Fast, multiple-site optical recording of voltage-sensitive dye (VSD) signals and intracellular microelectrode recordings were combined to characterize visually evoked neuronal responses in the visual cortex of the pond turtle, Pseudemys scripta. By using an in vitro, eye-brain preparation stained with the merocyanine oxazolone voltage-sensitive dye, NK-2495 or a close analog, NK-2761, large VSD signals relatively free of vibrational noise could be recorded in single trials following a stroboscopic light flash to the contralateral eye. VSD signals recorded from the same cortical location in repeated trials exhibited considerable variability in the onset, duration, and amplitude of secondary depolarizations. Because of this variability, secondary depolarizations were largely absent in signal-averaged responses. Superposition of VSD signals with intracellular recordings obtained from cortical pyramidal cells revealed a close correspondence between their signal waveforms. The two signals were virtually identical in their onset, initial rate of rise, and time-to-peak. At later periods (>500 ms), the correspondence was less close, especially for large cortical depolarizations. Some of this disparity could be attributed to contamination of the VSD signal by a large intrinsic optical response. A second contribution was a failure of the VSD signal to register asynchronous regenerative effects occurring in single pyramidal cells. It is suggested that the close correspondence between the microelectrode and optical recordings in the early phase of the response may reflect the organization of pyramidal cells into clusters that receive virtually identical synaptic inputs.

The retinas of placental mammals appear to lack the large number and morphological diversity of cone subtypes found in diurnal reptiles. We have now studied the photoreceptor layer of a South American marsupial (Didelphis marsupialis aurita) by peanut agglutinin labeling of the cone sheath and by labeling of cone outer segments with monoclonal anti-visual pigment antibodies that have been proven to consistently label middle-to-long wavelength (COS-1) and short-wavelength (OS-2) cone subpopulations in placental mammals. Besides a dominant rod population (max. = 400,000/mm2) four subtypes of cones (max. = 3000/mm2) were identified. The outer segments of three cone subtypes were labeled by COS-1: a double cone with a principal cone containing a colorless oil droplet, a single cone with oil droplet, and another single cone. A second group of single cones lacking oil droplets was labeled by OS-2 antibody. The topography of these cone subtypes showed striking anisotropies. The COS-1 labeled single cones without oil droplets were found all over the retina and constituted the dominant population in the area centralis located in the temporal quadrant of the upper, tapetal hemisphere. The population of OS-2 labeled cones was also ubiquitous although slightly higher in the upper hemisphere (200/mm2). The COS-1 labeled cones bearing an oil droplet, including the principal member of double cones, were concentrated (800/mm2) in the inferior, non-tapetal half of the retina. The two spectral types of single cones resemble those of dichromatic photopic systems in most placental mammals. The additional set of COS-1 labeled cones is a distinct marsupial feature. The presence of oil droplets in this cone subpopulation, its absence in the area centralis, and the correlation with the non-tapetal inferior hemisphere suggest a functional specialization, possibly for mesopic conditions. Thus, sauropsid features have been retained but probably with a modified function.

Calbindin D-28K is a calcium-binding protein found in the cone but not rod photoreceptor cells in the retinas of a variety of species. Recent studies of the monkey retina indicated that calbindin D-28K may be expressed preferentially in non-foveal regions of the retina. In the current studies of human retinas, immunohistochemical experiments demonstrated that calbindin D-28K is reduced or absent in the fovea and parafovea, but prevalent in the perifovea and periphery. These findings were supported by the quantification of calbindin D-28K in 1-mm trephine punches obtained from different regions of the human retina. The specificity of the anti-calbindin D-28K antibodies used in these studies was confirmed by Western blot analysis using purified calbindin D-28K. The protein was purified from retinal tissue and its identity confirmed by partial amino-acid sequence analysis. The expression of calbindin D-28K did not correlate with the spectral properties of the cones, rather to their position in the retina. The study of spatially expressed genes, like the one encoding calbindin D-28K, may help explain the patterns of retinal degeneration seen in some human cone-rod dystrophies.

We have compared the responses of simple cells to laterally moving sinusoidal gratings and to stationary temporally-modulated gratings. From the amplitudes and temporal phases of the responses to stationary gratings of different spatial phases, it should be possible to predict the preferred direction of movement, the amplitudes of the responses to gratings moving in the preferred and nonpreferred directions and, thence, the degree of directional preference (Reid et al., 1987). The preferred direction can be predicted reliably. However, the magnitude of the directional preference cannot be predicted, since the measured amplitude of the response in the nonpreferred direction of movement is very much less than that predicted by a linear theory. Nonlinearities in the relationship between response amplitude and contrast may contribute to the failure of the predictions, but this contribution is small. We conclude that the magnitude of the directional preference seems to be determined predominantly by nonlinear suppression of the response in the nonpreferred direction of movement.

The spectral mechanisms of the ferret (Mustela putorious furo) were studied with electroretinogram (ERG) flicker photometry. Variations in adaptation state and flicker rate were used to define corneally based spectral sensitivities for the three classes of receptor present in the retina of this mustelid—rods (λmax = 505 nm), S cones (430 nm), and L cones (558 nm). The retinal distributions of the two classes of cone were determined using opsin antibody labeling. Ferret retinas contain a total of about 1.3 million cones with L cones outnumbering S cones in a ratio of approximately 14:1. ERGs were also recorded using 18.75-Hz flickering stimuli that were designed to isolate signals from individual cone classes. The contrast/response functions for signals originating from both S and L cones were linear over low-to-moderate levels of contrast, but with greatly different slopes for the two cone types. The L:S contrast gain ratio derived from a comparison of these slopes, as well as inferences drawn from another experiment in which responses to various combinations of L- and S-cone activation were analyzed, suggest that contributions of these two cone types to the flicker ERG have a relative weighting of about 4:1 to 5:1 (L/S).

The phenomenon of apparent motion can arise when two spatially separated visual tokens are presented in temporal sequence. If tokens at opposite corners of a hypothetical square are presented simultaneously followed by simultaneous presentation of tokens at the remaining two corners, an apparent motion percept may occur along either the vertical or horizontal axis. The display is perceptually metastable since most observers will perceive motion along only one axis at a time. The metastable display, however, produces anisotropic results, in that with central fixation, vertical motion is seen more frequently than horizontal motion. The ratio of the vertical to horizontal length of the sides of a rectangle needed to achieve equal frequencies of motion judgments along the respective axes falls in the range of 1.18–1.92 for different observers in our experiments. It appears that signal transmission across the vertical midline is a major determinant of the vertical bias, since the anisotropic effects disappear when the fixation point is sufficiently offset along the horizontal meridian so as to cause a fully homonymous representation of all of the metastable tokens. One of the factors may be signal degradation or delay in callosal transmission which could reduce the strength of the motion signal along the horizontal axis. In addition, there appears to be a strip along the vertical midline with a width of 30–50 min are within which reduced levels of anisotropy are found. The possibility that this strip is a consequence of a zone of naso-temporal overlap in the projection of the retina to the brain along the vertical meridian will be discussed.

In humans, an increasing density of foveal cone photoreceptors occurs slowly over several years after birth, and accounts for a region that subserves high visual acuity. Concurrently, inner retinal cells move centrifugally away from the foveal center. Such developmental rearrangements reflect complex cellular remodeling after the retinal neuronal cells have differentiated and have formed synapses. Explaining foveal morphogenesis is difficult, because differentiated neuronal cells seem incapable of moving actively. Presented here is a biomechanical explanation of how the above events occur. This hypothesis assumes that the cellular movements throughout the retinal layers occur passively as the eye grows and the retina is stretched. Retinal stretch was simulated using virtual engineering models that were analyzed with finite element analysis. A pit combined with retinal stretch causes the retinal layers to deform in a way that accounts for both the centrifugal and centripetal movement of various retinal cell types. Axially directed, tensile forces associated with stretching the retinal tissue surrounding the pit also accounts for the elongated morphology of foveal cone photoreceptors. These simulations suggest that a pit is required for both the centripetal displacement of cone cells toward the center of the fovea, and for the elongated foveal cone morphology. Since the primate fovea may have minimal impact on acuity, its primary role may be to initiate foveal morphogenesis in slowly developing eyes.

The retinal synaptic network continues its development after birth in mammals. Recent studies show that postnatal development of retinal circuitry depends on visual stimulation. We sought to determine whether there is a time period during which the retina shows evidence of increased plasticity. We examined the effects of light deprivation on the retinal light response of mouse retina using electroretinogram (ERG) measurements. Our results showed that dark rearing mice from birth to postnatal day (P) 30, 60, and 90 suppressed the amplitudes of oscillatory potentials (OPs) and the magnitudes of suppression were age independent. In addition, dark-rearing-produced suppression of OP amplitudes can be completely reversed in both young and adult mice by returning them to cyclic light/dark conditions for 1 to 2 weeks. However, the recovery time course was age dependent with younger animals needing a longer time to achieve a full recovery. Furthermore, dark rearing of P60 mice raised under cyclic light/dark conditions for 30 days resulted in a similar magnitude of suppression of OP amplitudes as in age-matched mice dark reared from birth. These findings demonstrate that both the normal developmental changes and the maintenance of mature inner retinal light response in adult animals require visual stimulation. These results indicate a degree of activity-dependent plasticity in mouse retina that has not been previously described.

Two types of field potentials were identified in cat visual cortex using contrast reversal of oriented bar gratings: a short-latency fast-local component with a retinotopic organization similar to that seen with single-unit discharges at the same cortical site, and a slow, nonretinotopic component with a longer peak latency. The slow-distributed component had an extensive receptive field mapped by measuring the amplitude of binary kernels and showed strong inhibitory interactions within the receptive field. The peak latency of the slow-local component increased with distance from the retinotopic center, suggesting a possible conduction delay. Both components showed some orientation bias depending on the laminar location, but the bias could be independent of the orientation preferred by single units in the immediate vicinity. The present findings indicate that locally generated field potentials reflect cortical mechanisms for nonlinear integration over wide areas of the visual field.

The quality of the foveal cone mosaic in human and primate retinas is a basic parameter of spatial vision function. The present study uses digital-texture analysis procedures to analyze the crystalline order of inner segment sections containing the rod-free portions of foveal cone mosaics. Definition of the cone cross-sectional centers made possible by adequate preprocessing allows precise mapping of lattice vertices and differentiation of hexagonal positions by procedures for direct neighbor recongnition.

In a further step, the existence of subunits within the hexagonal areas is revealed by the determination of axial orientation. The lattice of the subunits is characterized by similar orientation and high positional correlation of its hexagonal units.

The axial orientation of the areas differs from that of neighboring subunits by angular shifts of 10–15 deg and linear series of nonhexagonal irregularities demarcate the borders. Although larger patches with continuous hexagonal order occur in the surrounding rod-free regions, elevated degrees of disorder (30%) are found within the foveolar center (ca. 300 cones). Analysis of a mosaic showing labeled B cones (Szél et al., 1988) demonstrates that lattice disorder is in part associated with the blue cone subpopulation. The foveal mosaic from a glaucomatuous eye reveals severe lattice degradation throughout the rod-free zone, presumably due to extensive receptor loss.

The low-frequency superstructure results in local sets of sampling grids (5'–8') with differing orientational bias. Besides a horizontal/vertical difference of mosaic compression (ca. 1:1.15), the present analysis gives no hints for the existence of systematic meridional anisotropies at the receptor mosaic level. The study reveals a discontinuous organization of the foveal mosaic and points to possible sources for the induction and location of lattice disorder.

We studied rod function in the chicken by recording corneal electroretinograms (ERGs). The following experiments were performed to demonstrate rod function during daytime: (1) determining the dark-adaptation function; (2) measuring the spectral sensitivity by a a–b-wave amplitude criterion in response to monochromatic flickering light of different frequencies ranging from 6.5–40.8 Hz (duty cycle 1: I); (3) analyzing the response vs. log stimulus intensity (V–log I) function in order to reveal a possible two phase process; and (4) determining the spectral sensitivity function either in a non-dark adapted state or after dark adaptation of the animals for I and 24 h. None of these experiments demonstrated clear evidence of rod function during daytime. On the other hand, we found rods histologically by light- and electron microscopy. Therefore, we repeated our ERG recordings during the night (between midnight and 3:00 A.M.). Without previous dark adaptation, rod function could be seen immediately in the same experiments described above. The result shows that, in the chicken, rods are turned on endogenously during the night but are scarcely functional during the day.

There is increasing evidence that GABAC receptors are composed of GABA ρ subunits. In this study, we compared the properties of native GABAC receptors with those of receptors composed of a GABA ρ subunit. A homologue of the GABA ρ gene was cloned from a white perch (Roccus americana) retinal cDNA library. The clone (perch-s) has an open reading frame of 1422 nucleotide base pairs and encodes a predicted protein of 473 amino acids. It is highly homologous to GABA ρ subunits cloned from human and rat retinas. The receptors (perch-s receptor) expressed by this gene in Xenopus oocytes show properties similar to those of the GABAC receptors present on white perch retinal neurons. GABA induced a sustained response that had a reversal potential of –27.1 +minus; 3.6 mV. The EC50 for the response was 1.74 +− 1.25 μM, a value similar to that reported for GABAC receptors. Pharmacologically, the responses were bicuculline insensitive and not modulated by either diazepam or pentobarbital as is the case for GABAc receptors. There were, however, some distinct differences between native GABAc and perch-s receptors. I4AA acts as a partial agonist on perch-s receptors whereas it is strictly an antagonist on native GABAC receptors. Picrotoxin inhibition is noncompetitive on perch-s receptors, but both competitive and noncompetitive on GABAC receptors. We conclude that GABAC receptors are formed by GABA p subunits but that native GABAc receptors probably consist of a mixture of GABA ρ subunits.

We quantified and compared the effect of element spacing on contour integration between the achromatic (Ach), red–green (RG), and blue–yellow (BY) mechanisms. The task requires the linking of orientation across space to detect a contour in a stimulus composed of randomly oriented Gabor elements (1.5 cpd, σ = 0.17 deg), measured using a temporal 2AFC method. A contour of ten elements was pasted into a 10 × 10 cells array, and background elements were randomly positioned within the available cells. The effect of element spacing was investigated by varying the mean interelement distance between two and six times the period of the Gabor elements (λ = 0.66 deg) while the total number of elements was fixed. Contour detection was measured as a function of its curvature for jagged contours and for closed contours. At all curvatures, we found that performance for chromatic mechanisms declines more steeply with the increase in element separation than does performance for the achromatic mechanism. Averaged critical element separations were 4.6 ± 0.7, 3.6 ± 0.4, and 2.9 ± 0.2 deg for Ach, BY, and RG mechanisms, respectively. These results suggest that contour integration by the chromatic mechanisms relies more on short-range interactions in comparison to the achromatic mechanism. In a further experiment, we looked at the combined effect of element size and element separation in contour integration for the Ach mechanism.

Foveal dark adaptation in 58 subjects and photopigment regeneration in 60 subjects from 10–78 years of age exhibit parallel slowing of recovery rate with increasing age, with significant correlation of the two functions among individuals. The data are suggestive of an initial slight decline in rate before age 50, followed by a greater decline occuring at different ages in different ages in different individuals. Longitudinal data for one subject from age 40–65 show an increase in pigment regeneration time constant consistent with this idea. Foveal sensitivity and photopigment density both decrease with increasing age and are significantly correlated among individuals, although sensitivity declines more with age than does photopigment density. In contrast to earlier proposals based upon the Rushton-Dowling equation, we found no universal constant of proportionality to relate log relative threshold to photopigment within our population.

Alpha ganglion cells from the midperiphery of the rabbit retina were recorded intracellularly under visual control, in a superfused everted eyecup, and labeled with HRP. Their physiology and large somata with broad dendritic arbors identified them as uniform populations of ON- and OFF-center alpha ganglion cells, which typically displayed transient/sustained light-evoked responses. When dark adapted, the light-evoked responses from both ON- and OFF-center alpha ganglion cells were more sustained than those generally seen under light-adapted conditions. During dark-adapted (scotopic) conditions, stimulation with dim full-field illumination and small spots, either positioned over the soma or displaced 450 μm from the soma, all elicited pure center responses. After light adaptation (photopic conditions), the displaced small spots that previously evoked center responses elicited antagonistic surround responses from both ON- and OFF-center cells. Thus, as originally described in cat retina (Barlow et al., 1957), the receptive-field organization of ganglion cells changed between dark and light adaptation, and an absence or presence of surround antagonism was indicative of scotopic versus photopic states.

Glutamate and γ-aminobutyric acid (GABA) are the dominant amino acids in the retina and brain. The manufacturing and degradation pathways of both of these amino acids are intricately linked with the tricarboxylic acid cycle leading to rapid redistribution of these amino acids after metabolic insult. Postmortem ischemia in mammalian retina predominantly results in a loss of glutamate and GABA from neurons and accumulation of these amino acids within Müller cells. This accumulation of glutamate and GABA in Müller cells may occur as a result of increased release of these neurotransmitters from neurons, and decreased degradation. Quantification of the semisaturation value (half-maximal response) for glutamate and GABA Müller cell loading during postmortem ischemia indicated a shorter semisaturation value for GABA than glutamate. Such changes are consistent with a single aerobically dependent GABA-degradation pathway, and the existence of multiple glutamate-degradation pathways. Comparison with the in vitro ischemic model showed similar qualitative characteristics, but a markedly increased semisaturation time for glutamate and GABA Müller cell loading (a factor of 5–10) in the postmortem ischemia model. We interpret these differences to indicate that the in vitro condition provides a more immediate and/or severe ischemic insult. In the postmortem ischemia model, the delayed glial cell loading implies the availability of internal stores of both glucose and/or oxygen. Increased glial and neuronal immunoreactivity for the amino acids involved in transamination reactions, aspartate, alanine, leucine, and ornithine was observed, indicating a potential shift in the equilibrium of transamination reactions associated with glutamate production. These findings provide evidence that, in the rat retina, there are multiple pathways subserving glutamate production/degradation that include a multitude of transamination reactions. Further evidence is therefore provided to support a role for all four amino acids in glutamate metabolism within a variety of retinal neurons and glia.

We have employed two paradigms to investigate the mechanisms of contrast gain control in cat striate cortex. In the first paradigm, optimal drifting gratings were presented in three consecutive periods. The contrast was near threshold in the first and third periods and accompanied by iontophoretic pulses of glutamate or glutamate receptor (GluR) agonists. The contrast was set to evoke a higher firing rate in the second period. Although both visual and iontophoretic conditions were identical in the first and third periods, responses to glutamate, N-methyl-D-aspartic acid (NMDA), and (1S, 3R)-1-Aminocyclopentane-1, 3-dicarboxylic acid (ACPD) were reduced following the adapting interval. (S)-α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) responses were not reduced. Administration of ionotropic GluR antagonists did not affect adaptation to the high-contrast grating. The metabotropic GluR antagonist (±)-α-Methyl-4-carboxyphenylglycine (MCPG), which acts at presynaptic glutamate autoreceptors, decreased the degree of adaptation exhibited by striate cells. In a second paradigm, contrast response functions (CRFs) were obtained at various adapting contrasts and least-squares fits to a hyperbolic ratio equation generated for each adapting level. Similar to previous reports, DL-2-amino-5-phosphonovaleric acid (APV) reduced the slope of the CRF and increased the responsiveness of the cells but did not affect the semisaturation constant, σ, or the exponent of the CRF, n. Only MCPG significantly altered the distribution of σ and n for 19 cells. The effect on α suggests that this drug can interfere with the cell's ability to shift its operating point to match the adapting contrast. These results suggest the involvement of a presynaptic mechanism for contrast adaptation. The decrease in neuronal responsiveness immediately following the high-contrast period may reflect an additional, postsynaptic effect in which there is a decrease in the NMDA-mediated component of the visual response.

Immunocytochemical methods were used to visualize glutamate immunoreactivity in the cat retina and to compare its localization with that of aspartate, GABA, and glycine. The cellular and subcellular distribution of glutamate was analyzed at the light-microscopic level by optical densitometry and at the electron-microscopic level by immunogold quantification. The findings were consistent with the proposed role for glutamate as the neurotransmitter of photoreceptors and bipolar cells as particularly high concentrations of staining were found in synaptic terminals of these cells. Ganglion cells were also consistently stained. Aspartate was totally colocalized with glutamate in neuronal cell bodies but the synaptic levels of aspartate were much lower than for glutamate. In addition to the staining of photoreceptor, bipolar, and ganglion cells, glutamate immunoreactivity was also observed in approximately 60% of the amacrine cells. These cells exhibited colocalization with either GABA or glycine. The elevated levels of Glu in amacrine cells may reflect its role as a transmitter precursor in GABAergic cells and as an energy source for mitochondria in glycinergic cells.

Responses to full-field colored flashes (red, blue, and green) were compared with those to illuminancematched white flashes in area V1, optic radiations, and the lateral geniculate nucleus of two alert macaques. Laminar profiles of visual evoked potentials (VEPs), current source density, and multiunit activity were obtained using multicontact electrodes capable of sampling from all layers of cortex or lateral geniculate nucleus, simultaneously. In striate cortex, stimulation with colored flash enhanced transmembrane current flow dramatically in both layer 4c and the supragranular laminae. Stimulation with red evoked the largest enhancement in every electrode penetration. The mean peak amplitudes of current sinks evoked by red were 203% and 537% of those evoked by white light in layer 4c and the supragranular laminae, respectively. Color effects in VI were preceded by an initial epoch of wavelength-insensitive activity. In layer 4c, the red effect reached significance, on average, at 47 ms, or ≈24 ms after the onset of transmembrane current flow. In the supragranular layers, the red effect reached significance, on average, at 55 ms, or ≈14 ms after the onset of current flow. Recordings from optic radiations in the white matter below V1 and from lateral geniculate nucleus showed no significant difference in the responses to color and illuminance-matched white light. Enhancement of supragranular current flow with color stimulation increased the contribution of these laminae to the generation of the surface VHP. Comparison of the surface VHP wave forms evoked by white and color stimuli may, therefore, help to differentiate the responses of the granular and supragranular laminae.

Amblyopia is usually considered to be a deficit in spatial vision. But there is evidence that amblyopes may also suffer specific deficits in motion sensitivity as opposed to losses that can be explained by the known deficits in spatial vision. We measured sensitivity to visual motion in random dot displays for strabismic and anisometropic amblyopic monkeys. We used a wide range of spatial and temporal offsets and compared the performance of the fellow and amblyopic eye for each monkey. The amblyopes were severely impaired at detecting motion at fine spatial and long temporal offsets, corresponding to fine spatial scale and slow speeds. This impairment was also evident for the untreated fellow eyes of strabismic but not anisometropic amblyopes. Motion sensitivity functions for amblyopic eyes were shifted toward large spatial scales for amblyopic compared to fellow eyes, to a degree that was correlated with the shift in scale of the spatial contrast sensitivity function. Amblyopic losses in motion sensitivity, however, were not correlated with losses in spatial contrast sensitivity. This, combined with the specific impairment for detecting long temporal offsets, reveals a deficit in spatiotemporal integration in amblyopia which cannot be explained by the lower spatial resolution of amblyopic vision.