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.
In his seminal paper, Donders proposed that Choice reaction time (RT) tasks differ from Go/No-go RT tasks only by the insertion of a response decision operation. We evaluated this possibility by comparing the time course of Laplacian-transformed ERPs, recorded over the primary (M1s) and supplementary motor areas (SMAs) in a Choice and in a Go/No-go task. Laplacian-transformed ERPs showed that a component that develops over the SMAs during the RT of Choice tasks vanishes in our Go/No-go task. This indicates that a process, absent in the Go/No-go task, was "inserted" in the Choice task. The Choice versus Go/No-go manipulation also modified the motor command: the activity recorded over M1s and the delay separating EMG onset from response completion depended on the nature of the task. This indicates that, although a process was inserted in the Choice task, it was not "purely" inserted, contrary to Donders' initial assumption.
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