Based on comparative anatomical studies and electrophysiological experiments, we have identified a conserved subset of neurons in the lamina, medulla, and lobula of dipterous insects that are involved in retinotopic visual motion direction selectivity. Working from the photoreceptors inward, this neuronal subset includes lamina amacrine (α) cells, lamina monopolar (L2) cells, the basket T-cell (T1 or β), the transmedullary cell Tm1, and the T5 bushy T-cell. Two GABA-immunoreactive neurons, the transmedullary cell Tm9 and a local interneuron at the level of T5 dendrites, are also implicated in the motion computation. We suggest that these neurons comprise the small-field elementary motion detector circuits the outputs of which are integrated by wide-field lobula plate tangential cells. We show that a computational model based on the available data about these neurons is consistent with existing models of biological elementary motion detection, and present a comparable version of the Hassenstein-Reichardt (HR) correlation model. Further, by using the model to synthesize a generic tangential cell, we show that it can account for the responses of lobula plate tangential cells to a wide range of transient stimuli, including responses which cannot be predicted using the HR model. This computational model of elementary motion detection is the first which derives specifically from the functional organization of a subset of retinotopic neurons supplying the lobula plate. A key prediction of this model is that elementary motion detector circuits respond quite differently to small-field transient stimulation than do spatially integrated motion processing neurons as observed in the lobula plate. In addition, this model suggests that the retinotopic motion information provided to wide-field motion-sensitive cells in the lobula is derived from a less refined stage of processing than motion inputs to the lobula plate.

We have localized glycine-like immunoreactivity to provide new anatomical detail about glycinergic neurons in the turtle retina. A rabbit antiserum directed against a glycine/albumin conjugate was used with standard fluorescent and avidin-biotin labeling techniques. Some processes in the outer plexiform layer and many processes in the inner plexiform layer, numerous somata in the inner nuclear layer, and isolated somata in the ganglion cell layer were immunoreactive.

The vast majority of labeled neurons were amacrine cells. One class of amacrine cells had well-labeled somata near the inner nuclear/inner plexiform layer border, which gave rise to thick primary processes that entered the inner plexiform layer and arborized near the border of strata 1 and 2 and in stratum 3. A second class of glycinergic neurons, consisting of putative interplexiform cells, was unique in that it gave rise to dendritic arborizations in both the outer plexiform layer and the inner plexiform layer. Some of the immunoreactive neurons in the ganglion cell layer were apparently displaced amacrine cells, while others were probably true ganglion cells because they gave rise to labeled axons, and many labeled axons were visible in the ganglion cell axon layer. These results suggested that glycine played an extensive role in the turtle retina, and that it was involved in many diverse synaptic interactions in both the outer plexiform layer and the inner plexiform layer.

Like many diurnal raptors, the American kestrel or sparrow hawk, Falco sparverius, possesses two foveae in each eye. In this study, we used fundus photography and reversed ophthalmoscopy to plot the projection of these foveae onto the visual field, together with other retinal landmarks such as the pecten and ora terminali. From such data, it was determined that the central monocular fovea (CMF) and temporal binocular fovea (TBF) were separated by 36 deg and that the kestrel has 58 deg of binocular overlap when the eyes are appropriately converged. Single-cell and multi-unit recordings were used to systematically explore the tectal surface and map receptive fields (RF) onto a hemispheric screen. A retinotopic map of the tectal surface was produced from such data and revealed an expanded representation for each fovea on the tectum and a systematic increase in RF size from fovea to periphery. The functional significance of this organization is discussed.

The purpose of this study was to evaluate the use of slow multifocal m-sequence stimulation in analyzing the topographic distribution and underlying mechanisms (including nonlinearities) of the retinal oscillatory potentials (OPs). In giving us access to the response topography and the nonlinear characteristics of the OP,, the m-sequence technique provides us with two important means for the identification and characterization of the signal sources. In this study, we analyzed the OPs into the first- and second-order components and investigated their topographies and luminance dependence. The distribution of both the first- and second-order OP components differed significantly from that of the nicker ERG investigated by Sutter and Tran (1992). At eccentricities and luminance levels favoring activity by both rods and cones, the second-order OPs were particularly prominent showing the most clear-defined and complex waveform The topographic distribution of the second-order OPs showed combined features of both rod and cone distributions On a strong rod-bleaching background the second-order OPs were eliminated and the first-order OPs showed a reduced amplitude and a shifted latency These results are consistent with the notion that the second-order component of the OPs is dominated by contributions from rod-cone interactions.

Areas V3 and V4 are commonly thought of as individual entities in the primate visual system, based on definition criteria such as their representation of visual space, connectivity, functional response properties, and relative anatomical location in cortex. Yet, large-scale functional and anatomical organization patterns not only emphasize distinctions within each area, but also links across visual cortex. Specifically, the visuotopic organization of V3 and V4 appears to be part of a larger, supra-areal organization, clustering these areas with early visual areas V1 and V2. In addition, connectivity patterns across visual cortex appear to vary within these areas as a function of their supra-areal eccentricity organization. This complicates the traditional view of these regions as individual functional “areas.” Here, we will review the criteria for defining areas V3 and V4 and will discuss functional and anatomical studies in humans and monkeys that emphasize the integration of individual visual areas into broad, supra-areal clusters that work in concert for a common computational goal. Specifically, we propose that the visuotopic organization of V3 and V4, which provides the criteria for differentiating these areas, also unifies these areas into the supra-areal organization of early visual cortex. We propose that V3 and V4 play a critical role in this supra-areal organization by filtering information about the visual environment along parallel pathways across higher-order cortex.

The receptive-field properties of retinal ganglion cells (RGCs) provide information about early visual processing. In the primate retina, P cells form the largest class of RGCs (Rodieck, 1988). A detailed exploration of the dynamics of the two subdivisions of the P-cell receptive field—the center and the surround—was undertaken. In the preceding paper (Benardete & Kaplan, 1996), the first-order responses of the center and the surround of P cells were described, which were obtained with a new technique, the multiple m-sequence stimulus (Benardete & Victor, 1994). In this paper, the investigation of P-cell responses measured as S-potentials in the lateral geniculate nucleus (LGN) is continued, and significant nonlinear, second-order responses from the center and the surround are described. These responses are quantified by fitting a mathematical model, the linear-nonlinear-linear (LNL) model (Korenberg, 1973; Korenberg & Hunter, 1986; Victor, 1988) to the data. In a second series of experiments, demonstration that steady illumination of the surround modifies the gain of the center to contrast signals (see also Kaplan & Shapley, 1989) is made. In P ON cells, increasing the steady illumination of the surround decreases the gain and speeds up the center's first-order response. In P OFF cells, increasing the steady illumination of the surround increases the gain of the center while speeding up the response. The results of both sets of experiments are related to the known anatomy and physiology of the P cell.

Glutamate and γ-aminobutyric acid (GABA) are two of the dominant neurotransmitters in the retina and brain. The production/degradation of glutamate and GABA involves an intricate interrelationship between neurons and glia, as well as aerobic and anaerobic metabolic pathways. The aim of this work was to develop an in vitro model of retinal ischemia/anoxia and determine the changes in cellular localization of glutamate and GABA and the time course for such changes. After anoxic/ischemic insult, glutamate and GABA rapidly accumulate within glia with GABA showing a quicker time course and larger magnitude change. The accumulation time constant for both glutamate and GABA under anoxic conditions was dependent upon glucose concentration: high glucose levels resulted in delayed glial amino acid loading. The differences in time constants between GABA and glutamate glial loading most likely reflect the multitude of glutamate degradation pathways compared to the single aerobically dependent GABA pathway. Oxygen availability and reduced glucose (hypoglycemia) lead to an almost immediate increase (within 1 min) of glutamate and GABA labelling within glia. In addition, altered labelling patterns were found under anoxic/ischemic conditions for amino acids involved in glutamate transamination reactions: aspartate, leucine, alanine, and ornithine. These changes are consistent with alterations of equilibria of enzymatic reactions involved in glutamate metabolism, and thus support a role for all four amino acids in glutamate metabolism within a variety of retinal neurons.

Patch-clamp recordings were used to investigate the properties of the regenerative activity in acutely isolated bipolar cells from the rat retina. Spontaneous, pacemaker-like membrane potential oscillations were observed in all rod bipolar cells and the majority of cone bipolar cells. The waveform of the regenerative potential was stereotypical but distinct among different bipolar cell groups, especially between rod and cone bipolar cells. The spontaneous activity was completely blocked by Co2+, suggesting that Ca2+ influx through voltage-dependent Ca2+ channels was required for initiating such activity. Ca2+-induced Ca2+ release, however, was not found to be involved. The spontaneous activity was also blocked by mibefradil, a T-type Ca2+ channel antagonist. In contrast, application of nimodipine, an L-type Ca2+ current antagonist, affected mainly the waveform of the regenerative potential. This study shows that mammalian retinal bipolar cells in isolation are also capable of generating Ca2+-dependent spontaneous regenerative potential. However, T-type Ca2+ channels appear to be essential for the initiation of the spontaneous activity in mammalian bipolar cells.

Nitric oxide (NO) acts as a neuronal messenger which activates soluble guanylyl cyclase (SGC) in neighboring cells and produces a wide range of physiological effects in the central nervous system (CNS). Using immunocytochemical and histochemical stains, we have characterized the NO/SGC system in the rabbit retina and to a lesser extent, in monkey retina. Based on staining patterns observed with an antibody to nitric oxide synthase (NOS) type I and a histochemical marker for NADPH diaphorase, a metabolic intermediate required for NOS activity, three major classes of neurons appear to generate NO in the rabbit retina. These include two subclasses of sparsely distributed wide field amacrine cells, rod and cone photoreceptors, and a subpopulation of ganglion cells. Equivalent cell populations were labled in monkey retina. An antibody to SGC (tested only in rabbit retina), labeled large arrays of cone photoreceptors in the outer nuclear layer, both amacrine and bipolar cells in the inner nuclear layer (INL), as well as populations of neurons in the ganglion cell layer. These data suggest that the ability to generate NO is restricted to relatively few neurons in the inner retina and to photoreceptor cells in the outer retina; while presumptive target cells, containing pools of SGC, are widespread and form contiguous fields across the inner and outer nuclear layers (ONL) as well as the ganglion cell layer.

Previous work suggests that photoreceptor synaptic ribbon length and absolute dark-adapted threshold may vary during a 24-h diurnal cycle. To test this hypothesis, we examined the length of photoreceptor synaptic ribbons and the dark-adapted threshold in black (+/+) and albino (c2J/c2J) C57BL/6J mice at six times over a 24-h period. Testing began 2 h after light onset (ZT 2:00) and continued at successive 4-h intervals (12 h:12 h light:dark). We determined the length of the synaptic ribbons in frozen sections by labeling them with an antibody specific for synaptic ribbons. Synaptic ribbons vary in length at different points in the diurnal cycle in both types of mice, but the synaptic ribbons in black mice are longer than those in albino mice by an average of 0.33 μm. The synaptic ribbons of black mice also have a larger response to changes in the light cycle. Ribbon length in black mice ranges from 1.66 μm to 1.4 μm, whereas ribbon length in albino mice ranges from 1.32 μm to 1.25 μm. The shortest ribbons are evident 6 h after light onset in both types of mice, whereas the longest ribbons appear within 2 h after light onset. These changes in synaptic ribbon length support the idea that photoreceptor synaptic ribbons are dynamic structures whose length changes over a 24-h diurnal cycle. Examining black and albino mice with a water-maze behavioral assay showed that visual thresholds in black and albino mice vary over the 24-h diurnal cycle. The visual thresholds of albino mice are elevated compared with black mice at all times tested. This is consistent with previous findings of visual thresholds in hypopigmented mice. The lowest threshold (greatest sensitivity) is present 2 h after light onset (ZT 2:00) and corresponds to the time when synaptic ribbons are the longest. The highest threshold is observed 6 h after light onset, the time when synaptic ribbons are shortest. These results show that synaptic ribbon length and visual sensitivity vary together in relation to the time.

We studied the morphology and center-surround organization of Lucifer Yellow injected OFF- and ON-center bipolar cells in the light-adapted Xenopus retina and the effects of glycine and GABA on their cone-mediated light responses. In both classes of cell, prominent antagonistic surround responses up to 20 mV in amplitude could be evoked without first suppressing the center responses with steady illumination. An additional feature of the light-evoked bipolar cell response was a pronounced (up to –24 mV) delayed hyperpolarizing after potential (DHAP) which followed the depolarizing responses of both classes of bipolar cell.

The morphological features of dye-injected bipolar cells conformed to the general idea of segregation of ON and OFF pathways in the inner and outer interplexiform layer, however, the morphology of axonal arborizations was different for both classes. OFF-center cells ramified symmetrically around the primary branchpoint, whereas ON-center cells had a strongly asymmetrical arrangement of their axonal tree.

The center and surround responses were differentially sensitive to glycine and GABA. Glycine eliminated the antagonistic surround responses in both OFF and ON cells; the center responses were reduced to some extent but were not eliminated. In contrast, GABA affected the hyperpolarizing responses much more strongly than the depolarizing response components. That is, the amplitude of the center response in the OFF cell and the surround response in the ON cell was reduced 80–90% during exposure to GABA, whereas the surround and center depolarizations of OFF and ON cells, respectively, were reduced only 0–10%.

Our findings implicate a role for GABAergic and glycinergic pathways in the center-surround organization of bipolar cells in Xenopus retina. In addition, the results suggest that the pathways mediating center-surround antagonism may be different in OFF-bipolar cells vs. ON-bipolar cells.

Simultaneous recordings from relay cells in the lateral geniculate nucleus (LGN) and their retinal afferents were used to examine the rules governing the transmission of spikes across the retino-geniculate synapse. Retinal spikes that terminate short retinal interspike intervals are much more likely to be transmitted across the synapse than spikes terminating longer intervals. This facilitation can be observed for interspike intervals as long as 50 ms when retinal firing rates are low, but the range of effective intervals decreases exponentially as retinal firing rate increases. Contribution, the fraction of LGN spikes triggered by an individual retinal afferent, is typically much higher during visual stimulation than during maintained activity, and these differences are unrelated to presynaptic or postsynaptic firing rate. It is suggested that this effect is a manifestation of increased synchronization of spikes among retinal afferents to the geniculate cell during structured visual stimulation, and that this synchronization offers a means of enhancing signal-to-noise ratio at the retino-geniculate synapse. Cross-correlograms between geniculate burst spikes and retinal afferents often contain two distinct peaks; a short latency peak that results from direct coupling between burst spikes and retinal input spikes, and a longer latency peak that represents indirect coupling in which retinal spikes trigger the calcium spike underlying the burst. Direct coupling is most likely to occur for the later spikes in the burst, and is present regardless of whether the calcium spike underlying the burst is triggered by the same or a different retinal afferent. These results further illuminate the relationship between tonic and burst modes of retino-geniculate transmission and indicate that bursts in LGN relay cells can be viewed as a mechanism of signal amplification, producing signals whose timing is potentially related to the temporal structure of a stimulus, independent of presynaptic and postsynaptic firing rate. This mechanism also appears to capitalize on the synchronization that is present among parallel retinal afferents to a geniculate cell.

Retinal bipolar cells convey visual information from photoreceptors to retinal third-order neurons, amacrine and ganglion cells, with graded potentials through diversified cell types. To understand the possible role of voltage-dependent T-type Ca2+ currents in retinal bipolar cells, we investigated the pharmacological and biophysical properties of T-type Ca2+ currents in acutely dissociated retinal cone bipolar cells from rats using whole-cell patch-clamp recordings. We observed a broad group of cone bipolar cells with prominent T-type Ca2+ currents (T-rich) and another group with prominent L-type Ca2+ currents (L-rich). Based on the pharmacological and biophysical properties of the T-type Ca2+ currents, T-rich cone bipolar cells could be divided into three subgroups. Each subgroup appeared to express a single dominant T-type Ca2+ channel subunit. The T-type calcium currents could generate low-threshold regenerative potentials or spikes. Our results suggest that T-type Ca2+ channels may play an active and distinct signaling role in second-order neurons of the visual system, in contrast to the common signaling by L-rich bipolar cells.

The spatiotemporal structure of cortical activity evoked by diffuse light flashes was investigated in an isolated eyecup-brain preparation of the pond turtle, Pseudemys scripta. By combining a photomicroscopic image of the preparation with voltage-sensitive dye signals recorded by a 464-element photodiode array, the spread of depolarization within different cortical areas could be directly visualized with millisecond temporal resolution. Diffuse stimulation of the contralateral eyecup initially depolarized the visual cortex at the junction between its lateral and medial divisions in a small area rostral of the ventricular eminence. From this point, the depolarization spread at different velocities (10–100 μm/ms) depending upon the direction of travel. Since the initial depolarization was always in the rostral pole, the largest spread invariably occurred in a rostral → caudal direction. Within the confines of the medial visual cortex, depolarization spread at a constant velocity but slowed after entering the adjoining medial cortex. Increasing the stimulus illuminance increased the velocity of spread. Rostrocaudal spread of depolarization was also observed in response to electrical stimulation of the geniculocortical pathway and by direct focal stimulation of the cortical sheet. These data suggest that excitatory connections between pyramidal cell clusters play a prominent role in the initial activation of the cortex by diffuse retinal stimulation.

Immunohistochemical techiniques were used to survey the distribution of several conventional transmitters, receptors, and neuropeptides in the pigeon nucleus of the basal optic root (nBOR), a component of the accessory optic ststem. Amongst the conventional neurotransmitts'modulators, the most intense labeling of fibers/terminals within the nBOR was obtained with antisera directed against glutamic acid decarboxylase (GAD) and serotonin (5-HT). Moderately dense fiber plexuses were seen to label with antibodies directed against tyrosine hydroxylase (TH) and choline acetltransferase (ChAT). GAD-like immunoractivity (GAD-L1) was found in many small and medium-size perikarya within the nBOR. Some of the medium-sized cells were occasionally positive for ChAT-L1. Cell body and dendritic staining was also commonly seen with the two tested antisera against receptors–anti-GABA-A receptor and anti-nicotinic acetylcholine receptor.

The antisera directed against various neuropetides produced only fiber labelling within the nBOR. The densest fiber plexus staining was observed with antiserum against neuropeptide Y (NPY-L1), while intermediate fiber densities were seen for substance P (SP-L1) and cholecystokinin (CCK-L1). A few varicose fibers were labeled with antisera against neurotensin (NT), leucine-enkephalin (L-KNK), and the vasoactive intestinal polypeptide (VIP).

Unilateral enucleation produced an almost complete elimination of TH-L1 in the contralateral nBOR. SP-L1 and CCK-L1 were also decreased after enucleation. No apparent changes were seen for all other substances.

These results indicate that a wide variety of chemically-specific systems arborize within the nBOR. Three of the immunohistochemically defined fiber systems (TH-LI, SP-LI, and CCK-LI fibers) were reduced after removal of the retina, which may indicate the presence of these substances in retinal ganglion cells. In contrast, the fibers exhibiting ChAT-LI, GAD-LI, 5-HT-Ll, NPY-Ll, NT-LI, L-ENK-LI, and VIP-LI appear to be of nonretinal origin. Two different populations of nBOR neurons exhibited GAD-LI and ChAT-LI. However, these two populations together constituted only about 20% of the nBOR neurons.

The electroretinogram (ERG) of the cone system provides a useful noninvasive measure of the activity of the cone pathway. Despite a wide application of the cone ERG in the study of rodent models of human hereditary retinal disease, the cellular origins of the rat cone ERG have not been well defined. Here, we address this issue using a pharmacological approach that has been used previously to derive ERG response components. Agents that impair synaptic transmission at well-defined retinal loci were dissolved in saline and injected into the vitreous of adult Sprague-Dawley rats anesthetized with ketamine/xylazine, and cone ERGs were recorded approximately 2 h later. Analysis of the resulting waveforms indicated that the rat cone ERG includes a relatively small-amplitude component of negative polarity that is derived from the activity of cone photoreceptors, and perhaps retinal glial (Müller) cells. The cone depolarizing bipolar cell pathway contributes a positive potential of large amplitude to the rat cone ERG. In comparison, the contribution of hyperpolarizing bipolar cells is of negative polarity and of much smaller amplitude. The inner retina contributes a negative wave upon which higher frequency oscillations are superimposed. These results provide a foundation for interpreting changes in the waveform of the rat cone ERG that may be observed following genetic alteration or other experimental treatment.

The distribution of the carbohydrate epitope CD 15, a putative cell adhesion molecule, was studied in adult vertebrate retinas by light-microscopic immunohistochemistry. Except for Old World primates, in which no immunoreactivity was detectable, all other species expressed the epitope on retinal interneurones. Subpopulations of stratified amacrine cells were found in all species with the exception of bats and marmoset monkeys, and bipolar cells were immunoreactive in frogs and all amniotic animals. Ganglion cells were labelled in urodelian, in all sauromorphian, as well as in some mammalian species. In some species, the distribution of immunoreactive neurones was correlated to areas of retinal specialization such as the visual streak in frogs and the dorsotemporal field in birds. In these parts of the retina with enhanced visual acuity, more CD 15 glycosylated bipolar cells were found than in other parts. Among mammals, labelled bipolar cells were found exclusively in species with cone-dominated retinas. This comparative study shows that CD 15 expression is consistently membrane associated in morphologically defined subsets of amacrine, bipolar, and ganglion cells throughout the vertebrate phylum. Its distribution pattern was found to depend more on the visual behavior of a given species (cone-dominated or rod-dominated retina) than on phylogenetic proximity between species.

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.

In the human rod visual system, self-cancellation of flicker signals is observed at high rod intensity levels near 15 Hz, both perceptually and in the electroretinogram (ERG). This and other evidence suggests that two rod signals are transmitted through the human retina with different speeds of transmission. Here we report a series of flicker ERG recordings from a normal observer and an observer who lacks cone vision. From these results, we propose a quantitative model of the two rod signals, which assumes (1) that the amplitude of the slow signal grows linearly with log intensity but then saturates at ~1 scot, td; (2) that the amplitude of the fast signal grows linearly with intensity; (3) that there is a difference in time delay of ~33 ms between two rod signals of the same polarity (or of ~67 ms if the signals are of inverted polarity); and (4) that the time delay of both signals declines linearly with log intensity (by ~10 ms per log scot. td). These simple assumptions provide a remarkably good account of the experimental data. Our results and model are relevant to current anatomical theories of the mammalian rod visual system. We speculate that the slower signal in the human ERG may reflect the transmission of the rod response via the rod bipolars and the An amacrine cells, while the faster signal may reflect its transmission via the rod-cone gap junctions and the cone bipolars. There are, however, several objections to this simple correspondence.

Bipolar cells are not only important for visual processing but input from these cells may underlie the reorganization of ganglion cell dendrites in the inner plexiform layer (IPL) during development. Because little is known about the development of bipolar cells, here we have used immunocytochemical markers and dye labeling to identify and follow their differentiation in the neonatal ferret retina. Putative cone bipolar cells were immunoreacted for calbindin and recoverin, and rod bipolar cells were immunostained for protein kinase C (PKC). Our results show that calbindin-immunoreactive cone bipolar cells appear at postnatal day 15 (P15), at which time their axonal terminals are already localized to the inner half of the IPL. By contrast, recoverin-immunoreactive cells with terminals in the IPL are present at birth, but many of these cells may be immature photoreceptors. By the second postnatal week, recoverin-positive cells resembling cone bipolar cells were clearly present, and with increasing age, two distinct strata of immunolabeled processes occupied the IPL. PKC-containing rod bipolar cells emerged by the fourth postnatal week and at this age have stratified arbors in the inner IPL. The early bias of bipolar axonal arbors in terminating in the inner or outer half of the IPL is confirmed by dye labeling of cells with somata in the inner nuclear layer. At P10, several days before ribbon synapses have been previously observed in the ferret IPL, the axon terminals of all dye-labeled bipolar cells were clearly stratified. The results suggest that bipolar cells could provide spatially localized interactions that are suitable for guiding dendritic lamination in the inner retina.