The laminar structure and cellular distribution of cytochrome-oxidase (CO) reactivity in supragranular puffs of striate cortex was examined in adult macaque monkeys surviving various periods of monocular enucleation, lid suture, and retinal impulse blockage with tetrodotoxin (TTX). Enucleation and TTX produced a rapid and severe loss in the size of the CO reactive region in puffs dominated by the removed or treated eye compared to slower and less marked reductions obtained in deprived puffs of lid-sutured monkeys. In all deprived animals, the cross-sectional areas of deprived puffs decreased most rapidly in the upper layers (2 and 3A). In long-term enucleated (60 wks) and TTX-treated (4 wks) monkeys, puff area was severely reduced in layer 3B, while reactivity in layer 3B appeared partially spared in lid-sutured monkeys. The density of the CO reaction product was significantly and evenly reduced throughout deprived puffs for all of the monkeys examined; however, this decrease was less severe in adult monkeys lid-sutured for 11 wks. Although no evidence for cell loss was obtained, all three forms of visual deprivation led to lower counts of neuronal perikarya with high levels of CO reaction product in both deprived puff and interpuff areas. This effect was less marked in the deprived puffs of monkeys lid-sutured for 2.5 and 3 yrs, suggesting recovery of CO activity in some neurons. Neurons in deprived puffs and interpuffs were generally similar in size to those in nondeprived regions, although CO-reactive cells were significantly smaller in the deprived puffs of monkeys enucleated for 28.5 or 60 wks. These results indicate that the metabolic response of neuronal elements in supragranular striate cortex depends upon the nature of the visual deficit. The partial sparing of CO reactivity in deprived puffs of lid-sutured monkeys may reflect the continued transmission of certain types of visual stimuli through a closed eyelid.

The projection from striate and prestriate visual cortex to the pontine nuclei has been studied in the macaque monkey by means of anterograde tracer techniques in order to assess the contribution of anatomically and functionally distinct visual cortical areas to the cortico-ponto-cerebellar loop. No projection to the pons was found from central or paracentral visual-field representations of V1 (striate cortex) or prestriate visual areas V2, and V4. Small patches of terminal labeling occurred after injections of tracer into more peripheral parts of V1, V2 and V3, and into V3A. The terminal fields were located most dorsolaterally in the anterior to middle third of the pons and were quite restricted in their rostro-caudal extent. Injections of V5, however, yielded substantial terminal labeling, stretching longitudinally throughout almost the entire pons. This projection could be demonstrated to arise from parts of V5 receiving input from central visual-field representations of striate cortex, whereas parts of V4 receiving similarly central visual-field input had no detectable projection to the pons. Its distribution may overlap to a large extent with the termination of tecto-pontine fibers and with the termination of fibers from visual areas in the medial bank (area V6 or P0) and lateral bank (area LIP) of the intraparietal sulcus, as well as from frontal eye fields (FEF). It appears that the main information relayed to the cerebellum by the visual corticopontine projection is related to movement in the field of view.

The present study examines the differential effects of three excitotoxins, kainic acid (KA), N-methyl-D-aspartate (NMDA), and α-amino-2,3-amino-2,3-dihydro-5- methyl-3-oxo-4- isoxazolepropanoic acid (AMPA) on neurons within the genglion cell layer (GCL) of the chick retina. Two-day-old chicks were given a single, 5 μl, intravitreal injection of KA, NMDA, or AMPA at a range of doses. Following treatment with 40 nmol KA, there was a 21% loss of neurons in the GCL. At 200 nmol KA, the loss increased to 46%. Exposure to KA eliminated mainly small neurons of soma area 5–15μm2, and medium-sized ganglion cells of soma area 15–25μm2. Large ganglion cells (>25μ,2) remained unaffected. The vast majority of small cells were probably displaced amarcrine cells. At a does of 3000 nmol NMDA, no further loss of cells was evident. Exposure to 200 nmol AMPA resulted in a 30% loss of large and some medium-sized ganglion cells. In a further series of experiments, exposure to excitotoxin was followed by a retinal scratch, which eliminated retinal ganglion cells within the axotomized region. The results indicate that only a small proportion of displaced amacrine cells are destroyed by NMDA and AMPA, whereas virtually all displaced amarine cells are sensitive to KA. The findings of this study indicate the existence of subclasses of ganglion cells with specificity towards different types of excitatory amino acids (EAA).

An antiserum directed against tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of dopamine, was used to study the pigeon retina. Labeled cells were observed in both the inner nuclear layer (INL) and ganglion cell layer (GCL). Two populations of TH-immunoreactive neurons were observed in the INL. Some of these cells were 7−10 μ in diameter and gave rise to processes that arborized in three Layers of the inner plexiform layer (IPL). These cells appeared similar to the dopaminergic amacrine cells described previously (Marc, 1988). Other labeled cells in the INL were 12−20 μ in diameter and were recognizable as a previously described subpopulation of TH-immunoreactive displaced ganglion cells (Britto et al., 1988).

A population of labeled cells was observed in the GCL. Counts of these cells in two retinae revealed 5000 and 7000 cells, respectively. They ranged in size from 8−15 μ in diameter in the central retina and from 8−20 m in diameter in the peripheral retina. The density of labeled cells was highest in the central retina and red field and lowest in the retinal periphery. The difference in cell size and cell density as a function of eccentricity is characteristic of the total population of ganglion cells in the avian retina (Ehrlich, 1981; Hayes, 1982). Some of the TH-positive cells in the GCL could be classified as ganglion cells for two reasons: (1) The axons of many of the TH-positive cells in the GCL were TH-immunoreactive as well and could be followed to the optic nerve head. (2) The injection of rhodamine-labeled microspheres into the nucleus geniculatus lateralis, pars ventralis (GLv), resulted in the retrograde labeling of many of the TH-positive cells in the contralateral retina.

The radial and longitudinal distribution of retinular screening pigment in the lateral eye of the horseshoe crab Limulus polyphemus was quantified under a variety of experimental conditions. Pigment position was characterized by the center and width of the radial distribution at four levels in the ommatidium.

Under diurnal lighting, intact animals show movement of pigment granules from the periphery of the retinular cell at night towards the junction of the arhabdomeral and rhabdomeral segments of the retinular cell in the day. In constant darkness, intact animals exhibit the same circadian rhythm in pigment migration. Animals with bilaterally cut optic nerves do not receive circadian efferent input from the brain and show little pigment movement in diurnal lighting. In all of these cases, pigment was either aggregated in a band just peripheral to the rays of the rhabdom or dispersed to the periphery of the retinular cell.

When dark-adapted animals are exposed to a sudden large light increment, pigment moves inward between the rays of the rhabdom. During the day, this inward response begins immediately and reverses as the ommatidial aperture begins to close. At night, the onset of the inward movement is delayed, but then occurs more rapidly than during the day. No significant longitudinal movement of photoreceptor screening pigment was detected under any of these experimental conditions.

Two opposing mechanisms control the movement of screening pigment in these cells. Release of neurotransmitters from the circadian efferents causes outward movement; large increments of light cause inward movement. In the absence of sudden changes in light intensity, circadian efferent input, not cyclic lighting, appears to be the major determinant of screening pigment position. A sudden and large increment of light triggers the rapid inward movement which appears to be a protective mechanism optimized for daytime performance.

We have measured orientation-discrimination thresholds of 4 deg in the cat, confirming an earlier study of Vandenbussche and Orban (1983). Unlike Vandenbussche and Orban (1983), we found that orientation-discrimination performance is not better at principal, as compared to oblique, reference orientations (no oblique effect). Despite the absence of the oblique effect, and despite the discrimination thresholds which were elevated by a factor of 4 compared to humans, orientation-discrimination performance of cats and humans is qualitatively similar in a number of aspects. First, orientation-discrimination performance as a function of length and contrast is qualitatively similar to human performance. Second, as in humans, detection and discrimination of the stimuli are closely related. Finally, randomizing the contrast between the stimuli does not affect orientation-discrimination performance. This suggests that similar computations underlay orientation-discrimination performance in both species. In summary, our results confirm that the cat is a useful model for human orientation-discrimination performance.

The distribution of cytochrome oxidase (CO) in the lateral geniculate nucleus (LGN) of normal adult macaque monkeys was analyzed in quantitative light- and electron-microscope (EM) studies. Both reactive and nonreactive neurons were found throughout laminae 1–6. Darkly reactive neurons were more numerous in laminae 1, 2, and 6. Within each lamina, there was a positive correlation between cell size and level of CO activity in neurons. The cell density was greater in laminae 1, 4, and 6, which received input from the contralateral eye, than in corresponding laminae representing the ipsilateral eye (2, 3, and 5). The cell density in each of the magnocellular laminae was less than that in each of the parvicellular laminae.

Three types of neurons could be distinguished within magnocellular laminae at the EM level. Types I and II were interpreted to be relay neurons, while the small size of type III qualified them to be interneurons. Two populations were found within parvicellular laminae, corresponding to relay neurons and interneurons, respectively. EM quantitative analyses revealed that the darkly reactive neurons contained a high proportion of moderate to darkly reactive mitochondria, which comprised 44.2%, 42.5%, and 39.2% of the mitochondrial population within neurons of laminae 1, 2, and 6 respectively. In contrast, a relatively low proportion of reactive mitochondria were found in neurons within laminae 3 (22.8%), 4 (26.4%), and 5 (32.4%). Moreover, magnocellular laminae contained a higher density of synapses on cell bodies than parvicellular laminae. All laminae, except layer 5, contained predominantly symmetric synapses on cell bodies. However, the proportion of symmetrical synapses in the magnocellular laminae (65.6%) was greater than that in the parvicellular laminae (59%).

Our quantitative light and EM results indicate that the level of metabolic activity in neurons of the monkey LGN is higher in magnocellular, ON-center, and contralateral than in parvicellular, OFF-center, and ipsilateral visual pathways, respectively. The metabolic activity of neurons is likely to reflect the chronic level of spontaneous and synaptically evoked discharges of these cells.

Saccades evoked electrically from the deep layers of the superior colliculus have been examined in the alert cat with its head fixed. Amplitudes of the vertical and horizontal components varied linearly with the starting position of the eye. The slopes of the linear-regression lines provided an estimate of the sensitivity of these components to initial eye position. In observations on 29 sites in nine cats, the vertical and horizontal components of saccades evoked from a given site were rarely influenced to the same degree by initial eye position. For most sites, the horizontal component was more sensitive than the vertical component. Sensitivities of vertical and horizontal components were lowest near the representations of the horizontal and vertical meridians, respectively, of the collicular retinotopic map, but otherwise exhibited no systematic retinotopic dependence. Estimates of component amplitudes for saccades evoked from the center of the oculomotor range also diverged significantly from those predicted from the retinotopic map. The results of this and previous studies indicate that electrical stimulation of the cat's superior colliculus cannot yield a unique oculomotor map or one that is in register everywhere with the sensory retinotopic map. Several features of these observations suggest that electrical stimulation of the colliculus produces faulty activation of a saccadic control system that computes target position with respect to the head and that small and large saccades are controlled differently.

The glycinergic system in goldfish retina was studied by immunocytochemical localization of glycine antiserum at the light-microscopical level. Numerous amacrine cells, a type of interplexiform cell, interstitial cell, and displaced amacrine cell were glycine-immunoreactive (IR). Amacrine cells, accounting for 97% of the glycine-IR neurons, were of four types based solely on their level of dendritic stratification: stratified amacrine cells of the first, third, and fifth sublayers and bistratified amacrine cells of the first and fifth sublayers. Double-labeling experiments were carried out to determine possible co-localization of glycine-IR with GABA-IR, serotonin-IR, substance P-IR and somatostatin-IR. No evidence for co-localization of glycine-IR with these other transmitter substances was found, despite reports of co-localization of these substances in retinas of other species. Glycinergic neurons in goldfish retina appear to consist of a heterogeneous population of at least seven morphologically distinct subtypes that are also neurochemically distinct in regard to GABA, serotonin, substance P, and somatostatin. Since dendritic stratification in the inner plexiform layer is correlated with ON-, OFF-response types, we suggest that the subtypes of glycine-IR amacrine cells play different roles in the encoding of visual information.