Male horseshoe crabs, Limulus polyphemus, use their eyes to locate mates day and night. We investigated their ability to detect targets of different size and contrast in a mating area of Buzzards Bay, Cape Cod, MA. We found that males can see large, high-contrast targets better than small, low-contrast ones. For targets of the same size, animals must be about 0.1 m closer to a low-contrast target to see it as well as a high-contrast one. For targets of the same contrast, animals must be approximately 0.2 m closer to a small target to see it as well as one twice as large. A decrease of 0.05 steradians in the size of the retinal image of a target can be compensated by a four-fold increase in contrast. About 60% of the animals detect black targets subtending 0.110 steradians (equivalent to an adult female viewed from about 0.56 m), while only 20% detect targets subtending 0.039 steradians. This study shows that horseshoe crabs maintain about constant contrast sensitivity under diurnal changes in light intensity in their natural environment. As a consequence of circadian and adaptive mechanisms in the retina, male horseshoe crabs can detect female-size objects about equally well day and night.

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 mammalian superior colliculus is involved in the transformation of sensory signals into orienting behaviors. Sensory and motor signals are integrated in the colliculus to produce movements of the eyes, head, and neck. While there is a considerable amount of information available on the afferent and efferent connections of the colliculus, almost nothing is known about its intrinsic circuitry, particularly that of its deepest layers. It is likely that intrinsic connections in these deeper layers of the colliculus participate in the sensory-motor transformations leading to orienting movements. In this study, we used the neuroanatomical tracer biocytin to label small groups of neurons in the deeper layers of the cat superior colliculus and examine the distribution of their axons and terminals. We found a broadly distributed network of intrinsic projections throughout the deep layers of the superior colliculus. While the majority of terminals were found in a 1–2 mm radius around the injection site, labeled terminals were found throughout the deep layers of the colliculus up to 5 mm from the injection site. In addition, these injections sometimes labeled terminals in the superficial tectum. Extensive projections were demonstrated by the more superficial injections, but few terminals were found when injections were confined to the deepest layers of the colliculus. There was no evidence of anisotropy in the distribution of terminals from injections made at different rostrocaudal or mediolateral locations; neurons located in any one region in the colliculus could potentially influence any other region. This network of intrinsic connections in the cat superior colliculus could provide a means for deeper-layer efferent neurons to associate, and to modulate or coordinate their output. Interneurons could also provide a substrate for mutual inhibition between neurons at the rostral pole of the colliculus that are active during fixation, and more caudally located neurons whose activity is associated with saccadic eye movements.

Previous authors have hypothesized that retinotopic projections may be influenced by ‘preordering’ of the axons as they grow towards their targets. In some nonmammalian species, axons are reorganized at or near the optic nerve head to establish a retinotopic order. Data are ambiguous concerning the retinotopy of the mammalian retinal nerve fiber layer and whether fibers become reorganized at the optic nerve head. We have examined this question in fetal and newborn ferrets (Mustela putorius furo) by comparing the arrangement of axons in the retinal nerve fiber layer with that in the optic nerve. Dil or DiA crystals were implanted into fixed tissue in the innermost layers of the retinal periphery, or at a location midway between the periphery and the optic nerve head. Fluorescence labelling was examined in 100–200 μm Vibratome sections, or the eyecup and nerve were photooxidized and 1–2 μm longitudinal or transverse sections were examined. Regardless of fetal age, eccentricity or quadrant of the implant site, a segregation of labelled peripheral axons from unlabelled central ones was not detected within the nerve fiber layer. Axons coursed into the nerve head along the margin of their retinal quadrant of origin, often entering the optic nerve as a radial wedge, thus preserving a rough map of retinal circumference. However, peripheral axons were in no way restricted to the peripheral (nor central) portions of the nerve head or nerve, indicating that the optic axons do not establish a map of retinal eccentricity. Our results demonstrate that (1) the nerve fiber layer is retinotopic only with respect to circumferential position and (2) optic axons are not actively reorganized to establish a retinotopic ordering at the nerve head. The present results suggest that any degree of order present within the optic nerve is a passive consequence of combining the fascicles of the retinal nerve fiber layer; optic axons are not instructed to establish, nor constrained to maintain, a retinotopic order within the optic nerve.

Occipital visual evoked potentials (VEPs) were recorded in response to low-contrast, low spatial-frequency chromatic, and achromatic gratings. Fast Fourier Transform (FFT) and time-domain analysis were used to reveal differences in harmonic content and amplitude of chromatic and achromatic response components over a wide range of temporal frequencies. The chromatic ON/OFF VHP is dominated by the fundamental component indicating that onset and offset responses are different. This type of response is typical of neurons with sustained type response characteristics. Conversely, the achromatic onset VEP contains a predominant second harmonic component in addition to the fundamental. This similarity between onset and offset responses suggests that transient mechanisms are responsible for the generation of achromatic components. Frequency analysis of VEPs elicited by phase-reversing stimuli reveals that all of the response energy is concentrated at the second harmonic of the stimulating frequency. The magnitude of the second harmonic component is maximal for achromatic stimuli and undergoes a distinct minimum for isoluminant, chromatic stimuli. This behavior indicates that under the stimulus conditions used, magnocellular neurons with transient characteristics dominate the reversal VEP.

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.

Retinas of the nocturnal geckos, Hemidactylus turcicus, Hemidactylus garnotii, and Teratoscincus scincus, were studied with microspectrophotometry and immunocytochemistry against various visual pigment epitopes to reveal UV-sensitive photoreceptors. From 6–20% of the thinner members of type C double photoreceptors, earlier believed to be blue-sensitive, were found to contain a UV-absorbing visual pigment with λmax at 363–366 nm. The pigment had bleaching and dichroic properties typical of other photoreceptor cell types of the retina. Presumptive UV-sensitive cells in retinal sections were “negatively” labeled as they did not react with either the cone-specific monoclonal antibody COS-1 or with the anti-rhodopsin polyclonal serum AO, which together labeled all of the remaining photoreceptor types (green-sensitive A singles, B doubles, and thicker members of C doubles, as well as the blue-sensitive majority of thinner members of C doubles). UV cells were moderately stained with the mAb K42–41 produced against the 5–6 loop of bovine rhodopsin, which also moderately labeled blue-sensitive cells. mAb OS-2 strongly stained all outer segments, including the UV-sensitive ones. Similarities between gecko UV visual pigments, and UV visual pigments of other vertebrates, as well as possible functional significance of these cells are discussed.

NADPH diaphorase histochemistry is commonly used to identify cells containing nitric oxide synthase (NOS), the enzyme catalyzing the production of nitric oxide from L-arginine. NADPH diaphorase activity and NOS immunostaining was demonstrated in different cells of the vertebrate retina; photoreceptors, horizontal cells, amacrine cells, ganglion cells, and Müller cells. However, the physiological role of nitric oxide (NO) in the retina has yet to be elucidated. In this study, we tested the assumption that NADPH diaphorase activity in the retinas of rabbits and rats depended on the state of visual adaptation. In the rabbit, light adaptation enhanced NADPH diaphorase activity in amacrine cells and practically eliminated it in horizontal cells. Dark adaptation induced the opposite effects; the NADPH diaphorase activity was reduced in amacrine cells and enhanced in horizontal cells. Retinas from eyes that were injected intravitreally with L-glutamate exhibited a pattern of NADPH diaphorase activity that was similar to that seen in dark-adapted retinas. In rats, the NADPH diaphorase activity of amacrine and horizontal cells exhibited adaptation dependency similar to that of the rabbit retina. But, the most pronounced effect of dark adaptation in the rat's retina was an enhancement of NADPH diaphorase activity in Müller cells, especially of the endfoot region. Assuming that NADPH diaphorase activity is a marker for NOS, these findings suggest that NO production in the mammalian retina is modulated by the level of ambient illumination and support the notion that NO plays a physiological role in the retina.

We have investigated the role of calcium in the regulation of pigment granule migration in photoreceptors of the semi-terrestrial crab, Sesarma cinereum. Isolated crab eyes (eyecup plus eyestalk) were maintained in crustacean Ringer either prepared normally or calcium-free plus 50 mM EGTA. Pigment granule movement was indirectly observed by monitoring reflectance from the eye during light stimuli using intracellular optical physiological techniques. Electroretinograms (ERGs) were also measured during light stimuli. EGTA treatment caused gradual loss of centripetal migration of pigment granules (normally leading to pupillary closure), and photoreceptors eventually became locked in the open-pupil, dark-adapted state despite repeated stimuli. In contrast, ERG responses continued throughout EGTA treatment, although the size and shape ofthe response was altered. Normal ERG responses and pigment granule movements returned after replacing EGTA-Ringer with normal-calcium medium. These results suggest that centripetal migration of pigment granules in crustacean photoreceptors requires calcium.

The isthmo-optic nucleus (ION) is the main source of efferents to the retina in birds. Isthmo-optic neurons project in topographical order on amacrine cells in the ventral parts of the retina, and a subclass of these known as proprioretinal neurons project onto the dorsal retina. We propose that, through the intermediary of the amacrine target cells, activity in the isthmo-optic pathway excites ganglion cells locally in the ventral retina but inhibits those in dorsal regions. This circuit would thereby mediate centrifugally controlled switches in attention between the dorsal retina, involved in feeding, and the more ventral parts, involved in scanning for predators. This hypothesis accounts for a wide range of disparate data from behavior, comparative anatomy, endocrinology, hodology, and neurophysiology.

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.

A model of contrast discrimination performance in human observers is developed and then extended to cover effects on performance of anticholinergic drugs. It is shown that it is necessary to assume that neural noise increases at high spatial frequencies in order to provide a satisfactory model of variations in discrimination performance with spatial frequency. The model results are compared with the results of empirical studies in which the effects of the muscarinic antagonist scopolamine (hyoscine) on contrast discrimination performance in human observers are examined. The purpose of the pharmacological work is to test the hypothesis that the differential contrast gain found psychophysically at different spatial frequencies might reflect differential facilitation by extrinsic cholinergic neurons. Contrast discrimination and contrast increment detection are found to be impaired by scopolamine in a manner that depends on both spatial frequency and base contrast. By comparing the empirical data with the predictions of the model, it is concluded that contrast constancy may reflect differential cholinergic modulation.

lbotenic acid lesions were made in four macaque monkeys in a region of cortical area V4 that corresponds to the lower quadrant of one hemifield. For visual testing, fixation locus was monitoredwith scleral search coils and controlled behaviorally to place test stimuli either in the lesionedquadrant or in a control location in the opposite hemifield. Some basic visual capacities were slightly altered by the lesions; there was a two-fold reduction of luminance contrast sensitivity as well as red-green chromatic contrast sensitivity, both tested with stationary gratings. On the other hand, little or no loss was found when contrast sensitivity for detection or direction discrimination was tested with 10–Hz drifting gratings nor was there a reliable change in visual acuity. Hue and luminance matching were tested with a spatially more complex matching-to-sample task, but monkeys could not learn this task in the visual field locus of a V4 lesion. If previously trained at this locus, performance was not affected by the lesion. In contrast to the small effects on basic visual capabilities, performance on two form discrimination tasks was devastated by V4 lesions. The first involved discriminating the orientation of colinear groups of dots on a background of randomly placed dots. The second involved discriminating the orientation of a group of three line segments surrounded by differently oriented line segments. Some selectivity of the deficitsfor form discrimination was shown by the lack of an effect of the lesions on a global motion discrimination. These results show that while V4 lesions cause only slight disruptions of basic visual capacities, they profoundly disrupt form discriminations.

Electrical stimulation in pigeons of the input from the medial subdivision of the nucleus of Edinger-Westphal (EWM) to the choroidal neurons of the ipsilateral ciliary ganglion, which themselves have input to the choroidal blood vessels of the ipsilateral eye, increases choroidal blood flow (ChBF). Since the EWM receives input from the contralateral suprachiasmatic nucleus (SCN), which in turn receives contralateral retinal input, the present study sought to determine if activation of the SCN by microstimulation or by retinal illumination of the contralateral eye would also yield increases in ChBF in that same eye. Using laser Doppler flowmetry (LDF) to measure ChBF, we found that electrical activation of the contralateral SCN by 100-Hz anodal pulse trains yielded increases in ChBF that were stimulus related and proportional to the stimulating current. These increases in ChBF elicited by the SCN stimulation were accompanied by increases in choroidal volume (vasodilation), but not by increases in systemic blood pressure. Furthermore, the increases could be blocked reversibly by lidocaine injection into the EWM. These results suggest that the increases in ChBF in the eye contralateral to the SCN stimulation were specifically mediated by the SCN-EWM pathway. Retinal illumination with a fiber optic light source was also found to increase ChBF in the illuminated eye, and these effects too could be blocked reversibly with lidocaine injection into the EWM or permanently by the EWM lesion. Control studies confirmed that the light-elicited increases were mediated by increases in choroidal volume (i.e. vasodilation), were not accompanied by systemic blood pressure increases, and were not artifactually generated by transocular illumination of the LDF probe. Thus, the SCN-EWM circuit may be involved in regulating ChBF in response to the level of retinal illumination and/or the visual patterns falling on the retina.

To further characterize the properties of retinal horizontal cell electrical synapses, we have studied the gating characteristics of gap junctions between cone-driven horizontal cells from the hybrid striped bass retina using double whole-cell voltage-clamp techniques. In a total of 105 cell pairs, the macroscopic conductance ranged from 0.4–100 nS with most cell pairs exhibiting junctional conductances between 10 and 30 nS. The junctional current-voltage relationship showed that peak or instantaneous currents (Iinst) were linear within the Vj range of ±100 mV and that steady-state junctional currents (Iss) exhibited rectification with increasing voltage beginning around ±30–40 mV Vj. The normalized junctional current-voltage relationship was well fit by a two-state Boltzmann distribution, with an effective gating charge of 1.9 charges/channel, a half-maximal voltage of approximately ±55 mV, and a normalized residual conductance of 0.28. The decay of junctional current followed a single exponential time course with the time constant decreasing with increasing Vj. Recovery of junctional current from voltage-dependent inactivation takes about 1 s following a pulse of 80 mV, and is about five times slower than the inactivation time course at the same Vj. Single-channel analysis showed that the unitary conductance of junctional channels is 50–70 pS. The overall open probability decreased in a voltage-dependent manner. Both the mean channel open time and the frequency of channel opening decreased, while the channel closure time increased. The ratio of closed time/total recording time significantly increased as Vj increased. Increased Vj reduced the number of events at all levels and shifted the unitary conductance to a lower level. Kinetic analysis of channel open duration showed that the distribution of channel open times was best fit by two exponentials and increased Vj significantly reduced the slower time constant. These results indicate that bass retina horizontal cells exhibit voltage-dependent inactivation of macroscopic junctional current. The inactivation occurs at the single-channel level mainly by increasing the rate of closure of voltage-sensitive channels.

The light-driven pupil mechanism, consisting of an assembly of mobile pigment granules inside the photoreceptor cells, has been investigated by in vivo reflection microspectrophotometry in wild type (WT) Drosophila and in the photoreceptor mutants inaC and trp. The pupillary response of a dark-adapted WT eye to a step in light is a monophasic reflectance increase reaching a plateau after ca. 15-s light adaptation. This reflectance change is due to photoreceptor pigment granules that accumulate near the tips of the rhabdomeres under light adaptation and that are withdrawn towards the periphery in the dark (Franceschini & Kirschfeld, 1976). The step response of the pupil mechanism of inaC is triphasic. Strikingly, the reflectance level at light onset is distinctly higher than that in WT, due to a partly aggregated state of the photoreceptor pigment granules near the rhabdomere tips that persists in the dark-adapted state, in line with direct calcium measurements of Peretz et al. (1994b). The step response of the pupil mechanism of inaC is slightly elevated compared to that of WT. The step response in trp is a transient, biphasic reflectance change, approximating a log normal function. This function is also a good approximation of the pulse response in WT and inaC. The intensity range of pupillary sensitivity is about 4 log unit. The range of inaC compared to that of WT is slightly (≈0.5 log unit) shifted towards lower intensities, but that in trp is strongly shifted to higher intensities (≈2.5 log unit). The results can be interpreted with the present knowledge of the primary steps in fly phototransduction and the hypothesis that the local intracellular calcium concentration determines the position of the pigment granules, and hence are in line with the notion that the pupil can be used as a qualitative Ca2+ probe.

Previous studies report that background luminance flicker, which is asynchronous with signal averaging, reduces the amplitude and increases the latency of the pattern-onset visual evoked potential (VEP). This effect has been attributed to saturation of the magnocellular (m-) pathway by the flicker stimulus. In the current study, we evaluate this hypothesis and further characterize this effect. We found that flicker had similar effects on the pattern-onset and pattern-reversal VEP, suggesting that the reversal and onset responses have similar generators. Chromatic flicker decreased latency of the chromatic VEP whereas luminance flicker increased peak latency to luminance targets. This result indicates that luminance flicker saturates a rapidly conducting m-pathway whereas chromatic flicker saturates a more slowly conducting parvocellular (p-) pathway. Finally, evoked potentials to chromatic and luminance stimuli were recorded from 34 electrodes over the scalp in the presence of static and asynchronously modulated backgrounds. An equivalent dipole model was used to assess occipital, parietal, and temporal lobe components of the surface response topography. Results showed that chromatic flicker reduced activity to a greater extent in the ventral visual pathway whereas luminance flicker reduced activity to a greater extent in the dorsal visual pathway to parietal lobe. We conclude that the VEP to isoluminant color and luminance stimuli contains both m- and p-pathway components. Asynchronous flicker can be used to selectively reduce the contribution of these pathways to the surface recorded VEP. Our results provide evidence of parallel pathways in the human visual system, with a dorsal luminance channel projecting predominantly to the posterior parietal lobe and a ventral color channel projecting predominantly to inferior temporal lobe.

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.

This paper describes changes in the appearance and distribution of microglia in postnatal cat retina as demonstrated by a new antibody, H386F. This fractionated IgM antibody was created via an intrasplenic immunization of a single BALB/C mouse with about 2–3 × 105 large, whole cells isolated from 46 minced cat retinae. To confirm that the labeled cells are microglia, the staining properties of H386F were compared with those of four commercially available antibodies, OX-33, OX-41, OX-42, and ED-1, that have been used by others to distinguish between microglia and other cells in rat brain. These experiments show that H386F is the only antibody of the five to label only microglia in both the cat retina and hippocampus.

We have studied the presence and distribution of α-amino-3–hydroxy-5–methyl-4–isoxazolepropionate (AMPA)-selective glutamate receptor subunits (GluR 1, 2, 3, and 4) in the adult cat visual cortical areas 17, 18, 19, and the lateral suprasylvian areas (LSA). Reverse transcription-polymerase chain reaction (RT-PCR) amplification indicated that the genes encoding GluR 1, 2, 3, and 4 are expressed in these areas and Western blot analysis revealed that the size of the corresponding peptides is similar to those described in the rat brain. In situ hybridization (ISH) using digoxigenin-labeled riboprobes showed that mRNAs coding for GluR1 and GluR3 were located in cells in all layers of the areas examined and also in the underlying white matter. GluR1 mRNA was relatively abundant throughout layers II–VI while GluR3 mRNA revealed a more laminated pattern of expression, preferentially labeling cells in layers II, III, V, and VI. The distribution of AMPA-selective receptor subunit peptides was studied by immunohistochemistry using subunit specific antibodies and found to be consistent with ISH results. In addition, we observed that most of the cells strongly labeled by the anti-GluR1 antibody were non-pyramidal neurons and that intense GluR2/3 immunoreactivity was seen preferentially in pyramidal neurons. Interestingly, double-labeling experiments indicated that neurons expressing γ-aminobutyric acid (GABA) as well as the GluR1 subunit were particularly abundant in deeper layers. The GluR4 peptide was predominantly found in a relatively low number of layer III and layer V neurons with either pyramidal or non-pyramidal morphology. Finally, the distribution of neurons expressing the various receptor subunits was similar in all the visual cortical areas studied. These findings indicate a high expression of GluR1–3 subunits in the cat visual cortex and that GluR1 and GluR2/3 subunits are particularly abundant in non-pyramidal and pyramidal neurons, respectively. In addition, the results described here provide a reference for future studies dealing with the effect of visual deprivation on the expression of this receptor type.