Beyond variability: subjective timing and the neurophysiology of motor cognition

BACKGROUND: Movement simulation helps increasing the chances to reach goals. A cognitive task used to study the neuro-behavioral aspects of movement simulation is mental rotation: people mentally re-orient rotated pictures of hands. However, the involvement of the primary motor cortex (M1) in mental rotation is largely controversial. HYPOTHESIS: Such inconsistency could arise from potential methodological flaws in experimental procedures and data analysis. In particular, until now, the timing of M1 activity has been computed in absolute terms: from the onset of mental rotation (onset-locked), neglecting intra- and inter-subject variability. METHODS: A novel phase-locked approach is introduced to synchronize the same phases of cognitive processing among different subjects and sessions. This approach was validated in the particular case of corticospinal excitability of the motor cortex during mental rotation. RESULTS: We identified the relative time-windows during which the excitability of M1 is effector-specifically modulated by different features of mental rotation. These time windows correspond to the 55%-85% of the subjective timing. CONCLUSIONS: In sum, (i) we introduce a new method to study the neurophysiology of motor cognition, and (ii) validating this method, we shed new light on the involvement of M1 in movement simulation