Effects of bilateral vestibular loss on podokinetic after-rotation.

We asked what the role of the vestibular system is in adaptive control of locomotor trajectory in response to walking on a rotating disc. Subjects with bilateral vestibular loss (BVL) were compared to age- and gender-matched controls (CTRL). Subjects walked in place on the surface of a rotating disc for 15 min and then attempted to step in place without vision on a stationary surface for 30 min. CTRL subjects demonstrated podokinetic after-rotation (PKAR), involuntarily and unknowingly turning themselves in circles while attempting to step in place. PKAR in CTRLs was characterized by a rapid rise in turning velocity over the first 1-2 min, followed by a gradual decay over the remaining 28 min. Subjects with BVL also demonstrated PKAR and had no knowledge of their turning. However, PKAR in BVLs was characterized by an extremely rapid, essentially instantaneous rise. Subjects with BVL immediately turned at maximum velocity and exhibited a gradual decay throughout the entire 30 min period. Despite this difference in the initial portion of PKAR in BVLs, their responses were not significantly different from CTRLs during minutes 2 to 30 of the response. These results suggest that vestibular inputs normally suppress PKAR velocity over the first 1-2 min of the response, but do not greatly influence PKAR decay. PKAR is therefore a process mediated primarily by somatosensory information and vestibular inputs are not required for its expression. Additionally, the absence of vestibular inputs does not result in increased somatosensory sensitivity that alters podokinetic intensity or decay time constants.