Response recovery cycles in the visual cortex and superior colliculus following conditioning "ON" and "OFF"...

The responsiveness of visual cortex (VC) and superior colliculus (SC) was simultaneously compared following conditioning "ON" or "OFF" stimulation, in the rabbit. Average evoked responses were recorded simultaneously from the visual cortex ans superior colliculus. "ON" or "OFF" steps constituted the conditioning stimuli whereas the test stimulus consisted of optic nerve stimulation. All evoked responses exhibited a reversal of their polarity when the electrode was moved in the dorsoventral direction (Negative-Positive in the SC, Positive-Negative in the VC). This assured the somato-dentritic origin of the potentials. The results showed that responsiveness in both structures was significantly higher following an "OFF" simulus than after an "ON" step. Collicular responsiveness was higher than in the VC when the same conditioning stimulus was applied. The spatial distribution of the source of "OFF" responses was circumscribed to the ventral part of the superficial layer of the superior colliculus. These results suggest specific properties associated with the brightening and dimming systems.