Optokinetic nystagmus and after-nystagmus during a 6 hour bedrest study.

Conclusions: A lengthy alteration of gravity direction produced different effects on the intrinsic horizontal and vertical optokinetic oculomotor systems.

Objective: To examine both optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) in a 6 h 6 degrees head-down bedrest study, in which the subjects were kept lying under simulated micro-gravity conditions.

Subjects And Methods: In six normal healthy adults, we repeatedly (five times) and comparatively studied OKN and OKAN evoked by horizontal and vertical stimuli. Stage 1 was an upright sitting position. During the 6 h bedrest condition, we studied OKN and OKAN in 90 degrees recumbent lateral positions (stages 2, 3, and 4). In stage 5 the subject returned to an upright position.

Results: We confirmed that the change in gravity direction had various effects on the condition of OKN and OKAN. Also, we found that it took more than 3 h to reach a desirable level of systemic adaptive modification to the unique environmental condition. We considered that the early change was basically due to the changes in sensory inputs through the otolith organs, and the latter changes represented the adaptive process of the spatial orientation system. During the tilt, the occurrence rates of both horizontal and vertical OKANs were decreased; however, the conditions of these changes were different.