Fast gamma oscillations in areas MT and MST occur during visual stimulation, but not during visually guided manual tracking.

We studied the incidence of oscillatory activity in the gamma range (35-110 Hz) in single cell and multi-unit activity recorded from extrastriate areas MT (middle temporal) and MST (superior middle temporal) while rhesus monkeys performed different behavioural tasks. During full field stimulation by coherent motion of random dots, we observed gamma oscillations in approximately 20% of the cells. The average oscillation frequencies differed considerably between both animals (60 Hz vs 100 Hz). In both animals, oscillatory modulation was particularly strong at sites that showed a strong directional bias to visual stimulation. The amount of oscillatory activity was roughly the same whether stimulus movement was presented during fixation or whether the animal had to perform pursuit movements across a stationary visual pattern. If cells were engaged in gamma oscillations during visual stimulation, the amount of oscillatory modulation was dependent on stimulus direction, stimulus velocity and stimulus contrast. During a visually guided manual tracking task no gamma activity was detectable. Cells with clear oscillatory modulation during the full field stimulation failed to show oscillatory activity when the animal was involved in a motor task in which the visual motion information had to be evaluated for the correct movement of the hand. Our results reaffirm the ubiquitous presence of stimulus-induced gamma oscillation in extrastriate areas MT and MST of the awake monkey during various stimulus conditions, but they fail to support the notion that high-frequency gamma oscillations in this area play a specific role during cortical control of a motor response to visual stimulation.