The role of the primary motor cortex in mental rotation: a TMS study

Mental rotation (MR) is sustained by a network of brain regions, including parietal, pre-motor and primary motor (M1) cortices. However it is still not clear whether M1 is recruited only when individuals mentally rotate hands or whether it is also enhanced by MR of non-body parts. Here we report two experiments in which the involvement of M1 in MR of hands and letters was tested using TMS. In Experiments 1a and 1b participants were asked to judge whether two line drawings, depicting either hands or letters, were the same or mirror images of each other (N = 112). Subjects were presented with pairs of stimuli with the same orientation (baseline condition) in half of the trials, while in the other half the stimulus in the right visual field was rotated (rotation condition). They performed the same-different task in three experimental situations: TMS of the primary motor hand area delivered at 400 ms after stimulus onset, sham TMS, and no-TMS. We stimulated the left M1 in Experiment 1a, and the right in Experiment 1b. Results showed that in Experiment 1a participants were slower after TMS when they performed MR of hands but not of letters. In Experiment 1b we failed to find an effect of TMS on MR of hands and letters. While in Experiment 1 the stimulus to be rotated was always presented in the right visual field, in Experiment 2 it was presented either in the left or in the right visual field. Results showed that only when TMS was delivered to the left M1, participants' ability to mentally rotate right and left hands slowed down. Taken together, these findings suggest that the left but not the right M1 plays a critical role in MR of hands.