Neural mechanisms of movement speed and tau as revealed by magnetoencephalography.

A fundamental aspect of goal-directed behavior concerns the closure of motion-gaps in a timely fashion. In this context, the critical variable is the time-to-closure, called tau (Lee in Perception 5:437-459, 1976), and is defined as the ratio of the current distance-to-goal gap over the current instantaneous speed towards the goal. In this study, we investigated the neural mechanisms of speed and tau in pointing hand movements by recording MEG activity from the whole brain of 20 right-handed healthy human subjects operating a joystick with their right hand. The relations between neural signals and speed and tau were analyzed using an autoregressive multiple regression model, where the time-varying MEG signal was the dependent variable and the corresponding value of speed and tau were the independent variables. With respect to speed, we found that 81% of sensors showed significant relations over the left frontal-parietal, left parieto-temporal, and, less prominently, the right temporo-occipital sensor space. These results document the widespread involvement of brain areas with movement speed, especially in the left hemisphere (i.e., contralateral to the moving limb), in accord with previous studies. With respect to tau, 22% of sensors showed significant relations over the parietal (bilaterally), right parietal-temporal, and, less prominently, the left temporo-occipital sensor space. The tau effects often occurred concurrently with speed effects and spatially overlapped in the left fronto-parietal sensors. These findings document for the first time the time-varying, dynamic processing of information regarding tau in specific brain areas, including the right parietal cortex. This is of special interest, for that area has been found to be involved in processing information concerning the duration of time intervals in perceptual tasks (Harrington et al. in J Neurosci 18:1085-1095, 1998; Rao et al. in Nat Neurosci 4:317-323, 2001). Since tau is itself a time interval, we hypothesize that the right parietal focus of tau processing observed in this study reflects the ongoing processing of tau as an interval for a timely arrival of the hand to the target.