Static and dynamic vestibulo-cervico-ocular responses after prolonged exposure to microgravity

Abstract

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.

The Role of Different Afferent Systems in the Modulation of the Otolith-Ocular Reflex After Long-Term Space Flights

“…Previous research has shown that the ocular counter roll induced by head tilt does not differ from whole body tilt ( Zingler et al, 2006 ). Results of assessment of changes in vestibular function and, particularly in otolith function after long-term space flight, obtained by Russian researchers ( Kornilova, 1997 , 2001 ; Clarke et al, 2000 ; Kornilova et al, 2002 , 2007a , b , 2017 ; Hallgren et al, 2015 , 2016 ; Naumov et al, 2015 ). It was found that there is a decrease in OCR early after return from space, and that this reflex regains the preflight amplitudes approximately 1–2 weeks after return in the normal gravitational environment of Earth.…”

“…During the SA experiments, described earlier by prof. L. Kornilova and her team ( Kornilova et al, 2007a , b , 2011b , 2012 ), the otolith mediated ocular reflex, representing the vestibular function in cosmonauts, was evaluated using two different methodologies ( Kornilova et al, 2007c ).…”

Ocular counter-roll is less affected in experienced versus novice space crew after long-duration spaceflight

“…Another study, by Kornilova et al, studied a group of 13 cosmonauts with long-duration missions (126–196 days), of whom they obtained OCR measurements by having the cosmonauts tilt their heads sideways 30–35°. They observed a decrease in OCR as measured on the first 2 days after return compared to preflight, which recovered to baseline 8–9 days after landing 16 . Lastly, our group has also demonstrated a decrease in OCR by off-axis centrifugation 3 days after landing, which returned to baseline values as measured 9 days postflight.…”

The effect of spaceflight on the otolith-mediated ocular counter-roll

“…The lack of gravitational input will cause a deconditioning of the otoliths by decreasing the gain (ratio of eye torsion over head tilt) of otolith-mediated reflexes [4]. Also, the assessment of the real vertical will be impaired when there is a loss of otolith input to the brain in microgravity [7]. Deconditioning of otolithmediated reflexes following microgravity exposure has been proposed as one of the multiple causing factors of the postural, locomotor, and gaze control problems experienced by returning astronauts [5].…”

“…The OCR has previously been used as a measure for microgravity's effect on the otoliths [7,8,9,10]. However, these studies overall provide conflicting results, which is most likely explained by variations in mission duration, methodological choices, and sample size.…”