The nature of unilateral motor commands in between-hand choice tasks as revealed by surface Laplacian estimation

From electroencephalographic recordings, we estimated the surface Laplacian over motor areas in a Stroop-like between-hand choice reaction time task in humans. Response-locked averages showed a (negative) "motor potential" over the primary motor areas contralateral to the response. At the same time, a positive wave was observed over the primary motor areas ipsilateral to the response. These data suggest that, when a between-hand choice is required, an inhibition of the primary motor cortex ipsilateral to the nonresponding hand is implemented. This observation is relevant to the interpretation of the lateralized readiness potential (LRP) because the LRP is blind to the respective contribution of the contralateral and ipsilateral motor cortices. In addition, a negative wave beginning about 200 ms before EMG onset and peaking about 50 ms before it occurred over the supplementary motor areas (FCz). This wave preceded the motor potential, which supports the view that the supplementary motor areas are upstream in a hierarchy of the motor command.