Attitude control is associated with coordinated limb, body, head, and ocular movements through vestibular interoception. During straight walking, walking speed for velocities ranging from 1.2–1.8 m/s. Over this range of walking velocities, there is both translation and rotation of the head in the sagittal plane, reaching translational frequencies close to 2 Hz, peak vertical linear accelerations of 0.3–0.5 g, and peak pitch velocities of 15–22°/s. The magnitude and frequency of the pitch angular and vertical translational head movements are sufficient to activate both the angular and linear vestibulo-ocular (aVOR and lVOR) and vestibulo-collic (aVCR and lVCR) reflexes, which may play important roles in directing gaze and in stabilizing gait. In support of this, unilateral vestibular disease is associated with instability of gait, and astronauts, whose vestibular function has been adapted to microgravity, also experience difficulty when walking a curved path after flight. The purpose of this study was to infer a possible role for the vestibular interoception in producing head and eye movements in three dimensions as the body moves in space.
Head and body movements were measured with a video-based motion analysis system and horizontal, vertical, and torsional eye movements with video-oculography. During straight walking, the head pointed along the forward motion of the body during straight walking. When turning corners, yaw head deviations moved smoothly through the turn, anticipating the shift in lateral body trajectory by as much as 25°. The trunk did not anticipate the change in trajectory. Gaze was stable in space during the slow phases and jumped forward in saccades along the trajectory, leading it by larger angles when the angular velocity of turning was greater. The anticipatory roll head movements during turning are likely to be utilized to overcome inertial forces that would destabilize balance during turning. The data show that compensatory mechanisms stabilize gaze, while orienting mechanisms direct the eyes, head, and body to tilts of the gravito-inertial acceleration in space through vestibular interocetpion.