We investigated the contribution of the projections from area MT to the receptive field properties of cells in visual area V2 in anesthetized and paralyzed Cebus apella monkeys. We recorded extracellular single-unit activity using tungsten microelectrodes in three monkeys before and after pressure injection of a 0.25-mol/l GABA solution. The visual stimulus consisted of a single bar moving in one of eight directions. In total, 72 V2 neurons were studied in 18 sessions of GABA injection into area MT. A group of 22 neurons was investigated over a shorter period of time ranging from 15 to 60 min, during which the activity did not return to baseline levels. The remaining 50 neurons were studied over a period of at least 2 h, and no statistical difference was observed in the neuronal response before and long after GABA inactivation. The effects on these 50 neurons consisted of an early (1–20 min) significant general decrease in excitability with changes in either orientation or direction selectivity. The differential decrease in excitability resulted in an intermediate improvement (20–40 min) of the signal-to-noise ratio for the stimulus-driven activity. The inactivation depended on the quantity of GABA injected into area MT and persisted for a period of 2 h. The GABA inactivation in area MT produced inhibition of most cells (72%) and a significant change of direction tuning in the majority (56%) of V2 neurons. Both increases and also decreases in the direction tuning of V2 neurons were observed. These feedback projections are capable of modulating not only the levels of spontaneous and driven activity of V2 neurons but also the V2 receptive field properties, such as direction selectivity.

The effects of the dopamine D1 antagonist (+)-SCH 23390 on the responses of ganglion cells in the superfused rabbit retinal preparation were studied by intracellular recording. At low micromolar concentrations, (+)-SCH 23390 hyperpolarized OFF-center brisk ganglion cells and reduced or abolished any spontaneous spike activity that was present. The light-evoked EPSPs at the onset of a spot or annulus were reduced or abolished, while the EPSPs at light offset were in most cases potentiated. (+)-SCH 23390 depolarized ON-center brisk ganglion cells and increased spontaneous spike activity. The light-evoked EPSPs to spots and annuli were either unaffected or reduced roughly to the same extent. The findings in this study are compared with findings in a recent study in which the effects of (+)-SCH 23390 on the extracellularly recorded responses of ganglion cells in the rabbit retina were examined.

Thresholds for detection of light by a dark-adapted test eye were measured while the other, non-test eye was either similarly dark adapted or while it was exposed to an intense red adapting field. An interocular effect that depends on the retinal location of the test was found: compared to the threshold during binocular dark adaptation, sensitivity decreased during contralateral light adaptation when the test was presented to the foveola and up to 4 deg above it; but sensitivity increased when the test was between 7 and 12 deg, showing a reversal at 5 deg.

Synchronized oscillatory discharge in the visual cortex has been proposed to underlie the linking of retinotopically disparate features into perceptually coherent objects. These proposals have largely relied on the premise that the oscillations arise from intracortical circuitry. However, strong oscillations within both the retina and the lateral geniculate nucleus (LGN) have been reported recently. To evaluate the possibility that cortical oscillations arise from peripheral pathways, we have developed two plausible models of single cell oscillatory discharge that specifically exclude intracortical networks. In the first model, cortical oscillatory discharge near 50 Hz in frequency arises from the integration of signals from strongly oscillatory cells within the LGN. The model also predicts the incidence of 50-Hz oscillatory cells within the cortex. Oscillatory discharge around 30 Hz is explained in a second model by the presence of intrinsically oscillatory cells within cortical layer 5. Both models generate spike trains whose power spectra and mean firing rates are in close agreement with experimental observations of simple and complex cells. Considered together, the two models can largely account for the nature and incidence of oscillatory discharge in the cat's visual cortex. The validity of these models is consistent with the possibility that oscillations are generated independently of intracortical interactions. Because these models rely on intrinsic stimulus-independent oscillators within the retina and cortex, the results further suggest that oscillatory activity within the cortex is not necessarily associated with the processing of high-order visual information.

The morphology of horizontal cells in ox, sheep, and pig retinae as observed after Lucifer Yellow injections are described and compared with the descriptions of Golgi-stained cells by Ramón y Cajal (1893). Horizontal cells in the retinae of less domesticated species, wild pig, fallow and sika deer, mouflon, and aurochs were also examined. All these retinae have two types of horizontal cell; their morphologies are in common, although with some familial differences. Their basic appearance is as Cajal described; except in one important respect, a single axon-like process could not be identified on the external horizontal cells. It is concluded that external horizontal cells of artiodactyls correspond to the axonless (A-type) cells of other mammals. Cajal's internal horizontal cells have a single axon which contacts rods. This type corresponds to the B-type cells of other mammalian retinae. Artiodactyl A- and B-type horizontal cells differ from those of many other mammals in that the B-type dendritic tree is robust and the A-type dendritic tree is delicate. Historically, this morphological difference between orders of mammals has led to some confusion. The comparisons presented here suggest that the morphological types of primate horizontal cells can be integrated into a general mammalian classification.

The molecules which control the patterns of cell division, growth, and precise interconnections characteristic of the central nervous system still remain largely unidentified. The protein platelet-derived growth factor (PDGF) has been shown to mediate interactions among glial cells in vitro. More recent evidence has indicated that PDGF may also be involved in controlling communication between neurons and glial cells and among neurons. The presence of receptors for PDGF on neurons of the developing nervous system is an essential piece of evidence in this chain of events. Ganglion cells are labeled with antibodies to PDGF receptor only during the period of active process outgrowth. These findings suggest that PDGF is used as a mediator of intercellular signaling during neuronal development.

Light causes a rapid, 1.7-fold increase in cyclic GMP concentration in intact squid retinas (Johnson et al. (1986)). To determine whether light-induced changes in cyclic GMP concentration result from activation of guanylate cyclase, we have studied the regulation of guanylate cyclase activity in squid (Loligo pealei) photoreceptors. The enzyme is membrane-associated and activity is enhanced by the detergents Triton X-100 or digitonin. The enzyme requires divalent cations, Mn2+ being preferred over Mg2+. The dependence of enzyme activity on the MnGTP concentration deviates from simple Michaelis-Menten kinetics. Under conditions where a light-induced binding of GTP to the guanine nucleotide regulatory protein can be observed, no light-induced change in guanylate cyclase could be detected.

The distribution of glycine receptors in the turtle retina was studied with the aid of a monoclonal antibody that detects the 93-kD protein associated with the strychnine-sensitive glycine receptor. Light microscopically, receptors were found in the inner plexiform layer and, more sparsely, in the innermost parts of the inner nuclear layer. No receptors were seen to be associated with photoreceptor cells, horizontal cells, or any other structures in the distal inner nuclear layer or outer plexiform layer. Ultrastructurally, glycine receptors were found on the inner face of postsynaptic membranes of processes from amacrine and presumed ganglion cells and always involved amacrine cell processes as the presynaptic element. Such glycine receptor immunoreactive synapses onto amacrine cell processes were distributed throughout the inner plexiform layer with a peak density near the middle. On the other hand, output synapses onto ganglion cell processes displaying immunoreactive glycine receptor sites showed a bimodal distribution in the inner plexiform layer. Glycine receptor immunoreactivity was not detected on bipolar cells, but presumed glycine-utilizing processes (i.e. those presynaptic to immunoreactive glycine receptors) were occasionally found to be postsynaptic in bipolar cell dyads. The majority of the synaptic input to the presumed glycine-utilizing amacrine cell processes was from other amacrine processes, some of which were themselves glycine utilizing. The observations suggest that glycinergic synapses in the turtle retina are, to a large extent, engaged in processing interamacrine signals.

Inhibition shapes activity and signal processing in neural networks through numerous mechanisms mediated by many different cell types. Here, we examined how one type of GABAergic interneuron in the retina, the A17 amacrine cell, influences visual information processing. Our results suggest that A17s, which make reciprocal feedback inhibitory synapses onto rod bipolar cell (RBC) synaptic terminals, extend the luminance range over which RBC synapses compute temporal contrast and enhance the reliability of contrast signals over this range. Inhibition from other amacrine cells does not influence these computational features. Although A17-mediated feedback is mediated by both GABAA and GABAC receptors, the latter plays the primary role in extending the range of contrast computation. These results identify specific functions for an inhibitory interneuron subtype, as well as specific synaptic receptors, in a behaviorally relevant neural computation.

The number of L cones in the retina normally exceeds that of the M cones. Because normal color vision does not depend on the ratio of L- and M-photoreceptors, their signals must undergo an alteration in gain before being analyzed in the cortex. Previous studies have shown that this gain must take place before the cortex, but after the bipolar/amacrine cell layer of the retina. The aim of this study was to obtain topographical information about L- and M-cone activity at the ganglion cell layer using multifocal pattern electroretinography (mfPERG). A standard (black and white) stimulus was used, as well as stimuli modulating only the long wavelength-sensitive (L) or only the middle wavelength-sensitive (M) cones. The L:M ratio was calculated from the amplitude of the L-cone isolating mfPERG to that of the M-cone isolating mfPERG of 10 trichromats. Both the positive and negative components of the waveform were analyzed. Additional recordings of single cone modulated mfERGs were obtained from nine of the 10 subjects. We also recorded from one protanope and one deuteranope. The L:M cone amplitude ratios for both deflections of the mfPERG in the trichromats were around unity (medians 1.18 and 1.16, respectively) for the central 8° of retina. In the peripheral retina between 12.8° and 26°, this ratio increased to 1.42 for the positive component, and 1.37 for the negative component. The median L:M cone amplitude ratios for the mfPERG were higher and ranged between 1.00–2.78 in the central 8° and 1.29–2.78 in the periphery. The results indicate that a major gain adjustment of the retinal signals takes place at the ganglion cell level, and that the ratio is higher at eccentric locations than in the central retinal area.

When normal binocular visual experience is disrupted during postnatal development, it affects the maturation of cortical circuits and often results in the development of poor visual acuity known as amblyopia. Two main factors contribute to the development of amblyopia: visual deprivation and reduced binocular competition. We investigated the affect of these two amblyogenic factors on the expression of the NMDAR1 subunit in the visual cortex because activation of the NMDA receptor is a key mechanism of developmental neural plasticity. We found that disruption of binocular correlations by monocular deprivation promoted a topographic loss of NMDAR1 expression within the cortical representations of the central visual field and the vertical and horizontal meridians. In contrast, binocular deprivation, which primarily affects visual deprivation, promoted an increase in NMDAR1 expression throughout the visual cortex. These different changes in NMDAR1 expression can be described as topographic and homeostatic plasticity of NMDA expression, respectively. In addition, the changes in NMDA expression in the visual cortex provide a greater understanding of the neural mechanisms that underlie the development of amblyopia and the potential for visual recovery.

The ventral intraparietal area (VIP) is located at the end of the dorsal stream. Its neurons are known to have receptive-field characteristics similar to those of MT and MST neurons, but little is known about the temporal characteristics of VIP cells' responses. How fast are directionally selective responses evoked in the ventral intraparietal area after viewing optic flow patterns, and what are the temporal properties of these neuronal responses? To examine these questions, we recorded the activity of 37 directionally selective ventral intraparietal area (VIP) neurons in two awake macaque monkeys in response to optic flow stimuli with presentation times ranging from 17 ms to 2000 ms. We found a minimum response latency of 45 ms, and a median latency of 152 ms. Of all neurons, 10% showed early response components only (response latency < 150 ms and no activity in 500–2000 ms interval after stimulus onset), 55% only late response components (response latency >150 ms and sustained activity in 500–2000 ms interval), and 35% both early and late response components. Early responses appeared to very brief stimulus presentations (33-ms duration), while the late responses required longer stimulus durations. The directional selectivity was independent of optic flow duration in all cells. These results suggest that only a subset of neurons in area VIP may contribute to the fast processing of optic flow, while showing that the temporal properties of VIP responses clearly differ from the temporal characteristics of neurons in areas MT and MST.

An open question in color rivalry is whether alternation between two colors is caused by a difference in receptoral stimulation or a difference in the neural representation of color appearance. This question was examined with binocular rivalry between physically identical lights that differed in appearance due to chromatic induction. Perceptual alternation was measured between gratings of the same chromaticity; each one was presented within a different patterned surround that caused the gratings, one to each eye, to appear unequal in hue because of chromatic induction. The gratings were presented dichoptically with binocular disparity so the rivalrous gratings appeared in front of the surround. Perceptual alternation in hue was found for the two physically identical chromaticities. Stereoscopic depth also was perceived, corroborating binocular neural combination despite color rivalry (Treisman, 1962). The results show that color rivalry is resolved after color-appearance shifts caused by chromatic context, and that color rivalry does not require competing unequal cone excitations from the rivalrous stimuli.

Neither discrete peripheral retinal lesions nor the normal optic disk produces obvious holes in one's percept of the world because the visual brain appears to perceptually “fill in” these blind spots. Where in the visual brain or how this filling in occurs is not well understood. A prevailing hypothesis states that topographic map of visual cortex reorganizes after retinal lesions, which “sews up” the hole in the topographic map representing the deprived area of cortex (cortical scotoma) and may lead to perceptual filling in. Since the map reorganization does not typically occur unless retinotopically matched lesions are made in both eyes, we investigated the conditions in which monocular retinal lesions can induce comparable map reorganization. We found that following monocular retinal lesions, deprived neurons in cat area 17 can acquire new receptive fields if the lesion occurred relatively early in life (8 weeks of age) and the lesioned cats experienced a substantial period of recovery (>3 years). Quantitative determination of the monocular and binocular response properties of reactivated units indicated that responses to the lesioned eye for such neurons were remarkably robust, and that the receptive-field properties for the two eyes were generally similar. Moreover, excitatory or inhibitory binocular interactions were found in the majority of experimental units when the two eyes were activated together. These results are consistent with the hypothesis that map reorganization after monocular retinal lesions require experience-dependent plasticity and may be involved in the perceptual filling in of blind spots due to retinal lesions early in life.

The image on the retina is almost never static. Eye, head, and body movements, and externally generated motion create rapid and continual changes in the retinal image (“active vision”). Virtually all vision in animals such as primates, which make saccades as often as 3–4 times/s, is based on information that must be derived from the first few hundred milliseconds after sudden, global changes in the retinal image. These changes may be accompanied by large changes in area mean luminance, as well as higher order image contrast statistics. This study investigated how retinal ganglion cell responses, whose response properties have been typically studied and defined in a stable stimulus regime, are affected by sudden changes in mean luminance that are characteristic of active vision. Specifically, the steady-state responses of retinal ganglion cells to static or moving square-wave grating stimuli were recorded in an isolated, superfused rabbit eyecup preparation and compared to responses after saccade-like changes in luminance. The manner of coding after luminance changes was different for different ganglion cell classes; both suppression and enhancement of responses to patterns following luminance changes were found. Brisk-transient Off cells unambiguously signaled the darkening of the overall image, but were also modulated by the subsequently appearing grating stimulus. Several types of On-center cell behavior were observed, ranging from strong suppression of the subsequent response by luminance changes, to strong enhancement. Overall, most ganglion cells distinguished static patterns after a luminance change via differences in their spike discharges nearly as well as before, although there were clear asymmetries between the On and Off pathways. Changes in mean luminance in some ganglion cells, such as On–Off directionally selective ganglion cells, could create large phase shifts in the response to patterned, moving stimuli, although these stimuli were still detected immediately after luminance changes. The results of this study show that the image dynamics of active vision may be a fundamental challenge for the visual system because of strong effects on retinal ganglion cell function. However, rapid extraction of unambiguous information after luminance changes appears to be encoded in differences in the spike discharges in different retinal ganglion cell classes. Asymmetries among ganglion cell classes in sensitivity to luminance changes may provide a basis by which some provide the “context” for interpreting the firing of others.

We serendipitously discovered that the preferred-direction responses of ON–OFF directionally selective (DS) ganglion cells in the rabbit retina fall as a function of contrast when the contrast of a moving bar exceeds about 100%. Null-direction responses did not fall for contrasts up to 400%. Because the non-monotonic (rise-then-fall) behavior as a function of contrast occurred only for preferred-direction responses, it must depend on the mechanism of directional selectivity. It became thus of interest to investigate how this non-monotonicity depends on the major synapses involved in directional selectivity. Blockades of nicotinic acetylcholine (ACh) and NMDA glutamate receptors reduced responses without eliminating preferred-response non-monotonicity. Blocking GABAergic inhibition, however, did eliminate non-monotonicity. These results pose a difficult puzzle, since in the accompanying paper (Grzywacz et al., 1998), we showed that residual responses under combined nicotinic and NMDA blockades are not statistically significantly directionally selective. How is it possible that null-direction GABAergic inhibition affects non-nicotinic-non-NMDA residual responses without generating directional selectivity? This may happen if there exists an asymmetric GABAergic input to distal dendrites of the DS cell while the excitatory, non-nicotinic-non-NMDA input is to proximal dendrites. In support of this hypothesis, bath-applied GABA reduces responses to exogenous ACh under synaptic block, providing for the first time in the rabbit's retina, direct evidence of GABA receptors on DS cells.

A stimulus display was devised that enabled us to examine how effectively monkeys and humans can process shading and disparity cues for depth perception. The display allowed us to present these cues separately, in concert and in conflict with each other. An oddities discrimination task was used. Humans as well as monkeys were able to utilize both shading and disparity cues but shading cues were more effectively processed by humans. Humans and monkeys performed better and faster when the two cues were presented conjointly rather than singly. Performance was significantly degraded when the two cues were presented in conflict with each other suggesting that these cues are processed interactively at higher levels in the visual system. The fact that monkeys can effectively utilize depth information derived from shading and disparity indicates that they are a good animal model for the study of the neural mechanisms that underlie the processing of these two depth cues.

There is controversy in the literature concerning whether or not neurons in the cat's posteromedial lateral suprasylvian (PMLS) visual cortex are orientation selective. Previous studies that have tested cells with simple bar stimuli have found that few, if any, PMLS cells are orientation selective. Conversely, studies that have used repetitive stimuli such as gratings have found that most or all PMLS cells are orientation selective. It is not known whether this difference in results is due to the stimuli used or the laboratories using them. The present experiments were designed to answer this question by testing individual PMLS neurons for orientation sensitivity with both bar and grating stimuli. Using quantitative response measures, we found that most PMLS neurons respond well enough to stationary flashed stimuli to use such stimuli to test for orientation sensitivity. On the basis of these tests, we found that about 85% of the cells with well-defined receptive fields are orientation sensitive to flashed gratings, and a similar percentage are orientation sensitive to flashed bars. About 80% of the cells were orientation sensitive to both types of stimuli. The preferred orientations typically were similar for the two tests, and they were orthogonal to the preferred direction of movement. The strength of the orientation sensitivity (measured as the ratio of discharge to the preferred and nonpreferred orientations) was similar to both types of stimuli. However, the width of the orientation tuning curves was systematically broader to bars than to gratings. Several hypotheses are considered as to why previous studies using bars failed to find evidence for orientation sensitivity. In addition, a mechanism for the difference in orientation tuning to bars and gratings is suggested.

The existence of multiple areas of extrastriate visual cortex raises the question of how the response properties of each area are derived from its visual input. This question was investigated for one such area in the cat, referred to here as the Clare-Bishop area (Hubel & Wiesel, 1969); it is the region of lateral suprasylvian cortex that receives input from area 17. A novel approach was used, in which kainic acid was injected locally into the Clare-Bishop area, making it possible to record directly from afferent inputs.

The response properties of the great majority of a sample of 424 presumed afferents resembled cells in areas 17 and 18. Thus, a systematic comparison was made with cells from area 17's upper layers, the source of its projection to the Clare-Bishop area (Gilbert & Kelly, 1975), to see whether these afferents had distinctive properties that might distinguish them from cells projecting to areas 18 or 19. Some differences did emerge: (1) The smallest receptive fields typical of area 17 were relatively scarce among afferents. (2) Direction-selective afferents were more abundant than were such cells in area 17. (3) End-stopped afferents were extremely rare, although end-stopped cells were common in area 17's upper layers.

Despite these differences, afferents were far more similar in their properties to cells in areas 17 and 18 than to cells in the Clare-Bishop area. Compared to the latter, afferents showed major discrepancies in receptive-field size, in direction selectivity, in end-stopping, and in ocular dominance distribution. These differences seem most likely to stem from circuitry intrinsic to the Clare-Bishop area.

Three interneurons were recorded from and then injected with horseradish peroxidase in the parvocellular laminae of the squirrel monkey's (Saimiri sciureus) dorsal lateral geniculate nucleus. They were then examined using the electron microscope for their synaptic contacts, both the afferent contacts onto their dendrites and their presynaptic dendritic contacts onto presumptive projection (relay) neuron dendrites. The somata of these interneurons were small (mean = 178 μm2), but the dendritic trees were large compared with those of projection neurons. All three interneurons had similar synaptic patterns onto their dendrites with about equal numbers of retinal, cortical, and GABAergic contacts. The distribution of these contacts was more uniform compared with the same types of contacts made onto projection neurons. The presynaptic dendrites were observed to contact only the dendrites of presumptive projection neurons, and these contacts were nearly all in the form of geniculate triads. None of the three interneurons displayed an axon. The receptive fields of these interneurons were similar to those of projection cells, but were larger and had center-response signs that were the opposite of the projection neurons around them (e.g. OFF center for the dorsal part of the parvocellular mass where ON-center projection neurons reside). The squirrel monkey data provides additional evidence that one aspect of the laminar pattern observed in the parvocellular pathway of the primate's dLGN might be related to a segregation of projection neurons of one center-response sign with interneurons of the opposite center-response sign.