Perceived versus actual head-on-trunk orientation during arm movement control.

Static roll head tilt induces bias in the trajectory of upper limb voluntary movements. The aim of the experiment was to investigate whether this bias is dependant on the perception of body configuration rather than on its actual configuration. We used the 'return' phenomenon as a method to produce dissociation between perceived and actual head tilt. Static roll head tilt in supine subjects was sustained for 15 min during which subjects were periodically required to estimate verbally the tilt of their head respective to their trunk and draw, with their right index finger, straight lines aligned with their trunk. After 15 min, subjects' head were realigned with the trunk, and subjects continued to give verbal estimate of head position and perform the motor task. Results showed that the initial angular deviation of the lines in the direction opposite to head tilt gradually diminished. The adaptation was noticeable within the first 3-5 min of tilt and subsequently diminished. Verbal estimates confirmed the return phenomenon, i.e. subjects perceived their head as slowly returning towards its neutral position after a few minutes of sustained tilt. When realigned with the trunk, subjects experienced the illusion that their head was tilted in the opposite direction to the initial head tilt and a line deviation in the opposite direction to those made on initial exposure was observed (after-effect). These results indicate that the angular deviation in motor production observed in condition of static head tilt were largely related to the perceived body configuration and therefore favour the hypothesis that the conscious perception of body configuration plays a key role in organising sensorimotor tasks.