal axons with Dil applied to the eye, and then examining the labeled axons in flatmount preparations of the rostral brain stem. This technique permits a panoramic view of the entire retinal projection, from the chiasm to the caudal end of the superior colliculus. In the Ell embryo, retinal axons have reached the chiasm. They defasciculate as they emerge from the nerve, prior to reaching the ventral midline of the diencephalon, then converge again as they pass over to the opposite side. At the midline, many axonal trajectories crisscross, implying some shuffling of relative positions. Retinal axons are tightly bundled within the optic tract. Upon reaching the ventral border of the lateral geniculate body (LGB), they splay out over the nucleus, revealing a wavefront of pioneer axons individually distributed across the rostro-caudal extent of the LGB. Later-emerging retinal axons course over the surface of the thalamus in waves; subsequent waves of axons interdigitate between the lead fibers without fasciculating along them. Past the LGB, the axons undergo a second change in relative positions as the ribbon of fibers swerves caudally, prior to entering the superior colliculus. Retinal axons are tipped with growth cones of varying morphologies. No strong correlation is evident between the Structural complexity of the growth cone and its position within the tract. In the majority of cases, ipsilaterally and contralaterally directed axons follow a similar developmental course along the optic tract, without any indication of a temporal lag in the ipsilateral projection as claimed in earlier reports. Understanding the changes in spatial distribution of embryonic retinal axons as they navigate along the optic tract provides a further step towards elucidating how point-to-point projections form in developing sensory systems.