In vitro mutagenesis and germline transformation were used to create a Drosophila mutant, ΔAsn20 lacking the N-linked glycosylation site near the amino terminus of the major rhodopsin (Asn20-Gly-Ser changed to Ile-Gly-Ser). Low opsin protein levels are detected in ΔAsn20 photoreceptors. Electroretinogram responses of mutant flies show that the residual rhodopsin found in this mutant is capable of initiating phototransduction. The organization of rhabdomeres, the photoreceptor organelle containing nearly all of the rhodopsin, is aberrant in the ΔAsn20 mutant and undergoes age-dependent deterioration. These results establish that an N-linked glycosylation site, and likely glycosylation itself, plays a critical role in the maturation of Drosophila rhodopsin.

Class A psychophysical observations are based on the linking hypothesis that perceptually distinguishable stimuli must correspond to different brain events. Class B observations are related to the appearance of stimuli not their discriminability. There is no clear linking hypothesis underlying Class B observations, but they are necessary for studying the effects of context on appearance, including a large class of phenomena known as “illusions.” Class B observations are necessarily measures of observer bias (Fechner’s “constant error”) as opposed to Class A measures of sensitivity (Fechner’s “variable error”). It is therefore important that Class B observations distinguish between response biases, decisional biases, and perceptual biases. This review argues that the commonly used method of single stimuli fails to do this, and that multiple-alternative forced choice (mAFC) methods can do a better job, particularly if combined with a roving pedestal.

GABA is a major inhibitory neurotransmitter in the mammalian retina and it acts at many different sites via a variety of postsynaptic receptors. These include GABAA receptors and bicuculline-resistant GABAC receptors. The release of acetylcholine (ACh) is inhibited by GABA and strongly potentiated by GABA antagonists. In addition, GABA appears to mediate the null inhibition which is responsible for the mechanism of directional selectivity in certain ganglion cells. We have used these two well-known examples of GABA inhibition to compare three GABA antagonists and assess the contributions of GABAA and GABAC receptors. All three GABA antagonists stimulated ACh release by as much as ten-fold. By this measure, the ED50s for SR-95531, bicuculline, and picrotoxin were 0.8, 7.0, and 14 μM, respectively. Muscimol, a potent GABAA agonist, blocked the effects of SR-95531 and bicuculline, but not picrotoxin. This indicates that SR-95531 and bicuculline are competitive antagonists at the GABAA receptor, while picrotoxin blocks GABAA responses by acting at a different, nonreceptor site such as the chloride channel. In the presence of a saturating dose of SR-95531 to completely block GABAA receptors, picrotoxin caused a further increase in the release of ACh. This indicates that picrotoxin potentiates ACh release by a mechanism in addition to the block of GABAA responses, possibly by also blocking GABAC receptors, which have been associated with bipolar cells. All three GABA antagonists abolished directionally selective responses from ON/OFF directional-selective (DS) ganglion cells. In this system, the ED50s for SR-95531, bicuculline, and picrotoxin were 0.7 μM, 8 μM, and 94.6 μM, respectively. The results with SR-95531 and bicuculline indicate that GABAA receptors mediate the inhibition responsible for directional selectivity. The addition of picrotoxin to a high dose of SR-95531 caused no further increase in firing rate. The comparatively high dose required for picrotoxin also suggests that GABAC receptors do not contribute to directional selectivity. This in turn suggests that feedforward GABAA inhibition, as opposed to feedback at bipolar terminals, is responsible for the null inhibition underlying directional selectivity.

The aim of the study was to determine whether memantine could slow down the changes seen in the rabbit and rat retina following ischemia/reperfusion. A “suction cup procedure,” which raises the intraocular pressure, was used to give an ischemic insult to the rabbit retina. The electroretinogram was recorded before ischemia and after 2 days of reperfusion. Memantine or saline (10 μl) was injected into the eye before ischemia. Immunohistochemistry was used to study the effect of ischemia/reperfusion on the GABA, ChAT, and αPKC immunoreactivities. Ischemia/reperfusion injury to the rat retina was induced by raising the intraocular pressure above the systolic blood pressure for 60 min, followed by reperfusion of 3–14 days. Memantine (5 mg/kg) or saline was injected i.p. at the onset of ischemia or reperfusion. Immunohistochemistry was used to study the effect of ischemia/reperfusion on the ChAT, αPKC, and Thy-1 immunoreactivities. In addition, morphometric analysis was carried out to determine the effects of ischemia/reperfusion on the thickness of the retina. Ischemia for 75 min caused a change in the nature of the normal GABA and ChAT immunoreactivities in the rabbit retina and a reduction in the b-wave of the electroretinogram. When memantine was injected into the vitreous humour at the onset of an ischemic insult, the changes in the GABA and ChAT immunoreactivities were reduced and the recovery of the reduced b-wave of the electroretinogram after 2 days reperfusion was enhanced significantly. Ischemia for 60 min followed by 3 days reperfusion showed a clear change in ChAT immunoreactivity in the rat retina. The Thy-1 immunoreactivity was only clearly altered after a reperfusion period of 7 days. Moreover, a measurable change in the thickness of the inner retinal layers was detected after 14 days of reperfusion. When given at the onset of ischemia, memantine counteracted the effect of ischemia/reperfusion to varying degrees. However, when memantine was given at the onset of the reperfusion this was not the case. The combined data show that a single injection of memantine given i.p. or intravitreally will protect the retina from a subsequent ischemic insult.

In this study, we investigated the potency of dopamine for being an intrinsic signal for cyclic events in the fish retina. Dopaminergic activity was measured during different light/dark cycles, during continuous darkness, and during short-term light and dark adaptation within 1 h. During a 12-h light/12-h dark cycle, the total content of endogenous dopamine was high during the dark phase and low during the light phase. The potassium-induced release of endogenous dopamine followed a parallel time course. The concentration of the dopamine breakdown product 3,4-dihydroxyphenylacetic acid (DOPAC), which reflects the endogenous dopaminergic activity, was high during the light phase and low during the dark phase. Similar alterations occurred in accelerated 6-h light/6-h dark cycles, again indicating a strong coupling of dopaminergic activity with light. The cyclic alterations in the total endogenous dopamine content persisted during continuous darkness after an entrainment of the fish to a 12-h light/12-h dark cycle. Although the magnitude of the change was weaker, changes in dopamine content, potassium-induced dopamine release, and DOPAC were also measured during 1 h of light or dark adaptation. During a 1-h period of dark adaptation, the total content of dopamine and the potassium-induced release of endogenous dopamine increased, while DOPAC values decreased. These values changed in the opposite direction during 1 h of light adaptation.

Our findings strongly suggest that dopamine is the intrinsic signal for light during both the light and dark phases and during short-term adaptation. Light seems to be the major trigger for dopaminergic activity within the fish retina. An endogenous clock might participate in the control of the dopaminergic activity. This has to be, however, confirmed by further investigations.

Electroretinography provides a useful noninvasive approach to evaluate cone pathway activity. Despite wide application of the cone ERG to characterize retinal function in transgenic mice and mouse models of human hereditary retinal disease, the cellular origins of the mouse cone ERG have not been well defined. Here, we address this issue using a pharmacological approach that has been previously applied to other species. Agents that block receptor activation at well-defined retinal loci were dissolved in saline and injected into the vitreous of anesthetized adult BALBc/ByJ mice; cone ERGs were recorded 1–2 h later. Analysis of the resulting waveforms indicated that the mouse cone ERG includes a cornea-negative component that is derived from the activity of cone photoreceptors and retinal glial (Müller) cells. Similar to other species, activity of cone depolarizing bipolar cells contributes a large amplitude cornea-positive potential to the mouse cone ERG. In contrast to primate but similar to rat, the mouse cone ERG includes only a small contribution from hyperpolarizing bipolar cell activity. The inner retina appears to contribute to both the a- and b-waves of the mouse cone ERG. These results provide a foundation for interpreting changes in the waveform of the mouse cone ERG that may be observed following genetic alteration or other experimental treatment.

Humans discriminate approaching objects from receding ones shortly after birth, and optic flow associated with self-motion may activate distinctive brain networks, including the human MT+ complex. We sought evidence for evoked brain activity that distinguished radial motion from other optic flow patterns, such as translation or rotation by recording steady-state visual evoked potentials (ssVEPs), in both adults and 4–6 month-old infants to direction-reversing optic flow patterns. In adults, radial flow evoked distinctive brain responses in both the time and frequency domains. Differences between expansion/contraction and both translation and rotation were especially strong in lateral channels (PO7 and PO8), and there was an asymmetry between responses to expansion and contraction. In contrast, infants' evoked response waveforms to all flow types were equivalent, and showed no evidence of the expansion/contraction asymmetry. Infants' responses were largest and most reliable for the translation patterns in which all dots moved in the same direction. This pattern of response is consistent with an account in which motion processing systems detecting locally uniform motion develop earlier than do systems specializing in complex, globally non-uniform patterns of motion, and with evidence suggesting that motion processing undergoes prolonged postnatal development.

The superfused retinal slice preparation was used to examine the morphology and glutamate-activated whole-cell currents of rabbit bipolar cells. There were six morphologically distinct types of cone bipolar cells and a rod bipolar cell that had axon terminals stratifying in stratum 3 to 5 of sublamina-b. All of these bipolar cell types exhibited an outward current in response to the application of the metabotropic glutamate receptor, mGluR6, agonist AP-4 (APB), and had I/V curves indicative of membrane channel closure. Conversely, there were no currents activated during the application of kainate, the AMPA/kainate receptor agonist. These data demonstrate they were on-bipolar cells. In addition, there were six morphologically distinct cone bipolar cells that stratified in sublamina-a. Every cell with axonal arborizations in stratum 1 and 2 exhibited an inward current when the ionotropic glutamate receptor agonist kainate was applied. This current was blocked by application of the AMPA/kainate receptor antagonist CNQX. These cells also decreased their membrane resistance in response to kainate, a characteristic of the opening of channels within the plasma membrane. Without exception, no cells stratifying in sublamina-a responded to the mGluR6 agonist AP-4, further identifying them as off-bipolar cells.

The first (V-I) and second (V-II) visual areas of primates contain three types of anatomical segregations of neurons as parts of hypothesized “P-B” or “color”, “P-I” or “form,” and “M” or “motion” processing channels. These channels remain distinct in relays of P-B and P-I information to the inferior temporal lobe via V-II and dorsolateral visual cortex for object recognition, and “M” information to posterior parietal cortex via the middle temporal visual area (MT) for visual tracking and attention. The present anatomical experiments demonstrate another channel where “P-B” modules in V-I and “P-B” and “M” modules in V-II merge in the projections to the dorsomedial visual area (DM), which relays to MT and posterior parietal cortex. This integrative area may function in unifying our perception of the visual world, and may allow “color” as well as “motion” to play a role in visual tracking and attention.

Bipolar cells were recorded in rat retinal slices to study the distribution of hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels. Patch-clamp whole cell measurements were combined with intracellular filling and recorded cells were morphologically identified. HCN channel isoforms HCN1-4 are differentially expressed in bipolar cells. Each bipolar cell type has a characteristic inventory of HCN channels. The combination of HCN channel currents and other voltage-gated currents can be used as a kind of “finger print” to electrophysiologically identify and classify bipolar cell types. Using this approach of combined electrophysiological and morphological classification we could identify a new ON-cone bipolar cell type.

A mathematical model is presented that obeys a strong form of Weber's law – over a range of adapting and stimulus intensities, equal contrast stimuli evoke identical responses. To account for the strong Weber's law, the adaptive stage in the proposed model employs a “delayed” reverse reaction along with a power-law input. It is suggested that this Weber's law mechanism is responsible for a slow, voltage-uncorrelated component of adaptation in the vertebrate photoreceptor. A plausible biochemical mechanism is the G-protein cycle with phosphorylation of photoactivated photopigment (and binding of arrestin to the phosphorylated photopigment) as the adaptive process. In an Appendix, features of the general model and implications of a specific biochemical model are examined by computer simulation.

Goldfish (Carassius auratus) were trained to discriminate triangles and squares using a two choice procedure. In the first experiment, three goldfish were trained with food reward on a black outline triangle on a white background, while a black outline square was shown for comparison. In transfer tests, a Kanizsa triangle and a Kanizsa square were presented, perceived by humans as an illusory triangle- or square-shaped surface of slightly higher brightness than the background. The choice behavior in this situation indicates that goldfish are able to discriminate between both figures in almost the same way as in the training situation. In control experiments goldfish did not discriminate between shapes in which humans do not perceive the illusion. A series of generalization experiments was performed indicating the similarity between the tested shapes and the training triangle. From all these findings we conclude that goldfish are able to perceive an illusory triangle or square within the Kanizsa figures. In a second experiment, four goldfish were trained on a white outline triangle versus a white outline square, both on black background with white diagonal lines. In transfer tests in which the shapes were replaced by gaps within the white diagonal lines, goldfish were clearly able to discriminate between the two patterns based on the illusory contours. This was not the case in tranfer tests with phase shifted abutting lines.

The objective of this study was to determine when the retina, lateral geniculate nucleus (LGN), and striate cortex first send out axons, and first connect with each other, during embryonic development in the ferret. Specifically, we were interested in the timing relationship between axon outgrowth and known temporal patterns of neurogenesis in the LGN and striate cortex. Ferrets (Mustela putorius furo) were selected for study because of their immature developmental state in late gestation and relatively large litters.

We examined axon outgrowth from the retina, and anlagen of presumptive LGN and striate cortex between embryonic day 21–30 (E21–E30) using in situ inoculations of two fluorescent lipophilic dyes, Dil and DiA. Dil inoculations were made into the cortex and contralateral thalamus, and DiA inoculations were made into the contralateral eye. Retinal axon termination zones in the diencephalon following the DiA inoculations were used to validate the location of the LGN.

Visual cortex and LGN neurogenesis begins at E20 in ferrets. No axon outgrowth could be documented from retina or anlagen of striate cortex and LGN until E24. At E24 some retinal axons reach and cross the chiasm, cortical axons extend some distance within the cortical radiations, and thalamic axons are within the internal capsule. Retinogeniculate, geniculocortical, and corticogeniculate axons extend to their target structures by E27, as evidenced by retrograde labeling in cells of origin.

These data suggest that in the ferret retina, and developing LGN and striate cortex, (1) axon outgrowth from each visual area begins within 24-h of each other, after neurogenesis has begun at the source but before it is complete in the target; (2) axons may be generated before parent cell bodies have completed migration; and (3) arriving axons are in a position to influence target structures almost from their inception.

The phosphorylation of photoexcited rhodopsin (Rho*) is thought to inactivate this receptor by inhibiting its interaction with the GTP-binding protein transducin (Gt). Here we report that the time course of phosphorylation-dephosphorylation after bright illumination of intact rod outer and inner segments (ROS-RIS) incubated in 33Pi can be altered if the ROS-RIS are first exposed to levels of dim illumination that cause light adaptation in these ROS-RIS. The dephosphorylation of >107 phosphorylated rhodopsin molecules/ROS following a bright flash can be blocked by prior dim continuous illumination (generating 103 Rho*/ROS/s) that cumulatively bleaches ≍ 105 rhodopsin molecules/ROS. The phenomenon has not been previously noted because these low levels of light are emitted as a result of Cerenkov radiation from the 32P isotope that is usually employed to monitor rhodopsin phosphorylation. The inhibition of rhodopsin dephosphorylation by dim conditioning illumination is observed in intact ROS-RIS but is lost when ROS-RIS are electropermeabilized or fragmented.

In the present work, we explore the perceptual bases of infants' spontaneous looking preferences among isoluminant chromatic stimuli (Bornstein, 1975). Three experiments were conducted. In Experiment 1, adult subjects made brightness matches between a white standard and each of six isoluminant chromatic stimuli. The classic variations of brightness with chromaticity were found. In Experiment 2, 12-week-old infants' spontaneous looking preferences were measured for white lights of different luminances. Preference increased with increasing luminance, suggesting that brightness differences are sufficient to create looking preferences among isochromatic stimuli. In Experiment 3, infants' preferences were tested for each of the six chromatic stimuli paired against white, at both isoluminance and (adult) isobrightness. All chromatic stimuli were preferred to white, and the pattern of preferences was similar for both isoluminance and isobrightness conditions. It is concluded that hue and/or saturation, rather than brightness, control infants' spontaneous looking preferences among chromatic stimuli.

Gap junctions are commonplace in retina, often between cells of the same morphological type, but sometimes linking different cell types. The strength of coupling between cells derives from the properties of the connexins, but also is regulated by the intracellular environment of each cell. We measured the relative coupling of two different gap junctions made by AII amacrine cells of the rabbit retina. Permeability to the tracer Neurobiotin was measured at different concentrations of the neuromodulators dopamine, nitric oxide, or cyclic adenosine monophosphate (cAMP) analogs. Diffusion coefficients were calculated separately for the gap junctions between pairs of AII amacrine cells and for those connecting AII amacrine cells with ON cone bipolar cells. Increased dopamine caused diffusion rates to decline more rapidly across the AII–AII gap junctions than across the AII–bipolar cell gap junctions. The rate of decline at these sites was well fit by a model proposing that dopamine modulates two independent gates in AII–AII channels, but only a single gate on the AII side of the AII–bipolar channel. However, a membrane-permeant cAMP agonist modulated both types of channel equally. Therefore, the major regulator of channel closure in this network is the local cAMP concentration within each cell, as regulated by dopamine, rather than different cAMP sensitivity of their respective gates. In contrast, nitric oxide preferentially reduced AII–bipolar cell permeabilities. Coupling from AII amacrine cells to the different bipolar cell subtypes was differentially affected by dopamine, indicating that light adaptation acting via dopamine release alters network coupling properties in multiple ways.

Amblyopia is characterized by losses in a variety of aspects of spatial vision, such as acuity and contrast sensitivity. Our goal was to learn whether those basic spatial deficits lead to impaired global perceptual processing in strabismic and anisometropic amblyopia. This question is unresolved by the current human psychophysical literature. We studied contour integration and contrast sensitivity in amblyopic monkeys. We found deficient contour integration in anisometropic as well as strabismic amblyopic monkeys. Some animals showed poor contour integration in the fellow eye as well as in the amblyopic eye. Orientation jitter of the elements in the contour systematically decreased contour-detection ability for control and fellow eyes, but had less effect on amblyopic eyes. The deficits were not clearly related to basic losses in contrast sensitivity and acuity for either type of amblyopia. We conclude that abnormal contour integration in amblyopes reflects disruption of mechanisms that are different from those that determine acuity and contrast sensitivity, and are likely to be central to V1.

The morphology and distribution of neurons labeled specifically by the lectin, Vicia villosa (VVA), were examined in striate cortex of adult macaque monkeys. Following incubation with VVA conjugated to histochemical markers, fine punctate reaction product appears to cover the surface of the soma and proximal dendrites of a population of cortical neurons. Although a small number of VVA-labeled cells are located in layers 2, 3A, 5, and 6, approximately 75% are located in a strip of cortex overlying layers 3B through 4Ca. Layers 1 and 4Cβ are virtually devoid of labeled cells. The morphology of labeled cells varies throughout the layers. In the supragranular layers, the labeled cells generally display a round or multipolar soma with a small number of radially disposed dendrites. In deeper layers, labeled cells are multipolar or horizontal, and their proximal dendrites are often more densely labeled. There is no clear correlation between the distribution of labeled cells and the pattern of cytochrome oxidase staining in supragranular layers.

Double labeling of single sections for VVA and for GABA (gamma-aminobutyric acid) immunoreactivity revealed that most VVA-labeled cells are also immunoreactive for GABA. The double-labeled cells comprise approximately 30% of all GABA immunoreactive cells. Soma size analysis of double-labeled cells shows that medium-to-large GABA cells in each layer are labeled by VVA. The soma size, laminar distribution, and morphology of the VVA-labeled GABA cells suggest that they include the large basket cells originally observed in Golgi preparations.

A 6.4–kb sequence upstream of the mouse blue visual pigment gene has been assayed in transgenic mice for the ability to direct cell-type-specific expression of a linked β-galactosidase (lacZ) reporter. The construct is expressed specifically in cone photoreceptors in three independent lines. Transgene expression is found in the developing retina on the first postnatal day, increases rapidly in subsequent days, and persists through adulthood. A gradient of transgene expression is observed across the retina, with the transgene-expressing cones found almost exclusively in the lower retina and rarely in the upper retina, a pattern that parallels the distribution of blue cones in the mouse retina. Double-labeling with anti-cone pigment antibodies shows that transgene expression is confined to blue cones. These results imply that all of the sequence elements necessary for the control of blue cone-specific expression are encoded within the 6.4–kb DNA fragment tested.

We have recorded steady-state visual evoked potentials (VEPs) from patients with vascular damage to their right brain hemispheres, some suffering from unilateral spatial neglect (n = 9), and some not (n = 7). VEPs were recorded in response to sinusoidal gratings of 0.56 cycle/deg contrast-reversed sinusoidally at temporal frequencies from 4–11 Hz. Stimuli were presented either to the left or to the right visual field, or to both. Confirming previous reports, reliable VEPs were recorded from stimuli in the left contralesional hemifield, of comparable amplitude to those of the ipsilesional hemifield and to those of both hemifields of brain damaged patients without neglect. However, analysis of apparent latency derived from phase data showed that the VEPs from the contralesional hemifield were systematically delayed by 30–40 ms compared with those of the ipsilesional hemifield, and compared with both hemifields of the nonneglect groups. This result suggests changes in neural processing in neglect patients.