Self-touch affects motor imagery: a study on posture interference effect.

Several studies showed that mental rotation of body parts is interfered with by manipulation of the subjects' posture. However, the experimental manipulations in such studies, e.g., to hold one arm flexed on one's own chest, activated not only proprioceptive but also self-tactile information. Here, we tested the hypothesis that the combination of self-touch and proprioception is more effective than proprioception alone in interfering with motor imagery. In Experiment 1 right- and left-handers were required to perform the hand laterality task, while holding one arm (right or left) flexed with the hand in direct contact with their chest (self-touch condition, STC) or with the hand placed on a wooden smooth surface in correspondence with their chest (no self-touch condition, NoSTC); in a third neutral condition, subjects kept both arms extended (neutral posture condition, NPC). Right-handers were slower when judging hand laterality in STC with respect to NoSTC and NPC, particularly when the sensory manipulation involved their dominant arm. No posture-related effect was observed in left-handers. In Experiment 2, by applying the same sensory manipulations as above to both arms, we verified that previous results were not due to a conflict between perceived position of the two hands. These data highlighted a complex interaction between body schema and motor imagery, and underlined the role of hand dominance in shaping such interaction.