Gaze control and vestibular-cervical-ocular responses after prolonged exposure to microgravity.

BACKGROUND Microgravity does not affect visual function directly. However, because of the altered afferentation from vestibular, support, and tactile-proprioceptive systems, it could lead to disturbances in visual tracking and inhibit the cosmonaut's activity. Therefore, it is necessary to obtain quantitative evaluations of spaceflight effects upon gaze control and vestibular-cervical-ocular responses. METHODS Examination of visual tracking with the head in a fixed position was performed in 26 Russian ISS cosmonauts before and after a prolonged spaceflight (129-215 d). As vestibular tests, we used several roll-tilts and yaw head rotations. Eye movements were recorded using both video-oculographic and electro-oculographic methods. RESULTS It was shown that until 9 d after landing (R+9) spontaneous eye movements were increased (spontaneous nystagmus, gaze nystagmus, square wave jerks); otolith function was suppressed (inversion, absence, or significant decrease of the compensatory torsional ocular counter-rolling); vestibular reactivity was elevated (an increased intensity of the vestibular nystagmus during head yaw rotations); amplitude and velocity characteristics of gaze control (saccades, smooth pursuit, gaze holding) were significantly decreased; total reaction time was significantly increased up to 2-3 times; and gaze holding ability was degraded. For several cosmonauts, smooth pursuit was collapsed and their gaze approached the stimulus or pursued its motion utilizing a sequence of saccades at least until R+5. DISCUSSION Prolonged exposure to microgravity considerably affects all forms of visual tracking. Revealed disturbances in precision of visual tracking and adoption of the new tracking strategy substantially prolong (up to 2-3 times) the period required to acquire, recognize, and to fixate gaze on the target.