1. Observations are presented on the physiological properties of W-, X-, and Y-type relay cells in the cat's lateral geniculate nucleus (LGN). Emphasis is placed on the most recently recognized type, W-cells; data are presented on X- and Y-cells by way of comparison. 2. Seventy-seven W-cells were recognized on 70 microelectrode penetrations through the LGN. They resembled W-type retinal ganglion cells in their responses to visual stimuli. Tonic (on-center and off-center) W-cells, phasic (on-, off- and on-off center) W-cells, suppressed-by-contrast, and color-coded cells were recognized. 3. W-type relay cells also resembled retinal W-cells in their maintained activity and receptive field-center diameters. 4. W-type relay cells comprised 11.5% X-cells 48.4%, and Y-cells 22.3% of all LGN cells encountered on a reference sample of 62 electrode tracks. W-cells were found in laminae C, C1, and C2, comprising 36.5% of the sample in these laminae, but were not encountered in laminae A or A1. X- and Y-cells were found in laminae A, A1, and C. Within lamina C there was a tendency for X- and Y-cells to be located dorsal to W-cells. There was thus a substantial dorsoventral segregation of W-cells from X- and Y-cells. W-cells being found in the ventral parvocellular component of the dorsal LGN. 5. Cells considered to be W-type relay cells were shown to respond to electrical stimulation of the optic nerve and chiasm at latencies which were longer than those of X- and Y-cells, and were consistent with their receiving monosynaptic input from retinal W-cells. Geniculate W-cells of all subtypes were activated antidromically from the visual cortex. Their antidromic latencies were, on the average, longer than for Y- or X-cells, indicating that W-type relay cells had slower axons as well as slower retinal afferents, than X- or Y-cells. 6. The visual cortex thus appears to receive input from all three major types of retinal ganlion cells (W-, X-, and Y-cells) relayed separately, in parallel, by different groups of relay cells.

12 rhesus monkeys, deprived of pattern vision from the day of birth to 20 or 60 days of age, were tested daily for various untrained visual responses and were trained on form, striation, and brightness discriminations. The 20and 60-day-old visually deprived Ss were similar to newborn monkeys in rate of learning visual discriminations and in untrained visual behaviors. Acuity, brightness difference thresholds, and generalization of learned discriminations changed as Ss gained experience. Ocular tracking of patterns, visual placing, and visual cliff avoidance developed only after hours or days in patterned light. Savings in hours required relative to the normal developmental time course gave evidence for maturational as well as experiential contributions.

On the basis of an electron microscopic examination of the optic nerve of the North American opossum it was estimated that the nerve has approximately 100,000 axons, of which 98% are myelinated. The myelinated axons ranged from 0.3 to 6.7 micrometers in diamter (mean 1.6 micrometers), while unmyelinated axons were 0.2 to 1.6 micrometers in diameter (mean 0.6 micrometers). Axoplasm and axon (axoplasm plus myelin) diameter spectrums were unimodal and positively skewed. The mean of the ratio of axoplasm to axon diameter was 0.69. However, this ratio varied widely across axons and was nonlinearly distributed, decreasing with axon diameter. An inverse relationship between axon density and a high proportion of large-caliber axons was located dorsally in a cross section obtained near the eye. However, in sections obtained near the optic chiasm, regions having the highest proportion of large-diameter axons were in the ventral periphery of the nerve. It is suggested that within the 40-mm length of the nerve, there may be a change from a retinotopic organization of axons according to their diameter and central targets.

The distribution of ganglion cells in the retina of the opossum was determined from whole-mounted retinae stained with cresyl violet. Isodensity lines were approximately circular with a peak density of 2,000 to 2,700 cells/mm2 in superior temporal retina (area centralis). The total number of retinal ganglion cells was estimated to be 72,000 to 135,000 (mean 101,026) in retinae ranging from 125 to 187 mm2 in total area. Three groups of ganglion cells were distinguished on the basis of soma size and retinal topography. Large cells (24 to 32 micrometer diameter) were fairly evenly distributed across the retina. Medium cells (12 to 23 micrometer diameter) were more numerous in the superior temporal quadrant than in other regions of the retina. Small cells (7 to 11 micrometer diameter) were prominent in all retinal regions, but particularly in nasal and inferior retina. An analysis of topographical differences in soma size distribution suggests that the medium size cells can be further subdivided into small-medium and large-medium groups.

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