Motion perception during tilt and translation after space flight

Abstract

Preliminary results of an ongoing study examining the effects of space flight on astronauts' motion perception induced by independent tilt and translation motions are presented. This experiment used a sled and a variable radius centrifuge that translated the subjects forward-backward or laterally, and simultaneously tilted them in pitch or roll, respectively. Tests were performed on the ground prior to and immediately after landing. The astronauts were asked to report about their perceived motion in response to different combinations of body tilt and translation in darkness. Their ability to manually control their own orientation was also evaluated using a joystick with which they nulled out the perceived tilt while the sled and centrifuge were in motion. Preliminary results confirm that the magnitude of perceived tilt increased during static tilt in roll after space flight. A deterioration in the crewmember to control tilt using non-visual inertial cues was also observed post-flight. However, the use of a tactile prosthesis indicating the direction of down on the subject's trunk improved manual control performance both before and after space flight.

Introduction

In darkness, the central nervous system must resolve the ambiguity of inertial motion sensory cues in order to derive an accurate representation of spatial orientation. The balance system in the inner ear (otoliths) senses both head translation and head tilt relative to gravity. During space flight, head tilt is not sensed; the brain must therefore learn new ways of orienting oneself in weightlessness, which can then lead to disturbances in perceived motion and balance control upon return to Earth's gravity [1], [2]. Adaptive changes during space flight in how the brain integrates vestibular cues with other sensory information can lead to impaired movement coordination, vertigo, spatial disorientation and perceptual illusions following gravity level transitions [3].

This ongoing study was designed to examine both the physiological basis and operational implications for disorientation and tilt-translation disturbances following short-duration space flights. Specifically, this study addressed three questions: (1) What adaptive changes occur in motion perception in response to different combinations of tilt and translation motion? (2) Do adaptive changes in tilt-translation responses impair the ability to manually control vehicle orientation? (3) Can sensory substitution aids, such as tactile cues, mitigate the risks associated with manual control of vehicle orientation?

The primary objective of this study was to evaluate how the brain adapts to conflicting motion cues by measuring changes in awareness of position. Before and after space flight, subjects were exposed to a combination of body tilt and translation on a sled or a centrifuge. Based on the results from previous post-flight studies [4], [5], [6] we hypothesized that perceived tilt will increase post-flight during static tilt, as a result of the in-flight decrease in the amplitude of the internal model of gravity. During dynamic tilt and translation, it was expected that motion perception will have specific frequency characteristics, with adaptive changes being greatest in the mid-frequency range where there is a crossover of tilt and translation [7].

The secondary objective of this experiment was to evaluate how a tactile prosthesis can be used to improve control performance. The tactile prosthesis is a simple four vibrotactile system that provide feedback when tilt position exceeds predetermined levels in either pitch or roll [8]. Subjects were tasked to use a joystick to null out tilt and translation motion disturbances with or without the help of the tactile prosthesis. We predicted that performance in the closed-loop tilt control task would be compromised following tilt-translation adaptation after space flight, with increased control errors corresponding to changes in self-motion perception [9]. Furthermore, that performance would improve with tactile feedback of tilt control errors.

In this paper we describe the equipment and experimental protocol used for this study and report the preliminary results obtained with astronaut-subjects.