Neuromagnetic correlates of sensorimotor synchronization

Sensorimotor synchronization tasks, in which subjects have to tap their fin ger in synchrony with an isochronous auditory click, typically reveal a syn chronization error with the tap preceding the click by about 20 to 50 msec. Although extensive behavioral studies and a number of different explanator y accounts have located the cause of this so-called "negative asynchrony" o n different levels of processing, the underlying mechanisms are stilt nor c ompletely 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 task s 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 t emporal coupling between "tap" and "click," evoked responses were averaged time-lacked to the auditory signal and the tap onset. Tap-related responses could he explained with a three dipole model: One source, peaking at appro ximately 77 msec before tap onset, was localized in contralateral primary m otor cortex (MI); the two other sources, peaking approximately at tap onset and 75 msec after tap onset, in contralateral primary somatosensory cortex (ST). Temporal coupling of these sources was compared in relation to diffe rent trigger points. The second SI source was equally well time-locked to t he 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 cli ck showed that the second event-irrespective whether tap or click-was decis ive in triggering the second SI source. This suggests that subjects use mai nly sensory feedback in judging and evaluating whether they are "keeping ti me."