Color-sensitive complex cells in monkey striate cortex.

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Abstract
With tungsten microelectrodes single cells in the monkey''s [Macaca mulatta] visual cortex were recorded, and those neurons that were sensitive to the color of the stimulus were specifically studied. In the primate striate cortex there are 4 classes of color-coded cells. The cells described in this paper were color-sensitive complex neurons. They had square or rectangular receptive fields that could not be mapped with either colored or white stationary stimuli. These cells responded only to moving monochromatic bars or edges of light. Some complex cells responded exclusively to 2-color edge stimuli. Stimulus width was critical, whereas length was not important. Most units were directionally selective. The fields varied in size from 0.5-5.degree. on a side. From cell to cell the peak spectral sensitivities ranged between 490-640 nm. Spectral sensitivity curves obtained with monochromatic backgrounds were similar to the absorption spectra of the monkey''s green-sensitive or red-sensitive cones. Thus these cells received excitatory connections from either red or green cones and silent antagonistic inputs from the other class. None of the cells tested were connected with rods. The orientation selectivity of the complex neurons to bar and edge stimuli was the result of spatial antagonistic interactions between cones of the same class rather than from opponent-color influences between different classes. In 53 instances recordings were made simultaneously with a single electrode from both a simple cell and a complex cell. The simple unit''s field was always in register with the complex cell''s field. The length of the simple field was always the same as the length of the complex field. Both units always had the same axis orientation and the same directional selectivity. Both cells always had similar spectral sensitivities and similar stimulus preferences. Most complex cells were driven by both eyes. In 46 of 53 instances of simple-complex multiple-unit recordings, the complex units were more strongly driven by 1 eye than the other. In every such case the simple cell was driven exclusively by the same eye as the one which dominated the complex neuron. Nearly all of the complex units were found in layers II, III, V, and VI. The multiple-unit recordings support the theory that these color-sensitive complex cells receive direct synaptic contacts from the dual opponent-color simple cells studied earlier. Thus the complex neurons are the 3rd stage in the cortical integration of color information.