Synchronizing effect of clock rhythm on the 'when to move' decision in repeated voluntary movements.

In 13 volunteers, surface EEGs from F(z), C(z), C(3), C(4), and P(z) electrodes and EMG from the flexor digitorum communis were recorded during an experimental task in which the performance of wrist flexion was linked with the selection of a clock sound. In the epoch from 4 to 1 s before movement, more than 95% of the records exhibited a small negative shift from the baseline. The position of these shifts on the time axis made it possible to create groups of time trials. The average EEG curves from these groups showed one prominent negative shift from the baseline, which could be related to the position of particular clock sounds. The slope values of these shifts were highest under the F(z) and C(z) electrodes. We suggested that these negative shifts coincided with 'when to move' decisions, which preceded the execution of the movement at varying intervals.