Updating visual space during passive and voluntary head-in-space movements.

The accuracy of our spatially oriented behaviors largely depends on the precision of monitoring the change in body position with respect to space during self-motion. We investigated observers' capacity to determine, before and after head rotations about the yaw axis, the position of a memorized earth-fixed visual target positioned 21 degrees laterally. The subjects (n=6) showed small errors (mean=-0.6 degrees) and little variability (mean=0.9 degrees) in determining the position of an extinguished visual-target position when the head (and gaze) remained in a straight-ahead position. This accuracy was preserved when subjects voluntary rotated the head by various magnitudes in the direction of the memorized visual target (head rotations ranged between 5 degrees and 60 degrees). However, when the chair on which the subjects were seated was unexpectedly rotated about the yaw axis in the direction of the target (chair rotations ranged between 6 degrees and 36 degrees ) during the head-on-trunk rotations, the performance was markedly decreased, both in terms of spatial precision (mean error=5.6 degrees ) and variability (mean=5.7 degrees). A control experiment showed that the prior knowledge of chair rotation occurrence had no effect on the perceived target position after head-trunk movements. Updating an earth-fixed target position during head-on-trunk rotations could be achieved through both cervical and vestibular signals processing, but, in the present experiment, the vestibular output was the only signal that had the potentiality to contribute to accurate coding of the target position after simultaneous head and trunk movements. Our results therefore suggest that the vestibular output is a noisy signal for the central nervous signal to update the visual space during head-in-space motion.