[Pattern of vestibular reactions and sensory interactions in weightlessness (data of the experiment "Optokinesis")]

This paper presents results of studying spontaneous and evoked optokinetic, opto-oculomotor and vestibulo-oculomotor reactions in microgravity. The examinations were performed during the 7-day Soviet-Indian and 237-day Salyut-7 missions. During an early stage of adaptation to microgravity the following changes occurred: enhanced spontaneous activity of the eyes, including spontaneous nystagmus in one of the crewmembers, decline of the efficiency of saccadic pursuit (disappearance of saccades in the course of pursuit of a moving stimulus, decrease of the saccadic amplitude, emergence of correction microsaccades in response to fixation events), and decrease of the threshold of the optokinetic nystagmus. The compensatory counter-rotation of the eyeballs during head side-to-side movements with open eyes in dark glasses did not disappear but slightly decreased, being slightly accompanied by low-amplitude nystagmus. When rocking the head with open eyes, one of the cosmonauts showed a complete eye destabilization and other cosmonauts displayed high-amplitude nystagmus. In response to head rotation with open eyes they developed clonic nystagmus. Vestibular stimulation in the form of active head movements helped to normalize the pattern of opto-ocular responses.