Unilateral partial ablation of the superior colliculus in the hamster results in a compression of the retinotopic map onto the remaining tectal fragment. In a previous electrophysiological study (Pallas & Finlay, 1989a), we demonstrated that receptive-field properties of single tectal units (including receptive-field size) remain unchanged, despite the increased afferent/target convergence ratios in the compressed tecta. The present study was done to investigate the mechanism that produces increased convergence from retina to tectum at the population level while maintaining apparent stability of convergence at the single neuron level. We injected comparable quantities of horseradish peroxidase into the tecta of normal adult hamsters and adult hamsters that had received neonatal partial tectal ablations of varying magnitude. We then compared the area of retina backfilled from the injection and the number and density of labeled retinal ganglion cells within it to the size of the remaining tectal fragment.

As expected from earlier anatomical (Jhaveri & Schneider, 1974) and physiological (Finlay et al., 1979a; Pallas & Finlay, 1989a) studies demonstrating compression of the retinotectal projection, we found that the area of retina labeled from a single tectal injection site increases linearly with decreasing tectal fragment size. However, for fragment sizes down to 30% of normal, total number of retinal ganglion cells projecting to the injection site remains in or above the normal range. For large lesions (less than 30% of tectum remaining), total number of labeled retinal ganglion cells declines from normal, despite the fact that a larger absolute area of retina is represented on each unit of tectum under these conditions. Comparison of retinal ganglion cell density with tectal fragment size shows an initial decline with decreasing fragment size, which becomes sharper with very large lesions (small tectal fragments).

The maintenance of the normal number of retinal ganglion cells innervating each patch of tectum could be accomplished by an elimination of the tectal collaterals of some retinal ganglion cells. Our results suggest that, in addition to collateral elimination, reduction in the size of ganglion cell arbors is occurring, since the peak density of backfilled ganglion cells declines less rapidly than backfilled retinal area increases, especially for small lesions. However, arbor reduction and collateral elimination must occur in such a way that individual tectal cells represent the same amount of visual space as normal.

Thus, collateral elimination and arbor reduction are two mechanisms that operate to maintain afferent/target convergence ratios (and thus receptive-field properties) over large variations in afferent availability. This compensation may occur through an activity-dependent stabilization mechanism that does not change its selectivity even when excess afferents are available. For very large lesion sizes, receptive-field size and innervating ganglion cell number and density are not preserved, thus demonstrating a limit to the afferent/target matching mechanism. The same ontogenetic mechanisms might provide a buffer for normal variations in afferent populations, and could help to align topographic maps with differing numbers of afferents.

Direction-selective responses were recorded from neurons in the dorsal terminal nucleus (DTN) of the cat accessory optic system before and after intravitreal injections of tetrodotoxin (TTX) into the contralateral eye. After approximately 100 min, direction-selective responses driven through stimulation of the contralateral, injected eye were reduced on average by 90%, while direction-selective responses driven through stimulation of the ipsilateral, uninjected eye were not significantly reduced. By 200 min postinjection, ipsilateral direction-selective responses were either equal to or sometimes greater than control values. In the final stages of these experiments (i.e. between 390–830 min after contralateral eye injections), ipsilateral eye responses were on average 30% higher than control. The effects of retinal blockade of the contralateral eye by TTX show that input from the ipsilateral eye alone is sufficient to mediate direction-selective responses in DTN cells. These results and those observed following bicuculline eye injections reported previously (Grasse et al. 1990) demonstrate that direction-selective responses in the DTN driven through stimulation of the contralateral and ipsilateral eyes arise from independent neural mechanisms located in the retina and visual cortex, respectively. Moreover, these findings also suggest that the contralateral eye exerts an inhibitory influence over ipsilateral eye responses which is diminished by TTX injections into the contralateral eye.

Total RNA was purified from freshly isolated retinas of adult carp and injected into oocytes of Xenopus laevis (stage 5–6). Two to six days after injection, depolarizing voltage-clamp steps evoked a slowly activated outward currents as large as 3 μA. This current inactivated slowly with a single time constant (τ= 3.1 ± 0.24 S.E.M., for Vm= +30 mV). The current was inhibited by tetraethylammonium (3.8 mM for half-maximal inhibition). In the presence of Co2+ (1 mM) or barium methanesulfonate (40 mM), the current-voltage relationship shifted to slightly more depolarized values (5–10 mV); the maximal value of the current that was sensitive to Co2+ or Ba2+ treatments was only a small fraction (about 10%) of the TEA-sensitive current, and its current-voltage relationship was similar to that for uninjected oocytes. The reversal potential of the membrane current was studied with [K+]0 of 1–77 mM. For [K+]0 > 20 mM, the reversal potential changed with a slope of 63 mV (±;2 mV S.E.M.) per 10-fold change in [K+]0. The conductance was induced half-maximally at 17 mV (±;0.9 mV s.e.m.). The depolarization required for an e−fold increase in conductance was 13 mV (±;0.6 mV s.e.m.). From these results, we conclude that the injection of total RNA from carp retinas induces the formation of a membrane K+ channel in Xenopus oocytes. The channel formed has many of properties reported for the maintained outward current of goldfish horizontal and bipolar cells.

Reversible inactivation by cooling of the transcallosal projecting neurons in areas 17 and 18 of one hemisphere bring about complex changes in the spontaneous and evoked activity of neurons in the callosal receiving zone of the opposite hemisphere. These changes include increase and decreases in evoked and spotaneous activities. Overall, 90% of neurons in alyers II and III, 50% in layer IV, and 100% in layers V and VI were affected by the block of transcallosal input. The complexity of the changes was greatest in layers II and III, which are the major callosal recipient layers. The results indicate that many excitatory and inhibitory circuits are under the direct control of transcallosal fibers in the normally fuctioning brain.

The effects of stray light upon retinal pigment densitometric measurements were evaluated by adding specified amounts of stray light to fundus reflections. To allow for interpolations and extrapolations, a computer simulation was devised and validated against the empirical data. The results demonstrate that measured density is greatly decreased by increases in stray light but the time constant of pigment regeneration is little affected.

In the South African clawed-toed frog Xenopus laevis, visual experience plays a crucial role in the formation of matching binocular maps in the tectum. The ipsilateral eye's projection, relayed through the crossed isthmotectal projection, displays marked plasticity in response to altered visual input during a critical period of development. This plasticity and the events responsible for the end of the critical period are mediated by N-methyl-D-aspartate (NMDA) receptor function. We have previously reported that chronic blockade of tectal NMDA receptors with the NMDA antagonist 5-amino-phosphonovaleric acid (APV) prevents plasticity of the crossed isthmotectal projection during the critical period, while chronic treatment with NMDA restores this plasticity after the end of the critical period. These results raise the question of whether the effects on plasticity are due to changes in electrical responsiveness of the treated tissue. In this study, we have quantitatively assessed the actions of APV and NMDA on certain aspects of tectal cell activity in Xenopus during and after the critical period by recording the output of the nucleus isthmi cells that are activated by the tectum after three weeks of treatment. We have found that chronic APV treatment does not alter tectal output, as indicated by the firing of isthmotectal axons, during the critical period and that chronic NMDA treatment increases tectal output in postcritical period Xenopus. Tectal output does not differ between normal Xenopus during and after the end of the critical period.

These results indicate that the effect of APV on blocking isthmotectal plasticity is not due to a nonspecific inhibition of the segment of the retinotectal relay that activates the nucleus isthmi. The enhancement of tectal output in postcritical period Xenopus by chronic NMDA treatment may promote the effectiveness of NMDA in restoring isthmotectal plasticity after the end of the critical period, but the finding that tectal activity does not differ between normal Xenopus during and after the critical period implies that a reduction in tectal activity in not the cause of the loss of plasticity at the end of the critical period.

This study describes a phenomenon of transient expression of corticotropin releasing factor-like immunoreactivity (CRF-LI) in immature horizontal cells of the developing rat retina. These cells could be distinguished from those destined to become CRF-LI amacrine cells in the adult by their location within the outer portion of the neuroblastic layer (NBL) and by their ontogenetic pattern. Upon initial detection on postnatal day 3 (PD-3), faint CRF-LI cellular profiles were found in the outer portion of the NBL, limited to the central region of the retina. Subsequently, on PD-5, these profiles began to appear in the periphery, forming a single horizontal row along the outermost aspect of the developing inner nuclear layer (INL), concomitant with the establishment of the outer plexiform layer (OPL). The results of our birth-dating study combining immunohistochemistry and [3H]-thymidine autoradiography indicated that these cells were generated between embryonic day 14 and 18. These findings are consistent with them being horizontal cells. Between PD-7 and PD-9, CRF-LI in horizontal cells began to diminish progressively following a center-to-periphery gradient such that only sporadic, faintly immunoreactive patches of cells could be seen by the time of eye opening (PD-15). Around PD-19, it declined to levels below immunohistochemical detection. However, when rats were reared in complete darkness beginning at birth until PD-21, the period of CRF-LI expression in horizontal cells was prolonged and persisted throughout the first three postnatal weeks of development.

Pigeons were trained on four different visual discrimination tasks: (1) concept of natural stimuli (food vs. non-food object discrimination); (2) arbitrary classification of natural stimuli (pseudoconcept); (3) concept of artificial stimuli (triangles generated by computer graphics); and (4) discrimination of one pair of artificial stimuli. Then, lesions of the ectostriatum were carried out. The ectostriatal lesions impaired the arbitrary classification of natural stimuli and the concept of artificial pattern but did not impair the natural concept or the simple discrimination of fixed two stimuli. Lesions in the neostriatum did not cause deficits in any discrimination task. The birds had to learn individual stimuli for the arbitrary classification of stimuli and the stimulus generalization test after the artificial pattern concept discrimination indicated that the pigeons formed a concept more complicated than “triangle” in human language. These results suggest that the ectostriatum plays a role in task discrimination that requires much visual processing to classify stimuli.

The effects of a variable-phase synaptic modification rule on the experience-dependent development of a simulated visual cortex were investigated. Of interest were the process of optimization of the internal representation with respect to orientation, through which the weakly tuned neurons of visually inexperienced animals attain their tightly tuned adult characteristics, and the process of association by which identical stimuli presented to either eye come to evoke identical cortical representations. In its general form, the synaptic modification rule was Hebbian. However, it was not assumed that positive correlation of presynaptic and postsynaptic activity would lead to an increase in synaptic weight. The relative phase of presynaptic vs. postsynaptic activity that would effect an increase in synaptic weight was a parameter of the modification rule. When this parameter was zero, synaptic modification conformed to the standard Hebbian type. With a value of 180 deg, or the reverse-Hebb condition, negative correlation between presynaptic and postsynaptic activity led to increased synaptic weight. It was found that a synaptic modification rule of the reverse-Hebb type not only optimized the cortical representation, and associated the representations from the two eyes, but was quite stable with respect to retaining the optimized state for long periods of learning.

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is found in the superior colliculus (SC) of many mammalian species. In cat, several distinct classes of putative GABAergic neuron have been identified using antibodies directed against the neurotransmitter. It is not known whether these classes are found in other species. To study this, we examined the distribution, morphology, ultrastructure, and synaptic organization of GABA immunoreactive neurons in the SC of the Rhesus monkey (Macaca mulatta). Antibody-labeled neurons were distributed throughout the monkey SC, but were most densely concentrated within the zonal and superficial gray layers (32.5% of the total). These neurons were all small cells ranging from 6.6–16.3 μm in average diameter, and had granule, pyriform, and horizontal morphologies. Four types of labeled profile were identified in single ultrathin sections with the electron microscope. Presynaptic dendrites (PSDs) contained pleomorphic vesicles, received synaptic input from unlabeled axon terminals, and sometimes formed symmeytric synaptic contacts with postsynaptic profiles. Two subtypes were found. One type contained loose accumulations of synaptic vesicles throughout the profile and had a distinctive varicose shape. The other type contained small discrete clusters of synaptic vesicles near the site of synaptic apposition. The former were much more common. Profiles with typical axon terminal morphology were also found. These profiles usually contained numerous flattened vesicles and formed symmetric synapses with postsynaptic profiles, both dendrites and cell bodies. Some conventional dendrites and myelinated axons were also labeled. Serial ultrathin section reconstructions revealed that PSDs formed complex synaptic relationships with other elements. Retinal terminals, identified by their characteristic pale mitochondria, established synaptic contacts with both types of PSD. These PSDs also established contact with each other, providing a possible anatomical substrate for disinhibition. We conclude that the monkey SC has multiple GABAergic cell types, similar to those found in cat may represent an organization common to both mammals and some other vertebrate species. The circuitry established by these cell types may provide a mechanism for disinhibition as well as inhibition in the mammalian SC.

We have examined the retinal decussation patterns in pigmented ferrets that were either wild-type sable or heterozygous with one albino gene. Unilateral injections of horseradish peroxidase were made into the optic tract and labeled ganglion cells visualized in retinal wholemounts. In both wild-type and heterozygous ferrets, those ganglion cells in the temporal retina with the largest cell bodies projected only to the contralateral side of the brain. The total number of ipsilaterally projecting ganglion cells did not differ with the genotype of the animal. The numbers ranged from 5471–6759 cells. Unlike the cat, there is no difference in retinal decussation patterns in wild-type sable ferrets and heterozygous ferrets carrying one albino gene.

We investigated the fine structure of receptive field centers of X and Y cells of the retina and lateral geniculate nucleus of the cat using sinusoidal grating stimuli of high spatial frequency. By measuring orientation tuning and spatial-frequency tuning at multiple orientations, the two-dimensional sensitivity distribution was examined. We found that receptive-field centers typically have multiple sensitivity peaks that can be modeled as several spatially offset subunits. A subunit structure was found in both X and Y cells, with an average number of subunits per receptive-field center of approximately 2.9 in X cells and approximately 4.6 in Y cells. In X cells these subunits may correspond to individual cone bipolar inputs. In Y cells, the subunits may reflect the structure of the dendritic tree. The observation of the subunit structure of the receptive-field center, in conjunction with manipulation of the retinal wiring through pharmacological intervention, may provide a new tool for probing the circuitry of the retina.

When negotiating a narrow gap, honeybees tend to fly through the middle of the gap, balancing the distances to the boundary on either side. To investigate the basis of this “centering response,” bees were trained to fly through a tunnel on their way to a feeding site and back, while their flight trajectories were filmed from above. The wall on either side carried a visual pattern. When the patterns were stationary vertical gratings, bees tended to fly through the middle of the tunnel, i.e. along its longitudinal axis. However, when one of the gratings was in motion, bees flying in the same direction as the moving grating tended to fly closer to while bees flying in the opposite direction tended to fly closer to the stationary grating. This demonstrates, directly and unequivocally, that flying bees estimate the distances of surfaces in terms of the apparent motion of their images. A series of further experiments revealed that the distance to the gratings is gauged in terms of their apparent angular speeds, and that the visual system of the bee is capable of measuring angular speed largely independently of the spatial period, intensity profile, or contrast of the grating. Thus, the motion-sensitive mechanisms mediating range perception appear to be qualitatively different from those that mediate the well-known optomotor response in insects, or those involved in motion detection and ocular tracking in man.

GABA-induced whole-cell currents were measured in rod bipolar cells dissociated from the adult rat retina. The patch-clamp electrode contained nystatin, which made the cell membrane electrically permeable without rupture, thus retarding the rate of diffusion of Cl– ions from the patch pipette to the cell interior. The reversal potential of the GABA-induced currents was around –70 mV at an extracellular Cl-–concentration of 149 mM. We conclude that GABA generates hyperpolarizing responses in rod bipolar cells of the rat retina.

The tree shrew has a cone-dominated retina with a rod proportion of 5%, in contrast to the common mammalian pattern of rod-dominated retinae. As a first step to elucidate the rod pathway in the tree shrew retina, we have demonstrated the presence of rod bipolar cells and studied their morphology and distribution by light and electron microscopy.

Rod bipolar cells were labeled with an antiserum against the protein kinase C (PKC), a phosphorylating enzyme. Intense PKC immunoreactivity was found in perikarya, axons, and dendrites of rod bipolar cells. The cell bodies are located in the sclerad part of the inner nuclear layer, the dendrites ascend to the outer plexiform layer where they are postsynaptic to rod spherules, and an axon descends towards the inner plexiform layer (IPL). The axons branch, and terminate in the vitread third of the IPL where mammalian rod bipolar cells are known to terminate. Two amacrine cell processes are always seen as the postsynaptic elements (dyads). Dendritic and axonal arbors of rod bipolar cells are rather large, up to 100 μm in diameter. The topographical distribution of the rod bipolar cells was analyzed quantitatively in tangential sections.Their density ranges from 300 cells/mm2 in peripheral retina to 900 cells/mm2 more centrally. The distribution is rather flat with no local extremes. Consistent with the low rod proportion in tree shrew, the rod bipolar cell density is low compared to the rod-dominated cat retina for example (36,000-47,000 rod bipolar cells/mm2). Rod-to-rod bipolar cell ratios in the tree shrew retina range from smaller than 1 to about 7, and thus are also lower than in cat.