Segregation of form, color, and stereopsis in primate area 18.

Primate visual cortical area 18 (visual area 2), when stained for the enzyme cytochrome oxidase, shows a pattern of alternating dark and light stripes; in squirrel monkeys, the dark stripes are clearly of 2 alternating types, thick and thin. We have recorded from these 3 subdivisions in macaques and squirrel monkeys, and find that each has distinctive physiological properties: (1) Cells in one set of dark stripes, in squirrel monkeys the thin stripes, are not orientation-selective; a high proportion show color-opponency. (2) Cells in the other set of dark stripes (thick stripes) are orientation-selective; most of them are also selective for binocular disparity, suggesting that they are concerned with stereoscopic depth. (3) Cells in the pale stripes are also orientation-selective and more than half of them are end-stopped. Each of the 3 subdivisions receives a different input from area 17: the thin stripes from the blobs, the pale stripes from the interblobs, the thick stripes from layer 4B. The pale stripes are thus part of the parvocellular system, and the thick stripes part of the magnocellular system. The physiological properties of the cells in the thin and pale stripes reflect the properties of their antecedent cells in 17, but nevertheless exhibit differences that suggest the kinds of processing that might occur at this stage.