A letter on a hand: Rotating together or separately?

The motor system plays a role in some object mental rotation tasks, and researchers have reported that people may use a strategy of motor simulation to mentally rotate objects. In this study, we used images of a hand with a letter printed on the palm to directly determine whether a hand image can be automatically rotated during the deliberate mental rotation of an object and whether the hand and object are rotated in the same trajectory. A total of 41 participants were shown the stimuli and asked to decide whether the letters, which were upright or tilted at specific degrees, were normal or mirrored. The hand images in the background showed either a left or a right hand in the palm view, with fingers pointing upwards, medial, downwards, or lateral. Reaction times and error rates were measured to determine the speed and accuracy of mental rotation. A complex interaction between the hand posture and letter orientation revealed that the hand image was mentally rotated automatically, together with the deliberate mental rotation of the letter. The biomechanical constraints of the hand also influenced reaction times, suggesting the involvement of the motor system in the concomitant mental rotation of the hand image. Consistent with the motor simulation theory, the participants seemed to imagine the hand carrying the object in its movement. These behavioural data support the motor simulation theory and elucidate specific processes of mental rotation that have not been addressed by neuroimaging studies.