Visual desynchronization of cortical EEG impaired by lesions of superior colliculus in rats.

It is unclear whether rats with lesions of the superior colliculus (SC) neglect visual stimuli because a) they fail to notice the stimuli or b) they notice the stimuli but fail to make specific orienting movements of the eyes and head toward them. To investigate this issue, we used an index of "noticing" that does not involve overt orienting movements, namely, desynchronization of the cortical electroencephalogram (EEG) in drowsy animals. In the first experiment, large collicular lesions with some invasion of surrounding areas severely impaired EEG desynchronization to a brief overhead flash of light but did not significantly affect desynchronization to brief bursts of white noise. No impairment was found after control lesions of cerebral cortex overlying the superior colliculus or of striate cortex. A subsequent experiment found that the visual impairment was still present when lesions were confined to the superior colliculus. It appears that rats with lesions of the superior colliculus can, in certain circumstances, fail to notice visual stimuli and that in rats, the superior colliculus is concerned not only with specific orienting movements to visual stimuli but also with visual desynchronization of the cortical EEG.