Tonic retinal influences in primates.

A type of unit discharge, termed "luxotonic," has been found in the striate cortex of unanesthetized squirrel monkeys and macaques.6, 21 The firing frequency of these units shows relatively little adaptation, continues indefinitely (hours), and reflects the level of diffuse illumination of the eye. The more numerous "photergic" units discharge more rapidly in the light, whereas "scotergic" units fire fastest in the dark (or at luminance levels below threshold for cones). Luxotonic activity is abolished by anesthesia and has not been described for striate cortex of other species. Primates also display a profound alteration in the EEG of striate cortex following elimination of all retinal input.32 Since this change is far more drastic than that produced by blindness in other species, it is natural to inquire whether it is related to the loss of the normally prominent luxotonic activity. When the blind monkey sleeps, the bizarre EEG is replaced by patterns wholly normal in appearance,32 indicating that some nonvisual system has extensive access to striate cortex in this state.