Horizontal segregation of color information in the middle layers of foveal striate cortex

Abstract

The present study examines the chromatic organization of foveal striate cortex in the awake monkey by means of a series of microelectrode penetrations made as perpendicular as possible to the layers. The study includes 79 penetrations and 261 cells, of which 218 were tested systematically for color selectivity. Detailed analyses are conducted on a subset of 41 penetrations, which included 164 color-tested cells, an average of four cells per penetration. The penetrations were divided into two major categories on the basis of orientation selectivity testing. One group of penetrations contained at least one nonoriented cell in the first 600 microns of microelectrode trajectory (upper layers), whereas the other group of penetrations contained only oriented cells in the first 600 microns of microelectrode trajectory. The two groups were called N (nonoriented) and O (oriented), respectively. Analysis of the color properties of cells in N and O penetrations revealed that middle layer cells in N penetrations showed poor responses to white light, and color preferences for endspectral wavelengths, i.e., red or blue. Middle layer cells in O penetrations, by contrast, responded well to white light and to midspectral wavelengths. There were two subgroups of N penetrations, characterized by predominantly red (NR) or blue (NB) sensitivity in the middle layers. O penetrations could likewise be divided up into three subgroups (OG, OY, OW), characterized, respectively, by predominant sensitivity to greenish wavelengths (490-540 nm), yellowish wavelengths (550-600 nm), or white (i.e., all colors). Despite the identification of five subgroups, similarities between NR and OY, between NB and OG, and between OY and OW subgroups might be consistent with a continuum. The middle layers of N penetrations contained a unique class of cells with excitatory responses restricted to the two spectral ends, endspectral double-peak cells. A model is proposed for the color organization of layer 4 in foveal striate cortex, with red and blue zones in register with alternate cytochrome oxidase "blobs" of layers 2 and 3, white zones in register with interblob centers, and yellow and green zones in between.