UPDATING VISUAL SPACE DURING PASSIVE AND VOLUNTARY HEAD-IN-SPACE MOVEMENTS

The accuracy of our spatially oriented behaviors largely depends on th e precision of monitoring the change in body position with respect to space during self-motion. We investigated observers' capacity to deter mine, before and after head rotations about the yaw axis, the position of a memorized earth-fixed visual target positioned 21 degrees latera lly. The subjects (n = 6) showed small errors (mean = 0.6 degrees) and little variability (mean = 0.9 degrees) in determining the position o f an extinguished visual-target position when the head (and gaze) rema ined in a straight-ahead position. This accuracy was preserved when su bjects voluntary rotated the head by various magnitudes in the directi on of the memorized visual target (head rotations ranged between 5 deg rees and 60 degrees). However, when the chair on which the subjects we re seated was unexpectedly rotated about the yaw axis in the direction of the target (chair rotations ranged between 6 degrees and 36 degree s) during the head-on-trunk rotations, the performance was markedly de creased, both in terms of spatial precision (mean error = 5.6 degrees) and variability (mean = 5.7 degrees). A control experiment showed tha t the prior knowledge of chair rotation occurrence had no effect on th e perceived target position after head-trunk movements. Updating an ea rth-fixed target position during head-on-trunk rotations could be achi eved 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 p osition after simultaneous head and trunk movements. Our results there fore suggest that the vestibular output is a noisy signal for the cent ral nervous signal to update the visual space during head-in-space mot ion.