Perceptual responses to linear acceleration after spaceflight: human neurovestibular studies on SLS-2

Perceptual responses of four astronauts were measured before and after a 14-day Spacelab Life Sciences-2 mission during interaural (y-axis) and rostrocaudal (z-axis) linear acceleration to measure adaptive changes in perceptual responses to inertial cues. In one test, subjects used a joystick to null a pseudorandom velocity disturbance. Postflight, two of three subjects showed a significantly enhanced ability to null linear self-motion in the y-axis and z-axis orientations. In another test, the subjects used a joystick to indicate their direction of motion during a series of low-acceleration steps. The postflight responses of three of the four subjects showed a significant increase in the response latency for both y-axis and z-axis orientations. In a third test, subjects were asked to track a stationary but unseen target with their eyes while they translated linearly in the dark. No significant changes were observed in the postflight responses. The observed changes, when present, may be due to a reinterpretation of inertial cues that are functionally adaptive for the microgravity environment but are not optimal for responses on Earth.