We have localized glycine-like immunoreactivity to provide new anatomical detail about glycinergic neurons in the turtle retina. A rabbit antiserum directed against a glycine/albumin conjugate was used with standard fluorescent and avidin-biotin labeling techniques. Some processes in the outer plexiform layer and many processes in the inner plexiform layer, numerous somata in the inner nuclear layer, and isolated somata in the ganglion cell layer were immunoreactive.

The vast majority of labeled neurons were amacrine cells. One class of amacrine cells had well-labeled somata near the inner nuclear/inner plexiform layer border, which gave rise to thick primary processes that entered the inner plexiform layer and arborized near the border of strata 1 and 2 and in stratum 3. A second class of glycinergic neurons, consisting of putative interplexiform cells, was unique in that it gave rise to dendritic arborizations in both the outer plexiform layer and the inner plexiform layer. Some of the immunoreactive neurons in the ganglion cell layer were apparently displaced amacrine cells, while others were probably true ganglion cells because they gave rise to labeled axons, and many labeled axons were visible in the ganglion cell axon layer. These results suggested that glycine played an extensive role in the turtle retina, and that it was involved in many diverse synaptic interactions in both the outer plexiform layer and the inner plexiform layer.

The survival of retinal ganglion cells (GCs) in the left eye was studied on retinal wholemounts from 2–33 weeks after the surgical removal of the right tectum in juvenile Xenopus. Two to five weeks after tectal removal, about 76% of neurons of the retinal ganglion cell (GC) layer showed signs of retrograde degeneration: swelling of their somata and chromatolysis. Neurons that were not affected by the operation were taken to be either displaced amacrine cells (DAs) or GCs not projecting to the tectum. A portion of GCs showing retrograde degeneration became pyknotic and died within the period of 2–16 weeks after operation. Counts of surviving GCs 20–33 weeks after tectal removal amounted to about 55% of the corresponding neuron number in the right intact retina of the same animal. No discernible GC loss was observed in animals where only the optic fibers were cut at their entry point to the tectum indicating that axotomy alone, followed by rapid regrowth to the target, does not adversely influence the survival of GCs. In long-surviving animals, the left optic nerve was exposed to cobaltic-lysine complex and the position of filled optic axons within the brain determined. Optic axons whose tectal target had been removed were seen to cross over to the left intact tectum via the posterior and pretectal commissures. Aberrant projections were detected to the ipsilateral tectum and the diencephalic periventricular grey in addition to an increased projection to the accessory optic nucleus. It is concluded that the removal of the tectum, the main target of optic fiber projection, induces a very substantial GC death. Since only a portion of optic fibers were able to grow to alternative targets, the surviving GCs may have also included those with main projection areas to the diencephalic visual centers.

We have used antibodies raised against a cytoskeletal protein, microtubule-associated protein 1A (MAP 1A), to stain adult rat retina. In cryostat and polyethylene glycol-embedded radial sections, the fiber layer, ganglion cell layer, and inner plexiform layer were highly immunoreactive, a finding that suggested that the ganglion cell somata, axons, and dendrites were recognized by these antibodies. Retrograde labeling of retinal cell somata from the superior colliculus and dorso-lateral geniculate nucleus to identify ganglion cells showed colocalization of the tracer and immunoreactive cells. Double labeling with nuclear stains revealed that many cells in the ganglion cell layer, which are likely displaced amacrine cells, were not recognized by these antibodies. Furthermore, transection of ganglion cell axons, a procedure that causes retrograde degeneration of many of the axotomized ganglion cells, led to a decrease in the number of anti-MAP 1A immunoreactive cells in retinal wholemounts. Thus, MAP 1A antibodies preferentially stain ganglion cell somata and dendrites but not amacrine cells. These antibodies should be useful ganglion cell markers.

The specific role of ON and OFF retinal information channels in the generation of the horizontal optokinetic nystagmus (OKN) of the frog was studied. Coil recordings of monocular eye and head OKN were obtained before and after intravitreal injection of two drugs that block either ON or OFF channels. The intravitreal injection of 2-amino-4-phosphonobutyrate (APB), a glutamate analog that selectively blocks the ON retinal channel, strongly reduced or even cancelled the monocular OKN of the head and of the eye. The intravitreal injection of another glutamate analog, the cis-2, 3-piperidine dicarboxylic acid (PDA) that especially blocks the OFF retinal channel, did not affect the gain velocity of the slow phase of both the horizontal monocular head and eye OKN, for low stimulus velocities. Our results suggest that the retinal ON information channel, but not the OFF channel, is involved in the generation of the slow phase of the OKN of the frog, at least at low drum velocities.

The putative neural pacemaker controlling circadian rhythms in mammals is contained in the suprachiasmatic nuclei of the hypothalamus. These nuclei receive a projection, the geniculo-hypothalamic tract (GHT), from neurons in the intergeniculate leaflet (IGL) and portions of the ventral lateral geniculate nucleus (vLGN) of the thalamus. We examined the responses of putative GHT neurons to diffuse illumination using extracellular electrophysiological recordings. The great majority of IGL neurons showed sustained ON responses to diffuse retinal illumination; vLGN neurons showed more variation in their responses. Discharge rates of sustained ON neurons increased monotonically as light intensity was increased and saturated over 2–3 log units of intensity changes. Many IGL neurons had binocular input, and input from the ipsilateral eye was often inhibitory. These results indicate that GHT neurons may provide information about ambient light intensity to the suprachiasmatic nuclei.

As in rabbit, gerbil, and rat, the guinea pig interstitial nucleus of the superior fasciculus, posterior bundle (INSFp) is a sparse assemblage of neurons scattered among the fibers forming the fasciculus bearing this name. Most of the INSFp neurons are small and are ovoid in shape. Interspersed among these, are a few larger, elongated neurons whose density becomes greater and whose shape becomes fusiform in correspondence to the zone of transition from the superior fasciculus to the ventral part of the medial terminal nucleus (MTN). Like the MTN, the INSFp is activated by retinal-slip signals evoked by whole-field visual patterns moving in the vertical direction, as shown by the increase of 14C-2-deoxyglucose (2DG) uptake into this nucleus. At the same level of luminous flux, neither pattern moving in the horizontal direction nor the same pattern held stationary can elicit increases in the INSFp 2DG assumption. The specificity of the observed increases in metabolic rates in INSFp following vertical whole-field motion suggests that this assemblage of neurons relays visual signals used in the control of vertical optokinetic nystagmus.

Using two microelectrodes, recordings were made from pairs of like-signed cells at different depths in single layers (A or Al) of the cat's lateral geniculate nucleus (LGN). The cells were chosen to have near or overlapping receptive fields so that they could be stimulated simultaneously with a single spot or bar of light. Under these controlled conditions, paired X cells (n = 32 pairs) showed differences in latency from less than 10 to about 80 ms, but no latency differences were observed between paired Y cells (n = 3 pairs). Within XY pairs (n = 11) the Y cells responded faster than the X cells. A separate analysis of previously reported, singly recorded X cells (n= 131) also showed wide differences in X cell latencies.

The results confirm that temporal differences in signalling occur within the X pathway in the cat LGN (Mastronarde, 1981, 1987a; Humphrey & Weller, 1988a) and they show explicitly that the differences occur simultaneously, in single layers and at single retinotopic loci. No consistent relationship was found between timing and depth, but the possibility of weak, depth-dependent trends is considered in the Discussion.

The effect of binocular competition on the development of ocular activation columns in areas 17 and 18 of cats was studied using the 14C-2-deoxyglucose (14C-2DG) technique to visualize the regions of cortex activated by one eye in cats reared with equal alternating monocular exposure (equal AME), unequal AME, or monocular deprivation (MD). The average size of the ocular activation columns of the eye stimulated during administration of 2DG was positively correlated with the competitive advantage during rearing. In order of increasing percentage of visual cortex activated, the eyes were (1) deprived eye of MD cats, (2) less experienced eye of unequal AME cats, (3) either eye of equal AME cats, (4) more experienced eye of unequal AME cats, and (5) experienced eye of MD cats. In area 17, the shape of the activation columns also was affected by the relative experience of the eye. The columns of the deprived eye of MD cats were widest in layer IV, where they were about the same width as those of the less experienced eye of the unequal AME cats; in other layers they were narrower, sometimes disappearing altogether. In contrast, the activation columns of the less experienced eye of the unequal AME cats were about the same width in all layers. These results suggest that when one eye is placed at a severe disadvantage and receives no patterned input, as in MD, both geniculocortical connections and intracortical connections may be disrupted, but when the disadvantage is less, as in unequal AME, only the geniculocortical connections are disrupted.

Binocular competition also affected the intensity of activation within columns in area 17. We used video densitometry to determine ratios of the amount of label in cortical and thalamic structures. Both the ratio of label in area 17 to that in the lateral geniculate nucleus (LGN) and the ratio of label in the binocular segment of area 17 to that in the monocular segment were significantly less for the deprived eye of MD cats than for any other group. These results suggest that even within the smaller activation columns, deprived geniculocortical afferents are relatively ineffective at driving cortical cells. This finding is consistent with earlier reports that the synapses from the deprived pathway are both morphologically abnormal and reduced in number. The cortical labeling for the less experienced eye of the unequal AME cats and the experienced eye of the MD cats were also significantly less than that in equal AME cats. The decreased labeling for the experienced eye activation columns suggests that, in order to cover an abnormally large area, afferents representing the experienced eye must make fewer synaptic contacts within that area and that there are intrinsic limits on the number of synapses that one axon can make.

The responses of neurones in the cat's visual cortex are very variable in amplitude. Thus, although the average response amplitude of a single neurone depends closely upon the contrast of a sinusoidal grating, the instantaneous amplitude of the response can convey little information about the grating's contrast. This paper shows that a typical cortical neurone can convey less than one bit of information about contrast in 0.5 s. The amount of information that a neurone can convey is closely correlated with the neurone's responsivity.

Studies of the physiology of the central visual system and visual behavior frequently require large tangent screens, and it has been difficult to create a smooth surface with uniform illumination. We have developed a technique for holding drafting paper on a sheet of acrylic so that it can be used as a tangent screen for both front and rear projections. The process utilizes a vacuum system that holds the paper firmly and produces a uniformly smooth surface with good light diffusion; it is simple, flexible, and inexpensive and can be used for any size screens.