Interaction of the body, head, and eyes during walking and turning

Body, head, and eye movements were measured in five subjects during straigh t walking and while turning corners. The purpose was to determine how well the head and eyes followed the linear trajectory of the body in space and w hether head orientation followed changes in the gravito-inertial accelerati on vector (GIA). Head and body movements were measured with a video-based m otion analysis system and horizontal, vertical, and torsional eye movements with video-oculography. During straight walking, there was lateral body mo tion at the stride frequency, which was at half the frequency of stepping. The GIA oscillated about the direction of heading, according to the acceler ation and deceleration associated with heel strike and toe flexion, and the body yawed in concert with stepping. Despite the linear and rotatory motio ns of the head and body, the head pointed along the forward motion of the b ody during straight walking. The head pitch/roll component appeared to comp ensate for vertical and horizontal acceleration of the head rather than ori enting to the tilt of the GIA or anticipating it. When turning corners, sub jects walked on a 50-cm radius over two steps or on a 200-cm radius in five to seven steps. Maximum centripetal accelerations in sharp turns were ca.0 .4 g, which tilted the GIA ca.21 degrees with regard to the heading. This w as anticipated by a roll tilt of the head of up to 8 degrees. The eyes roll ed 1-1.5 degrees and moved down into the direction of linear acceleration d uring the tilts of the GIA. Yaw head deviations moved smoothly through the turn, anticipating the shift in lateral body trajectory by as much as 25 de grees. The trunk did not anticipate the change in trajectory. Thus, in cont rast to straight walking, the tilt axes of the head and the GIA tended to a lign during turns. Gaze was stable in space during the slow phases and jump ed forward in saccades along the trajectory, leading it by larger angles wh en the angular velocity of turning was greater. The anticipatory roll head movements during turning are likely to be utilized to overcome inertial for ces that would destabilize balance during turning. The data show that compe nsatory eye, head, and body movements stabilize gaze during straight walkin g, while orienting mechanisms direct the eyes, head, and body to tilts of t he GIA in space during turning.