Affiliations: [a] Department of Neurology, Mount Sinai School of Medicine, New York, NY, USA | [b] Laboratory of Biological Anthropology, Osaka University, Osaka, Japan | [c] Department of Computer and Information Science, Brooklyn College, Brooklyn, NY, USA
Correspondence:
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Corresponding author: Steven Moore Ph.D., Mount Sinai School of Medicine, Department of Neurology, Box 1135, 1 E 100th St., New York NY 10029, USA. Tel.: +1 212 241 9306; Fax: +1 212 831 1610; E-mail: steven.moore@mssm.edu
Abstract: Rotation axes were calculated during active head movements using a motion analysis system. The mean rotation axis for 1 Hz head pitch when seated was posterior (6 mm) and inferior (21 mm) to the interaural axis, shifting 16 mm downwards when standing. During seated 2 Hz head pitch the rotation axis was close to the interaural axis, shifting downwards 15 mm when standing. This downward shift suggests that cervical vertebrae were recruited during head pitch with the trunk unsupported. The proximity of the pitch axis to the otoliths implies minimal otolith activation during small-amplitude, high-frequency pitch rotations, such as those encountered during locomotion. The mean rotation axis for 1 Hz yaw rotation was located slightly posterior (10 mm) to the interaural axis at the midpoint between the vestibular labyrinths when both seated and standing. In addition, the orientation of the plane of yaw rotation relative to the stereotaxic horizontal plane (pitched 5° nose-down) was essentially fixed in head coordinates, regardless of the pitch orientation of the head, suggesting that yaw movements occur about an axis restricted by the mechanical structure of the atlanto-axial joint. The results demonstrate that the instantaneous rotation axes technique overcomes the inherent instability of the helical-axis representation for small head movements.
Keywords: rotation axis, head rotation, locomotion, VOR, otoliths
