Alpha and beta oscillatory changes during stimulus-induced movement paradigms: effect of stimulus predictability.

We studied the effect of stimulus predictability on the alpha and beta changes observed in central regions during stimulus-induced movement paradigms. Six young volunteers were instructed to extend briskly their dominant wrist as soon as possible after hearing a 2000 Hz sound. Two sequences of stimuli were presented to each subject, the first rhythmic at 1/6 s and the second with random intervals between 5 and 13s. A time-frequency analysis of nonphase-locked activity in the 7-37 Hz range was performed on stimulus-centred EEG sweeps using wavelet filters and Gabor transforms. During the sequence of predictable rhythmic stimuli, stimulus-induced movements were accompanied by a decrease in beta activity that began contralaterally about 1 s prior to the stimulus and extended to both sides later on. This decrease was followed by a rebound after the end of the movement. In the alpha band, the decrease observed started just after the sound. During the sequence of non-predictable, random stimuli, stimulus-induced movements were accompanied by a shorter and smaller alpha and beta-ERD, that started after the stimulus. The presence of a pre-stimulus beta ERD only in the rhythmic predictable paradigm suggests that central pre-movement beta ERD may be an indicator of motor preparation, and could be used for objective evaluation of time estimation and motor timing. The minimal differences observed in the alpha changes in both paradigms suggest that alpha-ERD may not be linked to motor preparation.