The synaptic organization of dopaminergic interplexiform cells (DA-IPC) in the goldfish retina was studied by a combined double-label electron-microscopical (EM) immunocytochemical/autoradiographical study. DA-IPCs were labeled with antisera against tyrosine hydroxylase. The possibility of synaptic contact with GABAergic amacrine cells in the proximal inner plexiform layer (IPL) was studied by using 3H-GABA uptake. Most synaptic input and output from DA-IPC processes involved amacrine cell processes. In addition, synaptic interactions were observed between DA-IPC processes and bipolar cell terminals, other DA-IPC processes, very small dendrites in the IPL, ganglion cell and optic fiber layers (OFL), and cell bodies in the ganglion cell layer (GCL). Input and output synapses with GABAergic amacrine processes also were observed. Two-thirds of the DA-IPC boutons in the proximal IPL were involved in “junctional appositions,” that is, the junctions appeared to be specialized but they were different than classical chemical synapses. The synaptic organization of DA-IPCs in the goldfish IPL appears to be far more complex than previously thought. Although earlier studies have attempted to explain the action of dopamine in terms of interaction only with amacrine cells, the present study shows that effects involving bipolar cells, other DA-IPCs, unidentified processes and cell bodies in the GCL and OFL must be considered as well.

Smooth pursuit eye movements allow primates to keep gaze pointed at small objects moving across stationary surroundings. In monkeys trained to track a small moving target, we have injected brief perturbations of target motion under different initial conditions as probes to read out the state of the visuo-motor pathways that guide pursuit. A large eye movement response was evoked if the perturbation was applied to a moving target the monkey was tracking. A small response was evoked if the same perturbation was applied to a stationary target the monkey was fixating. The gain of the response to the perturbation increased as a function of the initial speed of target motion and as a function of the interval from the onset of target motion to the time of the perturbation. The response to the perturbation also was direction selective. Gain was largest if the perturbation was along the axis of ongoing target motion and smallest if the perturbation was orthogonal to the axis of target motion. We suggest that two parallel sets of visual motion pathways through the extrastriate visual cortex may mediate, respectively, the visuo-motor processing for pursuit and the modulation of the gain of transmission through those pathways.

If surfaces in a scene are to be distinguished by their color, their neural representation at some level should ideally vary little with the color of the illumination. Four possible neural codes were considered: von-Kries-scaled cone responses from single points in a scene, spatial ratios of cone responses produced by light reflected from pairs of points, and these quantities obtained with sharpened (opponent-cone) responses. The effectiveness of these codes in identifying surfaces was quantified by information-theoretic measures. Data were drawn from a sample of 25 rural and urban scenes imaged with a hyperspectral camera, which provided estimates of surface reflectance at 10-nm intervals at each of 1344 × 1024 pixels for each scene. In computer simulations, scenes were illuminated separately by daylights of correlated color temperatures 4000 K, 6500 K, and 25,000 K. Points were sampled randomly in each scene and identified according to each of the codes. It was found that the maximum information preserved under illuminant changes varied with the code, but for a particular code it was remarkably stable across the different scenes. The standard deviation over the 25 scenes was, on average, approximately 1 bit, suggesting that the neural coding of surface color can be optimized independent of location for any particular range of illuminants.

Ganglion cells in an isolated wholemount preparation of the rat retina were labeled using the “DiOlistic” labeling method (Gan et al., 2000) and were classified according to their morphological properties. Tungsten particles coated with a lipophilic dye (DiI) were propelled into the wholemount retina using a gene gun. When a dye-coated particle contacted the cell membrane, the entire cell was labeled. The ganglion cells were classified into four types based on their soma size, dendritic-field size, branching pattern, and level of stratification. Broadly monostratified cells were classified into three types: RGA cells (large soma, large dendritic field); RGB cells (small- to medium-sized soma, small- to medium-sized dendritic field); and RGC cells (small- to medium-sized soma, medium-to-large dendritic field). Bistratified cells were classified as RGD. Several subtypes were identified within each ganglion cell group. A number of new subtypes were discovered and added into the existing catalog, among them were two types of bistratified cells. This study therefore represents the most complete morphological classification of rat retinal ganglion cells available to date.

As more human retinas affected with genetic or immune-based diseases become available for morphological analysis, it is important to identify immunocytochemical markers for specific subtypes of retinal neurons. In this study, we have focused on bipolar cell markers in central retina. We have done single and double labeling using several antisera previously utilized in macaque monkey or human retinal studies and two new antisera (1) to correlate combinations of antisera labeling with morphological types of bipolar cells in human retina, and (2) to compare human labeling patterns with those in monkey retina. Human bipolar cells showed a wide range of labeling patterns with at least ten different bipolar cell types identified from their anatomy and marker content. Many bipolar cell bodies in the outer part of the inner nuclear layer contained combinations of protein kinase C alpha (PKCα), Islet-1, glycine, and Goα. Bipolar cells labeled with these markers had axons terminating in the inner half of the inner plexiform layer (IPL), consistent with ON bipolar cells. Bipolar cell bodies adjacent to the amacrine cells and with axons in the outer half of the IPL contained combinations of recoverin, glutamate transporter-1, and PKCβ, or CD15 and calbindin. Bipolar cells labeled with these markers were presumed OFF bipolar cells. Calcium-binding protein 5 (CaB5) labeled both putative ON and OFF bipolar cells. Using this cell labeling as a criteria, most cell bodies close to the horizontal cells were ON bipolar cells and almost all bipolar cells adjacent to the amacrine cells were OFF with a band in the middle 2–3 cell bodies thick containing intermixed ON and OFF bipolar cells. Differences were found between human and monkey bipolar cell types labeled by calbindin, CaB5, and CD15. Two new types were identified. One was morphologically similar to the DB3, but labeled for CD15 and CaB5. The other had a calbindin-labeled cell body adjacent to the horizontal cell bodies, but did not contain any accepted ON markers. These results support the use of macaque monkey retina as a model for human, but caution against the assumption that all labeling patterns are identical in the two primates.

We have recently developed a behavioral assay, based on the escape response of fish to a threatening object, to analyze quantitatively the visual sensitivity of zebrafish. During the course of dark adaptation, we measure the threshold light intensity required to evoke an escape response. Under a normal light–dark (LD) cycle, thresholds for both the cone and rod systems are considerably lower in late afternoon hours than in early morning hours. Over a period of 24 h, zebrafish are most sensitive to visual stimuli prior to light off and least sensitive prior to light on. Under conditions of constant illumination, this rhythm of visual sensitivity persists for several days but is gradually lost. In constant light (LL), the rhythm persists 1–2 days; thereafter, visual thresholds at all times of the day converge at a level similar to thresholds measured in late afternoon hours in control animals. In constant darkness (DD), the rhythm persists at least 5 days; thereafter, it dampens to a level about a half-log unit less sensitive to that measured in the late afternoon hours in control animals. These data suggest that visual sensitivity in zebrafish is regulated by an endogenous circadian clock which functions to decrease the visual sensitivity.

lbotenic acid lesions were made in four macaque monkeys in a region of cortical area V4 that corresponds to the lower quadrant of one hemifield. For visual testing, fixation locus was monitoredwith scleral search coils and controlled behaviorally to place test stimuli either in the lesionedquadrant or in a control location in the opposite hemifield. Some basic visual capacities were slightly altered by the lesions; there was a two-fold reduction of luminance contrast sensitivity as well as red-green chromatic contrast sensitivity, both tested with stationary gratings. On the other hand, little or no loss was found when contrast sensitivity for detection or direction discrimination was tested with 10–Hz drifting gratings nor was there a reliable change in visual acuity. Hue and luminance matching were tested with a spatially more complex matching-to-sample task, but monkeys could not learn this task in the visual field locus of a V4 lesion. If previously trained at this locus, performance was not affected by the lesion. In contrast to the small effects on basic visual capabilities, performance on two form discrimination tasks was devastated by V4 lesions. The first involved discriminating the orientation of colinear groups of dots on a background of randomly placed dots. The second involved discriminating the orientation of a group of three line segments surrounded by differently oriented line segments. Some selectivity of the deficitsfor form discrimination was shown by the lack of an effect of the lesions on a global motion discrimination. These results show that while V4 lesions cause only slight disruptions of basic visual capacities, they profoundly disrupt form discriminations.

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.

Visual stimulation of a region outside the receptive field of single cells in visual cortex often results in the modulation of their responses. The modulatory effects are thought to be mediated through lateral connections within visual cortex. Research on lateral interactions commonly shows suppression. There has been no systematic study of the optimal conditions for facilitation. Here we have studied the nature of the modulation using a new type of compound stimulus: contrast reversal of pattern stimuli made of three discrete grating patches. The middle patch, optimally fitted to the receptive field in orientation, size, and spatial as well as temporal frequencies, was flanked by two similar patches presented well outside the receptive field. We found that (1) both facilitation and suppression occurred often in the same cells, when orientations of the target and flankers matched the receptive-field's optimal orientation; (2) facilitation with collinear flankers occurred most frequently at target contrasts just above the cell's firing threshold and suppression prevailed at high contrasts; (3) facilitative or suppressive modulation was obtained with target-flankers separation of up to 12 deg or more; (4) collinear facilitation was lost when flankers' orientation was rotated by 90 deg, while keeping all other parameters the same; and (5) neither the modulation mode nor the proportion of modulated cells was related to the cell types (simple vs. complex cells) and cells' laminar locations. Here we have provided physiological evidence for contrast-dependent, collinear facilitation probably underlying perceptual grouping in humans.

We studied the light-evoked responses of AII amacrine cells in the rabbit retina under dark- and light-adapted conditions. In contrast to the results of previous studies, we found that AII cells display robust responses to light over a 6–7 log unit intensity range, well beyond the operating range of rod photoreceptors. Under dark adaptation, AII cells showed an ON-center/OFF-surround receptive-field organization. The intensity–response profile of the center-mediated response component followed a dual-limbed sigmoidal function indicating a transition from rod to cone mediation as stimulus intensities were increased. Following light adaptation, the receptive-field organization of AII cells changed dramatically. Light-adapted AII cells showed both ON- and OFF-responses to stimulation of the center receptive field, but we found no evidence for an antagonistic surround. Interestingly, the OFF-center response appeared first following rapid light adaptation and was then replaced gradually over a 1–4 min period by the emerging ON-center response component. Application of the metabotropic glutamate receptor agonist APB, the ionotropic glutamate blocker CNQX, 8-bromo-cGMP, and the nitric oxide donor SNAP all showed differential effects on the various center-mediated responses displayed by dark- and light-adapted AII cells. Taken together, these pharmacological results indicated that different synaptic circuits are responsible for the generation of the different AII cell responses. Specifically, the rod-driven ON-center responses are apparently derived from rod bipolar cell synaptic inputs, whereas the cone-driven ON-center responses arise from signals crossing the gap junctions between AII cells and ON-center cone bipolar cells. Additionally, the OFF-center response of light-adapted AII cells reflects direct synaptic inputs from OFF-center cone bipolar cells to AII dendritic processes in the distal inner plexiform layer.

All mammalian cone photopigments are derived from the operation of representatives from two opsin gene families (SWS1 and LWS in marsupial and eutherian mammals; SWS2 and LWS in monotremes), a process that produces cone pigments with respective peak sensitivities in the short and middle-to-long wavelengths. With the exception of a number of primate taxa, the modal pattern for mammals is to have two types of cone photopigment, one drawn from each of the gene families. In recent years, it has been discovered that the SWS1 opsin genes of a widely divergent collection of eutherian mammals have accumulated mutational changes that render them nonfunctional. This alteration reduces the retinal complements of these species to a single cone type, thus rendering ordinary color vision impossible. At present, several dozen species from five mammalian orders have been identified as falling into this category, but the total number of mammalian species that have lost short-wavelength cones in this way is certain to be much larger, perhaps reaching as high as 10% of all species. A number of circumstances that might be used to explain this widespread cone loss can be identified. Among these, the single consistent fact is that the species so affected are nocturnal or, if they are not technically nocturnal, they at least feature retinal organizations that are typically associated with that lifestyle. At the same time, however, there are many nocturnal mammals that retain functional short-wavelength cones. Nocturnality thus appears to set the stage for loss of functional SWS1 opsin genes in mammals, but it cannot be the sole circumstance.

The present study examines the changes in ultraviolet (UV) photosensitivity that occur during the growth of rainbow trout (Salmo gairdneri). A comparison of the ocular media transmission of small (n = 3) and large (n = 3) trout eyes did not reveal large changes in the transmission of UV radiation through the eye. We used the heart-rate conditioning technique to measure spectral sensitivity in immobilized trout. Four trout, each weighing less than 30 g, exhibited a UV-sensitivity peak at 360 nm while four additional trout weighing more than 60 g each exhibited no evidence of UV sensitivity. Spectral-sensitivity measurements of two trout weighing 44 g and 60 g revealed UV sensitivity, but when measured one month later (after a 25% increase in body weight) both fish exhibited no UV-sensitivity peak. At this time their sensitivity appeared to conform to the known blue-sensitive cone mechanism.

Retinal input to the hypothalamic suprachiasmatic nucleus (SCN) synchronizes the SCN circadian oscillator to the external day/night cycle. Retinal ganglion cells that innervate the SCN via the retinohypothalamic tract are intrinsically light sensitive and express melanopsin. In this study, we provide data indicating that not all SCN-projecting retinal ganglion cells express melanopsin. To determine the proportion of ganglion cells afferent to the SCN that express melanopsin, ganglion cells were labeled following transsynaptic retrograde transport of a recombinant of the Bartha strain of pseudorabies virus (PRV152) constructed to express the enhanced green fluorescent protein (EGFP). PRV152 injected into the anterior chamber of the eye retrogradely infects four retinorecipient nuclei in the brain via autonomic circuits to the eye, resulting in transneuronally labeled ganglion cells in the contralateral retina 96 h after intraocular infection. In animals with large bilateral lesions of the lateral geniculate body/optic tract, ganglion cells labeled with PRV152 are retrogradely infected from only the SCN. In these animals, most PRV152-infected ganglion cells were immunoreactive for melanopsin. However, a significant percentage (10–20%) of EGFP-labeled ganglion cells did not express melanopsin. These data suggest that in addition to the intrinsically light-sensitive melanopsin-expressing ganglion cells, conventional ganglion cells also innervate the SCN. Thus, it appears that the rod/cone system of photoreceptors may provide signals to the SCN circadian system independent of intrinsically light-sensitive melanopsin ganglion cells.

The genetic locus for incomplete congenital stationary night blindness (CSNB2) has been identified as the CACNA1f gene, encoding the α1F calcium channel subunit, a member of the L-type family of calcium channels. The electroretinogram associated with CSNB2 implicates α1F in synaptic transmission between retinal photoreceptors and bipolar cells. Using a recently developed monoclonal antibody to α1F, we localize the channel to ribbon active zones in rod photoreceptor terminals of the mouse retina, supporting a role for α1F in mediating glutamate release from rods. Detergent extraction experiments indicate that α1F is part of a detergent-resistant active zone complex, which also includes the synaptic ribbons. Comparison of native mouse rod calcium currents with recombinant α1F currents reveals that the current–voltage relationship for the native current is shifted approximately 30 mV to more hyperpolarized potentials than for the recombinant α1F current, suggesting modulation of the native channel by intracellular factors. Lastly, we present evidence for L-type α1D calcium channel subunits in cone terminals of the mouse retina. The presence of α1D channels in cones may explain the residual visual abilities of individuals with CSNB2.

The distribution of GABA-like immunoreactivity in the cat retina was studied through the use of preembedding immunocytochemistry for light microscopy and by postembedding immunogold techniques for electron microscopy. Staining was observed in both inner and outer plexiform layers. Approximately 30% of the somata in the amacrine portion of the inner nuclear layer were immunoreactive and included amacrine and interplexiform cells. Horizontal cells and a subpopulation of cone bipolar cells were also stained. In the ganglion cell layer, staining was observed in both small- and medium-sized neurons. GABA-labeled amacrine cells were presynaptic to somata of amacrine cells and to dendrites of amacrine, bipolar, and ganglion cells. Bipolar cells were a major target, receiving more than 60% of all labeled synapses in the inner plexiform layer. Many of these contacts were reciprocal synapses. These findings support a major role for GABA-labeled amacrines in providing feedback inhibition to bipolar cells in the inner retina.

Positive- and negative-contrast stimuli yield the perceptions of brightness and darkness, respectively, and are processed separately by ON and OFF neural pathways. The properties of these morphologically and pharmacologically distinct subsystems were measured in humans by recording visual evoked potentials (VEPs). These electrical responses from the visual cortex were elicited by novel positive- and negative-contrast stimuli, designed to emphasize, selectively, contributions from ON and OFF pathways. Results revealed differential processing of the two types of contrast information, suggesting asymmetries in ON and OFF subsystems; OFF subsystems have finer spatial tuning and greater contrast gain than ON subsystems. These VEPs may be useful in diagnosing neurological disorders that involve primarily one subsystem.

We examined the laminar distribution of corticogeniculate neurons in the macaque striate cortex labeled by axonal transport following injections of retrograde tracers into the lateral geniculate nucleus (LGN). Large injections of retrograde tracers involving all layers of the LGN resulted in a distinctive bilaminar distribution of labeled cells in cortical layer 6. One tier of labeled neurons was located along the layer 5–6 border and a second was located near the bottom of the layer, leaving the middle of layer 6 largely free of labeled neurons. Following injections of tracers that were restricted to the magnocellular layers of the LGN, almost all of the labeled neurons were located in the lower tier. In contrast, following injections of retrograde tracers confined to the parvocellular layers of the LGN, labeled cells were found in both tiers, with the greatest number in the upper tier. Thus, layer 6 of macaque striate cortex consists of three distinct sublayers only two of which are the source of descending projections to the LGN: an upper tier that projects exclusively to the parvocellular layers and a lower tier that projects to both magnocellular and parvocellular layers.

The human visual system is remarkably tolerant to degradation in image resolution: human performance in scene categorization remains high no matter whether low-resolution images or multimegapixel images are used. This observation raises the question of how many pixels are required to form a meaningful representation of an image and identify the objects it contains. In this article, we show that very small thumbnail images at the spatial resolution of 32 × 32 color pixels provide enough information to identify the semantic category of real-world scenes. Most strikingly, this low resolution permits observers to report, with 80% accuracy, four to five of the objects that the scene contains, despite the fact that some of these objects are unrecognizable in isolation. The robustness of the information available at very low resolution for describing semantic content of natural images could be an important asset to explain the speed and efficiently at which the human brain comprehends the gist of visual scenes.

A region of dorsomedial frontal cortex (DMFC) has been implicated in planning and executing saccadic eye movements; hence it has been referred to as a supplementary eye field (SEF). Recently, activity related to executing smooth-pursuit eye movements has been recorded from the DMFC, and microstimulation here has been shown to evoke smooth eye movements. This report documents neuronal activity present in smooth-pursuit tasks where the predictability of target motion was manipulated. The activity of many neurons in the DMFC reached a peak when a predictable change in target motion occurred. Furthermore, the peak activity of some cells was systematically shifted by manipulating the duration of the target event, indicating that the network these neurons were in could learn the temporal characteristics of new target motion. Finally, the activity of most neurons tested was greater when target motion was predictable than when it was unpredictable. The results suggest that the DMFC participates in planning smooth-pursuit eye movements based on past stimulus history.

Form vision is traditionally regarded as processing primarily achromatic information. Previous investigations into the statistics of color and luminance in natural scenes have claimed that luminance and chromatic edges are not independent of each other and that any chromatic edge most likely occurs together with a luminance edge of similar strength. Here we computed the joint statistics of luminance and chromatic edges in over 700 calibrated color images from natural scenes. We found that isoluminant edges exist in natural scenes and were not rarer than pure luminance edges. Most edges combined luminance and chromatic information but to varying degrees such that luminance and chromatic edges were statistically independent of each other. Independence increased along successive stages of visual processing from cones via postreceptoral color-opponent channels to edges. The results show that chromatic edge contrast is an independent source of information that can be linearly combined with other cues for the proper segmentation of objects in natural and artificial vision systems. Color vision may have evolved in response to the natural scene statistics to gain access to this independent information.