We propose that there exists within the avian, and perhaps more generally in the vertebrate retina, a two-state nonadapting flip-flop circuit, based on reciprocal inhibitory interactions between the photoreceptors, releasing melatonin, the dopaminergic amacrine cells, and amacrine cells which contain enkephalin-, neurotensin-, and somatostatin-like immunoreactivity (the ENSL1 amacrine cells). This circuit consists of two loops, one based on the photoreceptors and dopaminergic amacrine cells, and the other on the dopaminergic and ENSLI amacrine cells. In the dark, the photoreceptors and ENSL1 amacrine cells are active, with the dopaminergic amacrine cells inactive. In the light, the dopaminergic amacrine cells are active, with the photoreceptors and ENSLI amacrine cells inactive. The transition from dark to light state occurs over a narrow (<1 log unit) range of low light intensities, and we postulate that this transition is driven by a graded, adapting pathway from photoreceptors, releasing glutamate, to ON-bipolar cells to dopaminergic amacrine cells. The properties of this pathway suggest that, once released from the reciprocal inhibitory controls of the dark state, dopamine release will show graded, adapting characteristics. Thus, we postulate that retinal function will be divided into two phases: a dopamine-independent phase at low light intensities, and a dopamine-dependent phase at higher light intensities. Dopamine-dependent functions may show two-state properties, or two-state properties on which are superimposed graded, adapting characteristics. Functions dependent upon melatonin, the enkephalins, neurotensin, and somatostatin may tend to show simpler two-state properties. We propose that the dark-light switch may have a role in a range of light-adaptive phenomena, in signalling night-day transitions to the suprachiasmatic nucleus and the pineal, and in the control of eye growth during development.

Photocurrents of cones in the retinas of a small fish, Danio aequipinnatus (Cyprinidae) were recorded with suction pipette electrodes. Spectral sensitivity was measured between 277 and 697 nm. Four spectral classes of cone were found, with λmax at 560, 480, 408, and 358 nm. For the latter, we provide the first complete characterization of spectral sensitivity of a vertebrate ultraviolet (UV) photoreceptor. All cones responded with similar kinetics, except for a subset of the 560-nm cones, which were distinctly faster. The a-bands of the three cones absorbing maximally in the visible have the same bandwidth when log sensitivity is plotted versus normalized frequency, and in this respect they are indistinguishable from primate cones (“Mansfield's rule’). An eighth-degree polynomial in λmax/λ based on this combined data set (fish, primate) is presented as a template that is likely to have predictive value in describing cone spectra from other vertebrates. The α−band of the UV cone, however, is somewhat narrower than predicted by this function, is similar to other UV visual pigments, and an eighth-degree polynomial that describes its shape is also presented. These measurements also provide information on the β−band (i.e. cis peak region), difficult to obtain by microspectrophotometry. The β−band of cone pigments is found at longer wavelengths as the α−band shifts toward the red. A secondary rise in cone sensitivity around 280 nm indicates that photons absorbed by aromatic amino acids in the opsin (γ−band) excite the transduction cascade, but the quantum efficiency is not as high as when absorption occurs in the retinal-protein chromophore.

We examined the effects of picrotoxin and pentylenetetrazol (PTZ) on the responses to motions of ON-OFF directionally selective (DS) ganglion cells of the turtle's retina. These drugs are antagonists of the inhibitory neurotransmitter GABA. For continuous motions, picrotoxin markedly reduced the overall directionality of the cells. In 21% of the cells, directional selectivity was lost regardless of speed and contrast. However, other cells maintained their preferred direction despite saturating concentrations of picrotoxin. And in most cells, loss, maintenance, or even reversal of preferred and null directions could occur as speed and contrast were modulated. In 50% of the cells, reversal of preferred and null directions occurred at some condition of visual stimuli. However, picrotoxin did not tend to alter the preferred-null axis for directional selectivity. For apparent motions, picrotoxin made motion facilitation, which normally occurs exclusively in preferred-direction responses, to become erratic and often occur during null-direction motions. Finally, PTZ had effects similar to picrotoxin but with less potency. The results in this paper indicated that models of directional selectivity based solely on a GABAergic implementation of Barlow and Levick's asymmetric-inhibition model do not apply to the turtle retina. Alternative models may comprise multiple directional mechanisms and/or a symmetric inhibitory one, but not asymmetric facilitation.

Previous studies have indicated that γ-aminobutyric acid (GABA) plays an important trophic role in the synapse formation between horizontal cells and photoreceptors in postnatal rabbit retina. However, the mechanism of the GABA effect has not been identified. Using fluo-3 Ca2+ imaging and confocal laser scanning microscopy we examined the effect of GABA on [Ca2+]i during postnatal retinal development. GABA (100 μM) evoked a fast and transient increase of [Ca2+]i in selected populations of freshly dissociated retinal cells from postnatal rabbits. This increase was apparent on postnatal day 1 and reached a maximum on day 5. Little increase in [Ca2+]i, was observed in retinal cells isolated from adult rabbits. GABA receptor antagonists, picrotoxin and bicuculline, significantly reduced the response. The GABAU agonist, baclofen, did not evoke any [Ca2+]i changes. The GABA-induced increase in [Ca2+]i, was observed in all retinal layers in neonatal retinal whole-mount explants. In the outer retina, the increase was seen in cone photoreceptors which were specifically labeled with peanut agglutinin (PNA). The GABA-induced increase in [Ca2+]i may provide an important mechanism for regulating cone synaptogenesis in the outer plexiform layer of the postnatal retina.

The morphological features of retinal terminals in cat brain were examined at sites where projections of W-type ganglion cells predominate. These included the parvicellular C laminae of the dorsal lateral geniculate nucleus, the ventral lateral geniculate nucleus, stratum griseum superficiale of the superior colliculus, and the suprachiasmatic nucleus. Positive identification of retinal terminals was achieved following anterograde transport of intravitreally injected native or wheat germ agglutinin-conjugated horseradish peroxidase. In contrast to the classic features of retinal terminals as defined from sites where X- and Y-type ganglion cells predominate, i.e. round synaptic vesicles, large profiles, and pale mitochondria, substantial numbers of terminals in W-cell rich areas were found to contain dark mitochondria. Synaptic vesicles, although consistently round, were typically smaller in terminals with dark mitochondria than in those with pale mitochondria. These findings indicate a diversity among terminals of W-cells and suggest that such terminals cannot be distinguished on the basis of limited morphological criteria.

While much attention has been given to the correlation between cytochrome-oxidase (CO) compartments and patterns of cortico-cortical projections originating from supragranular layers in the striate cortex, little is known in this regard about patterns of cortico-subcortical projections originating from infragranular cortex. We studied the tangential distribution of the striate cortex neurons projecting to the superior colliculus and used two approaches to analyze the relationship of this distribution to the arrangement of CO “blobs.” First, chi-square analysis indicated that significantly fewer labeled neurons were found within the CO blob compartment than the number expected for a random distribution. Second, spatial cross-correlation analysis – which circumvents the inherent subjectivity of delineating blob boundaries – revealed an area around blob centers in which there was a decreased probability of encountering labeled cells. The size of this area compared well with that of our outlines of CO blobs. We conclude that corticotectal projection neurons in the striate cortex are distributed preferentially within the interblob compartment of the infragranular striate cortex. These results demonstrate that the spatial distribution of cortico-subcortical projection neurons within infragranular cortex can correlate with the CO architecture of the primary visual cortex.

The ground level visual field of each eye of the leopard frog includes the entire ipsilateral 180-deg field and approximately 60 deg into the frontal contralateral field. When one eye is covered with an opaque patch, a frog responds to prey stimuli over the entire field of the other eye. Nevertheless, when one optic nerve is cut, the animal responds to prey in the ipsilateral hemifield of the connected eye, but only responds as far as about 30 deg past the frontal midline. If one optic tract is cut, the animal does not respond to prey past the frontal midline. We hypothesized that the responses past the frontal midline might be mediated by input from contralaterally projecting isthmotectal fibers. These fibers originate in the nucleus isthmi, a posterior midbrain structure. We found that when we placed an opaque patch over one eye and either ablated the ipsilateral nucleus isthmi, or cut crossing isthmotectal fibers in the optic chiasm, or blocked input to nucleus isthmi by ablating the ipsilateral tectal lobe, animals did not respond to prey stimuli past the frontal midline. We found that when we placed an opaque patch over one eye and cut crossing optic fibers in the anterior part of the optic chiasm (sparing crossing isthmotectal fibers), animals responded to prey stimuli in the nasal half of the seeing eye's contralateral frontal field. Our results suggest that contralaterally projecting isthmotectal fibers enable the frog to respond to stimuli past the frontal midline. We suggest a one-dimensional model of how nucleus isthmi influences tectal function.

We investigated the dynamics of neurons in the striate cortex (V1) and the lateral geniculate nucleus (LGN) to study the transformation in temporal-frequency tuning between the LGN and V1. Furthermore, we compared the temporal-frequency tuning of simple with that of complex cells and direction-selective cells with nondirection-selective cells, in order to determine whether there are significant differences in temporal-frequency tuning among distinct functional classes of cells within V1. In addition, we compared the cells in the primary input layers of V1 (4a, 4cα, and 4cβ) with cells in the layers that are predominantly second and higher order (2, 3, 4b, 5, and 6). We measured temporal-frequency responses to drifting sinusoidal gratings. For LGN neurons and simple cells, we used the amplitude and phase of the fundamental response. For complex cells, the elevation of impulse rate (F0) to a drifting grating was the response measure. There is significant low-pass filtering between the LGN and the input layers of V1 accompanied by a small, 3-ms increase in visual delay. There is further low-pass filtering between V1 input layers and the second- and higher-order neurons in V1. This results in an average decrease in high cutoff temporal-frequency between the LGN and V1 output layers of about 20 Hz and an increase in average visual latency of about 12–14 ms. One of the most salient results is the increased diversity of the dynamic properties seen in V1 when compared to the cells of the lateral geniculate, possibly reflecting specialization of function among cells in V1. Simple and complex cells had distributions of temporal-frequency tuning properties that were similar to each other. Direction-selective and nondirection-selective cells had similar preferred and high cutoff temporal frequencies, but direction-selective cells were almost exclusively band-pass while nondirection-selective cells distributed equally between band-pass and low-pass categories. Integration time, a measure of visual delay, was about 10 ms longer for V1 than LGN. In V1 there was a relatively broad distribution of integration times from 40–80 ms for simple cells and 60–100 ms for complex cells while in the LGN the distribution was narrower.

Adult teleost retinas grow throughout life, in part by the addition of cells from an encircling, proliferative neuroepithelium. In some species, this proliferative growth is asymmetric around the retina. The present study evaluated two hypotheses regarding asymmetric proliferative growth in adult green sunfish retina: (1) the generation of rod photoreceptors in central retina from proliferative rod precursor cells is also asymmetric; and (2) asymmetric proliferative growth patterns are regulated by mechanisms that are organized retinotopically and are independent of body-axis coordinates. In all retinas examined, rod precursor distribution and rod production were asymmetric, and both were in coarse spatial register with the asymmetric pattern of cellular addition at the retinal margin. In adult eyes that were surgically rotated, the asymmetric patterns of proliferative growth were present and appropriate for the retina's prerotation orientation. The results suggest that proliferative growth at both marginal and central adult sunfish retina is asymmetric, and that these asymmetric growth patterns are regulated by a retinotopic mechanism that is independent of body-axis coordinates.

We have used light- and electron-microscopic immunohistochemistry to identify the presence of immunoreactivity to neuropeptide Y (NPY) within Müller cells in the retina of the cane toad, Bufo marinus. Müller cells containing NPY-like immunoreactivity (NPY-LI) were identified at the light-microscopic level by the coexistence with immunoreactivity to glial fibrillary acidic protein (GFAP) and at the ultrastructural level by their characteristic relationship to neuron cell bodies and processes. At the light-microscopic level, those cells which contained both NPY-LI and GFAP-LI usually had small cell bodies in the inner nuclear layer, while those cells which contained only NPY-LI were identified as large and small amacrine cells. The radially oriented primary processes in the inner plexiform layer and the vitreal end feet of GFAP-LI Müller cells also expressed NPY-LI. At the ultrastructural level, thin lamellar processes of Müller cells with NPY-LI enclosed some amacrine cell bodies in the inner nuclear layer and amacrine cell dendrites in the inner plexiform layer. These observations suggest that NPY-LI is localized in Müller cells in addition to two types of amacrine cells previously identified in the Bufo retina. This study provides the first evidence that glial elements in the vertebrate retina express NPY-LI.

We have employed the tetrode technique, which allows accurate discrimination of individual neuronal spike trains from multiunit recordings, in order to examine the variation of orientation selectivity among local groups of neurons. We recorded a total of 321 cells from 62 sites in area 17 of halothane-anesthetized cats; each site contained between three to ten neurons that were estimated to be less than 65 μm away from the tetrode tip. For each cell, we determined the orientation tuning in response to moving bars. Of the cells tested, 8.4% were unresponsive, 22.7% had no preferential response to any particular orientation, while 68.8% were tuned. The average difference in preferred orientation between cell pairs recorded at the same site was 10.7 deg, but the variance in preferred orientation differences differed significantly among sites. Some clusters of cells exhibited the same or nearly the same orientation preference, while others had orientation preferences that differed by as much as 90 deg. Our data demonstrate that the tuning for orientation is more heterogeneously distributed at a local level than previous studies have suggested.

Biplexiform ganglion cells were labelled by retrograde transport of HRP in five species of marine fish from the neoteleost acanthopterygian orders Perciformes and Scorpaeniformes. Their forms and spatial distributions were studied in retinal flatmounts and thick sections. Biplexiform ganglion cells possessed sparsely branched, often varicose, dendrites that ramified through the inner nuclear layer (INL) to reach the outer plexiform layer (OPL), as well as conventional arborizations in the most sclerad part of the inner plexiform layer (IPL). Their somata were of above-average size and were displaced into the vitread border of the INL. Mean soma areas ranged from 99 ± 6 μm2 in Bathymaster derjugini (Perciformes) to 241 ± 12 μm2 in Hexagrammos stelleri (Scorpaeniformes), but were similar in each species to those of the outer-stratified alpha-like ganglion cells, whose dendritic trees occupied the same IPL sublamina. In the best-labelled specimens, biplexiform cells formed clear mosaics with spacings and degrees of regularity much like those of other large ganglion cells, but spatially independent of them. Biplexiform mosaics were plotted in three species, and analyzed by nearest-neighbor distance and spatial correlogram methods. The exclusion radius, an estimate of minimum mosaic spacing, ranged from 113 urn in Hexagrammos stelleri, through 150 μm in Ernogrammus hexagraminus (Perciformes), to 240 μm in Myoxocephalus stelleri (Scorpaeniformes). A spatial cross-correlogram analysis of the distributions of biplexiform and outer-stratified alpha-like cells in Hexagrammos demonstrated the spatial independence of their mosaics. Similar cells were previously observed not only in the freshwater cichlid Oreochromis spilurus (Perciformes) but also in the goldfish Carassius auratiis (Cypriniformes) which, being an ostariophysan teleost, is only distantly related. Thus, biplexiform ganglion cells may be regular elements of all teleost fish retinae. Their functional role remains unknown.

To examine the role of visual area V4 in pattern vision, we tested two monkeys with lesions of V4 on tasks that required them to discriminate the orientation of contours defined by several different cues. The cues used to separate the contours from their background included luminance, color, motion, and texture, as well as phase-shifted abutting gratings that created an “illusory” contour. The monkeys were trained to maintain fixation on a fixation target while discriminating extrafoveal stimuli, which were located in either a normal control quadrant of the visual field or in a quadrant affected by a lesion of area V4 in one hemisphere. Comparing performance in the two quadrants, we found significant deficits for contours defined by texture and for the illusory contour, but smaller or no deficits for motion-, color-, and luminance-defined contours. The data suggest a specific role of V4 in the perception of illusory contours and contours defined by texture.

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

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

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

Visual localization was studied by flashing small stimuli on a green background and requiring observers to press keys to indicate whether the stimulus appeared to the left or right of fixation. The results suggest that, for small (0.25 deg) briefly flashed (17 ms) stimuli at an eccentric location (10 deg), color contrast is not useable and localization presumably must rely on the magnocellular pathway. When stimulus size and duration were increased at 10-deg eccentricity, isochromatic stimuli could be localized at less than 10% luminance contrast (again suggesting use of the magnocellular high sensitivity luminance-contrast system), but isoluminant color-contrast stimuli could also be localized (suggesting use of the color-contrast sensitive parvocellular system). Thus, the results indicate that, dependent on stimulus conditions, both magnocellular and parvocellular pathways were utilized by normal observers in this localization task.

Multiunit activity was recorded in the optic tectum of awake pigeons with two electrodes at sites varying in depth and separated by 0.3 to 3.0 mm. Autocorrelation and cross-correlation functions were computed from the recorded spike trains to determine temporal relationships in the neuronal firing patterns. Cross-correlation analysis revealed that spatially separate groups of cells in the tectum show synchronous responses to a visual stimulus. Strong synchronization occurred in both superficial and deep layers of the tectum, in general with zero-phase shift. The response synchronization in the avian optic tectum resembles that observed in the mammalian cortex, suggesting that it may subserve common functions in visual processing.

We examined the numerical correlation between total populations of neurons in the lateral geniculate nucleus (LGN) and the primary visual cortex (area 17 of Brodmann) in ten cerebral hemispheres of five normal rhesus monkeys using an unbiased three-dimensional counting method. There were 1.4 ± 0.2 million and 341 ±54 million neurons in the LGN and area 17, respectively. In each animal, a larger LGN on one side was in register with a larger area 17 of the cortex on the same side. Furthermore, asymmetry in the number of neurons in both the LGN and area 17 favored the right side. However, because of small variations across subjects, correlation between the total neuron number in LGN and area 17 was weak (r = 0.29). These results suggest that the final numbers of neurons in these visual centers may be established independently or by multiple factors controlling elimination of initially overproduced neurons.

Color appearance judgments revealed fundamental differences in visual processing of incremental and decremental lights. First, the balance of cone activation required for a light to appear achromatic was different for increments and decrements (Judd, 1940; Helson & Michels, 1948). Second, adaptation-the visual system's adjustment to background light – affected achromatic decrements more than increments. Third, the regulation of adaptation for incremental and decremental stimuli depended differently on background signals from the three cone types. We interpret these asymmetries as differences in mechanisms of adaptation in the ON and OFF pathways, and suggest that they evolved to accommodate the range and physical sources of color signals in the two pathways.