In choice reaction time tasks (RT), only one response being correct on a given trial, the possible responses are thus in competition in order to be emitted first. Several models assume that inhibition of the non required responses is needed to resolve this competition. Using Laplacian-transformed EEG recordings in two-choice RT tasks, we show here that shortly before EMG onset, the activation of the (contralateral) primary motor cortex involved in the response is concurrently associated with the covert inhibition of the (ipsilateral) primary motor cortex not involved in the response. We demonstrate that while contralateral activation is mandatory, ipsilateral inhibition is optional and context-dependent. We propose that ipsilateral inhibition is implemented to prevent the emission of errors, which is consistent with the fact that the strength of ipsilateral inhibition depends on the risk of committing an error. This would indicate that ipsilateral inhibition is a sign of the implementation of a proactive action monitoring process in choice RT conditions.