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In mental rotation tasks (MRT), people show a remarkably different pattern of responses to hand stimuli compared to geometric 3D objects. However, the mechanisms of these effects remain unclear. In order to provide a more solid understanding of the cognitive processes involved in the mental rotation of hands, I tested both left-handed and right-handed subjects on a modified Shepard & Metzler task with hand stimuli. I crossed two orthogonal hypothesis axes to yield four competing hypotheses. One axis of the hypothesis space contrasted i) embodied experience (people’s experience with their own hands) versus ii) world knowledge of a right-handed world. The other hypothesis axis contrasted a) the holistic motor imagery matching between the visual image of a hand on the screen and one’s own hand versus b) the resemblance of only the shape outline information from the hand stimuli with the proprioception of one’s own hands. The results suggest that, for mixed-handed people, embodied experience is important in the mental rotation of hands and the information is likely processed through a visual-proprioceptive integration cognitive mechanism. However, for extreme-handed people, the results only showed that extreme right-handers had overall better performance than extreme left-handers. World knowledge might independently influence performance for left hand stimuli while the performance for right hand stimuli is influenced by a combination of world knowledge and embodied experience. Finally, I discuss potential future studies that could further test embodied experience versus world knowledge in left-handed and right-handed people.

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.