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Journal of Vestibular Research, Vol. 7, No. 6, pp. 453-457,1997
Copyright 4:1 1997 Elsevier Science Inc .
Printed in the USA. All rights reserved
0957-4271/97 $17.00 + .00
I:
PII S0957.4271(96)00167-X
Short Communication
MICROGRAVITY VESTIBULAR INVESTIGATIONS: PERCEPTION OF
SELF-ORIENTATION AND SELF-MOTION
Alan J. Benson,* Fred E. Guedry,t Donald E. Parker,+ and Millard F. Reschke§
...
til
*RAF School of Aviation Medicine, Famborough Hants, England, UK; tUniversity of West Florida, and
Naval Aerospace Medical Research Laboratory, Pensacola, Florida, USA; tDepartment of
Otolaryngology - HNS, University of Washington, Seattle, Washington, USA;
'Spase Biomedical §eseacgh IpstU!!te .!ghp59P spRre gemer H9'!5!on Teves I'SA
Reprint address: Donald E. Parker, Box 357923, Department of Otolaryngology HNS,
University of Washington, Seattle, Washington 98195-7923. Tel: (206) 285-7528;
Fax: (206) 616-1828; E-mail: deparker@u .washington.edu
o Abstract - Four astronauts experienced passive
'Whole-body rotation in a number of test sessions
during a 7-day orbital mission. Pitch (V-axis) and
roll (X-axis) rotation required subject orientations
on the rotator in which the otolith system was at
radius of 0.5 m. Thus subjects experienced a constant -0.22 Gz stimulus to the otoliths during the
60 s constant-velocity segments of ''pitch'' and ''roD''
ramp profiles. The Gz stimulus, a radius-dependent vector ranging from -0.22 Gz at the otoliths
to +0.36 Gz at the feet, generated sensory information that was not interpreted as inversion in
any of the 16 tests carried out in flight (12 in pitch
and 4 in roll orientation). None of the subjects was
rotated with head off-center during the first 33 h
of the mission. In the state of orbital adaptation of'
these sub.iect~. a -0.22 G1. otolith stimulus did not
provide E. vertical reference ir. the presence 0:' a
gradient 0:' -G 1 stimuli t(l the trunk amI leg!>.
© 19Yi Elsevier Science Inc ..
o Keywords - perception; weightlessness;
self-motion: self-orientation.
II
1992, and returned to Edwards Air Force Base
on January 30, 1992.
The restraint system on the MVI rotator required off-center head position for X (roll) or Y
(pitch) canal stimulation. Because ramp profiles
included periods of constant angular velocity
between the angular acceleration and deceleration, the otolith organs received sustained abovethreshold stimulation in a direction that would
be expected to yield perception of self-inversion.
This report deals with orientation perception
duri~g these pitch and roll ramp profiles.
Methods
SLlI~i(' cts
Four as[ronaut~ who were member~ of the
IML- J crew participatec in this investigation.
Three of the four had been on orbi t in previous
missions.
Introduction
Stimulus Profile
This paper describes part of one experiment conducted during the Microgravity Vestibular Investigations (MVI) which were included in the First
International Microgravity Laboratory (IML-1)
Spacelab. IML-1 was launched on January 22,
RECEIVED
I
The rotational stimulus profile of interest for
this report was an upramp acceleration at 200/S2
to 1200/s, speed held constant for 60 s before
downramp df'C'eleration at 1200/S2.
4 October 1996; ACCEPTED 8 October 1996.
453
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A. J. Benson et al
454
Stimuli to Vestibular Receptors
When the subject was in the pitch and roll
orientations, the vestibular apparatus received a
complex stimulus of both angular and linear accelerations. The physical stimulus to the semicircular canals is independent of their distance
from the axis of rotarion (l). In contrast. the
"timulus to the otolith organs depends on their
distance from the rotation :.lxis. During angular
acceleration. rhe otoliths were qimubted by the
'e~u ltaI1l of d1t: [~mgen[(ai J I1U :eomoetul au:elcrati om . At the end ,)1" upramp. c.: nrriperal .ICceleration was maximum and langential :'lCceler:.ltion was nil. The centripetal ~inear acceleration
(radial acceleration) during the 60 s of constant
velocity was aligned with the longitudinal (Z)
body axis and had a magnitude of 2.2 mls 2
(-0.22 G z) at the vestibule.
orientation perceptions. They practiced using the
vocabulary to describe their motion and orientation during preflight data collection sessions.
Data Recording
Inflight testing began as early as 18 hours after launch and continued throughout the 7-day
mission. but the first run with head off-center. a
pitch :un. occurred 33 hours .tfter launch. The
L': rs , ~ead 'lff-'.::::nter run 0:' ;he -;ubject without
:J1'ior orbital 'nission experience occllrred 54 hour"
..ther bunch. Intlight pen:epwai reports were
voice-recordetl after test sessions. During postnight debrier's (voice-and video-taped) the astronauts were asked to clarify or expand upon
their flight experiences.
Results
Stimuli to Other Graviceptors
Linear accelerations stimulate cutaneous, visceral, and other graviceptors. During rotation,
there was a radius-dependent gradient of the force
environment in headward and footward directions away from the rotation axis. Accelerations
reached their highest value at the feet, which at
a radius of 0.8 m experienced accelerations of
3.5 mls 2 (+0.36 G z). Movement of the lower
limbs during rotation was minimized by foot
plates and straps, and movement of the trunk and
head by a harness and helmet. These restraints
In particular, on earth, movement of the torso
was reduced by frictional forces between it and
bit, prevention of centrifugal movement of the
torso and legs relied on pressure exerted by the
shoulder straps, helmet and foot plate.
Perceptual Report Training
The astronaut subjects were taught a vocabulary, which included body coordinates and metrics, to be used in reporting their motion and
In the fIrst complete ramp profile in the pitch
orientation, on the second day of the flight (33 h
after liftoff), the astronaut reported no change in
the perceived plane of rotation during either
clockwise or anticlockwise rotations, but commented that the post-rotational self-rotation perception was more intense and of longer duration
than during preflight tests. Over subsequent flight
days, the four subjects were tested 11 additional
times in the pitch orientation and 4 times in roll.
On none of these occasions did any of the four
subjects report a perception of inversion or change
.
the
Three of the four subjects made at least one
report that indicated that they felt they were
tumblin "head-over-heels" rather than rotating
in pitch while lying on one side. One subject reported the plane of motion to be truly vertical
during pitch runs throughout the mission. The
fourth subject reported brief headward translation during the acceleration portion of a ramp
stimulus. Thus, perceived orientation of the
path of movement (the plane of angular and linear velocity) seemed to be influenced by linear
acceleration vectors in the pitch plane, but the
sustained - 2.2 Gz vector did not yield percep-
'
Microgravity Vestibular Investigations: t-'erceptlon
tion of static inversion in the interval after the
canal-mediated rotation sensation was expected
to abate. If perceived rotation were to be lengthened during the 60 s of constant velocity, as might
be inferred from the reported lengthening of the
post-rotation response, then little or no time would
remain for static inversion perception to develop.
Discussion
s
Perception of self-orientation and self-motion with respect to the environment is dependent upon central nervous system integration of
e
zontal when the head was some 0.2 m from the
axis of rotation. At greater radii they had a sensation of head-down inclination, and at smaller
radii, of head-up tilt. From this and other observations, Mittelstaedt argued that perception of
static orientation is governed by the summation
of inputs from the otolith and visceral gravity
receptors. the latter having a centroid at the height
of the last ribs.
On the MVI rotator, with the head 0.5 m from
the axis of rotation and the centroid of visceral
graviceptors close to the axis of rotation, a perception of head-down inclination (or inversion)
was a reasonable expectation based on the sustained -0.22 Gz stimulus to the otolith system.
----~~· ---+--~~~~~~~~~~~~~~~~~~~~~~~~~~ml~a~bro~ra~t~o~ry~te~s~ts~,~i~n~w~hi~·c~h~th~e~. . . .~
about orientation to the gravitational vertical is
provided by the otolith organs and by less specialized mechanoceptors that respond to inertial
and hydrostatic forces. These somatic graviceptors include receptors in the skin, joints, muscles, vascular system, viscera, and perhaps even
the kidneys (2,3).
It is well established, from experiments conducted on centrifuges and in aircraft, that subjects exposed to a sustained linear acceleration
report an illusory perception of tilt: the somatogravic illusion. The resultant of the imposed acceleration and gravity becomes the perceptual
vertical, and the subject experiences tilt if the
imposed acceleration is sustained for more than
a few seconds and is not aligned with gravitational vertical (4). If during aerobatic maneuvers the imposed acceleration is aligned with
gravity but acts in the opposite direction and is
greater than 1.0 G. so tha: brief -G, is expen enced. :,unjects often report feelin~ inverted
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The essential ditferencl Deiw;;:,:;", I1'k lS; ~;; pe: ­
imental studies of the somatogravic illusion and
the !vIV! experiment i~ that in th e former. the
force environment was essential1) uniform (that
is, isogravic) over the subject's body. whereas
in the latter it varied linearly as a function of
distance from the axis of rotation. Nevertheless,
it is known from the work of Mittelstaedt (2)
that subjects lying on a turntable rotating at constant speed (3.77 rad/s) felt that they were hori-
m-
ear
the
ep-
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ity at higher speeds and longer durations than
those in the MVI ramp profile, three of the four
astronauts reported an illusory "somatogravic"
perception that the Z axis of the head and body
was no longer earth-horizontal, but was tilted in
a head-down direction. The fourth astronaut, more
experienced in orbital missions than the others,
reported only sustained horizontal position during -Gz stimuli to the otoliths far greater than
those that produced head-down tilt perception
in Mittlestaedt's subjects. During pre- and postflight tests, perhaps this subject retained cognitive assessment of orientation acquired in previous missions on orbit.
During formal preflight tests in which the
standard MVI flight ramp stimulus was used,
none of the subjects reported apparent tilt. However, the -0.22 Gz otolith stimulus was less than
the -0.29 G2. stimulus that was just strong enough
[0 yield perceived head-tl0wn inclination in
Mittleswedt' :- suhjects . 'V. \. rea~ () ned thai on 0 1'hI : withcm: the - !.O G~ \ ~C lOr thai (In eartn i ~
i .:!so:\:ec '1:1- th: -(\.2': I~ \'!:" ..:tfl C;; lI", e ~t ~ \h:()ulC:
experience inversion or at least some change in
inclin mion. However. the -G. stimulus of 2.2
m/s::! to the otoliths sustained for 60 s did not
yield a perception of inversion. A number of
possible and nonexclusive explanations for this
finding include:
',1
t
",
1) Within the first day on orbit, a significant redistribution of body fluids occurs. The pro-
A. J. Benson et al
456
2)
3)
Ir
4)
I
5)
cess of adaptation to headward fluid distribution could alter interplay between a -Gz
otolith stimulus and a gradient of +Gz stimuli
to somatic graviceptors.
On orbit, without the frictional forces between the body and the rotator that are
present on earth, a slight centrifugal shift of
the legs relative to the rotator in response to
the centripetal acceleration could yield contact cues on the soles of ,he feet (and perhaps
on the burtocks, [h .. ( ,.v UUJ.] :;ppu!-,\.' .)[ul;m
cues to .percei ved in ver-;ion ..-\Japr:.trion to
microgravjry invoi ves ~l greater jepenJenc~
on tactile cues: 7).
Signals [rom the otoliths may have been reinterpreted or their weighting reduced. On
orbit, static otolith information has no utility.
As astronauts intellectualize orientation
within the space craft, adaptive processes
may involve reduction in the sensitivity of
otolith-dependent tilt reflexes, such as ocular
counterolling (8), and may elevate thresholds
for tilt-position perception during low frequency, linear oscillation (9).
A vector of only 2.2 m1s 2 may be an insufficiently strong otolith stimulus to yield inversion perception, especially when coupled
with a reduction in the weighting of information from the otolith receptors.
The stimulus was of insufficient duration. In
the absence of concordant information from
the semicircular canals, the somatogravic illusion develops gradually following a change
ant force are required before the subject's
percei ved change in body orientation attains
a maximum (10). If perceived rotation during
the period of constant velocity persisted longer
than 30 s, little time would remain for development of a perception of static inversion.
Summary
?er.:ei ved .n vasion was not produced by a
stimulus to the otolith system of -0.22 Gz sustaincu for 00 ~ecoI1J~ . The failure of .1 ~u s tained
otolith stimulus to influence perceived orientation position may be attributable ro cues that on
orbit differ from those on earth; alternatively or
in addition, it may be attributable to the state of
orbital adaptation that subjects had attained when
they were tested on orbit. Perceptual studies of
spatial orientation during forces produced by
controlled centrifugation on orbit are probably
the only way to specify conditions that will yield
a stable vertical reference in astronauts who are
in various states of adaptation to the orbital environment. Such studies should also assist in the
design of "artificial gravity" countermeasures
that would ameliorate the deleterious effects of
long-duration space flight.
Acknowledgmellts - We thank the astronauts who
strove to provide data under arduous circumstances,
the MYI investigator team who discussed the findings, and the support staff who helped acquire the
30 s of steady-state tilt relative to the result-
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velyor
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:ed by
obably
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ects of
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