Linearity of canal-otolith interaction during eccentric rotation in humans

During natural behavior, the head may simultaneously undergo rotation, transduced by the semicircular canals, and translation, transduced by the otolith organs. It has been demonstrated in monkey that the vestibulo-ocular reflexes (VORs) elicited by both endorgans (i.e., the angular and linear VORs, or AVOR and LVOR) sum linearly during combined rotation and translation, but this finding has proven more elusive in humans. To investigate the combined AVOR/LVOR response, six human subjects underwent yaw eccentric rotation at 3 Hz in darkness while displaced from the axis of rotation. Responses to on-center yaw rotation (AVOR alone) and interaural translation (LVOR alone) were also recorded. During eccentric rotation with the subject facing away from the axis of rotation (i.e., nose out), in which a yaw to the right occurs simultaneously with a translation to the right (i.e., translation in phase with rotation), the AVOR and LVOR acted synergistically. Responses were always out of phase with rotation, and became larger in magnitude as vergence increased. For nose-in eccentric rotation, during which translation is out of phase with rotation, the LVOR acted antagonistically to the AVOR. During near viewing, the LVOR often dominated the overall response when eccentricity was sufficiently large, producing eye movements that were in phase with the rotational stimuli. As vergence decreased, the LVOR influence diminished, eventually resulting in responses that were out of phase with rotation at lowest vergence. When the response to pure yaw rotation was vectorially removed from the responses to eccentric rotation, the results proved statistically indistinguishable from the LVOR recorded during interaural translation, suggesting that the ocular response to combined angular and linear motion reflects the linear combination of the AVOR and LVOR.