Most astronauts experience perceptual disturbances, including illusory motion and impaired spatial orientation, and space motion sickness during exposure to microgravity. These symptoms result, at least in part, from alterations in the pattern of sensory input that occur in the microgravity environment. Prolonged exposure to an altered pattern of sensory input can produce adaptation, which should alleviate the frequency and/or severity of symptoms in microgravity. However, upon returning to normal gravity on earth, adaptation to microgravity transiently persists, causing a new complement of perceptual disturbances. These in-flight and post-flight distortions of perception reduce performance efficiency and can potentially compromise mission objectives, including safety. A strategy to minimize these disturbances is to preadapt astronauts to specific combinations of altered sensory input simulating the transition between normal and microgravity. One of the training systems currently under investigation at JSC to facilitate adaptation is the Tilt-Translation Device (TTD). The TTD allows pitch or roll motions of the head and body to be coupled with translation of the visual scene that mimics the relationship between otolith and visual information in microgravity.[1]

The Tilt Translation Device (TTD) at the Johnson Space Center allows pitch or roll motions of the head and body to mimic experience in microgravity. Experimenting with the apparatus are Dr. Harold Bedell (l.), Dr. Deborah L. Harm (r.), and Dr. Saumil Patel (seated).

One change that occurs during exposure to microgravity is a degradation of the coordination between eye and head movements in shifting gaze to peripheral visual targets.[2] Specifically, post-flight vertical head movements are delayed and reduced in amplitude, compared to preflight data. Consequently, reflexive vestibulo-ocular eye movements (VOR) pull the eyes off target, resulting in gaze destabilization and extrafoveal imaging of the target. In addition, smooth pursuit tracking movements were decreased in gain, resulting in poorer stabilization of gaze on moving targets. Similar oculomotor changes have been observed in ground-based personnel following relatively brief exposures to a pre-flight adaptation training protocols involving pitch head movements in the TTD.[1,3]

The aim of the proposed experiments is to evaluate the functional visual consequences of gaze destabilization that occurs when eye and head coordination or pursuit are degraded. Impairment is anticipated when gaze is destabilized because of excessive retinal image velocity and extrafoveal imaging of targets, both of which degrade visual functions such as visual acuity and stereopsis.[4,5] One proposed experiment to be integrated into ongoing studies of eye-head coordination and oculomotor performance using ground-based personnel exposed to the TTD pre-flight adaptation trainer, will include pre- and post-TTD adaptation visual acuity and stereoacuity tests. A second experiment will compare pre- and post-flight visual acuity in astronauts for targets in eccentric gaze.

If VOR and pursuit gains are abnormal, visual acuity is expected to be degraded until the retinal image velocity falls below a critical value and the target is imaged within a specific angular distance from the fovea. Similar expectation holds for the response times for stereopsis. However, stereopsis require that both eyes accurately fixate the stereotarget. Thus, discrepancies in the direction or velocity of gaze of the two eyes that do not decrease acuity can significantly impair stereo-performance.

Project Status
The initial steps towards conducting experiments proposed in this project have been successfully completed. These steps include experiment protocol design and development and configuration of software and hardware. An experiment system has been developed to measure visual acuity during large rapid gaze shifts. Various procedures exists to estimate visual acuity. The one chosen here, called Vernier acuity is defined in terms of the smallest offset (horizontal or vertical) between two nearby target lines that can be identified correctly 84 percent of the time. Vernier threshold which is inversely related to acuity is used synonymously with Vernier acuity.

The gaze of astronauts is impaired because of excessive retinal image velocity and extrafoveal imaging of targets. UH and NASA researchers experiment with protocol design and develop software and hardware configurations. One of the experiments deals with visual acuity during large rapid gaze shifts.

The experiment system allows simultaneous threshold measurements from multiple locations. This system is based on a projection scheme whereby a single video output (640x480 pixels) maps the entire visual field (10ft x 10ft equal to 105 x 85 deg visual field when viewed from 1 m). The advantage of such a scheme is mainly in the simplicity and modularity of the design. A disadvantage of the scheme is the need for a very large projection area which reduces the available spatial resolution.

The need for a large screen prohibits its use in conjunction with the TTD device; hence an alternative approach is under development. This system will use two computer monitors mounted above and below the straight-ahead gaze position. To test Vernier thresholds at +50 and -50 deg vertical locations from a viewing distance of 1m, the vertical separation between monitors will be 8 ft. An adjustable mechanical frame is currently under construction for monitor mounting. The advantage of this scheme is high spatial resolution in the area where targets would be presented. This high resolution is achieved at the cost of flexibility of positioning the targets. The same software supports both systems (projection v/s multi-monitor).

Vernier threshold data have been obtained using the projection based system from one astronaut (pre-flight) and three other normal subjects. Vernier thresholds were obtained from four locations: left (-50 deg), right (50 deg), top (40 deg), bottom (-40 deg) with respect to straight-ahead gaze. As an initial part of these investigations, a normative database is being created. These normative data can be used to evaluate the post-flight performance of the astronauts. For Vernier threshold to accurately indicate post-flight or post-TTD-adaptation perceptual disturbances, inter-subject variability must be small. As shown in Fig. 1, the initial data for four subjects suggests little inter-subject variability. In addition, there is no significant inter-location variability, thus allowing for pooling across locations to obtain a single precise threshold estimate.

Figure 1. Average Vernier thresholds for all subjects at the four locations. The inter-subject variations and inter-location variations were statistically insignificant.

We conclude that perceptual aspects of post-flight and post-TTD-adaptation can be assessed rapidly using Vernier thresholds. The small number of subjects tested so far show no significant variability in their average Vernier threshold, thus satisfying the requirement for a stable sensory variable. A significant post-flight or post-TTD-adaptation elevation in Vernier threshold should then reflect functional visuo-vestibular changes during rapid and large voluntary gaze shifts. Including measures of stereopsis may provide an additional and perhaps more sensitive indicator of these adaptive changes.

References
1D. L. Harm and D. E. Parker. "Preflight Adaptation Training for Spatial Orientation and Space Motion Sickness," J. Clinical Pharmacology 34 (1994): 618-27.
2M. F. Reschke et al. "Visual-Vestibular Integration as a Function of Adaptation," in: DSO Status Report-DSOs and DTOs on STS-60m 62, 59, 65, 64, 68, and 66. (NASA/ Internal Status Report), 1995. 50-58.
3D. L. Harm, L. M. Zografos, N. C. Skinner, and D. E. Parker. "Changes in Compensatory Eye Movements Associated with Simulated Stimulus Conditions of Space Flight," Aviation, Space, and Environmental Medicine 64 (1993): 820-26.
4J. L. Demer and F. Amjadi. "Dynamic Visual Acuity of Normal Subjects during Vertical Optotype and Head Motion," Investigative Ophthalmology & Visual Science 34.6 (1993): 1894-1906.
5G. Westheimer and S. P. McKee. "Stereoscopic Acuity for Moving Retinal Images," J. Optical Society of America 68 (1978): 450-55.