We have used antibodies raised against a cytoskeletal protein, microtubule-associated protein 1A (MAP 1A), to stain adult rat retina. In cryostat and polyethylene glycol-embedded radial sections, the fiber layer, ganglion cell layer, and inner plexiform layer were highly immunoreactive, a finding that suggested that the ganglion cell somata, axons, and dendrites were recognized by these antibodies. Retrograde labeling of retinal cell somata from the superior colliculus and dorso-lateral geniculate nucleus to identify ganglion cells showed colocalization of the tracer and immunoreactive cells. Double labeling with nuclear stains revealed that many cells in the ganglion cell layer, which are likely displaced amacrine cells, were not recognized by these antibodies. Furthermore, transection of ganglion cell axons, a procedure that causes retrograde degeneration of many of the axotomized ganglion cells, led to a decrease in the number of anti-MAP 1A immunoreactive cells in retinal wholemounts. Thus, MAP 1A antibodies preferentially stain ganglion cell somata and dendrites but not amacrine cells. These antibodies should be useful ganglion cell markers.

Cells of the cat's perigeniculate nucleus (PGN), part of the visual sector of the thalamic reticular nucleus (TRN), provide GABAergic inhibition to the A and A1 layers of the dorsal lateral geniculate nucleus (LGNd) and, therefore, may control information flow from the retina to the cortex. Previous electrophysiological experiments suggested that the PGN may be subdivided on the basis of ocular dominance thus reflecting the afferent and efferent projections with lamina A and A1 of the LGNd. The present study utilized the ability of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) to be transported transneuronally following intraocular injections in four cats to examine whether there is any anatomical evidence for eye specific layers within the PGN. Sections were processed with tetramethylbenzidine. Light WGA-HRP transneuronal labeling of LGNd collaterals and somata were seen in the PGN and very light labeling (but not somata) was seen in the TRN. Neither the cells of the PGN projecting to the LGNd nor the LGNd relay collaterals within the PGN were clearly organized into nonoverlapping laminae related to the eye specific layers of the LGNd. However, parts of the PGN immediately adjacent to the LGNd appear devoid of connections with lamina A1 thus creating a thin monocular segment for the contralateral eye.

Amodal completion is the most common form of visual completion occurring when portions of an object are hidden, due to their occlusion behind another object (Michotte, 1951). Just as a shape is completed amodally behind another occluding shape, so is a color behind another occluding color or behind a lighting: a bright light reflected by a three-dimensional object. Four possible phenomenal combinations related to the amodal completion of color are shown: amodal or modal coloration or discoloration. Purposes of four experiments were: (1) to demonstrate the amodal completion of color under different stimulus conditions and under chromatic and achromatic conditions and (2) to extract the general principles ruling the amodal completion of color: “which, among many, is the color that completes amodally?” and, consequently, “which is the region of an object that determines its color?” The results showed the effectiveness of the amodal completion of color and that chromatic and achromatic conditions reveal different results. Four general principles of the amodal completion of color, useful to understand the more general problem of phenomenal organization of color, are suggested.

Contrast encoding for sinusoidal modulations of luminance contrast was investigated by intracellular recording in the intact salamander retina. In what appears to be the first study of this kind for vertebrate bipolar cells, responses of the central receptive-field mechanism of cone-driven cells to modulation of 3 Hz were analyzed quantitatively via both signal averaging and a Fast Fourier Transform (FFT) while the retina was light adapted to 20 cd/m2. Depolarizing and hyperpolarizing bipolar cells showed very similar encoding. Both responded with sinusoidal waveforms whose amplitude varied linearly with modulation depths ranging up to 7–8%. The slope of the modulation/response curve was very steep in this range. Thus, the contrast gain was high, reaching values of 6–7, and the half-maximal response was achieved at modulations of 9% or less. At modulations above ∼15%, the responses typically showed strong compressive nonlinearity and the waveform was increasingly distorted. At maximum modulation, the higher harmonics of the FFT constituted about 30% of the amplitude of the fundamental. Measurements were also made for cones and horizontal cells. Both cell types showed predominantly linear responses and low contrast gain, in marked contrast to bipolar cells. These results suggest that the high contrast gain and strong nonlinearity of bipolar cells largely arise postsynaptic to cone transmitter release. Further experiments were performed to compare responses to contrast steps versus those to sinusoidal modulation. In the linear range, we show that the contrast gains of cones and horizontal cells are low and virtually identical for both steps and sinusoidal modulations. In bipolar cells, on the other hand, the contrast gain is about two times greater for steps than that for the 3-Hz sine waves. These results suggest that mechanisms intrinsic to bipolar cells act like a high-pass filter with a short time constant to selectively emphasize contrast transients over slower changes in contrast.

It has been shown that the visual acuity loss experienced by the deprived eye of kittens following an early period of monocular deprivation (MD) can be alleviated rapidly following 10 days of complete darkness when imposed even as late as 14 weeks of age. To examine whether 10 days of darkness conferred benefits at any age, we measured the extent of recovery of the visual acuity of the deprived eye following the darkness imposed on adult cats that had received the same early period of MD as used in prior experiments conducted on kittens. Parallel studies conducted on different animals examined the extent to which darkness changed the magnitude of the MD-induced laminar differences of the cell soma size and immunoreactivity for the neurofilament (NF) protein in the dorsal lateral geniculate nucleus (dLGN). The results indicated that 10 days of darkness imposed at one year of age neither alleviated the acuity loss of the deprived eye induced by an earlier period of MD nor did it decrease the concurrent lamina differences of the soma size or NF loss in the dLGN.

Amacrine cells are a diverse set of local circuit neurons of the inner retina, and they all release either GABA or glycine, amino acid neurotransmitters that are generally inhibitory. But some types of amacrine cells have another function besides inhibiting other neurons. One glycinergic amacrine cell, the Aii type, excites a subset of bipolar cells via extensive gap junctions while inhibiting others at chemical synapses. Many types of GABAergic amacrine cells also release monoamines, acetylcholine, or neuropeptides. There is now good evidence that another type of amacrine cell releases glycine at some of its synapses and releases the excitatory amino acid glutamate at others. The glutamatergic synapses are made onto a subset of retinal ganglion cells and amacrine cells and have the asymmetric postsynaptic densities characteristic of central excitatory synapses. The glycinergic synapses are made onto other types of ganglion cells and have the symmetric postsynaptic densities characteristic of central inhibitory synapses. These amacrine cells, which contain vesicular glutamate transporter 3, will be the focus of this brief review.

Based on clinical findings, diabetic retinopathy (DR) has traditionally been defined as a retinal microvasculopathy. Retinal neuronal dysfunction is now recognized as an early event in the diabetic retina before development of overt DR. While detrimental effects of diabetes on the survival and function of inner retinal cells, such as retinal ganglion cells and amacrine cells, are widely recognized, evidence that photoreceptors in the outer retina undergo early alterations in diabetes has emerged more recently. We review data from preclinical and clinical studies demonstrating a conserved reduction of electrophysiological function in diabetic retinas, as well as evidence for photoreceptor loss. Complementing in vivo studies, we discuss the ex vivo electroretinography technique as a useful method to investigate photoreceptor function in isolated retinas from diabetic animal models. Finally, we consider the possibility that early photoreceptor pathology contributes to the progression of DR, and discuss possible mechanisms of photoreceptor damage in the diabetic retina, such as enhanced production of reactive oxygen species and other inflammatory factors whose detrimental effects may be augmented by phototransduction.

Brn-3a, Brn-3b, and Brn-3c are highly homologous POU-domain transcription factors that are expressed in subsets of retinal ganglion cells. From each of the mouse Brn-3 genes, a DNA segment ranging in size from 4.6 to 13.4 kb and located immediately upstream of the start site of translation was joined to a human placental alkaline phosphatase (AP) reporter cDNA. Following the introduction of each construct into the mouse germline, a total of 19 transgenic lines were obtained, of which 16 expressed the AP reporter in the retina. Unexpectedly, at least 14 of the 16 expressing lines showed AP activity in subsets of amacrine cells, and these subsets typically differed among mouse lines injected with the same construct. Transgene expression was also found in ganglion cells in four lines and bipolar cells in seven lines. In all cases AP activity was confined to cells in the inner nuclear layer and the ganglion cell layer. The expression of Brn-3 transgenes in multiple cell types in the inner retina is reminiscent of earlier experiments in which visual pigment transgenes were found to be expressed in multiple cell types in the outer retina. Taken together, these observations suggest that anatomically and/or functionally related retinal neurons contain partially overlapping transcriptional regulatory specificities.

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.

We recorded visual evoked potentials in response to square-wave contrast-reversal checkerboards undergoing a transition in the mean contrast level. Checkerboards were modulated at 4.22 Hz (8.45-Hz reversal rate). After each set of 16 cycles of reversals, stimulus contrast abruptly switched between a “high” contrast level (0.06 to 1.0) to a “low” contrast level (0.03 to 0.5). Higher contrasts attenuated responses to lower contrasts by up to a factor of 2 during the period immediately following the contrast change. Contrast-response functions derived from the initial second following a conditioning contrast shifted by a factor of 2–4 along the contrast axis. For low-contrast stimuli, response phase was an advancing function of the contrast level in the immediately preceding second. For high-contrast stimuli, response phase was independent of the prior contrast history. Steady stimulation for periods as long as 1 min produced only minor effects on response amplitude, and no detectable effects on response phase. These observations delineate the dynamics of a contrast gain control in human vision.

The perception of dynamic random-dot stereograms (RDS) depends on the physiological fusion of horizontally disparate binocular visual input. Thus, the use of RDS offers the possibility to study selectively cortical processing of visual information in man. We investigated the influence of horizontal disparity on the scalp topography of RDS evoked brain activity in 33 healthy subjects. Stereoscopic checkerboard patterns were presented in the center or lateralized in the left or right visual field with horizontal disparities changing at temporal frequencies of six or eight depth reversals/s using different disparity values ranging from 3.5 to 28 min of arc. In 11 subjects evoked potential fields were recorded from 16 electrodes, and 21 subjects participated in 30-channel recordings with electrodes located over the parietal and occipital brain areas. Stimulation frequency-related brain activity was obtained with all disparity values; however, with large or small disparities the potential field strength decreased significantly while largest responses were obtained with intermediate disparities. Significant differences were observed in RDS evoked brain activity when central and lateralized stimulus locations were compared. With lateral stimuli (extending from the fovea to 17.1-deg eccentricity) maximal amplitudes were obtained at larger disparities than with central stimuli. In addition there were pronounced differences between brain activity evoked with stimuli presented in the left or right visual field; however, there were very similar evoked potential signals recorded from electrodes located over the left and right hemispheres. Our findings indicate that the processing of disparity information with lateralized stimuli is different from the processing in the center of the visual field. In addition, lateralized stimulation yields a significant disparity tuning mainly with stereoscopic targets occurring to the right from the fixation point (but not with stimuli to the left) suggesting a functional difference between the visual half-fields.

The effects of dopamine on luminosity-type horizontal cells have been documented in different vertebrate retinas, both in vivo and in vitro. Some of these effects may reflect direct action of dopamine onto these cells, but indirect effects mediated by presynaptic neurons cannot be ruled out. Furthermore, direct effects of dopamine on horizontal cells may affect other, postsynaptic neurons in the outer plexiform layer. To test these possibilities, we studied the effects of dopamine on photoreceptors and all types of horizontal cells in the turtle (Pseudemys scripta elegans) retina. Receptive-field properties, responsiveness to light, and time course of light responses were monitored with intracellular recordings. Dopamine at a concentration of 40 μM exerted effects with two different time courses. “Short-term” effects were fully developed after 3 min of dopamine application and reversed within 30 min of washout of the drug. “Long-term” effects were fully developed after about 7–10 min and could not be washed out during the course of our experiments. Only the “short-term” effects were studied in detail in this paper. These were expressed in a reduction of the receptive-field size of all types of horizontal cells studied; L1 and L2 luminosity types as well as Red/Green and Yellow/Blue chromaticity types. The L1 horizontal cells did not exhibit signs of reduced responsiveness to light under dopamine, while in the L2 cells and the two types of chromaticity cells responsiveness decreased. None of the rods, long-wavelength-sensitive, or medium-wavelength-sensitive cones exhibited any apparent reduction in their receptive-field sizes or responsiveness to light. The present results suggest that the “short-term” effects of dopamine are not mediated by photoreceptors and are probably due to direct action of dopamine on horizontal cells.

The retinal pigment epithelium (RPE) of the quokka wallaby, Setonix brachyurus, grows and changes throughout life. To investigate factors that determine changes in the quokka RPE, we have examined topography of this tissue in experimentally enlarged eyes. Unilateral eyelid suture was conducted at the time of normal eye opening, postnatal day (P) 110, and animals were examined at 1 or 1½ years of age. The numbers and densities of RPE cells and the extent of multinucleation were compared with those in normal animals. Eyelid suture resulted in a 9.8% and 17.4% increase in retinal area at 1 and 1½ years, respectively; a significant degree of myopia was associated with this enlargement. Cell density topography in experimental eyes was not the same as in controls. Cells from central retina were disproportionately larger in the experimental than control eyes. However, the RPE cell topography in sutured eyes was not the same as that of aged retinae of a similar size. Notably, in sutured eyes there was no development of the high or highest cell densities seen in equatorial and temporal central RPE in aged retinae, respectively. Furthermore, the degree of cell enlargement in peripheral regions was slight compared with that observed in similar-sized, aged retinae. There was no increase in RPE cell number; rather, average cell area increased accompanied by no change or a slight decrease in RPE thickness. Consequently, overall volume of cells did not change significantly. The large number of multinucleate cells normally seen in aged animals was not observed in experimentally enlarged eyes, implying that an increase in cell volume may be the trigger for multinucleation.

Isolated dopaminergic amacrine (DA) cells in mouse retina fire rhythmic, spontaneous action potentials and respond to depolarizing current with trains of low-frequency action potentials. To investigate the roles of voltage-gated ion channels in these processes, the transient A-type K+ current (IK,A) and Ca2+ current (ICa) in isolated mouse DA cells were analyzed by voltage clamp. The IK,A activated at −60 mV and inactivated rapidly. ICa activated at around −30 mV and reached a peak at 10 mV without apparent inactivation. We also extended our previous computational model of the mouse DA cell to include the new electrophysiological data. The model consisted of a membrane capacitance in parallel with eight currents: Na+ transient (INa,T), Na+ persistent (INa,P), delayed rectifier potassium (IKdr), IK,A, calcium-dependent potassium (IK,Ca), L-type Ca2+ ICa, hyperpolarization-activated cation current (Ih), and a leak current (IL). Hodgkin-Huxley type equations were used to define the voltage- and time-dependent activation and inactivation. The simulations were implemented using the neurosimulator SNNAP. The model DA cell was spontaneously active from a wide range of initial membrane potentials. The spontaneous action potentials reached 35 mV at the peak and hyperpolarized to −76 mV between spikes. The spontaneous firing frequency in the model was 6 Hz. The model DA cell responded to prolonged depolarizing current injection by increasing its spiking frequency and eventually reaching a depolarization block at membrane potentials greater than −10 mV. The most important current for determining the firing rate was IK,A. When the amplitude of IK,A was decreased, the firing rate increased. ICa and IK,Ca also affected the width of action potentials but had only minor effects on the firing rate. Ih affected the firing rate slightly but did not change the waveform of the action potentials.

Amphibian retinas are commonly used for electrophysiological studies on neural function and transduction because they share the same general properties as higher vertebrate retinas. Glycinergic synapses have been well described in amphibian retinas. However, the role of glycine transporters in the synapses is largely unknown. We studied the distribution and function of glycine transporters in the retinas from tiger salamanders, mudpuppies, and leopard frogs by immunofluorescence labeling and whole-cell recording methods. Our results indicated that GlyT1- and GlyT2-like transporters were present in Müller cells and neurons, respectively. GlyT1 labeling was present in Müller glial cells and co-localized with Glial fibrillary acidic protein (GFAP), a Müller cell marker, whereas the GlyT2 immunoreactivity was present in the somas of amacrine cells (ACs) and processes in the inner plexiform layer (IPL) and the outer plexiform layer (OPL). Because the axon processes of glycinergic interplexiform cells (IPCs) are the only source of glycine input in the OPL, GlyT2 staining revealed a spatial pattern of the axon processes of IPCs in the OPL. The function of GlyT2 in the IPCs was studied in tiger salamander retinal horizontal cells (HCs) by whole-cell gramicidin perforated recording. The results demonstrated that inhibition of GlyT2 by a specific inhibitor, amoxapine, increased a tonic glycine input to HCs. Thus, the GlyT2 transporter is responsible for uptake of synaptic glycine in the outer retina. We also compared the distribution of glycine transporters in other amphibian species: salamander, mudpuppy, and frog. The results are consistent with the general pattern that GlyT1-like transporters are present in Müller cells and GlyT2-like transporters in neurons in amphibian retinas.

During development, the projection from the lateral geniculate nucleus to striate cortex becomes segregated into monocular regions called ocular dominance columns. Prior studies in cats have suggested that experimental strabismus or alternating monocular occlusion increases the width and segregation of columns. In the squirrel monkey, strabismus has been reported to induce the formation of ocular dominance columns. However, these studies are difficult to interpret because no animal can serve as its own control and the degree of inter-individual variability among normal subjects is considerable. We have re-examined the effect of strabismus on ocular dominance columns in a large group of strabismic and normal squirrel monkeys. Five animals rendered strabismic at age one week had well-developed, widely spaced columns. Among 16 control animals, a wide spectrum of column morphology was encountered. Some control animals lacked ocular dominance columns, whereas others had columns similar to those observed in strabismic animals. Natural variation in column expression in normal squirrel monkeys, and potential uncontrolled genetic influences, made it impossible to determine if strabismus affects ocular dominance columns. It was evident however, that strabismus does not affect the binocular projection from the lateral geniculate nucleus to each CO patch in the upper layers. In strabismic monkeys, just as in normal animals, each patch received input from geniculate afferents serving both the left eye and the right eye. In addition, in strabismic monkeys, as in normal animals, patches were not aligned with ocular dominance columns.

Gamma aminobutyric acid (GABA) has been established as an important developmental signal in a number of regions of the central nervous system (CNS), including retina. Our previous studies have shown that GABAergic horizontal cells act as the initial synaptic target for developing cone photoreceptors in neonatal rabbit retina. Since intraocular injections of the GABAA receptor antagonists, picrotoxin or bicuculline, disrupt cone synaptogenesis in vivo, GABA released from horizontal cells may provide a necessary signal for cone axon growth and/or synapse formation. In the current report, we have used cultured retinal explants to examine the effects of GABAA receptor antagonists on other aspects of developing cones. These include the distribution pattern of cone cell bodies across the outer surface of the retina and the expression of GABAA receptors within both cone cell bodies and axonal processes. Peanut agglutinin (PNA), a plant lectin that specifically labels cone plasma membrane and extracellular matrix, was used to monitor cone development, and a GABAA receptor antibody against the β2/3 subunits of the protein was used to label GABAA receptors. Results showed that cones maintained in the explant culture express GABAA receptors in a temporal and spatial pattern similar to that observed in vivo, namely a low expression of receptors on cone cell bodies at postnatal day 1 (P1), peaking around P3 and diminishing by P7. Neonatal retinal explants exposed to the GABAA receptor antagonists, bicuculline (10 μM) or SR95531 (5 μM), for 24 h in culture showed disruption of the normal distribution of cone cell bodies. When GABA (100 μM) was added along with either antagonist, cone cell bodies appeared normal. Neither bicuculline nor SR95531 alone had any effect on the general morphology of other retinal layers, suggesting that these GABAA receptor antagonists at the concentrations used were not acting as nonspecific disruption agents. The effects of GABA antagonists were confined to the first week after birth with no disruption seen in P9 or adult explant cultures. These data provide a direct demonstration of the necessity for GABAergic input to cones during active synaptogenesis. As we have previously shown, GABAA receptor activation causes a substantial increase in intracellular calcium concentrations in cones and thereby could provide a mechanism by which GABA regulates cone maturation.

It is a matter of debate whether X-linked dichromacy is accompanied by enhanced achromatic processing. In the present study, we used sinusoidally modulated achromatic gratings under photopic conditions to compare the contrast sensitivity (CS) of protanopes, deuteranopes, and normal trichromats. 36 male volunteers were examined. CS was tested in static and dynamic conditions at nine different spatial frequencies. The results support the assumption that X-linked color-defective observers are at an advantage in terms of achromatic processing. Both protanopes and deuteranopes had significantly better CS than controls in both the static and the dynamic conditions. In the static condition, the advantage was observed especially at higher spatial frequencies, whereas in the dynamic condition, it was seen also at lower frequencies. The results are interpreted in terms of decreased chromatic modulation of the luminance channel and the early plasticity of the parvocellular system.

The turtle retina has been shown to have a variety of different morphological ganglion cell types as well as distinct physiological ganglion cell types. The major projection of the retina to the brain in nonmammalian vertebrates is to the optic tectum. In this study, we address the question of which retinal ganglion cell types project to the optic tectum in the turtle.

Fluorescent rhodamine-labeled microspheres were used to trace the retinal ganglion cell projection to the superficial layers of the optic tectum. The fluorescent ganglion cell somata, retrogradely marked by transport from the contralateral optic tectum, were impaled with micropipettes containing rhodamine-horseradish peroxidase solution and this dye was iontophoresed into the cells under visual control.

Most of the morphological ganglion cell types described in Golgi studies (Kolb, 1982; Kolb et al., 1988) were stained. Thus, the small cell types G1, G2, G3, G5, G6, and G7; the medium-sized types G10, G11, G12, G13, and G14; and the large-sized types G15, G16, G19, G20, and G21 project to the optic tectum in the turtle. We have added a new type, G2a, which proves to have some differences from the original G2 in branching pattern. We were unable to stain the small type G4, the medium-sized types G8 and G9, and the large cell types G17 and G18; this suggests that they might not project to the superficial layers of the dorsolateral optic tectum, at least, in the turtle.

The physiological actions of biogenic amine and amino-acid neurotransmitters are terminated by their removal from the synaptic cleft by specific high-affinity transport proteins. The members of the Na+- and Cl−-dependent neurotransmitter transporter family expressed in bovine retina and responsible for the uptake of biogenic amine and amino-acid neurotransmitters were identified using a reverse transcriptase-polymerase chain reaction-based approach. cDNA clones encoding bovine homologues of glycine (GLYT-1), γ-aminobutyric acid (GAT-1) creatine (CreaT), and orphan (NTT4) transporters were identified using this strategy. The expression pattern of mRNAs encoding these proteins in the retina was determined by in situ hybridization histochemistry GLYT-1 CreaT NTT4 and GAT-1 mRNAs were expressed in the retina by cells in the inner nuclear inner plex, iform and ganglion cell layers They were not expressed at detectable levels in the photoreceptor cells whose cell bodies are in the outer nuclear layer and are the most abundant cell type in the retina GLYT-1 mRNA was present exclusively in the proximal inner nuclear layer GAT-1 mRNA was localized to both the inner nuclear and ganglion cell layers CreaT mRNA was expressed in all cell types in the retina except photoreceptors and NTT4 mRNA was expressed by a sub subpoulation of cells in the ganglion cell laver. Elucidation of the expression pattern of these neurotransmitter transporter mRNAs in the retina provides a basis for studies of the role of glycine γ-aminobutyric acid and creatine transporters in retinal function.