Responses to diffuse monochromatic light were recorded from single units in the diencephalon of pigeon. Units were both excited and inhibited by light stimulation. Intensity-response functions based on latency measures to the first spike after stimulation were used to generate action spectra. One class of spectral sensitivity functions presumably from rods, showed peak sensitivities near 500 nm: these functions were unaffected by changing criterion values used to generate the functions. A second class of cone functions showed multiple peak sensitivities at 540 nm and 600–620 nm. These units shifted their peak sensitivities with a change in criterion values. Unit response types tended to be localized differentially in the nucleus rotundus. Excitatory units were located in the dorsal half of the nucleus, while inhibitory units were located in the ventral half, with a few exceptions. An attempt was made to integrate the present findings with previous behavioral, electrophysiological, photochemical, and anatomical data in the pigeon.

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