The role of gravitation-dependent systems in visual tracking

The effects of prolonged microgravity conditions on the performance of visual tracking functions such as fixational rotations of the eyes (saccades), smooth tracking of linear and curved movements of a foveal point stimulus, and following a vertical pendulum-like movement of foveoretinal optokinetic stimuli were studied. Experiments were performed on 31 cosmonauts in freefall conditions, in ten cases followed by additional studies after a cycle of head movements and in 14 after resting. These experiments showed that while intrinsic visual functions were retained in microgravity conditions, there were decreases in the precision and speed measures of all types of visual tracking (fixational rotations of the eyes, smooth tracking) and, in some cases, complete degradation of the smooth tracking reflex, an increase in the time taken to fix the gaze on a target (by factors of 2 or more), and decreases in the frequency of stimulus tracking. During the initial period of adaptation to the altered gravitational conditions and periodically during prolonged flight, the system of smooth visual tracking was found to undergo a transition to a strategy of saccadic approximation, in which gaze tracks the movement of the target using a set of macro- or microsaccadic movements. These impairments, seen in virtually all the cosmonauts, resulted from vestibular deprivation (functional deafferentation of the otolith input) in conditions of weightlessness, while in cosmonauts conceptualizing space on the basis of perceiving the positions of the feet and head additionally showed support-tactile deprivation.