Visual Performance Challenges To Low-Frequency Perturbations After
Long-Duration Space Flight and Countermeasure Development
A.P. Mulavara,1 S. J. Wood,1 M. J. Fiedler,2 I. Kofman,2 W. B. Kulecz,2 C. Miller,2 B. Peters,2 J. M. Serrador,3 H. Cohen,4 M. F. Reschke,5 and J. J. Bloomberg5
1Universities Space Research Association, Houston, TX, USA, 2Wyle Integrated Science and Engineering Group, Houston, TX, USA, 3Dept of Veterans Affairs NJ
Healthcare System, East Orange, NJ, USA, 4Baylor College of Medicine, Houston, TX, USA, and 5NASA Johnson Space Center, Houston, TX, USA.
Objective 2: Develop a countermeasure using stochastic resonance to enhance sensorimotor adaptation.
Introduction
Exposure to space flight
Central reinterpretation
vestibular information
Alteration in sensorimotor
responses
Thus, during exploration class missions the sensorimotor disturbances due to the crewmember's
adaptation to microgravity may lead to disruption in the ability to maintain postural stability and
perform functional egress tasks during the initial introduction to the Earth's gravitational
environment. The first objective of this study was to document human motor and visual performance
during simulated motion below 2.0 Hz. The second objective was to develop a countermeasure to
mitigate sensorimotor disturbances after space flight.
Reduction in performance of
visual /motor tasks
Stochastic resonance (SR) is a phenomenon whereby the response of a nonlinear system to a weak input
signal is optimized by the presence of a particular non-zero level of noise (Collins et al. 1995, for reviews
see Collins et al. 2003, Moss et al. 2004, McDonnell and Abott 2009 and Aihara et al. 2010). This
phenomenon of SR is based on the concept of maximizing the flow of information through a system by a
nonzero level of noise (Collins et al. 2003). Application of imperceptible SR noise coupled with sensory
input through the proprioceptive, visual, or vestibular sensory system has been shown to improve motor
function as well as cardiovascular responses (Soma et al. 2003; Yamamoto et al 2005, Priplata et al. 2006).
Figure
shows
subject
performing the standardized
balance task standing on an
unstable compliant surface.
Performance was measured
using IMU’s attached to the
head and trunk segments and
a force plate underneath the
foam surface.
 Performance decrements occur during and after g-transitions
Objective 1: Investigate performance of motor and visual tasks during varying vehicle motion (0.1-2Hz)
2” x 4” electrodes (Axelgaard
manufacturing Co., Fallbrook,
CA) used for delivering the
electrical stimulation to the
vestibular system
Visual performance during vertical linear motion at 2 Hz
Baseline Period
(20 seconds)
Stimulation Period
(20 seconds)
Stimulation signals
Two bipolar stochastic stimulation signals were
generated with frequencies in the bandwidth of
0-30 Hz and 1-2 Hz. Seven stimulation levels of
0 µA, ±100 µA, ±200 µA, ±300 µA, ±400 µA,
±500 µA, and ±700µA were tested. The fully
generated signal was checked for zero mean
(±1%) and RMS [(30 µA RMS/100µA) ±5%].
Mediolateral
Moments
(Mx, N - mm)
Mediolateral
Acceleration – Head
(Hay, m/s2)
Roll Angular
Velocity– Head
(Hrv, deg/sec)
Portable bipolar constant current stimulator
used in the stand alone mode.
U
Horizontal Plane
Subject Position
0
LogMar Acuity
Vertical Plane
1.5
-0.1
-0.15
Standing
-0.2
1
0.9
0.8
0.7
150
300
450
Presentation Time, ms
1.1
100
Accuracy, %
Accuracy, %
80
60
40
20
80
60
40
20
0
0
0.96
0.92
0.88
0.84
Horizontal
Plane
20/20 20/30 20/50
Size of C, Snellen ratio
100
150
300
450
Presentation Time, ms
20/20 20/30 20/50
Size of C, Snellen ratio
Presentation Time
Target size: 20/30
Target Size
Presentation time: 300 ms
Hrv
0.9
Optimal Stimulus
level
0.7
0.5
-100
0
5
4
Tay
3
Trv
2
Mx
100 200 300 400 500 600 700
Stimulation level range (±, µA)
Parameter ratios - 1-2 Hz (Subject 16)
2.4
1-2 Hz
0.08
0-30 Hz
0.06
0.04
0.02
2.1
Fy
1.8
Hay
1.5
Hrv
1.2
Tay
0.9
Optimal Stimulus level
0.6
-100
Trv
Mx
0
100 200 300 400 500 600 700
Stimulation level range (±, µA)
0.07
0.05
0.03
Baseline Stimulus
Baseline Stimulus
11
9
7
5
0.018
0.014
0.01
0.006
0.022
0.018
0.014
0.01
0.006
Baseline Stimulus
Baseline Stimulus
Trial Epochs
Trial Epochs
A multivariate repeated measures ANOVA was used for analysis, using all six variables with two
within subject factors: Period (2 levels – Baseline, Stimulation), Trials (2 levels – No stimulation and
Optimal amplitude) and one between subject factor: Frequency (2 levels – 1-2 Hz and 0-30 Hz). The
within subject factor Period was significant (Wilk’s Lambda = 0.694, p<0.0001) and the interaction
Trial * Period was also significant (Wilk’s Lambda = 0.277; p<0.012).
These results show that as a group normal healthy control subjects significantly improved their balance performance at the
optimal level of SRVS.
Conclusions
1
Accuracy, %
Target Location
Active
Average Visual Acuity
Reaction Time, s
1.1
Reaction Time, s
Reaction Time, s
1.08
1.04
1
0.96
0.92
0.88
Target Location
Hay
6
Static
-0.25
Subject in rotary chair
Fy
1.3
These data show that this subject had an
optimal response at an amplitude of
200 μA (0-30 Hz ) and 100 μA (1-2 Hz).
-0.05
Optimal Trial
1.7
Subjects younger than 50 showed no differences between control and patient
groups, p=0.33, while, in subjects older than age 50 patients had significantly
higher scores than controls, p=0.02 in static and dynamic conditions.
Visual Acuity
Control Trial
RMS Mx (N-m)
D
Landolt-C
Optimal Trial
RMS Hrv(deg/sec)
R
Control Trial
RMS Fy (N)
RMS ratio (stimulation period/
Baseline)
Parameter ratios - 0-30 Hz (Subject 16)
L
Display
The RMS values of the parameters for the stimulation period is less than that for the baseline
period showing improved balance control with SRVS.
RMS Trv(deg/sec)
Chair for Dynamic Visual Acuity threshold by decade during 2 Hz vertical
Visual Acuity Test
motion
Visual performance during horizontal rotational motion at 0.8 Hz
 The effect of target location was then measured during
horizontal rotation with the optotypes randomly presented
in one of nine different locations on the screen (offset upto
10 degrees).
 The optotype size was logMar 0, 0.2 or 0.4 corresponding
to Snellen ratios of 20/20, 20/30 or 20/50.
 Presentation duration was 150, 300 and 450 ms. All these
conditions were counterbalanced across 5 trials each
utilizing horizontal rotation at 0.8 Hz.
Onset Ramp Offset Ramp
Mediolateral Force
(Fy, N)
Roll Angular
Velocity – Trunk
(Trv, deg/sec)
Visual acuity is significantly reduced during 2 Hz vertical translational motion compared to static, across all age ranges
within and between control and clinical vestibular subjects.
 Visual acuity was measured in 12 healthy subjects as they
moved a hand held joystick to indicate the orientation of a
computer generated landholt C as quickly and as accurately
as possible.
 Acuity thresholds were established with optotypes
presented centrally on a wall-mounted LCD screen at 1.3 m
distance, first without motion (static condition) and then
while oscillating at 0.8 Hz (DVA, peak velocity 60 de/sec).
Acuity was also measured using a standard wall chart 10’
distance. All acuity measures were made using a forcedchoice strategy and determined by the smallest size in
which the subjects answered correctly in 3 of 5 responses.
Stimulus Window
Stimulus amplitude = 0-± 700μA
Mediolateral
Acceleration- Trunk
(Tay, m/s2)
RMS ratio (stimulation
period/Baseline)
 Subjects sat in a comfortable chair, and were strapped in
using a standard seat belt.
 A personal computer set upon a tripod was 2 m away
from the chair. The center of the computer screen was set
at eye level for each subject.
 In both the static and dynamic testing phases the Landolt
C flashed on the computer screen for 75 ms around peak
velocity of motion. The size of optotypes presented
ranged from 0.4 to 1.0 logMAR (log of the Minimum
Angle Resolvable) or 20/8 to 20/200 Snellen ratios, in
one of 8 configurations: up, down, right, left, up-right,
up-left, down-right and down-left.
 Subjects were instructed to state the orientation of the C.
 In the static testing phase, always performed first, the
chair was stationary.
 In the dynamic testing phase, the chair moved vertically,
at 2 Hz, ± 5 cm to mimic vertical oscillations during
walking.
 The Best PEST (i.e. parameter estimation by sequential
testing) psychophysical threshold detection algorithm
was used to determine the visual acuity threshold for
each condition.
Baseline Window
Stimulus amplitude = 0 μA
RMS Hay (m/s2)
g-transitions
X4
 Each transition requires adaptation to altered gravity environment
Stimulation periods
RMS Tay (m/s2)
Earth Postflight
1. No
Sensation
Typical curve of output performance (e.g. discrimination
index) vs noise magnitude - McDonnell MD and Abbott D.,
PLOS Computational Biology, May 2009, Vol 5 (5) and
Harry J, Niemi JB, Priplata AA, Collins JJ, IEEE Spectrum,
April 2005.
Optimization of stimulus amplitude and frequency characteristics for vestibular stochastic resonance countermeasure
 Inter planetary missions have 4 periods requiring sensorimotor adaptation
Earth
transit
4. Decreased
Sensation
2. Some
Sensation
SR phenomenon using vestibular electrical stimulation by imperceptible stochastic noise (SRVS), when
applied to normal young and elderly subjects, showed significantly improved ocular stabilization reflexes in
response to whole-body tilt as well as improved balance performance during postural disturbances
(Geraghty et al. 2008; Mulavara et al. 2011).
Sensorimotor Performance Profile During Inter-Planetary Missions
Inter
Earth Preflight planetary Lunar/Mars surface ops
transit
Discrimination Index
Astronauts experience disturbances in sensorimotor function following their return to Earth due to
adaptive responses that occur during exposure to the microgravity conditions of space flight.
Crewmembers adapted to the microgravity state may need to egress the vehicle within a few minutes
after landing for safety and operational reasons. Exposure to even low frequency motions coupled
with the varying environmental conditions can cause performance deficits by affecting the efficacy of
motor and visual acuity dependent skills in tasks critical to emergency egress activities such as visual
monitoring of displays, actuating discrete controls, operating auxiliary equipment and
communicating with Mission Control and recovery teams.
3. Peak
Sensation
Vertical
Plane
1. Dynamic visual acuity (DVA) is reduced in the vertical plane at frequencies of 2 Hz and in the horizontal plane at
frequencies of 0.8 Hz. DVA varies with target location, with acuity optimized for targets in the plane of motion.
Perturbations at low frequency motions (0.1-2 Hz) may exacerbate sensorimotor deficits after space flight.
2. Low imperceptible levels of white noise based electrical stimulation of the vestibular system improves balance
performance consistent with the stochastic resonance phenomenon in normal healthy control subjects. The
amplitude of optimal stimulus for improving balance performance was predominantly in the range of
100-400 μA.
92
88
84
80
76
72
68
Horizontal
Plane
Vertical
Plane
Target Location
Target size: 20/30
Presentation time: 300 ms
Dynamic visual acuity threshold was reduced relative to static acuity in 7 of 12 subjects by one step size. Both accuracy and
reaction time varied as a function of target location, with greater performance decrements when acquiring vertical targets.
3. An SRVS based device may be fielded, either as a training modality to enhance adaptability or skill acquisition,
or as a miniature patch type stimulator that may be worn by astronauts to enhance adaptation following
gravitational transitions including people with disabilities due to aging or disease, improving posture and
locomotion function.
Acknowledgements
This study was supported in part by a grant from the National Space Biomedical Research Institute through NASA NCC 9-58 and
National Institutes of Health grant RO1-DC009031.