From perception to action:: phase-locked gamma oscillations correlate with reaction times in a speeded response task

Background: Phase- locked gamma oscillations have so far mainly been described in relation to perceptual processes such as sensation, attention or memory matching. Due to its very short latency (approximate to 90 ms) such oscillations are a plausible candidate for very rapid integration of sensory and motor processes. Results: We measured EEG in 13 healthy participants in a speeded reaction task. Participants had to press a button as fast as possible whenever a visual stimulus was presented. The stimulus was always identical and did not have to be discriminated from other possible stimuli. In trials in which the participants showed a fast response, a slow negative potential over central electrodes starting approximately 800 ms before the response and highly phase-locked gamma oscillations over central and posterior electrodes between 90 and 140 ms after the stimulus were observed. In trials in which the participants showed a slow response, no slow negative potential was observed and phase- locked gamma oscillations were significantly reduced. Furthermore, for slow response trials the phase- locked gamma oscillations were significantly delayed with respect to fast response trials. Conclusion: These results indicate the relevance of phase- locked gamma oscillations for very fast ( not necessarily detailed) integration processes.