The nature of sensorimotor adaptation to altered G-levels: evidence from mass discrimination.

Mass discrimination is impaired under zero G compared to 1 G. This is partly due to loss of weight information (a reduction in z-axis pressure stimulation), and probably also to incomplete adaptation to the reduced G-level. Maladaptation to an increased G-level postflight causes objects to feel too heavy and weight discrimination to be impaired. Video tapes of hand movements while shaking test objects were made before, during, and after the D-1 Spacelab mission. Movements were faster inflight than preflight, slowed down postflight and returned to baseline by about the third day. Corresponding changes in movement amplitude also occurred. Errors of weight or mass perception during the maladaptation phase may be due to both inadequate monitoring of command signals and to inappropriate scaling of afferent signals. High-acceleration shaking was found to improve mass discrimination but impair weight discrimination.