Adapted head- and eye-movement responses to added-head inertia

Adaptation to inertia added to the head was studied in man by mounting masses on a rigidly attached helmet. Two- to ten-fold increases of inertia were thus produced, while an overhead suspension compensated for the weights. Eye and head positions and corresponding velocities were simultaneously recorded during eye-head tracking of a target stepping at 0.2 Hz in the horizontal direction. Without added inertia, fast gaze movements are type III, the accelerated head movement coming early and the resulting VOR truncating the simultaneous eye movement saccade in both amplitude and velocity. Head oscillations are fast and overcompensated by higher gain VOR. With added inertia, the adapted head movement is slowed and delayed. This permits the eye movement saccade to be completed before head movement begins and to escape truncation; the saccade is normal or slightly increased in amplitude. Head oscillations are slow and compensated by normal gain VOR. Either truncation of the saccade or overcompensation of the VOR leads to eye movement and gaze position error that is corrected for by secondary corrective saccades. These same two errors in gaze coordination could explain the cause of the perceived oscillopsia. Oscillopsia, or continual displacement or instability of the visual worlds, is a symptom of breakdown of space constancy, and was prominent and consistent in perceptual reports of our subjects. Adaptation resulting from adding inertia to the head occurred much faster than that induced by adding prisms or lenses.(ABSTRACT TRUNCATED AT 250 WORDS)