The vestibular function was investigated in 13 Russian crew members of the ISS missions on days 1(2), 4(5), and 8(9) after prolonged exposure to microgravity (126 to 195 days). The static torsional otolith-cervico-ocular reflex was studied, as well as the dynamic vestibulo-cervico-ocular responses, vestibular reactivity, and spontaneous oculomotor activity using videooculography (VOG) and electrooculography (EOG) for simultaneous recording of eye movements. On days 1–2 of return to the gravity (R+1−2), the cosmonauts were found to increase the spontaneous oculomotor activity (floating eye movements, both typical and atypical forms of spontaneous nystagmus, square wave jerks, gaze nystagmus) with the head held in the vertical position. The otolith function during static head inclinations to the right or left shoulder at 30° was suppressed as determined by the inversion or absence, or reduction by half of the amplitude of torsional compensatory eye counter-rolling and the vestibular reactivity during head yaw movements at 0.125 Hz was increased as revealed by a lowered threshold and an increased intensity of vestibular nystagmus. The pattern, depth, dynamics, and velocity of the vestibular function recovery varied with individual participants in the investigation. However, the suppressed otolith functioning in the period of readaptation to the normal gravity was, as a rule, accompanied by an exaggerated vestibular reactivity.

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