Ordinal position and afferent input of neurons in monkey striate cortex

From the extracellular recording of single units in the monkey striate cortex and electrical stimulation at two selected sites in the optic radiations it was possible to estimate 1) the ordinal position of striate neurons (i.e., whether they received a monosynaptic, disynaptic or polysynaptic input from the thalamus) and 2) the nature of the afferent input to these neurons (i.e., whether it came from the magnocellular or parvocellular subdivision of the lateral geniculate nucleus (LGN)). Based on receptive field properties six major classes of striate neuron were identified--three which lacked orientation specificity (the ON-center, the OFF-center, and the ON/OFF or nonoriented (N-0) receptive fields) and three with orientation specific responses (the S, the C, and the B categories of receptive field). Units lacking orientation specificity were concentrated in laminae 4A, 4C beta and 6 while, for the cells with orientation specificity, C cells were found in laminae 4B and 6, B cells in 2/3 and 5, and S cells predominantly in laminae 2/3, 4C alpha, and 5. The results of electrical stimulation indicated that cell-to-cell transmission time in the monkey striate cortex is 1.5 msec, and latency measures showed that cells with a monosynaptic drive from the thalamus were confined to laminae 4 and 6 while disynaptically driven cells were found principally in upper lamina 4 (4A and 4B). No cell class was identified exclusively with a given ordinal position and there were many types of potential first-order neurons. The conduction time from one stimulating electrode to the next in the optic radiation was used to identify the afferent input to each striate neuron. The input to color-coded neurones was found to come exclusively from parvocellular layers while the C cells and two subclasses of the S cell (S2 and S3) were driven predominantly by the magnocellular subdivision. For other cell types (those with ON-center, N-0, and S1 receptive fields) the input came from either type of LGN neuron. The laminar distribution of neurons receiving a direct input from the magnocellular and parvocellular streams is in accord with the results of anatomical studies into the site of termination of the LGN input. The cell types receiving these direct inputs vary in the two streams so that the parvocellular input terminates on cells with ON-center and N-0 receptive fields in lamina 4C beta while the magnocellular input goes to cells with S, ON-center, N-0, and C receptive fields in lamina 4C alpha and the lower part of 4B. Consideration is given to the influence of these results on models for neural processing in monkey striate cortex and a comparison is drawn with the results of similar studies in the cat.