Physiological organization of layer 4 in macaque striate cortex.

Numerous highly angled electrode penetrations through the opercular region of macaque striate cortex reveal that layers 4A, 4C alpha, and 4C beta--the primary input sublaminae for axons from the lateral geniculate nucleus (LGN)--are retinotopically organized on a fine scale and populated mostly by monocularly driven cells having small receptive fields and lacking orientation selectivity. Layer 4B, which does not receive a direct thalamic input, contains orientationally selective cells, and many of these are also direction selective. To a significant degree the response properties of cells in layers 4C alpha and 4C beta reflect the response properties of their respective afferent inputs, from the magno- and parvocellular laminae of the LGN. Accordingly, cells in layer 4C alpha have lower contrast thresholds and larger minimum response fields than do the cells in layer 4C beta. In contrast to this clear-cut separation, the cells of layer 4A (whose major source of direct LGN input arises from the parvocellular layers) exhibit both high and low contrast thresholds. With regard to the precision of retinotopic mapping that is seen in lamina 4C, it is noteworthy that there is substantial overlap among the minimum response fields of neighboring neurons. Due to a larger mean receptive field size, this overlap is greater in layer 4C alpha than it is in 4C beta. In either sublamina, however, the minimum cortical distance that separates different and nonoverlapping parts of the visual field corresponds closely--within a factor of 2--to the known arborizational spreads of single geniculate afferents.