FINAL ACCEPTED VERSION Phase-plane analysis of gaze stabilization to high acceleration head thrusts– a continuum across normal subjects and patients with loss of vestibular function

We investigated the vestibulo-ocular reflex (VOR) during high-acceleration, yaw-axis, head rotations in 12 normals and 15 patients with vestibular loss (7 unilateral (UVD) and 8 bilateral (BVD)). We analyzed gaze stabilization within a 200ms window after head rotation began, using phase planes, which allowed simultaneous analysis of gaze velocity and gaze position. These 'gaze planes' revealed critical dynamical information not easily gleaned from traditional gain measurements. We found linear relationships between peak gaze-velocity and peak gaze-position error when normalized to peak head speed and position, respectively. Values fell on a continuum, increasing from normals, to normals tested with very high acceleration (VHA = 10,000 – 20,000°/s 2), to UVD patients during rotations toward the intact side, to UVD patients during rotations toward the lesioned side, to BVD patients. We classified compensatory gaze corrections as gaze-position corrections (GPC) or gaze-velocity error corrections (GVC). We defined patients as better-compensated when the value of their end gaze position was low relative to peak gaze position. In the gaze plane this criterion corresponded to relatively stereotyped patterns over many rotations, and appearance of high velocity (100-400°/s) GPCs in the gaze plane ending quadrant (150-200ms after head movement onset). In less-compensated patients, and normals at VHA, more GVCs were generated, and GPCs were generated only after gaze-velocity error was minimized. These findings suggest that challenges to compensatory vestibular function can be from vestibular deficiency or novel stimuli not previously experienced. Similar patterns of challenge and compensation were observed in both patients with vestibular loss and normal subjects.