Title : Feasibility of use of galvanic vestibular stimulation device on gaze stabilization and as a countermeasure for microgravity induced space motion sickness
Abstract:
The peripheral vestibular system encodes head acceleration, and the central nervous system integrates this with other sensory inputs to maintain balance and gaze stability. This work discusses the challenges of space exploration, focusing on microgravity-induced physiological changes, particularly those affecting the vestibular system, which significantly alters human performance in space thus requiring effective countermeasures. In microgravity, astronauts experience disorientation and space motion sickness due to changes in vestibular organ input, leading to symptoms like vertigo and headache, that affect up to 70% of astronauts during the first few days of flight. Postflight, astronauts show various neurological alterations, similar to symptoms in individuals with vestibular disorders experiencing significant cognitive and perceptual difficulties. Studies have also shown that microgravity affects cortical and sensory responses, altering perception, motor function, and brain connectivity. In this work Galvanic Vestibular Stimulation (GVS) is explored as a countermeasure stabilizing posture and gaze in microgravity.
In the first part of this work, it is presented a compartmental mathematical model of the vestibule acceleration encoding dynamics that combines hair-cell mechanotransduction, synaptic transmission, and afferent neuron dynamics. Analysis of this model under applied GVS shows that the stimulation, within a certain range of amplitude, produces a modulation of the dynamics of vestibular afferent neurons, inducing a transition from a state of low sensitivity (a stable focus) to one of rhythmic, high-gain output (a limit cycle), revealing a potential mechanism for enhancing the detection of inertial cues.
This theoretical finding motivated our complementary experimental study in which we performed experiments analyzing the vestibulo-ocular reflex (VOR) in (N=5) volunteer subjects under control and when GVS was applied in coordination with a dynamic flight simulation based on a Steward 6 DOF platform system. The GVS (2 mA) were applied via transmastoid electrodes and lasted 8 seconds; its beginning was synchronized with the dynamic stand tilt. An eye tracking device was used to record the eye and head movements of the participants (Otometrics ICS® Impulse). Eye vertical and horizontal absolute position and angular displacements of the head were obtained from this device.
The analyses of the path travel of the eye showed that there are significant differences, while GVS was compared to pre-stimulation (sham-GVS) comparison of control vs cathode right GVS (4.2 vs 2.1, p = 0.04) and control vs cathode left GVS (4.2 vs 3.0, p = 0.14) significantly reducing the eye displacement from a target data from each experimental condition.
Our combined theoretical and experimental results demonstrate that GVS is a feasible non-invasive intervention for modulating vestibular function during dynamic conditions. The improvement in gaze stabilization provides a strong rationale for developing GVS as a practical countermeasure for Space Adaptation Syndrome.
