Perceptual and Motor Processes in Motor Imagery.

The degree to which motor imagery engages the motor system or relies on perceptual/cognitive processes is a continuing debate. Here, we used the size weight illusion to create dissociation between perception and action to address the nature of motor imagery. Participants alternated lifting bricks of equal mass but where one brick was larger than the other, resulting in a perceptual illusion. Fifty-seven participants were divided into three groups differing in the modality used for training (motor imagery, MI; and overt execution, OE) and exposure to the size weight illusion pretraining, one (MI-2) and five (MI-10 and OE) lifts of each brick. We hypothesized that the MI groups would use lifting dynamics post-training consistent with the illusion, whereas the OE group would maintain accurate lifting forces. Contrary to our hypothesis, the OE and MI-10 groups maintained the effect of the illusion post-training. In the MI-2 group, perception of the bricks' weight changed to reflect the participant's belief that large objects are heavy, and they correspondingly adjusted their lifting force post-training. These results demonstrate that perceptual and motor processes are engaged during motor imagery and that the simulation of the motor component of the movement during motor imagery guides the performed action.