Modeling the organization of the linear and angular vestibulo-ocular reflexes.

A one-dimensional mathematical model of the compensatory linear vestibuloocular reflex (lVOR) was developed. The model was based on the concept that to effect oculomotor compensation, linear head acceleration sensed by the otoliths must be integrated twice to form the angular position-related signal required by the motoneurons. This contradicts the postulate that linear acceleration is differentiated to generate "jerk," which is then used to drive the compensatory lVOR. The transfer characteristics of different otolith afferent classes were modeled by a transfer function with a common modal structure and different degrees of compensation. Both the time and frequency domain behavior of regular and irregular otolith afferents were simulated. The outputs of the various afferent classes were superposed by a linear filter to generate the velocity command which drives the oculomotor velocity-position integrator. The model was used to simulate the dominant gain and phase characteristics of the compensatory lVOR in monkey and the dynamic characteristics of the compensatory human lVOR response for brief periods of linear acceleration on a sled. The model was then combined with the velocity storage-based model of the angular vestibulo-ocular reflex (aVOR) to simulate the eye velocity response to centrifugation in monkey and man. The model suggests that the orientation response that modifies the time constants of the velocity storage integrator is the dominant aspect of the response to linear acceleration in monkey. Human responses, on the other hand, are dominated by an effect of the beating field, which modifies the eye velocity command to the oculomotor system.