Effects on Visual Acuity and Stereoacuity of Oculomotor Changes Produced by Pre-Flight Adaptation Training

CHANGES IN THE PATTERN OF SENSORY INPUT that accompany the transition from Earth to the microgravity environment of space can induce perceptual disturbances, such as illusory motion and impaired spatial orientation, and may contribute to space motion sickness. Continued exposure to the new pattern of sensory inputs in microgravity usually results in sensory adaptation and an alleviation of symptoms. The investigators on this project from NASA-JSC are evaluating the possibility of training astronauts with specific regimes of altered sensory input before flight, in order to facilitate this adaptation. One training system currently under investigation at JSC to promote adaptation to the microgravity environment is the Tilt-Translation Device (TTD). As described in greater detail below, the TTD combines pitch or roll motions of the head and body with linear translation of the visual scene. This combination simulates the relationship between information from the otolith organs and visual input that occurs in microgravity.

VISUAL ACUITY/STEREOACUITY-Harold Bedell, UH College of Optometry, sits in a device designed to test reflexes requiring eye-head coordination. This equipment, designed by NASA researchers, seeks answers to substantial changes that astronauts experience in their visual-vestibular calibration in microgravity. The transition from Earth to the microgravity environment of space can induce perceptual disturbances that result in impaired spatial orientation and contribute to space motion sickness.

During exposure to microgravity, the normal coordination that exists between eye and head movements when shifting gaze to peripheral visual targets is disrupted. In particular, vertical head movements measured post-flight are reduced in amplitude and delayed, compared to preflight measurements. As a result, reflexive vestibulo-ocular (VOR) eye movements in a 1g environment less accurately stabilize gaze, leading to transient increases of retinal image velocity and off-foveal imaging of targets. Smooth pursuit eye movements may also have reduced gain, so that the stabilization of gaze on moving targets is less accurate. The oculomotor changes that occur in ground-based personnel after brief exposure to pre-flight adaptation training in the TTD are similar to those observed after adaptation to microgravity.

Objective
If VOR and pursuit gains are abnormal, precise visual functions such as acuity and stereoacuity would be expected to be impaired during and immediately after gaze shifts. This expectation is based on the results of studies showing that both acuity and stereoacuity are degraded when the retinal image velocity exceeds approximately 2 to 3 deg/s or when the target is imaged off the fovea. Because stereopsis requires accurate fixation by both eyes simultaneously, it may be sensitive to smaller discrepancies in the direction or velocity of gaze of the two eyes than those that affect visual acuity. Below, we show that visual acuity (measured using a Vernier configuration) is impaired for targets in eccentric gaze, particularly if the gaze shift requires a combination of head and eye movements. We also present some preliminary data to indicate that stereothresholds for eccentrically placed targets are elevated in ground-based personnel after a brief adaptation period in the TTD.

We also plan to conduct experiments on a degradation of motion sensitivity that is anticipated to accompany gaze destabilization. Motion sensitivity should be impaired by gaze destabilization because the resulting instability of the retinal image produces a background of noise, within which motion of the stimulus must be detected. Motion is a highly relevant stimulus dimension for several reasons besides its high perceptual salience: it represents a principal input to oculomotor systems such as pursuit and optokinetic nystagmus, it is involved in maintaining accurate calibration of the VOR, and motion parallax provides a sensitive depth cue that supplements binocular stereopsis. Our pilot studies, described below, sought to determine the extent to which motion sensitivity is reduced during natural eye and head movements by 1) increased motion noise attributable to retinal image slip and 2) neural signals associated with the movements of the eyes and head.

Background and Methods
As described in detail in our previous progress reports (1996 and 1997), two types of tests were developed for the quick functional assessment of visual-vestibular performance. The first type of test (the ST method) reports the threshold spatial value of visual acuity or stereoacuity when subjects make combined eye-head movements toward Vernier or stereo target configurations that appear for a fixed duration. The second type of test (the TT method) reports the minimum duration of a Vernier or stereo target required to achieve a criterion level of acuity. Using the ST and TT methods, we obtain either the spatial offset or the time a subject needs to judge correctly a given Vernier offset or stereoscopic depth 84 percent of the time. Either test can be used to investigate the effect of gaze instabilities.

Previously, we showed that the Vernier thresholds measured using the TT method are equivalent to those obtained by using the ST method. We also reported measurements of stereothresholds for various disparities using the TT method. From the results of the stereothresholds measurements, we obtained a value of disparity (4 arc-min) that should be sensitive to changes in visual-vestibular coordination.

In the current report, we include stereothresholds obtained from preliminary experiments involving TTD stimulation, which is known to transiently alter visual-vestibular coordination.

Figure 1 illustrates a typical motion profile used to mimic the combination of visual and otolith inputs during microgravity in humans. Thirty minute exposure to this type of profile is known to induce changes in oculomotor parameters, such as VOR gain. The difference in stereothresholds before and after TTD stimulation therefore measures the sensitivity of our psychophysical test to altered oculomotor coordination.

We also compared Vernier thresholds for small gaze shifts, which can be accomplished by eye movements alone and large gaze shifts that require head, as well as eye, movements. This comparison indicates the ability of the TT method to discriminate between visual performance for these two types of gaze shifts.

In our current TT and ST methods, Vernier thresholds during large voluntary gaze shifts are measured using rectangular bar targets. To evaluate the feasibility of using complex targets, which may be more familiar to flight personnel, we used a variation of the ST method to compare Vernier thresholds in straight-ahead gaze for targets composed of irregular and regular shapes.

Finally, because we plan to determine how oculomotor adaptation to microgravity (simulated using the TTD) affects sensitivity to relative motion, we collected preliminary data on motion sensitivity during pursuit tracking and during voluntary oscillatory head movements. The motion to be detected was internal to the pursuit or fixation target and was either in the direction of eye or head movement, or orthogonal to it.

Results
TTD Adaptation. A preliminary TTD adaptation experiment was conducted to determine what duration of TTD stimulation is required to induce changes in stereothreshold. Pre- and post-adaptation stereothresholds were obtained for stimulation durations of 30 min and 60 min for two subjects. Stereothresholds were measured for targets in vertical up and down gaze of 40 deg.

As shown in Fig. 2, after averaging across directions of gaze, the temporal stereothreshold is increased by about 200 msec after 30 or 60 min of TTD stimulation. In subject S2, doubling the duration of TTD stimulation duration did not proportionally increase the difference between pre- and post-adaptation stereothresholds.

Vernier Thresholds During Small and Large Gaze Shifts. For these experiments, Vernier thresholds were obtained using the TT method under two conditions. In the first condition (inside), subjects made 20 deg vertical (up/down) or horizontal (left/right) gaze shifts from straight-ahead fixation using eye movements alone. In the second condition (outside), subjects made vertical gaze changes of 40 deg and horizontal gaze changes of 50 deg, using combined eye and head movements. The average data from five subjects for both of the conditions are shown in Fig. 3.

Overall, the average temporal Vernier threshold for the inside condition (276±69) is about 200 msec smaller than that for the outside condition (462±102). These data indicate that a 200 msec increase in exposure duration is required to acquire a target at 40-50 deg compared to one at 20 deg eccentricity.

Vernier Thresholds for Irregular Target Shapes. Vernier thresholds for vertically separated irregular shapes were compared with those for vertically separated lines and dots. The average results for three subjects indicate that Vernier thresholds are similar for random-shape and dot targets if the angular separation between the two targets is less than about 10 arc-min, but poorer for random-shape targets if the separation is larger than 10 arc min (Fig. 4). This experiment suggests that, within certain constraints, targets of any shape can be used for the TT method.

Preliminary Experiments on Motion Sensitivity During Eye and Head Movements. Motion thresholds were measured during binocular pursuit of smooth and sampled horizontal motion of a 1 deg x 1 deg grating patch at several velocities. The motion to be detected was a back and forth motion of the grating stripes within the patch, either in the same (horizontal) or orthogonal (vertical) direction as the direction of pursuit tracking.

Figures 5(a) and 5(c) show that the thresholds for vertical motion are elevated during pursuit tracking when the velocity of the horizontal tracking stimulus increases, both for smooth and sampled movement. The elevation is greater and occurs at much lower velocities for tracking of sampled than smooth movement. Note that optimal motion thresholds in the zero-velocity tracking condition are extremely good: a 3.5 deg phase shift corresponds to a spatial displacement of the grating stripes of about 15 arc sec. Comparison of Figs. 5(a) and 5(b) indicates that motion thresholds are elevated considerably more during pursuit tracking of sampled motion if the grating stripes move horizontally (i.e., in the same direction as pursuit tracking) than if they move vertically. Residual retinal image motion due to imperfect pursuit tracking is presumably responsible for the greater elevation of horizontal than vertical motion thresholds.

Thresholds for vertical motion were also measured during voluntary oscillatory head movements at various frequencies.

As shown in Fig. 6, thresholds for vertical motion of the grating stripes are elevated during horizontal head movements, suggesting that neural signals related to eye-head coordination also affect motion sensitivity. In conjunction with the results shown in Fig. 5, these results suggest that, during large gaze shifts, motion sensitivity should be degraded both by uncompensated retinal image motion due to imperfect eye-head coordination and by non-retinal signals associated with the eye and head movements themselves. The velocity of uncompensated retinal image motion should be higher and the duration of eye and head movements should be longer after adaptation to microgravity or TTD stimulation, suggesting that the impairment of motion sensitivity during gaze shifts should be larger and more protracted.

Acknowledgements
The experiments on small vs. large gaze shifts were conducted by NASA summer intern Carmarius Jackson and medical intern Chris Courtney. Scott Wood, Jody Cerisano, Denine Gasaway and Phil Bush provided invaluable technical assistance.

Deborah Harm, JSC PI in the Life Sciences Research Laboratories.	Dr. Bedell and ISSO post-doctoral fellow Saumil S. Patel are shown adjusting the apparatus.

Publications
Bedell, H. E. and S. S. Patel. "Comparison of Letter and Vernier Acuities with Dioptric Blur," abstract, American Academy of Optometry, 1997 Annual Mtg., San Antonio, TX; Optometry & Vision Science 74.12S (1997): 109.
Bedell, H. E., S. S. Patel, and S. T. L. Chung. "Comparison of Letter and Vernier Acuities with Dioptric and Diffusive Blur." Optometry & Vision Science. (In press.)
Patel, S. S. and H. E. Bedell. "Motion Thresholds within a Tracked Target Are Elevated During Smooth Pursuit and Vergence Tracking," abstract, Assoc. for Research in Vision & Ophthalmolog, 1998 Annual Mtg., Ft. Lauderdale, FL; Investigative Ophthalmology & Visual Science 39 (1998): S1077.
Patel, S. S., H. E. Bedell, and M. T. Ukwade. "Motion Thresholds During Smooth and Sampled Binocular Vergence and Pursuit Tracking." (In preparation.)
--. "Vernier Judgments for Targets of Arbitrary Shape," abstract, Assoc. for Research in Vision & Ophthalmology, 1997 Annual Mtg., Ft. Lauderdale, FL; Investigative Ophthalmology & Visual Science 38 (1997): S634.
--. "Vernier Judgments in the Absence of Regular shape Information." Vision Research. (In press.)
Patel, S. S., B.-C. Jiang, and H. Ogmen. "On the Neural Origin of Binocular Fixation Disparity," abstract, Soc. for Neuroscience, 1997 Annual Mtg., New Orleans, LA; Society for Neuroscience Abstracts 2 (1997): 1557.
--. "Vergence Dynamics Predict Fixation Disparity." (Submitted for publication.)
Patel, S. S., B.-C. Jiang, J. M. White, and H. Ogmen. "Non-Linear Alteration of Transient Vergence Dynamics after Prolonged Convergence." Optometry & Visual Science. (In press.)
Patel, S. S., H. E. Bedell, M. F. Reschke, D. L. Harm, C. Courtney, C. Jackson, S. Wood, J. Cerisano, and D. Gasaway. "Vernier and Stereo Thresholds During Large Voluntary Gaze Shifts," abstract, Soc. for Neuroscience, 1998 Annual Mtg., Los Angeles, CA.
Purushothaman, G., S. S. Patel, H. E. Bedell, and H. Ogmen. "Moving Ahead: Differential Visual Latency and Not Motion Extrapolation." Nature. (In press.)
--. "Perceived Spatial Lag of Flashed vs. Moving Lines Varies with Luminance and Timing," abstract, Assoc. for Research in Vision & Ophthalmology, 1997 Annual Mtg., Ft. Lauderdale, FL; Investigative Ophthalmology & Visual Science 3 (1998): S1077.
Ukwade, M. T., H. E. Bedell, and S. S. Patel. "Stereothresholds with Relative and Absolute Disparity Noise," abstract, American Academy of Optometry, 1997 Annual Mtg., San Antonio, TX; Optometry & Vision Science 74.12S (1997): 68.
Presentations
Patel, S. S. "Vernier Threshold as a Potential Indicator of Post-Space Gaze Instability," Graduate General Seminar, College of Optometry, Fall 1996.
Patel, S. S., H. E. Bedell, M. F. Reschke, D. L. Harm, D. Gasaway, and J. Cerisano. "Vernier Threshold as a Potential Indicator of Post-Space Flight Gaze Instability," Houston Conf. on Biomedical Engineering Research, Houston, TX, Feb. 1997.
Patel, S. S., H. E. Bedell, M. F. Reschke, D. L. Harm, C. Courtney, C. Jackson, S. Wood, J. Cerisano, and D. Gasaway. "Stereo and Vernier Thresholds During Large Voluntary Gaze Shifts," Houston Conf. on Biomedical Engineering Research, Houston, TX, May 1998.
Grants
"A Versatile Low-Cost, Binocular Head-Mounted Stimulation System for Visual and Visuo-Motor Testing and Research." Co-Principal Investigators: M. Reschke and D. L. Harm; NASA RUG, March 1997; not funded.

Contents
ISSO -- Institute for Space Systems Operations
1997-1998 Annual Report