Effects of physical practice on the duration of motor imagery.

Motor Simulation Theory proposes that imagined actions are produced using the brain's motor system, and should therefore always be temporally equivalent to physical movements. However, empirical results are not always consistent with this prediction. Studies indicate that the durations of unfamiliar imagined actions are over-estimated, whereas the durations of more familiar actions may be closer to (or even faster than) actual movement execution. We therefore examined the effects of different levels of practice on the durations of both physically performed and imagined actions. Participants (N=31) completed an initial assessment in which the durations of physically performed and imagined finger movement sequences were measured. Participants then completed three days of physical training in which different sequences received either extensive training (150 repetitions/session), minimal training (10 repetitions/session), or no training. In a subsequent assessment session, we found that the time taken to both physically execute and imagine performing sequences decreased with training. However, contrary to the predictions of Motor Simulation theory, imagined movement durations consistently over-estimated those of physically performed movements. While the difference in the timing of imagined and physically executed movements decreased between the initial and final assessment, this effect was not modulated by training. These results extend our understanding of the relationship between motor imagery and physical practice, and highlight a key limitation in the predictions of Motor Simulation Theory.