Visual–vestibular interaction in progressive supranuclear palsy

We measured the stability of gaze during horizontal head rotations at 1-3 Hz in four patients with progressive supranuclear palsy (PSP), while they viewed a stationary target. Median gain of compensatory eye movements was 0.94, similar to control subjects. During rotation in darkness, median gain of vestibulo-ocular reflex (VOR) was 0.88, similar to controls. Conversely, the median gain of smooth-pursuit eye movements at 1.0 Hz was 0.23, lower than controls. A simple superposition model of smooth pursuit and the VOR could not account for the observed gaze stability during fixation. Our results are further evidence that a visually mediated mechanism, independent of smooth pursuit, optimizes eye movements to compensate for head rotations.