Spectral response curves of postgeniculate neurons in the cat.

Unit discharges in response to a spot of colored light subtending 0.5° in visual angle were recorded from the lateral gyros of the cat immobilized with Flaxedil. 1. Two types of responses, “on” and “off”, were most frequently obtained from postgeniculate neurons, optic radiation fiber and cortical units. The cortical unit responded with a lower rate and had a smaller receptive field than the radiation unit. 2. All the units showed scotopic spectral sensitivity curves, having a maximum at about 500 mμ. 3. The peak of the spectral sensitivity curve could not be shifted by light adaptation, although the sensitivity was much reduced. 4. Spectral response curves (spike frequency-wavelength relation) obtained with a high intensity of stimulus showed a dominant peak at about 500 mμ and a submaximum or hump around 580 mμ. 5. Some units reversed its response type from “off” to “on” after light adaptation.