Effects of Visual Acuity and
Stereoacuity of Oculomotor Changes
Produced by Pre-Flight Adaptation Training
Harold E. Bedell, Ph.D., Professor, UH
Deborah L. Harm, Ph.D., JSC
Millard F. Reschke, Ph.D., JSC
Saumil S. Patel, Ph.D., Post-Doctoral Fellow, UH
MOST ASTRONAUTS experience perceptual disturbances, including illusory
motion, 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 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.
Above. Figure 1. TTD and visual target presentation system in the testing
configuration. The lightweight box (center) that surrounds the subject during adaptation
is elevated to allow viewing of the three rack-mounted monitors (left) on which visual
stimuli are presented. During adaptation, the TTD surround rolls back and forth on long
aluminum rails. The subject's chair is partly visible behind components of the hydraulic
system that tilts (chair and rails) and translates (surround only) the TTD. The monitor
for the computer that controls the TTD is at the bottom.
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.
Specifically, post-flight vertical head movements are delayed and reduced in amplitude,
compared to preflight data. Consequently, reflexive vestibulo-ocular (VOR) eye movements
pull the eyes off target, resulting in gaze destabilization and extrafoveal imaging of the
target. In addition, smooth pursuit tracking movements may be 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 pre-flight
adaptation training protocols involving pitch head movements in the TTD.
Objective
The aim of these experiments is to evaluate the functional visual consequences of gaze
destabilization that occur when eye and head coordination or pursuit is 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. One experiment, currently in progress, measures visual
acuity and stereoacuity in ground-based personnel pre- and post adaptation in the TTD. A
second planned experiment will compare pre- and post-flight visual acuity in astronauts
for targets in eccentric gaze. In addition, we plan to pursue clinical applications to
improve the diagnosis and characterization of patients with vestibular dysfunctions.
If VOR and pursuit gains are abnormal, visual acuity is expected to be degraded until
the retinal image velocity falls below a critical value of about 2 to 3 deg/s and the
target is imaged within a small angular distance from the fovea. A similar expectation
holds for stereopsis. However, because stereopsis requires that both eyes fixate
accurately, discrepancies in the direction or velocity of gaze of the two eyes that do not
decrease acuity can significantly impair stereo-performance.
 |
Figure 2. Dr. Harm, sitting in the TTD as a subject for
stereoacuity testing, is wearing glasses with a blue filter in front of her left eye and a
red filter in front of the right eye. She indicates the depth in the stimulus with the
joystick. The headset permits communication with the TTD operators during adaptation. The
black and white stripes that comprise the inside of the TTD surround are clearly visible.
A bank of four small lights (directly above Dr. Harm) illuminates the TTD surround during
adaptation, but are extinguished during testing. |
Methodology
Two types of tests were developed for quick assessment of visual-vestibular performance.
The first type of test (the ST method) reports the spatial value of visual or stereo
acuity while making large eye-head movements towards a Vernier or stereo target that
appears for a fixed duration. The second type of test (the TT method) reports the minimum
duration of the Vernier or stereo target to achieve a criterion level of acuity. Using the
ST and TT methods, we obtain either the spatial offset or the time that a subject needs to
judge a given Vernier offset or stereoscopic depth correctly 84 percent of the time. Both
tests yield equivalent results and either can be used to investigate the effect of gaze
instabilities.
The hardware and the experimental protocol used in our studies are described in the
following sections.
Hardware Required
- Tilt-Translation Device (TTD) Preflight Adaptation Trainer (PAT)
- Visual Target Presentation and Data Recording System
- Macintosh Computer with three 14-inch monitors rack-mounted in the vertical plane
- Joystick
Experimental Protocol
To test stereoacuity, each monitor is divided into two halves by velcro-mounted color
filters: red in front of the left half, blue in front of the right half (Fig. 3(a)). The
subject wears a blank spectacle frame that includes matching color filters: blue for the
left eye and red for the right eye. Consequently, targets presented on the right half of
each monitor are seen by only the left eye and targets on the left half of each monitor
are seen only by the right eye. People with normal binocular vision fuse the right and
left eye views and see a single set of targets.
 |
Figure 3. Targets used in the stereoacuity test. (a, top) On a dark
screen, two bright lines appear on each half of the screen. The left and right half
screens are covered, respectively, by red and blue filters. The black dot in the middle of
top lines identifies the fused top line as the reference. (b, bottom) When the left and
right images are binocularly fused, the subject sees two lines in 3-D. In this example,
the bottom line is seen in front of the top, reference line. If the two bottom lines in
(a) were closer together than the two top lines, the bottom line would be seen behind the
reference line. |
Before each trial, the central monitor presents a blue cross and a red cross, which the
subject sees as a single cross floating in front of the monitor. The stereotargets appear
at 50 deg up or down gaze on the upper or the lower monitor. The targets are a pair of
vertical bright lines, 6 arc-min vertically by 60 arc-min horizontally, separated by a
vertical gap of 30 arc-min (Fig. 3(b)). The relative positions of the lines
presented to the right and the left eyes are not exactly the same, resulting in a
stereo-offset or disparity that results in the perception of depth. Whether the
bottom line is perceived to be in front or behind the top line depends on the direction of
the disparity. In the test, the subject is required to make a combined eye-head movement
to look at the stereo-targets. Using a joystick, the subject indicates the direction of
the depth between the bottom line and the top reference line which is marked for quick
identification. The subject has up to two seconds to respond after the targets are
extinguished. In the TT method, the stereo-offset or disparity between the two targets is
fixed at a certain value, and the duration of each presentation is manipulated based on
the subject's previous responses using an adaptive 2-alternate forced choice procedure
called MOBS.[1] In our implementation of this procedure, the duration of the target is
initially halved if the subject judges the direction of depth correctly on two successive
trials. Conversely, the target duration on the next trial increases when the subject makes
an error. The target presentation sequence ends automatically when a set of threshold
criteria are met. If the MOBS algorithm fails to meet the termination criteria, the test
ends when a maximum number of 40 trials per gaze location is reached.
To test Vernier acuity, both eyes are shown a single pair of line targets with no
disparity (e.g., the right-hand only in Fig. 3(a)). Consequently, only a single
pair of lines is shown on each monitor and no colored filters are required. We described
the set up used to assess Vernier acuity in two horizontal and two vertical gaze positions
in the 1996 ISSO Annual Report.
Data Analysis
Analysis of the stereoacuity and Vernier acuity data obtained with the TT and ST methods
is performed in the same way.
As shown in Fig. 4, an s-shaped psychometric
function is fit to the binary data (for stereoacuity, test line in front = 0; test line
behind = 1) generated by the subject's responses. In the TT method, the psychometric
function has target durations on the x-axis and the binary responses on the y-axis.
The temporal threshold is defined as the target duration needed to obtain an 84 percent
probability of correct responses for the value of disparity presented. In Fig. 4, the
threshold is about 675 m sec, as indicated by the intersection of the vertical line with
the x-axis.
Results
The ST and TT Methods
Vernier thresholds were measured for various target durations using the ST method These
experiments were performed on the hardware setup described in a previous report.[2] Figure 5 shows the effect of target duration on the Vernier
thresholds obtained from four normal observers. Each panel corresponds to one of the four
gaze locations tested with this protocol: 50 degrees left, 50 degrees right, 40 degrees up
and 40 degrees down. The time-constant of the exponential function fit to each data set is
specified in the lower right corner of each panel. Notice that the data for all locations
are very similar, except for the 40 degrees down- gaze where the improvement with target
duration is more rapid.
Figure 6 demonstrates the equivalence between temporal
thresholds obtained using the TT method and those predicted from the exponential
functions fitted to the Vernier data from the ST method. The Vernier offset used for the
TT method was 4 arc-min. Note that there are negligible differences at each gaze position
between the measured temporal thresholds and those predicted from the ST method.
Consistent with the ST method, the measured thresholds at 40 degrees down gaze were
considerably lower than those at the other gaze positions.
Stereoacuity Results
Stereothresholds were obtained from four normal subjects using the TT method. Each subject
was tested twice. Five different disparities were used to obtain the thresholds.
Figure 7 shows the effect of disparity on the
duration thresholds for stereoacuity. The difference between the thresholds at large
disparities (greater than 8 arc-min) for up- vs. down-gaze is minimal. Although
duration thresholds increase more quickly in down-gaze as target duration is reduced, this
difference is not statistically significant. The best fit exponential functions indicate
that the temporal stop improving when the disparity is about 15.6 arc-min in up-gaze and
12.2 arc-min in down-gaze.
Preliminary TTD Experiment
A critical parameter for our experiments is the optimal duration of adaptation in the TTD.
Too short of a duration would not be sufficient to induce adaptive vestibulo-ocular
changes and too long of a duration would run the risk of causing motion sickness. From
past experience, 30 minutes of TTD stimulation results in noticeable oculomotor
alterations. These alterations were not quantified, so it is unknown whether a significant
elevation of stereoacuity should occur. Stimulation exceeding one hour causes motion
sickness symptoms in susceptible subjects.
The degradation of visual performance as a result of TTD-induced adaptation is being
assessed by comparing duration thresholds for stereoacuity before and after TTD
stimulation. Four TTD stimulation durations (20, 30, 45 and 60 min) are being tested.
Based on the results in Fig. 7, a 4 arc-min disparity is used for this experiment. The
subject's chair is positioned on a one degree-of-freedom moving base which tilts by a
maximum of ±22 degrees about a horizontal axis. Surrounding the subject is a lightweight
box, lined on the inside with black stripes that accentuate the 3-D structure of the box.
As the subject tilts back and forth, a computer-controlled hydraulic system synchronously
slides the visual scene that surrounds the subject with a motion profile designed to
elicit complex perceived self-motion. These experiments to identify the optimal duration
of TTD stimulation are currently in progress.
References
[1]R. A. Tyrell and D. A. Owens. "Modified Binary Search," Behav. Res. Meth.
Instrum. Comp. 20 (1988): 137-41.
[2]ISSO Annual Report, 1996.
Contents
ISSO -- Institute for Space Systems Operations
1996-1997 Annual Report
