Reorientation of visually evoked postural responses by different eye-in-orbit and head-on-trunk angular positions.

We examined the question of whether the position of the eyes in the head and of the head on the trunk influence the direction of visually elicited postural reactions. Normal subjects stood on a force platform viewing a large disc, rotating in the roll plane, always maintained orthogonal to the line of sight. The disc was presented at 0 degree, 30 degrees and 90 degrees to the right or left with respect to the mid-frontal plane of the subject's body and was viewed with various combinations of horizontal eye-in-orbit and head-on-trunk deviations. It was found that the main direction of body sway was always reoriented to be parallel to the disc (e.g. viewing the disc at 30 degrees oriented sway responses at a mean angle of 33 degrees). The largest sway responses were obtained when the disc was parallel to the sagittal plane of the body and was viewed with an ipsilateral eye-neck deviation totalling 90 degrees (head-on-trunk 60 degrees+eye-in-orbit 30 degrees). When eye and head deviations cancelled each other (i.e. eye-in-orbit +30 degrees combined with head-on-trunk -30 degrees), directional effects on sway also cancelled each other out. This result demonstrates that signals of eye-in-orbit and head-on-trunk position have the capability to redirect visuo-motor commands to the appropriate postural muscles. This allows vision to regulate postural balance whatever the position of the eyes in space. We speculate that this function is mediated by eye and neck proprioceptive signals (or alternatively by efference copy) with access to gain control mechanisms in the visuo-postural system.