Spectral Responses of Single Units in the Primate Visual Cortex

Spectral response behavior of single units was investigated in the visual cortex of the unanesthetized macaque monkeys with test stimuli of 0.5° in visual angle. 1. Cortical and optic radiation units were distinguished by the configuration of spikes and spontaneous discharge rate. 2. “On” units responding to a narrow-band of the spectrum were designated as “chromatic” or C type units. Their response maxima were found in three selected regions of the spectrum, orange-red (600-640 mμ), green (520-540 mμ) and blue (460 mμ) respectively. Most units showed one or two submaxima besides the dominant peak in their spectral response curves. 3. Some units responded to a certain limited region of the spectrum with “on” or “off”, but to another with “off” or “on”, thus changing discharge type depending on the wavelengths. These units were designated as “opponent” or O type units. 4. Some units responded to the whole parts of the spectrum, but showed no clear-cut response maxima anywhere in the visible parts of the spectrum. 5. Some “on-off” units and pure “off” units showed two response maxima towards both ends of the spectrum. 6. Two kinds of units, A and B, relating to rod receptors were distinguished. The common feature of these units was that the sensitivity maximum was found at about 500 mμ The unit A had a large receptive field and gave “on-off” discharges. Its photosensitivity is low compared with that of B. The unit B was always linked with photopic units and worked in a range of low intensities in which the linked photopic unit was almost inactive. Its receptive field was very small as compared with that of A.