Limb (hand vs. foot) and response conflict have similar effects on event-related potentials (ERPs) recorded during motor imagery and overt execution

Although there is substantial evidence that motor execution (M-Ex) and motor imagery (M-Im) share a common neural substrate, the role of the primary motor cortex (M1) during imagery is still a matter of debate. The present ERP study tries to clarify the functional similarity between the two processes in respect of (i) the engagement of the corresponding somatotopic M1 areas during execution and imagery of hand vs. foot movements; and (ii) the effect of conflicting information on response preparation. To this end, we recorded ERPs from 28 electrode sites in 19 participants while they performed a conflict task with congruent (target and flanker arrowheads pointing in the same direction) and incongruent (target pointing in the opposite direction to the flanker arrowheads) trials. We obtained the lateralized readiness potential (LRP), a component generated in M1, while subjects physically executed or mentally simulated the task. As expected by the somatotopic organization of M1, the LRP was of opposite polarity when foot, rather than hand, movements were prepared. The inversion of polarity also occurred during M-Im, a result that strongly argues in favour of the participation of M1 in motor imagery. In incongruent trials, longer LRP latencies, a premature preparation of the incorrect response (positive deflection in LRP waveform) and a fronto-central N2 component associated with response conflict appeared during both M-Ex and M-Im. Altogether, the results support the functional equivalence of the two processes and give support to the clinical use of M-Im for the improvement and recovery of motor functions.