Studies on light-sensitive units in the deep mesencephalon of blinded frogs

Unit responses susceptible to light stimulation of a small area on the frog's head were recorded in the deep encephalon of blinded specimens of Rana esculenta. The responses consisted of a spike discharge upon illumination. Using a threshold criterion the dark adaptation curves showed two parts, separated by a kink, the final dark threshold being complete after 20-30 min in darkness. Using a threshold criterion the spectral sensitivity curves under dark adapted conditions were broad with a peak at 548 nm. The dark adapted intensity threshold for a stimulus of 548 nm ranged between 0.15-1.4 microW/cm2.