Neuromagnetic correlates of sensorimotor synchronization.

Sensorimotor synchronization tasks, in which subjects have to tap their finger in synchrony with an isochronous auditory click, typically reveal a synchronization error with the tap preceding the click by about 20 to 50 msec. Although extensive behavioral studies and a number of different explanatory accounts have located the cause of this so-called "negative asynchrony" on different levels of processing, the underlying mechanisms are still not completely understood. Almost nothing is known about the central processes, in particular, which sensory or motor events are synchronized by subjects. The present study examined central-level processing in synchronization tasks with magnetoencephalography (MEG). Eight subjects synchronized taps with their right index finger to an isochronous binaural pacing signal presented at an interstimulus interval of 800 msec. To gain information on central temporal coupling between "tap" and "click," evoked responses were averaged time-locked to the auditory signal and the tap onset. Tap-related responses could be explained with a three dipole model: One source, peaking at approximately 77 msec before tap onset, was localized in contralateral primary motor cortex (MI); the two other sources, peaking approximately at tap onset and 75 msec after tap onset, in contralateral primary somatosensory cortex (SI). Temporal coupling of these sources was compared in relation to different trigger points. The second SI source was equally well time-locked to the tap and to the auditory click. Furthermore, analysis of the time locking of this source activity as a function of the temporal order of tap and click showed that the second event - irrespective whether tap or click - was decisive in triggering the second SI source. This suggests that subjects use mainly sensory feedback in judging and evaluating whether they are "keeping time."