Tyrosine hydroxylase-immunolabeled wholemounts and sections of rat and monkey retina were observed at both the light- and electron-microscopic level. Small processes derived from sclerally directed processes of dopaminergic interplexiform cells were observed ascending to the outer nuclear layer where they were distributed between photoreceptor cells. A role in the regulation of disc shedding and/or melatonin biosynthesis is proposed for dopamine released at this level.

Contrast sensitivity is a measure of the ability of an observer to detect contrast signals of particular spatial and temporal frequencies. A formal definition of contrast sensitivity that can be applied to individual linear visual neurons is derived. A neuron is modeled by a contrast transfer function and its modulus, contrast gain, and by a noise power spectrum. The distributions of neural responses to signal and blank presentations are derived, and from these, a definition of contrast sensitivity is obtained. This formal definition may be used to relate the sensitivities of various populations of neurons, and to relate the sensitivities of neurons to that of the behaving animal.

The projection of the perihypoglossal (PH) complex to the superior colliculus (SC) in the rhesus monkey was investigated using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Following physiological identification by electrical stimulation and multiunit recording, small injections of the tracer were placed within the SC of three monkeys. The largest numbers of retrogradely labeled neurons within the PH complex were found in the contralateral nucleus prepositus hypoglossi (NPH), in the laterally adjacent medial vestibular nucleus, and in the ventrally adjacent reticular formation (the nucleus reticularis supragigantocellularis). These labeled neurons are strikingly heterogeneous in size and morphology. The nuclei supragenualis and intercalatus also contain numerous labeled neurons in the 2 cases in which the injections involve the caudal SC. Large numbers of retrogradely labeled neurons as well as anterogradely transported WGA-HRP are observed alo throughout the pontine and medullary reticular formation, including the midline raphe. The PH complex, particularly the NPH, is known to be involved in the coding of eye position and has been hypothesized to be a critical component of the “neural integrator.” Our data demonstrate the existence of a robust projection from the PH complex to the contralateral SC in the rhesus monkey. This projection may serve as the anatomical substrate by which a corollary of eye position could reach the SC. Such a signal is a prerequisite for the computation, at the collicular level, of saccadic motor error signals observed in the SC of rhesus monkeys.

Electrical reponses of luminosity horizontal cells (L cells) to monochromatic stimuli were analyzed by intracellular recordings in the retinas of the freshwater turtle (Pseudemys scripta elegans) and of the sea turtle (Chelonia mydas mydas). Light intensity, duration, and wavelength were varied to assess temporal effects. For a given intensity of monochromatic light, response amplitude increased with stimulus duration until maximum amplitude occurred at a specific duration. This suprathreshold metric of temporal integration is called here summation time, and it is wavelength-dependent.

L cells always had some level of red-sensitive cone input, although in some cells inputs from green- and blue-sensitive cones were also observed. For these latter cells, summation times were shorter for 640-nm than for 540-nm or 450-nm lights. These results were most evident in cells that received dominant inputs from blue- or green-sensitive cones.

Responses of some other L cells were almost completely dominated by inputs from red-sensitive cones. Summation times of these cells were not wavelength-dependent. But when these inputs also included green-sensitive cones, shorter summation times were obtained to 640-nm light than to 540-nm light, even though dominant inputs were still from red-sensitive cones. These results, obtained from both retinal and 3,4-dehydroretinal photopigment systems, are consistent with reported observations in Pseudemys scripta elegans that show linear responses of red-sensitive cones to have shorter integration times and times-to-peak than green-sensitive cones.

Responses from horizontal cells dominated by blue-sensitive cone inputs were the most sensitive of all; they also had the longest summation times. These results support the hypothesis that a gain in sensitivity occurs from the integration of absorbed photons over longer periods of time.

These intracellular responses are of particular importance because behavioral critical durations in turtle, as defined by Bloch&'s law, are similarly wavelength-dependent.

A number of lines of evidence suggest that, in the macaque monkey, inferior parietal and inferotemporal cortices process different types of visual information. It has been suggested that visual information reaching these two subdivisions follows separate pathways from the striate cortex through the prestriate cortex. We examined directly this possibility by placing injections of the retrograde fluorescent tracers, fast blue and diamidino yellow, in inferior parietal and inferotemporal cortex and examining the spatial pattern of cortical areas containing labeled cells in two-dimensional reconstructions of the cortex.

The results of injections in inferotemporal cortex show that TEO receives afferents from areas V2, ventral V3, V3A, central V4, V4t, and DPL in prestriate cortex and from areas IPa, PGa, and FST in the superior temporal sulcus (STS). Area TEp receives afferents only from V4 in prestriate cortex and from IPa, PGa, and FST in the anterior STS. Area TEa receives no prestriate input and is innervated by IPa, PGa, FST, and TPO in the anterior STS.

The results of injections in inferior parietal cortex demonstrate that POa receives afferents from dorsal V3, V3A, peripheral V4, DPL, and PO in prestriate cortex, from MST and *VIP and from IPa, PGa, TPO, and FST in anterior STS. Area PGc (corresponding to 7a) is innervated by PO, MST, and by TPO in the anterior STS.

Examination of the two-dimensional reconstructions of the pattern of labeling after combined injections of fast blue and diamidino yellow in areas POa and TEO revealed that these areas are principally innervated by different prestriate areas. Only a small region, centered on area V3A and extending into V4 and DPL, contained cells labeled by either injection as well as a small number of double-labeled cells. In contrast, areas POa and TEO receive afferents from extensive common regions in the anterior STS corresponding to areas IPa, PGa, and FST.

These results directly demonstrate that visual information from the striate cortex reaches inferior parietal and inferotemporal cortices through largely separate prestriate cortical pathways. On the other hand, both parietal and inferotemporal cortices receive common inputs from extensive regions in the anterior STS which may play a role in linking the processing occurring in these two cortical subdivisions of the visual system.

A postembedding immunogold procedure was used to estimate quantitatively, at the electron-microscopical level, the intensity of glutamate (GLU) immunoreactivity in different identifiable profiles of the lateral geniculate nucleus (LGN) and perigeniculate nucleus (PGN) of the cat. Synaptic terminals of retinal and cortical origins in the LGN, and of axon collaterals of geniculo-cortical relay cells in the PGN, were identified by previously determined ultrastructural features. Processes of interneurons or relay cells were identified by being immunoreactive or non-immunoreactive, respectively, in serial thin section reacted with a GABA antibody.

The results showed that synaptic terminals of geniculo-cortical relay cells in the PGN have significantly higher levels of GLU immunoreactivity than their parent somata or dendrites in the LGN; this suggests transmitter storage of this amino acid in these terminals. By contrast, synaptic terminals of interneurons did not show enrichment of GLU relative to their parent somata. This argues against the possibility that the relative enrichment of GLU in relay cells terminals is due to factors other than presynaptic storage. In addition, axon collateral terminals of relay cells in the PGN, as well as retinal and cortical terminals in the LGN, showed significantly higher GLU immunoreactivity than GABAergic terminals. These immunocytochemical results suggest that GLU is a neurotransmitter in the retino-geniculate, cortico-geniculate, and geniculo-cortical pathways in the cat.

Using an antibody against tyrosine hydroxylase on newborn to 30-day kitten retinas, we have been able to follow the development of the dopaminergic amacrine cells of the cat retina by light-microscopical investigations of retinal wholemounts. The Type 1 or large Toh+ amacrine cells described by others (Oyster et al., 1985; Törk & Stone, 1979) and named A18 from a Golgi study (Kolb et al., 1981), is at birth (P1) an immature neuron with a small cell body and two or three simple thick radiating dendrites stratifying in stratum 1 with many of the dendrites ending in enlarged growth cones. With increasing postnatal age, the cell body size increases from 12.5 μm diameter to reach 15.5 μm diameter at P30. The dendritic fields also increase in size and complexity. At P1, cells of the central area exhibit dendritic appendages which then develop progressively until at P13 (after eye opening) they are part of rudimentary rings and by P30 the dendritic plexus of Toh+ dendrites and rings in stratum 1, typical of the adult cells, are complete. Toh+ stained processes with growth cones that run deep in stratum 5 of the inner plexiform layer and processes passing to the outer plexiform layer first become apparent at P1 in cells of central inferior retina but not till after P13 are these processes clearly expressed. At P1, the total number of Toh+ Type 1 cells is approximately 4000 and this number remains unchanged to the adult retina. However, the retina increases in size over the Pl–P30 stage and thus the mean density of Type 1 Toh+ cells decreases from 30/mm2 at P1 to 18/mm2 at P30. The maximum density of Type 1 Toh+ cells occurs in central retina 2–4 mm superior temporal to the area centralis, corresponding to the maximum rod photoreceptor concentration.

A second type of small Toh+ amacrine cell can be visualized at P1. This Type 2 cell is characterized by a much smaller cell body than Type 1 cells (9 μm diameter), and with faintly stained dendrites located in stratum 3 of the inner plexiform layer. During later postnatal days, Type 2 cells gradually become unstainable and only few are still seen in far peripheral retina by P23. Type 2 Toh+ cells form a total population of 40,000 cells at P1 with their highest density occurring in peripheral retina. By P13, they cannot be seen in central retina and are reduced to a total population of cells staining for the antibody of 7400 cells in far peripheral retina. Their density decreases from 233/mm2 at P1 to 0/mm2 at P30. The transiently staining population of Type 2 Toh+ immunoreactive cells probably correspond to the small THI-CA-cell seen in rat retina (Nguyen–Legros et al., 1983), and the second type of dopaminergic amacrine cell seen in macaque retina (Mariani & Hokoc, 1988).

The effects of cobalt ions on 502-nm rod- and 575-nm cone-driven components of the b-wave of the electroretinogram were studied in the isolated frog retina. Addition of 100–150 μM cobalt initially caused a suppression of rod-driven responses and an enhancement of cone-driven responses. In the continued presence of cobalt, however, the rod-driven responses gradually recovered and the cone-driven responses became suppressed. These concentrations of cobalt had no effect on the rod- and cone-driven mass receptor potentials which were isolated in the presence of 4 mM glutamate. At higher concentrations of cobalt (1 mM or greater), both rod- and cone-driven b-wave responses were eliminated and there was no recovery in the continued presence of cobalt. The results suggest that cobalt has markedly different, time-dependent effects on signal transmission from rods and cones to second-order cells.

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.

The exposure to bright light of dark-adapted (DKA) mouse retinas incubated in the dark (DI) in IBMX-containing medium causes a marked loss of cyclic AMP. This light response also occurs if the medium contains 10 mM aspartate or cobaltous ion, agents believed to confine the effects of light to photoreceptors. Thus, the action of light in the presence of either of these agents defines a light-sensitive pool of cyclic AMP in photoreceptors. This pool could also be reduced or eliminated in DKA-DI retinas by nanomolar to micromolar levels of dopamine (if the medium contained SCH23390, a potent antagonist of Dl receptors), thus indicating an agonistic action of dopamine at D2 receptors. The D2 agonists LY171555 (EC50 10 nM) or (+)-3-PPP also reduced the cyclic AMP level in the dark. Of the D2 antagonists tested, the butyrophenone spiperone (in the presence of the 5HT-2 blocker ketanserin) countered the action of the D2 agonists but substituted benzamides were ineffective. Consistently, the D2 agonists had no effect on cyclic AMP levels of mutant retinas lacking photoreceptors (rd'rd), but reduced cyclic AMP in DKA-Dl glutamate-modified retinas which exhibit a major loss of inner retinal neurons without apparent loss of photoreceptors. The Dl antagonist SCH23390 only reduced cyclic AMP levels of DKA-DI retinas when cyclic AMP levels had been elevated by adding dopamine to the incubation medium.

The representation of the visual field in the architectonically defined transition zone between areas 17 and 18 of cat cerebral cortex was assessed by recording the activities and plotting the receptive fields of neurons at 2327 sites along 148 electrode penetrations made in 19 cats. The results show that the transition zone contains a significant representation of the ipsilateral visual hemifield although not all elevations in the visual field represented to the same extent. The shape of the field region represented resembles an hour glass, for the region represented is narrowest on the 0-deg horizontal meridian and increasingly wider at progressively more positive and negative elevations. When receptive-field centers are considered, the extent of the representation reaches to -2.5 deg on the 0-deg horizontal meridian and to 10 or more degrees towards the field periphery. When receptive-field areas are considered, the representation at the 0-deg horizontal meridian extends to -3.6 deg and to beyond 20 deg at other elevations. In contrast, the visual-field representations in flanking areas 17 and 18 are essentially limited to the contralateral hemifield. The presence of a distinct representation of part of the ipsilateral hemifield in the transition zone suggests that the zone may have connections distinctly different from those of the adjacent areas. The observations bear on the problems of understanding the visual pathways in hypopigmented cats and binocular disparity mechanisms about the midline.

The eyes of growing chicks adjust to correct for myopia (eye relatively long for the focal length of its optics) or hyperopia (eye relatively short for the focal length of its optics). Eyes made functionally hyperopic with negative spectacle lenses become myopic and long, whereas eyes made functionally myopic with positive spectacle lenses become hyperopic and short. We report here that these compensatory growth adjustments occur not only in normal eyes but also in eyes unable to accommodate (focus) because of lesions to the Edinger-Westphal nuclei. Thus, at least in chicks, accommodation is not necessary for growth that reduces refractive errors during development, and may not be necessary for the normal control of eye growth.

The strength of the flanking inhibitory regions of the receptive fields of fly lamina cells (LMC) decreases as the mean luminance is lowered. Simultaneously, the biphasic temporal flash (impulse) response of the lamina cells becomes monophasic on lowering luminance. For a moving-edge stimulus at high mean luminance, this implies that the spatial integration by the larnina cell yields a temporal waveform which is congruent to the waveform of the temporal impulse response of the lamina cell. In other words, the temporal waveform generated by the moving edge is matched to the temporal waveform most preferred by the lamina cell. The edge is the stimulus causing the largest amplitude response at high (above 1 cd/m2) levels of luminance. On lowering luminance, the now monophasic nature of the spatial and temporal impulse responses of the lamina gives a preference not for the edges but for the center of a uniform region. We describe this theory and its behavioral corroboration in walking flies (Lucilia cuprina).

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 anatomy and physiology of the retinotectal pathway of the perch was investigated using physiological and histological techniques. Massed responses of the optic nerve to single shocks exhibited five distinct peaks. Single-unit responses to shocks indicate two groups of fast fibers correlating well with peaks I and II of the massed response. The flash-evoked response in nerve and tectum has three major phases (PSPI-III), with a marked low-threshold fast component. Patterns of flash-evoked response from single fibers vary, but the responses of fast transient fibers coincide with the timing of PSPI, and longer latency groups with PSPII-III. Units reflexly activated by efferents were also seen, and 12% of units were photically inexcitable.

Surprisingly, few fibers responded well to a scanned spot light, unlike tectal cells, and receptive fields were often large (>70 deg). ON/OFF responses, evoked either by whole field or local illumination, were much commoner than pure ON or OFF responses.

Effects of electrical stimulation or cautery of the tectum on the flash-evoked response of fiber bundles, via the efferents were marginal, but repetitive stimulation or section of the optic nerve produced clear-cut deficits in the slow components of the flash-evoked response of the nerve. Stimulation of the eighth nerve produced a complex long-latency, large-amplitude response in the optic nerve.

The fiber spectrum of the optic nerve taken from electron micrographs revealed the presence of a relatively small group (less than 1%) of thick fibers with diameters between 3 μm and 10 μm that could be correlated with fast responses recorded from the optic nerve, and the remainder with axon diameters down to 0.2 μm providing the slow responses. The distribution of cell-body diameters from sectioned and wholemount material indicated a marked distinction between small and large ganglion cells. The total number of fibers in the nerve was estimated 868,840.

A technique by which the retina can be isolated from the turtle eye is described. Scanning electron microscopy revealed morphological variability between preparations and also between regions of the same one. Large areas were often totally free of any pigment epithelial cells, yet contained a high proportion of photoreceptors with complete outer segments. However, adjacent regions may contain photoreceptors without outer segments or with fragmented ones. The physiological properties of the horizontal cells also demonstrated large variability between different preparations. In all cases, lowering calcium concentration from 2 mM to 0.1−0.5 mM depolarized the horizontal cells and augmented the amplitude of the maximum photoresponses. However, these effects were accompanied by changes in the photoresponse kinetics and by a reduction in the horizontal cell sensitivity to light. Moreover, prolonged exposure to low calcium induced permanent damage to the retina as was indicated by the reduction in the response amplitude after superfusion with 2 mM calcium solution had been resumed. The toxic effects of low calcium were most apparent when superfusion with 0.1−1.0 μM calcium concentration was performed. These solutions induced complex time-dependent effects on the resting potential of horizontal cells and on the amplitude and kinetics of the photoresponses. We conclude from these observations that the normal concentration of extracellular calcium in the turtle retina is in the 2 mM range.

The ability of freely-flying honeybees to track moving targets was examined by training to collect a reward on a target, and then videotaping their approach to the target while it was in motion. Training experiments were carried out with several groups of bees, using various colors for the target and the background. Computer-aided frame-by-frame analysis of video recordings was used to plot the instantaneous positions of the target, as well as the position and orientation of the approaching bee in three dimensions. The results show that bees are perfectly capable of tracking moving targets and landing on them. When the distance of the target is greater than 15 cm, approaching bees correct for angular deviations of the target from the midline, both in the horizontal and in the vertical plane. In either plane, the input vaariables that are important to the tracking system seem to be (1) the angular bearing of the target with respect and (2) the angular velocity of the target with respect to the eye. The tracking control system tends to orient the bee such that the target is located frontally, at an angle of Ca. 35 deg below the bee's long axis. The chromatic properties of tracking behavior were investigated by employing combinations of colors for the target and background such that the boundary between the target and the background presented a contrast that was visible either only to the green-sensitive receptors of the bee's eye, or only to theblue-sensitive receptors. The results of these experiments suggest that, in controlling tracking, the measurement of the angular velocity of the target is derived almost exclusively from signals from the green-sensitive receptors, as is the case with previously studied movement-sensitive behavior. However, the measurement of the angular bearing of the target is derived from the blue-sensitive receptors as well as the green-sensitive noes. When the target is closer than Ca. 15 cm, approaching bees use translational maneuvers, in addition to rotational ones, to track the moving target. Translational target tracking appears to be driven primarily by signals from the green-sensitive receptors.

GAP-43 levels have been determined by immunoassay in cat visual cortex during postnatal development to test the idea that GAP-43 expression could be related to the duration of the critical period for plasticity. For comparison, GAP-43 levels have also been assayed in primary motor cortex, primary somatosensory cortex, and cerebellum at each age. GAP-43 levels were high in all regions at 5 d (with concentrations ranging from 7−10 ng;/μg protein) and then declined 60−80% by 60 d of age. After 60 d of age, GAP-43 concentrations in each region continued a slow decline to adult values, which ranged from 0.5−2 ng/μg protein. To test for the involvement of GAP-43 in ocular dominance plasticity during the critical period, the effect of visual deprivation on GAP-43 levels was investigated. Monocular deprivation for 2−7 d, ending at either 27 or 35 d of age, had no effect on total membrane levels of GAP-43. The concentrations of membrane-associated GAP-43 prior to 40 d of age correlate with events that occur during postnatal development of the cat visual cortex. However, the slow decline in membrane-associated GAP-43 levels after 40 d of age may be an index of relative plasticity remaining after the peak of the critical period.

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.