291
Journal of Vestibular Research 12 (2002/2003) 291–299
IOS Press
Sensorimotor aspects of high-speed artificial
gravity: III. Sensorimotor adaptation
Paul DiZio∗ and James R. Lackner
Ashton Graybiel Spatial Orientation Laboratory, Brandeis University, MS033, Waltham, MA 02454, USA
Received 6 June 2002
Accepted 28 April 2003
Abstract. As a countermeasure to the debilitating physiological effects of weightlessness, astronauts could live continuously in
an artificial gravity environment created by slow rotation of an entire spacecraft or be exposed to brief daily “doses” in a short
radius centrifuge housed within a non-rotating spacecraft. A potential drawback to both approaches is that head movements made
during rotation may be disorienting and nauseogenic. These side effects are more severe at higher rotation rates, especially upon
first exposure. Head movements during rotation generate aberrant vestibular stimulation and Coriolis force perturbations of the
head-neck motor system. This article reviews our progress toward distinguishing vestibular and motor factors in side effects of
rotation, and presents new data concerning the rates of rotation up to which adaptation is possible. We have studied subjects
pointing to targets during constant velocity rotation, because these movements generate Coriolis motor perturbations of the arm
but do not involve unusual vestibular stimulation. Initially, reaching paths and endpoints are deviated in the direction of the
transient lateral Coriolis forces generated. With practice, subjects soon move in straighter paths and land on target once more. If
sight of the arm is permitted, adaptation is more rapid than in darkness. Initial arm movement trajectory and endpoint deviations
are proportional to Coriolis force magnitude over a range of rotation speeds from 5 to 20 rpm, and there is rapid, complete
motor adaptation at all speeds. These new results indicate that motor adaptation to high rotation rates is possible. Coriolis force
perturbations of head movements also occur in a rotating environment but adaptation gradually develops over the course of many
head movements.
Keywords: Coriolis force, artificial gravity, disorientation, motion sickness, sensorimotor, vestibular, head, arm
1. Introduction
Long duration human presence in space is taking
place in the International Space Station and is being
planned for a future Mars mission. The microgravity
aspect of the space environment causes bone and muscle structural alterations, cardiovascular deconditioning, fluid shifts, and sensorimotor recalibration [2,8,12,
19,21,26]. These adaptations to weightlessness could
present debilitating threats upon return to earth gravity or landing on Mars. Many approaches are being
taken to develop countermeasures, the most integrative
∗ Corresponding author: Paul DiZio, Ashton Graybiel Spatial Orientation Laboratory, Brandeis University, MS033, Waltham, MA
02454, USA. Tel.: +1 781 736 2033; E-mail: dizio@brandeis.edu.
and most likely to be successful approach would be a
spacecraft that rotates to generate “artificial gravity”.
The challenge is to develop a form of artificial gravity
for which the benefits outweigh the side effects.
We will briefly describe the possible benefits and
likely side effects of several proposed countermeasures
involving centrifugation. The basis for understanding
these side effects traditionally relies on studying perturbations of the vestibular system that occur in a rotating
reference frame. We will describe a series of experiments assessing the shared role of extra-vestibular sensorimotor factors in head and arm movement control
and adaptation to rotation. The studies reviewed show
that Coriolis forces perturb head and arm movements,
but rapid motor adaptation of both is possible with additional movements. New results indicate at least par-
ISSN 0957-4271/02/03/$8.00  2002/2003 – IOS Press. All rights reserved
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P. DiZio and J.R. Lackner / Sensorimotor adaptation at high rotation speeds
tial adaptation is possible up to the rotation rates required for the forms of artificial gravity being considered. Ultimately, side effects and aftereffects will be
best countered by tailoring the environment and the exposure history to take advantage of the capacities of the
vestibular and non-vestibular systems for adaptation.
2. Centrifugal rotation as a countermeasure to
space flight deconditioning
The ideal artificial gravity environment,from the perspective of human habitation, would be one that continuously generates 1 g of centripetal force by slow rotation of a large radius spacecraft [29]. It would certainly
be an effective countermeasure. The large radius would
permit space travelers to carry out normal activities,
such as are required in a 1 g environment for bone, muscle and carviovascular homeostasis and maintenance of
accurate sensorimotor calibration. The necessity of activity is demonstrated by the fact that bed rest on earth
mimics the debilitating effects of microgravity [10,27,
30]. A large radius permits a slow rotation rate, which
will minimize side effects. The most salient side effects
in a rotating environment are the severely disorienting
and nauseogenic consequences of head movements, errors in movement and gaze control, postural imbalance,
and locomotory ataxia. The severities of these side effects are proportional to rotation rate [11,18,20]. At rotation rates greater than 5–6 rpm, most people exposed
to living in a rotating environment initially experience
incapacitating motion sickness and disorientation [3,
13,15]. Adaptive reduction of motion sickness and disorientation can be achieved through a regime of making
thousands of head movements at progressively greater
rotation speeds, but this procedure leads to powerful
aftereffects upon return to a non-rotating environment,
including temporary evocation of motion sickness by
head movements [14]. A rotating environment would
have to have a radius of about 37 m to produce 1 g of
artificial gravity at 5 rpm.
An alternative countermeasure, for which the likelihood of success is very doubtful, is to give people
periodic exposure in a small rotating chamber within
a non-rotating weightless spacecraft. Extremely vigorous exercise would be required during brief “doses” of
centrifugation to replace the conditioning gained simply by postural support and normal activities in a 1 g
environment. Periodic exercise with bungee cord loading in orbital space flight has not proven to be an effective countermeasure [31]. Bone mineral loss in space
flight is about 1% per month despite the various forms
of exercise loading that have been employed, including hours of daily treadmill locomotion with bungee
cord loading [28]. In addition, very high rotation rates
would be needed with a short radius device. A rotating
capsule must be about 2 m in radius to permit a subject
to have their head on the rotation axis and their feet at
the “wall” of the device. This requires a rotation rate
of about 21 rpm to produce 1 g at the feet and about
36.5 rpm to produce 1 g at the heart. The side effects
of vigorous exercise will be severe at these speeds, and
subjects will have to go back and forth between a nonrotating and a rotating environment for every exposure
session.
Both artificial gravity options require active movements on the part of space travelers and will produce
side effects, including motion sickness and disorientation evoked by head movements as well as derangement
of locomotion and manual control.
3. Vestibular and extra-vestibular factors in side
effects of head movements during rotation
Research into the origins and possible amelioration
of the side effects due to head movements during rotation has focused primarily on vestibular function. During constant velocity body rotation, turning or tilting
the head about any axis other than that of body rotation elicits afferent activity from the semicircular canals
signaling simultaneous movement of the head about
multiple axes, not just the actual movement axis. This
bizarre pattern of vestibular stimulation is called Coriolis, cross-coupled stimulation (CCS). People without
functioning labyrinths are immune to its nauseogenic
side effects [17]. CCS contributes to disorientation and
motion sickness in normal individuals, but the Skylab
M-131 experiments and parabolic flight experiments
have shown that the severity of motion sickness and
disorientation evoked by CCS is a function of gravitoinertial force background, being most severe in hyper-g
environments and quite modest in weightlessness [7,
16]. This pattern suggests that otolithic and mechanical
factors influence the side effects experienced.
To compare side effects of head movements during
rotation in conditions of constant CCS but different
loading of neck muscles, one study had two groups of
subjects make active head movements during body rotation, with and without a 500 g weight attached to a
bite plate [24]. Mechanical guides kept the subjects’
head movements in the pitch plane and of the same
P. DiZio and J.R. Lackner / Sensorimotor adaptation at high rotation speeds
amplitude to ensure that CCS of the vestibular system
was identical in the two conditions. The group supporting the weight became much more motion sick. Both
groups perceived a curved and laterally deviated head
movement path involving illusory roll and yaw in addition to their physical pitch plane movements, and the
group with the additional load perceived significantly
more curvature and deviation. The increased requirements on the neck and trunk musculature for movement
and postural maintenance due to the 500 g weight made
the difference. These results indicate that efferent commands to the neck muscles and feedback from muscle spindles and other receptors, along with vestibular signals, can influence perceived head trajectory and
motion sickness.
The contribution of non-vestibular factors to the perception of head trajectory was also demonstrated in an
experiment where rotation had similar Coriolis influences on arm and head movements. Subjects with eyes
closed in a chair rotating at a constant velocity misperceived the apparent paths of their head and arm movements. Figure 1 shows the experienced paths when
they attempted to make a pitch head movement and
when they attempted to extend their forearm in a sagittal plane. Scalloped paths of the head and arm were
perceived [25]. In parabolic flight experiments, the
perceived curvatures of the paths of the head and arm
during rotation were greater in a 1.8 g force background
than in 1 g and less in 0 g then in 1 g. This pattern
implicates a non-vestibular sensorimotor factor in the
experienced lateral deviations of the head and arm (in
addition to the CCS vestibular stimulation contributing
to the experienced rotational movement of the head in
roll and yaw). To identify the potential role of efferent
and proprioceptive signals in the common illusions of
head and arm deviation, experiments were done with
arm movements, which do not have the added complication of vestibular activation.
4. Motor adaptation of arm movements in a
rotating environment
In initial studies, subjects made reaching movements
before during and after constant velocity, 10 rpm counterclockwise (CCW) rotation, in the center of a fully
enclosed rotating room [23]. They reached to a target light that went off at movement onset leaving them
in total darkness without visual guidance of their arm.
During rotation, their reaches generated Coriolis forces,
Fcor = −2m(w × v), where m is the mass of the mov-
293
ω
Fig. 1. When subjects attempt to make pitch forward head movements or sagittal plane extension movements of the forearm (dotted lines) during counterclockwise body rotation, they perceive their
movements paths to be curved and deviated rightward (solid lines).
The movements are in fact deviated and curved rightward (dashed
lines) but the perceived effect is larger than the actual effect.
ing arm, v its linear velocity, and w the angular velocity
of the room. This velocity dependence meant there was
not a Coriolis force acting on the arm at the very beginning nor at the very end of the reaches, but while the
arm was being thrust forward a rightward Coriolis force
acted on it (see Fig. 2A). Constant velocity periods
were maintained for 2 min before reaching movements
were made to allow semicircular canal activation to decay. Accordingly, vestibular signals about body rotation were absent before, during and after the reaches. A
position control class of motor control theories, the αand λ-equilibrium point theories [1,9], would predict
that under these conditions a movement’s path should
arch to the right but end accurately, on target. However,
the subjects’ first movements during rotation were abnormally curved and missed the target in the direction
of the Coriolis force. The subjects adapted completely
in 10–20 reaches so that their movements again were
straight and accurate. At first they consciously felt the
Coriolis force perturbing their arm but when adapted
they reported it no longer seemed present, their movements again felt completely normal. Adaptation was
more rapid if the room lights were left on so that the
subjects could see their reaching movements. When
rotation ceased, they initially made reaching errors with
the adapted arm, symmetric to the trajectory and end-
294
P. DiZio and J.R. Lackner / Sensorimotor adaptation at high rotation speeds
point errors that had originally occurred during rotation. With additional movements, they re-adapted to
stationary conditions at the same rate they had adapted
to rotation (see Figs 2B and 2C). Leg movements during rotation are also perturbed by Coriolis forces and
also show rapid motor adaptation and negative aftereffects [5].
These findings prove that equilibrium point theories
of motor control do not predict the errors in movement control a rotating artificial gravity environment
will cause and greatly underestimate the capacity for
adaptation. In fact, exposure to rotation causes functionally significant movement errors, but rapid motor
adaptation to Coriolis force perturbations is possible.
A detailed monitoring of reaching trajectory through
vision, muscle spindles, Golgi tendon organs and cutaneous afferent signals from the arm in relation to efferent commands is required for the trajectory straightening and resumption of endpoint accuracy achieved in
these experiments. The near mirror-image aftereffects
that occur when subjects carry over their adaptation
from a rotating environment into a stationary one indicate that the form of adaptation involves an internal
model of the anticipated perturbation.
Recently the trajectories of arm movements have
been measured in a fully enclosed rotating room, turning at three rotation velocities, 5, 10 and 20 rpm,
CCW [32]. These rotation rates encompass the approximate range likely to be used in a rotating artificial
gravity environment or a short radius centrifuge as a
countermeasure to microgravity. Subjects pointed in
darkness to a visual target that was extinguished as a
movement began. The reaching movements were made
pre-rotation, during rotation, and post-rotation. Four
naı̈ve subjects were rotated at each test velocity. The
rightward Coriolis forces generated by the motion of
the arm perturbed reaching paths and endpoints at all
room speeds. Initial per-rotation movement paths deviated rightward from baseline and then inflected leftward
but still landed right of the target position. The magnitudes of lateral endpoint errors and movement curvatures increased monotonically with rotation velocity.
Average endpoint errors for the first per-rotation movements were 50 mm at 10 rpm, 36% smaller at 5 rpm and
70% larger at 20 rpm. The initial per-rotation movement curvatures were 14 mm at 10 rpm, 31% smaller
at 5 rpm and 102% larger at 20 rpm. All three groups
showed rapid and complete adaptation to the Coriolis
forces and had negative post-rotation after effects. This
pattern proves that the initial perturbations and adaptive accommodations have common properties across
A
ωsrr = 60°/s
FCor
Target
Start
varm
Initial
B
Pre-rot
Per-rot
Post-rot
C
2 cm
Final
5 cm
5 cm
Endpoint
Curvature
2 cm
Fig. 2. A. Illustration of rightward Coriolis forces (Fcor) generated
during forward reaching movements made while the body is rotating
counterclockwise. B. A top view of initial reaching movements
made before, during and after 10 rpm counterclockwise rotation at
the center of a rotating room (N = 11). C. Average endpoints
and curvatures of 40 reaching movements before, during and after
counterclockwise rotation (N = 11).
rotation speeds relevant for countermeasures involving
a large artificial gravity environment or a short radius
centrifuge.
Five subjects with complete, bilateral loss of vestibu-
P. DiZio and J.R. Lackner / Sensorimotor adaptation at high rotation speeds
lar function were also tested making reaching movements in the center of the darkened room rotating at
constant velocity [4]. They were tested in order to identify any possible vestibular causes of the perturbation
of reaching movements during rotation and to determine whether their capacity for motor adaptation would
be similar to that of subjects with full vestibular function. Their per-rotation reaches were initially curvilinear and ended to the side of the target in the direction in
which the Coriolis force had acted, just like those of the
control subjects. This pattern of initial perturbations
indicates that the reaching errors during rotation are
non-vestibular, motor errors. Subsequent per-rotation
reaches became progressively straighter, just like those
of normal subjects. However, unlike the normal subjects, they failed to show complete movement endpoint adaptation. See Fig. 3A. During the per-rotation
period, their reaches remained deviated in the direction of the transient Coriolis forces acting during the
reaches. Thus, although their reaches straightened out,
they reached straight to the wrong place. Labyrinthine
loss subjects in darkness cannot sense body motion in
space nor their position relative to gravity. This did
not affect their pre-rotation reaches which were identical in curvatures and endpoints to those of control
subjects, but the reaches of the two groups terminated
in different fashions. The labyrinthine loss patients
brought their finger down vertically while the controls
approached the target with their finger at a shallow angle along the same line as the main transport phase of
their reach. The difference in termination styles disrupted the normal pattern of shear forces on the finger pad when it contacted the surface which, in turn,
resulted in less than full endpoint adaptation because
these shear forces provide information about finger position relative to the trunk [4]. This interpretation also
explains an earlier finding that control subjects show
significantly less adaptation to Coriolis forces when
they perform reaches that end in the air above a target
than when they touch the target surface [23]. These patterns of adaptation indicate that fingertip contact cues
are critical for complete adaptation of reaching movements to artificial gravity when sight of the arm is not
permitted.
In another experiment, subjects with normal vestibular function were tested on-center and off-center in the
rotating room, pointing without visual feedback to a
target that was extinguished at the onset of a reach.
The purpose was to evaluate whether the static level of
background artificial gravity affects the initial response
and adaptation to identical Coriolis forces [22]. In the
295
off-center condition, subjects were positioned 2.23 m
away from the rotation axis, which exposed them to
a centripetal force of 0.25 g during 10 rpm rotation.
This created a gravitoinertial resultant force tilted about
14◦ relative to the vertical. The subjects seated upright
felt inclined backward during rotation. The centripetal
force was present before, during and after per-rotation
reaching movements and was parallel with the reaching
movements which were directed toward the center of
the room. During reaching movements a Coriolis force
was generated to the right because the room rotated
counterclockwise.
The peak velocities and durations of movements
were the same in the on and off-center conditions, therefore, the same Coriolis forces were generated. The
initial reaches made off-center showed deviations of
curvature and endpoint to the right, the same pattern
shown by subjects tested at the center of the room. This
means that background force level does not modulate
motor errors introduced by Coriolis forces. This contrasts with the strong background force dependence of
reactions to vestibular CCS [cf. 7, 16]. However, the
adaptation pattern of the off-center group was not the
same as the subjects on-center but was similar to that
of the labyrinthine loss patients, described above. See
Fig. 3B. The per-rotation reaches of the normal subjects tested off-center became progressively straighter
and showed negative (leftward) trajectory curvature aftereffects post-rotation. However, their adaptive reduction of movement endpoint errors per-rotation leveled
off before resuming pre-rotation baseline, in contrast to
on-center subjects who adapted completely during rotation. Endpoint aftereffects were significantly smaller in
the off-center than in the on-center group reflecting the
lesser endpoint adaptation acquired per-rotation. The
subjects seated off-center experienced mild disorientation relative to the physically level work surface of
the target array due to the altered gravitoinertial force
magnitude and direction. This means that adaptation to
Coriolis motor perturbations can be influenced by force
background and suggests that fingertip contact cues are
one mediating factor. It is likely that allowed visual
feedback of their reaching movements subjects tested
off-center would show rapid and complete adaptation.
This summary of arm movement control in a rotating environment indicates that Coriolis forces will initially elicit reaching errors but rapid motor adaptation
is possible. The initial errors will scale with Coriolis
force magnitude across an operationally relevant range
of rotation speeds and background force levels and orientations. This is important because it is not yet clear
296
P. DiZio and J.R. Lackner / Sensorimotor adaptation at high rotation speeds
A
Labyrinth loss, on center
Initial
Final
B
Control subjects, off center
Initial
Final
5 cm
Endpoint
Endpoint
Curvature
2 cm
Pre-rotation
Curvature
Per-rotation
Fig. 3. Plots of reaching movements made before and during 10 rpm counterclockwise rotation in darkness by: A) subjects with complete
bilateral vestibular loss seated on the axis of rotation (N = 5) and B) subjects with normal vestibular function seated off-center in the rotating
room (N = 10).
whether an artificial gravity environment must provide
a full 1 g force background in order to be an effective countermeasure. Practically speaking, the lower
the force required to prevent physiological deconditioning, the lower the necessary rotation rate. The extravestibular factors that are involved in side effects of
head movements during rotation are considered next.
5. Vestibular and motor adaptation of head
movements during rotation
Previous studies of motion sickness, head-eye coordination and spatial disorientation in a rotating environment have emphasized vestibular factors and neglected the Coriolis force actions on the inertial mass
of the head/neck system. These studies have not even
measured the actual properties of the head movements.
We have reported measurements of unconstrained pitch
head movements made during constant velocity rotation [6]. Movements were measured in six degrees of
freedom with an Optotrak motion analysis system. The
initial movements made during rotation showed deviations from pre-rotation patterns. Figure 4 illustrates
this for a typical subject. The perturbations involved
lateral translatory deviations of path and endpoint in
the direction of the Coriolis forces on the head. In
addition, the initial movements were deviated in roll
and yaw. The roll axis deviation was in the direction
of the Coriolis force on the head, and the yaw axis
deviation was consistent with vestibulo-collic reflexes
elicited by CCS. The lateral translations usually abated
within 8 movements but the roll and yaw components
adapted more slowly and were still partially present after 24 movements. These results indicate that there are
dual adaptation processes, one involving rapid motor
P. DiZio and J.R. Lackner / Sensorimotor adaptation at high rotation speeds
A
297
ω
ω
ω’ r
FCor
ω’ y
vhead
Top view
B
fwd
Side view
up
Pre-rotation
Per-rotation
Front view
Left
up
up
1 cm
fwd
1 cm
fwd
left
1 cm
left
Fig. 4. Illustration of the Coriolis force (FCor ) on the inertial mass of the head and cross-coupled stimulation of idealized yaw (ωy ) and roll (ωr )
plane semicircular canals evoked by a pitch head movement made during constant velocity counterclockwise rotation (ω). The head movement
is made after the semicircular canals have had a chance to equilibrate to their resting state. B. Plots of a typical head movement trajectory from
several perspectives before and during 10 rpm counterclockwise rotation. Motion of a point between the eyes is presented.
adjustments of the head/neck system analogous to arm
and leg adaptation to Coriolis forces and the other a
slower process of adapting to vestibular CCS. The findings have several important implications for designing
exposure paradigms for adapting astronauts to CCS.
First, the overall recovery of normal function may be
hastened with controlled exposures that appropriately
engage each sub-system. In addition, the pattern of
CCS that will be elicited by a head movement is a function of the head movement kinematics. Most analyses
of CCS exposure assume that the head will move in
the intended path, but our findings show that this does
not occur until the subject has undergone motor adaptation to the Coriolis force perturbations generated by
the planned head movements. Studies are currently in
progress in our laboratory to segregate and to optimally
engage the motor and vestibular subsystems in head
movement adaptation to rotation
6. Conclusions
The reviewed studies demonstrate that both vestibular and motor adaptation are important elements of human accommodation to rotating environments. Perturbation of motor control by Coriolis forces is an important influence of rotation upon all motor activities,
not just those involving head movements. Head move-
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P. DiZio and J.R. Lackner / Sensorimotor adaptation at high rotation speeds
ments involve side effects due to motor and vestibular
factors. Studies of reaching movements have illuminated principles of motor control and adaptation: 1)
existing theories of motor control, such as equilibrium
point control [1,9], do not predict the observed Coriolis
force disturbances of arm, leg and head movements; 2)
Coriolis motor perturbations are proportional to rotation speed and rapid adaptation is possible between the
upper and lower bounds of speeds being considered for
artificial gravity countermeasures; 3) adaptation to rotation involves acquiring internal models of Coriolis perturbations, this results in aftereffects upon transition to
a stationary environment; 4) motor adaptation depends
upon monitoring fine details of movement trajectory
and dynamics, spatial orientation and contact forces via
proprioceptive, visual, cutaneous and vestibular channels in relation to efferent signals; 5) the magnitude
of initial Coriolis motor perturbations is not dependent
on background force level but the rate and extent of
adaptation are affected. Orientation-dependence may
open the possibility for context-specific adaptation to
Coriolis forces, which could minimize side effects of
transitions between multiple force environments. Coriolis motor perturbations will occur in all rotating environments and adaptation will be rapid, but there is
less need for adaptation in low force backgrounds because the vestibular side effects are less severe [6,7,
24]. Finally, an effective countermeasure against the
side effects of head movements must act upon both the
vestibular and neuromotor systems.
[5]
[6]
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[10]
[11]
[12]
[13]
[14]
[15]
Acknowledgement
[16]
This work was supported by NSBRI grant NCC958-7 and NASA grant NAG9-1263
[17]
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