Vestibular compensation and orientation during locomotion.

Body, head, and eye movements were studied in three dimensions while walking and turning to determine the role of the vestibular system in directing gaze and maintaining spatial orientation. The body, head, and eyes were represented as three-dimensional coordinate frames, and the movement of these frames was related to a trajectory frame that described the motion of the body on a terrestrial plane. The axis-angle of the body, head, and eye rotation were then compared to the axis-angle of the rotation of the gravitoinertial acceleration (GIA). We inferred the role of the vestibular system during locomotion and the contributions of the VCR and VOR by examining the interrelationship between these coordinate frames. Straight walking induced head and eye rotations in a compensatory manner to the linear accelerations, maintaining head pointing and gaze along the direction of forward motion. Turning generated a combination of compensation and orientation responses. The head leads and steers the turn while the eyes compensate to maintain stable horizontal gaze in space. Saccades shift horizontal gaze as the turn is executed. The head pitches, as during straight walking. It also rolls so that the head tends to align with the orientation of the GIA. Head orientation changes anticipate orientation changes of the GIA. Eye orientation follows the changes in GIA orientation so that the net orientation gaze is closer to the orientation of the GIA. The study indicates that the vestibular system utilizes compensatory and orienting mechanisms to stabilize spatial orientation and gaze during walking and turning.