Coriolis-force-induced trajectory and endpoint deviations in the reaching movements of labyrinthine-defective subjects

When reaching movements are made during passive constant velocity body rota tion, inertial Coriolis accelerations are generated that displace both move ment paths and endpoints in their direction. These findings directly contra dict equilibrium point theories of movement control. However, it has been a rgued that these movement errors relate to subjects sensing their body rota tion through continuing vestibular activity and making corrective movements . In the present study, we evaluated the reaching movements of five labyrin thine-defective subjects (lacking both semicircular canal and otolith funct ion) who cannot sense passive body rotation in the dark and five age-matche d, normal control subjects. Each pointed 40 times in complete darkness to t he location of a just extinguished visual target before, during, and after constant velocity rotation at 10 rpm in the center of a fully enclosed slow rotation room. All subjects, including the normal controls, always felt co mpletely stationary when making their movements. During rotation, both grou ps initially showed large deviations of their movement paths and endpoints in the direction of the transient Coriolis forces generated by their moveme nts. With additional per-rotation movements, both groups showed complete ad aptation of movement curvature (restoration of straight-line reaches) durin g rotation. The labyrinthine-defective subjects, however, failed to regain fully accurate movement endpoints after 40 reaches, unlike the control subj ects who did so within 11 reaches. Postrotation, both groups' movements ini tially had mirror image curvatures to their initial per-rotation reaches; t he endpoint aftereffects were significantly different from prerotation base line for the control subjects but not for the labyrinthine-defective subjec ts reflecting the smaller amount of endpoint adaptation they achieved durin g rotation. The labyrinthine-defective subjects' movements had significantl y lower peak velocity, higher peak elevation, lower terminal velocity, and a more vertical touchdown than those of the control subjects. Thus the way their reaches terminated denied them the somatosensory contact cues necessa ry for full endpoint adaptation. These findings fully contradict equilibriu m point theories of movement control. They emphasize the importance of cont act cues in adaptive movement control and indicate that movement errors gen erated by Coriolis perturbations of limb movements reveal characteristics o f motor planning and adaptation in both healthy and clinical populations.