DYNAMIC PROPERTIES OF THE HUMAN VESTIBULOOCULAR REFLEX DURING HEAD ROTATIONS IN ROLL

We investigated the dynamic properties of the human vestibule-ocular r eflex (VOR) during rob head rotations in three human subjects using th e magnetic search coil technique. In the first of two experiments, we quantify the behavior of the ocular motor plant in the torsional plane . The subject's eye was mechanically displaced into intorsion, extorsi on or abduction, and the dynamic course of return of the eye to its re sting position was measured. The mean predominant time constants of re turn were 210 msec from intorsion, 83 msec from extorsion, and 217 mse c from abduction, although there was considerable variability of resul ts from different trials and subjects. In the second experiment, we qu antify the efficacy of velocity-to-position integration of the vestibu lar signal. Position-step stimuli were used to test the torsional or h orizontal VOR, being applied with subjects heads erect or supine. Afte r a torsional position-step, the eye drifted back to its resting posit ion, but after a horizontal position-step the eye held its new horizon tal position. To interpret these responses we used a simple model of t he VOR with parameters of the ocular motor plant set to values determi ned during Expt 1. The time constant of the velocity-to-position neura l integrator was smaller (typically 2 sec) in the torsional plane than in the horizontal plane (> 20 sec). No disconjugacy of torsional eye movements was observed. Thus, the dynamic properties of the VOR in rob differ significantly from those of the VOR in yaw, reflecting differe nt visual demands placed on this reflex in these two planes.