Three groups of cortico-geniculate neurons and their distribution in binocular and monocular segments of cat striate cortex.

Among 409 neurons recorded from binocular and monocular segments of the cat striate cortex, 91 were identified as cells (C-G cells) projecting to the dorsal lateral geniculate nucleus (LGN) on the basis of antidromic activation from LGN and of histological localization of cortical layer VI. The axonal conduction velocity of these C-G cells was calculated from differences in latency between antidromic responses to electrical stimulation of LGN and the optic radiation. According to this velocity, 70 C-G cells from the binocular segment could be classified as fast (13--32 m/sec), intermediate (3.2--11 m/sec) and slow (0.3--1.6 m/sec) cells. The fast cells (47% of the total) were spontaneously active and had receptive fields of complex type. Histologically they were located mostly in the upper half of layer VI. The intermediate cells (31%) were mostly simple. The slow cells (21%) were completely silent, not driven by visual stimuli, and located mostly in the lower VI. From the monocular segment of the cortex, the intermediate cells could not be recorded, while the other two groups of cells were sampled with the same frequency as from the binocular segment. These findings suggest an existence of three functionally distinct groups of C-G cells and a possible participation of the intermediate cells in binocular vision.