There are several different groups of ganglion cells in the retina of the frog. Although their axons are thought to terminate in different layers of the optic tectum, little is known about the morphology of their terminal arbors or their synaptic targets. The present paper reports the results of a layer-by-layer study of horseradish peroxidase labeled retinal axons in the optic tectum of Rana pipiens. Light and electron microscopy was used to study the axon's laminar distribution, patterns of arborization, and synaptic contacts.

Labeled retinal axons were found in all of the superficial layers of the tectum (A-G). From layer to layer, the retinal axons differed markedly in the diameter of their parent axons (0.2−3.0 μm) and in the morphology and horizontal extent of their terminal arbors.

Five classes of synaptic terminals could be distinguished in the tectum. The retinal terminals belonged to class characterized by round, medium-sized synaptic vesicles. They made synaptic contact with dendrites and other axon terminals in each of the layers. They were always the presynaptic component. The postsynaptic dendrites were often the vertically oriented processes of cells located in the deeper layers. The postsynaptic terminals belonged to a class distinguished by their flat, medium-sized vesicles. These terminals in turn contacted what appeared to be dendrites. In layer eight, the retinal axons were often large, spoon-shaped boutons that ended in apposition with the somata of the layer.

The superficial layers of the frog's optic tectum, Potter's (1969) layers A-G, comprise a complex neuropil made up of many afferent axons, the somata of a few neurons, and many dendrites from the neurons located in the deeper layers. Different types of retinal axons are believed to terminate in different layers (Maturana et al., 1960; Kuljis & Karten, 1988; Sargent et al., 1989), but little is known about the relationships between each type of input and the dendrites of the deep tectal neurons that extend into these superficial layers. The present study used the method of retrograde transport of horseradish peroxidase to study the synaptic contacts on the dendrites of the neurons that give rise to the crossed tecto-bulbar pathway. These cells have apical dendrites that ascend through the superficial retino-recipient layers.

The somata of the cells that give rise to the crossed tecto-bulbar pathway are located in the superficial half of layer 6, preferentially clustered along the caudal, lateral, and rostral margins of the tectum. The somata of these cells range from 8−30 ¼m in diameter. Their axons are large (2−4 ¼m in diameter) myelinated fibers that arise from either their somata or proximal dendrites. Their axons travel within the deep medullary layer to leave the tectum at the lateral margin. Their dendritic arbors extend obliquely through the superficial layers to reach layer B where they turn and extend within the layer for up to 0.5 mm. The somata of these cells receive only a scant synaptic input. In contrast, their dendrites receive input in every layer, but the nature of this input varies from layer to layer. Synaptic terminals that resemble retinal ganglion cell boutons contact the labeled dendrites in layers B, F, and G. This indicates that the dendrites may receive monosynaptic input from several types of retinal ganglion cells. Terminals with small, flattened vesicles also contact the dendrites of these cells in each layer. In layer F and below, the terminals with flattened vesicles constitute 15% of the contacts; above layer F they constitute only 5−8% of the contacts. Terminals with medium-sized, flattened vesicles also contact the dendrites of these cells in every layer and constitute a large proportion of their input (33−95%). The latter terminals resemble those that are often postsynaptic to retinal terminals.

The distribution of ganglion cells in the retina of the silver lamprey, Ichthyomyzon unicuspis, was revealed by retrograde labeling from the optic nerve with horseradish peroxidase (HRP) and fluorescent-labeled dextrans in live animals and with the fluorescent dye DiI in aldehyde-fixed tissue. The majority of ganglion cells (74%) termed the “outer ganglion cells,” are multipolar and are located at the vitread boundary of the inner nuclear layer. The remaining ganglion cells (26%), termed the “inner ganglion cells” are bipolar and are distributed in a sublamina within the inner plexiform layer. The dense, dendritic meshwork of the outer ganglion cells is largely restricted to the sclerad half of the inner plexiform layer with some cells possessing dendrites which pass through the inner nuclear layer to terminate within the outer plexiform layer. The dendrites of the inner ganglion cells form a thin, dendritic network apposing the inner limiting membrane. Axons from both populations of ganglion cells originate from dendrites or the soma and form fascicles lying adjacent to the outer ganglion cell somata.

Retinopetal fibers, originating from bilaterally distributed neurons of the tegmental midbrain, were thin and varicose and ran parallel to the ganglion cell axons to terminate either with a varicose enlargement or a few short sidebranches in the sclerad third of the inner plexiform layer. The unusual organization of the lamprey retina and outgroup comparison with hagfish suggests that agnathans share a presumably primitive type of retinal ganglion cell organization compared to that of gnathostomes.

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.

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.

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).

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.

In albinos, the retinofugal projections to the ipsilateral side of the brain are reduced (e.g., see Guillery, 1969; LaVail et al., 1978; Lund, 1965). Although all ganglion cell types are affected, in mice the displaced ganglion cell population is the main target of the albino mutation (Dräger & Olsen, 1980). Here we tested whether this preferential effect on displaced ganglion cells is a general consequence of the melanin reduction or a pleiotropic effect unique to the albino locus, retrogradely tracing tracing retinal ganglion cells in normal C57BL/6J mice and in several non-allelic hypopigmentation mutants on the same background: albino (C57BL/6J-c2J), beige (C57BL/6J-bg), pale ear (C57BL/6J-ep), ruby-eye/haze (C57BL/6J-ru-2hz), and pearl (C57BL/6J-pe). All mutants have lower overall cell counts in the ipsilateral projection, but the displaced population is disproportionately affected: the albinos contain 42% of the normal number of displaced ganglion cells, and the other mutants have an average 57% of normal counts.

The reduction in uncrossed retinofugal projections in albinos affects the inputs to the lateral geniculate nucleus and the superior colliculus, but not to the suprachiasmatic nucleus (Dräger, 1974). To address the question in which way the susceptible uncrossed projections differ from the nonsusceptible one, we compared ganglion cells backfilled from the suprachiasmatic nucleus to ganglion cells backfilled from the optic tract at geniculate level. Whereas the uncrossed optic tract projection originates from the binocular region in the ventro-temporal retina and contains a high fraction of large and displaced ganglion cells (Dräger & Olsen, 1980), both the crossed and uncrossed inputs to the suprachiasmatic nucleus originate from the entire retina with a relative preference for the lower nasal region that corresponds to part of the monocular visual field; all ganglion cells projecting to the suprachiasmatic nucleus are of medium size, and they are located in the ganglion cell layer.

These observations allow the following conclusions: (1) All genetic mutants which cause a reduction in ocular melanin, regardless of the molecular or cell-biological mechanism underlying the pigment reduction, result in decreased uncrossed projections; this confirms previous reports (LaVail et al., 1978, Sanderson et al., 1974). (2) The decrease affects only projections involved in binocular vision. (3) In mice, the ganglion cells displaced to the inner nuclear layer, and hence located closer to the retinal pigment epithelium, are disproportionately affected by the melanin reductions. These observations may provide cues to the spatio-temporal mechanism of the melanin action in the embryonic visual system.

Extracellular recordings from single units in the dorsal terminal nucleus (DTN) of the cat accessory optic system (AOS) were before and after intravitreal injections of the GABA antagonist bicuculline methiodide (BMI). Direction-selective responses of DTN cells elicited through the contralateral, injected eye were aboloshed 7−12 h following the injection. For the concentrations tested, direction-selective responses through the contralateral (injected) eye did not recover within 26 h. Direction-selective responses through stimulation of the ipsilateral (uninjected) eye were also dramatically depressed for 1−9 h after contralateral eye injections. However, direction-selective responses through the ipsilateral eye eventually returned and were often more vigorous in the final stages. BMI injections into the ipsilateral eye failed to block direction-selective responses through the ipsilateral eye. The effects of intravitreal BMI on contralateral eye responses imply that DTN units receive input from direction-selective retinal ganglion cells. In addition, these results suggest that direction-selective input to the DTN from the visual cortex is independent of the retinal pathway. Using pharmacological methods described here, for the first time direction-selective responses of AOS units driven through the ipsilateral eye can be experimentally isolated.

We have examined the morphology and distribution of neurones that contain nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase in human retinae. NADPH-diaphorase reactivity was observed in three different classes of amacrine cells (ND1, ND2, ND3 cells) and in the cone photoreceptors. ND1 cells had relatively large somata (mean, 12.3 ¼m) located in the inner nuclear layer (INL) and in the ganglion cell layer (GCL). Their dendrites were often strongly labeled and spread into either the middle or outer strata of the inner plexiform layer (IPL). The somata of ND2 cells were medium-sized (mean, 8.2 ¼m) and located in the INL and in the GCL; their dendrites were usually beaded and often spread in either the middle or outer strata of the IPL. ND3 cells had small, round somata (mean, 5.2 ¼m) located in either the INL or GCL, and were without labeled processes. The total number of NADPH-diaphorase cells (all classes) was estimated at 118,000, with a mean density of about 100/mm2. The most striking fea ture of NADPH-diaphorase cells in humans was that their distribution was relatively uniform across the retina, with no evidence of a peak in density at the foveal rim.

When a cortical neuron receives synaptic input from both eyes, do the synaptic pathways that mediate the input from each eye match? In this study, inputs from the two eyes were compared by measuring the latencies of EPSPs and IPSPs evoked by electrical stimulation of the two optic nerves. For binocular neurons, these latencies invariably matched closely, indicating that the pathways from the two eyes contain the same number of synapses; monosynaptic input from lamina A of the lateral geniculate nucleus (LGN) is always matched by monosynaptic input from lamina A1. Conversely, polysynaptic input from one eye, either excitatory or inhibitory, is invariably accompanied by similar input from the other eye. In addition, the match between the two eyes in latency indicates that for each eye a synaptic potential is mediated by the same type of afferent, either X or Y.

Judging from intracellular recording, 75% of the neurons studied were binocular, that is, EPSPs could be evoked from either eye. In the remaining 25%, EPSPs could be evoked from only one eye, in agreement with extracellular receptive field studies in which 30% of cortical neurons are monocular.