Stimulus-dependent neuronal oscillations and local synchronization in striate cortex of the alert cat.

Neuronal responses to visual stimuli that are correlated on a millisecond time scale are well documented in several areas of the mammalian visual cortex. This coherent activity often takes the form of synchronous rhythmic discharges ranging in frequency from 20 to 70 Hz. We performed experiments to determine the incidence and properties of this rhythmic activity in the striate cortex of alert cats and to compare this activity to similar data collected in the striate cortex of anesthetized cats. The results demonstrate that optimal visual stimuli evoke robust, locally synchronous, 20-70 Hz oscillatory responses in the striate cortex of cats that are fully alert and performing a visual fixation task. The oscillatory activity is stimulus dependent, largely absent during periods of spontaneous activity, and shows a systematic increase in frequency with increasing stimulus velocity. Thus, the synchronous oscillatory activity observed in this and earlier studies cannot be explained as an artifact of anesthesia nor as a phenomenon that occurs independent of visual stimulation. Rather, it is a robust process that is present in the alert state and is dependent on the presence and specific properties of visual stimuli.