Characterization of Galvanic Vestibular Stimulation Effects for use in Astronaut Training

Astronaut sensorimotor impairment is a concern for human spaceflight missions, particularly during and immediately following gravity transitions. When astronauts transition back to a gravitational environment following microgravity exposure, they commonly experience two types of vestibular self-motion illusions termed the tilt-gain and tilt-translation illusions. These vestibular illusions occur when astronauts tilt their heads relative to gravity and result in a sense of tilt over-estimation and/or an illusory sense of translation. This may lead to an improper perception of self or vehicle orientation which may impact functional task performance and vehicle operation. To address this risk, training tools must be developed to prepare astronauts for the vestibular illusions they will encounter post-spaceflight. Galvanic Vestibular Stimulation (GVS) has been shown to produce illusions of tilt and degrade the performance of functional tasks by artificially stimulating the vestibular system. It still remains unclear, however, how well GVS can mimic the vestibular illusions experienced by astronauts post-spaceflight. In support of developing a training tool that can accurately mimic post-flight illusions, this study attempts to systematically quantify the magnitude of orientation perception alteration and the prevalence of side effects experienced during GVS.As a measure of GVS susceptibility, all subjects were seated unrestrained with electrodes attached to the two mastoids. Current waveforms with amplitudes of up to 4mA were applied. Subjects rated sensations of self-motion, skin tingling, metallic taste, and visual flashes at varying current levels. To assess the impact of GVS on tilt perceptions, subjects were also placed in the Tilt Translation Sled (TTS) with heads restrained and then passively tilted ±8° in the roll axis along a pseudorandom sum of sines tilt profile. GVS up to 4mA was applied, in a coupled manner, proportional to the tilt angle, angular velocity, or a combination of the two. Subjects reported their sense of roll tilt using a subjective haptic horizontal task, aligning a bar to their perceived horizontal with their fingertips. Results indicate that when subjects are unrestrained, GVS as low as 0.5mA can induce a roll tilt sensation and that 2mA is sufficient for all subjects to feel an effect. This is consistent with the results of other studies using GVS. Results from the other side effects, which have not been systematically assessed previously, indicate that skin tingling can also be perceived as low as 0.5mA and that 2mA is sufficient for all subjects to notice. Metallic taste and visual flashes were reported by some subjects, though less frequently. Additionally, when subjects were tilted within the TTS, perceptual reports of tilt were overestimated when GVS was applied in an amplifying manner compared to reports without GVS applied. Similarly, an underestimation of tilt occurred when the GVS polarity was flipped. While there is some variability in perception across subjects, amplifying GVS produces consistent alterations in orientation perception accordant with illusions experienced by astronauts returning from microgravity (i.e., the tilt-gain illusion). These results indicate that GVS can be used as a training tool to simulate astronaut post-flight illusions. The results also provide a foundation of knowledge for future GVS applications.

1. INTRODUCTION

It is well documented that astronauts experience a wide range of neurovestibular impairments when returning to Earth from microgravity [1]. Of interest for this study are reports of altered sensations of movement and tilt, or more generally, orientation perception. Many neurovestibular symptoms post-flight can be elicited by head movements, particularly head tilts away from upright [2], [3]. Several studies have investigated the occurrence of two vestibular illusions termed the tilt-gain and tilt translation illusions [4]–[7] where head tilt is either overestimated or perceived as translation, respectively. These illusions, along with other postflight neurovestibular symptoms, make it difficult for astronauts to perform locomotive tasks like emergency egress [2], [8]–[10] or manual control tasks like piloting and landing a spacecraft [2], [11] which occur before symptoms subside [12].