Interhemispheric inhibition during mental actions of different complexity.

Several investigations suggest that actual and mental actions trigger similar neural substrates. Yet, neurophysiological evidences on the nature of interhemispheric interactions during mental movements are still meagre. Here, we asked whether the content of mental images, investigated by task complexity, is finely represented in the inhibitory interactions between the two primary motor cortices (M1s). Subjects' left M1 was stimulated by means of transcranial magnetic stimulation (TMS) while they were performing actual or mental movements of increasing complexity with their right hand and exerting a maximum isometric force with their left thumb and index. Thus, we simultaneously assessed the corticospinal excitability in the right opponent pollicis muscle (OP) and the ipsilateral silent period (iSP) in the left OP during actual and mental movements. Corticospinal excitability in right OP increased during actual and mental movements, but task complexity-dependent changes were only observed during actual movements. Interhemispheric motor inhibition in the left OP was similarly modulated by task complexity in both mental and actual movements. Precisely, the duration and the area of the iSP increased with task complexity in both movement conditions. Our findings suggest that mental and actual movements share similar inhibitory neural circuits between the two homologous primary motor cortex areas