A set of seven experiments will study adaptation of the human nervous system to weightlessness. Particular emphasis will be placed on the vestibular and proprioceptive systems. The experiments are as follows: the sled/H-reflex; rotation/vestibulo-ocular reflex; the visual stimulator experiment; proprioception (relaxed) experiment; proprioception (active) experiment; proprioception (illusion) experiment; and tactile acuity

Watt, D.

English

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N92.2 ..
SPACE ADAPTATION SYNDROME EXPERIMENTS
(8-IML-1)
D. Watt
McGill University
Montreal, Quebec, Canada
This set of seven experiments will study adaptation of the human nervous system to
weightlessness, with particular emphasis on the vestibular and proprioceptive systems.
Sled/H-Reflex Experiment
The absence of gravity must cause human otolith organ function to be altered
significantly. These changes should be reflected in modified vestibulospinal reflexes. In this
experiment, subjects will be exposed to sinusoidal linear motion as a stimulus to the otolith
organs. H-reflex testing will be used to detect resulting modulation of spinal cord excitability.
This approach will make it possible to determine how vestibulospinal reflex function changes
during and after exposure to weightlessness (even if those changes are subthreshold), free of
contamination by voluntary motor activity.
Wearing ear plugs and a blindfold to eliminate sound and visual cues, a test subject for
the Sled Experiment is strapped into a seat on a device called the mini-sled (Figure 1). The seat
slides gently back and forth along two rails for a maximum distance of 100 cm (40 in.) to provide
a linear acceleration stimulus to the otolith organs. The test subject is further outfitted with
electrodes that stimulate reflexive muscle responses in the leg. One electrode placed behind the
subject's knee applies small electric shocks to the leg, and electrodes placed over the calf
muscles record the responses. Acting through usual reflex pathways, the acceleration stimulus to
the inner ear changes the response to the electrical shocks in the leg, and thus, it is possible to
measure otolith activity in an alert human being with a functional vestibular system. To study
the two independent receptors that make up each otolith organ, the subject is accelerated back
and forth while seated upright and while positioned on the back.
A computer records all results during the experiment, and the data are analyzed after
landing. If the nervous system learns to reinterpret the modified sensory information from the
otoliths, acceleration stimuli to the inner ear should continue to influence the response recorded
in the leg. If the nervous system learns to ignore that information, the variation in leg responses
should gradually disappear.
Rotation/VOR Gain Experiment
There are several reasons why the vestibulo-ocular reflex (VOR) and other components
of gaze control may be abnormal in weightlessness. These abnormalities could be an underlying
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cause of the Space Adaptation Syndrome since an unstable retinal image usually leads to motion
sickness. Subjects will use self-generated head movements to stimulate their semi-circular
canals. Eye and head rotations will be measured and the gain of the VOR calculated. The
subjects will also perform a gaze refixation task to evaluate eye-head coordination in
weightlessness.
Strapped into the stationary mini-sled seat, the test subject wears electrodes and
measuring devices that record head and eye movements (Figure 2). In the first test, the crew
member looks at a target, closes the eyes, rotates the head to the side while trying to keep the
eyes pointed at the unseen target, and then opens the eyes. The effectiveness of the
vestibulo-ocular reflex is determined by how close the subject's gaze is to the target when the
eyes are opened. In a complementary test, the subject, with eyes closed, oscillates the head from
side to side or up and down while trying to keep gazing at an imaginary fixed target. The
subject's ability to keep the eyes pointed at the target is a measure of the reflex effectiveness.
Finally, the subject is asked to move the head and eyes as necessary to look at a spot of light
which reappears in different locations on a screen.
In this experiment, all results are recorded on an analog tape and analyzed after flight.
Visual Stimulator Experiment
Subjects exposed to steady rotation of their visual field experience an illusion of
self-motion known as circularvection. There is now evidence that in the absence of a
gravitational reference, the phenomenon is enhanced as visual inputs become relatively more
important than vestibular inputs. This experiment builds on the previous studies of Young and
should lead to increased understanding of this important mechanism whereby the nervous system
compensates for disrupted vestibular function in space.
The subject on the stationary seat of the mini-sled faces a dome-shaped, randomly
patterned rotating visual field with its rim 2 inches ahead of his eyes. This position leaves a
narrow rim of the outside world still visible around the periphery of the dome, which has been
found to enhance the circularvection illusion, and to simplify the tracking task. The stimulus will
consist of constant speed rotation of the device at 30, 45, or 60 degrees per second, in one
direction or the other, for about 40 seconds. During the application of the visual stimulus, the
subject will develop a feeling of self-rotation, accompanied by apparent rotation of the small part
of the outside world which he can see in his peripheral vision, beyond the dome rim. The
subject's task will be to rotate a crank attached to a 360 degree precision potentiometer, matching
crank angular velocity to the apparent rotation velocity of the outside world.
A computer will monitor and record both dome speed and crank speed and the strength of
circularvection will be calculated by comparing these signals.
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Proprioception (Relaxed) Experiment
Anecdotal reports and the results of previous flight experiments provide a body of
evidence suggesting that proprioception is degraded in weightlessness.
In this experiment, an observer moves the head, arm, or leg of a blindfolded test subject,
who remains passive during the movements (Figure 4). The observer bends the joint fast to a
reference angle and then to a slightly different angle. The subject must determine whether the
second angle is larger or smaller than the reference angle, then move the joint and finally set it
back to the second angle. A goniometer measures the actual angles which are recorded by
computer.
The methods used will allow the determination of proprioceptive thresholds, and should
detect interactions occurring between the proprioceptive, vestibular, and motor systems, during
and after prolonged exposure to weightlessness.
Proprioception (Active) Experiment
There appears to be an unusual degree of loss of sense of orientation and of perception of
body image in the absence of both gravity and vision. Body image perception seems to improve
following active muscular contractions. Using a pointer similar to a flashlight, the subject will
point at a series of memorized targets during a prolonged absence of vision (Figure 5). A second
crew member records how closely the light beam comes to each target. This test is repeated with
the subject's eyes closed only during the extension, pointing, and retraction of the arm. This will
make it possible to separate errors caused by degraded proprioceptive ability from those resulting
from a distorted image of the outside world. This should determine if the perceptual errors are
the result of faulty sensory inputs or if they have a more complex cause.
Proprioception (Illusions) Experiment
Some crewmembers from previous space flight missions have described illusions in
which hopping up and down resulted in a feeling of the floor moving up and down like a
trampoline underneath them. This could be the result of inappropriate motor commands,
incorrect sensory feedback, or modified interactions between the two. In this experiment,
subjects will perform several types of rhythmical motor activity, note any resulting
proprioceptive illusions, and determine the effects of vision and tactile inputs on these illusions
(Figure 6).
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Tactile Acuity Experiment
During prolonged space flight, as unloaded intervertebral discs expand, astronauts
become several centimeters longer. This could lead to pressure blocks of nerve roots. In this
experiment, tactile acuity will be measured in one finger and one toe with the aid of metal blocks
having surfaces on which parallel ridges are spaced increasingly close together (Figure 7). The
subject is required to distinguish by touch alone the direction of the unseen ridges.
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Figure 1. SLED/H-Reflex Experiment.
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Figure 2. Rotation/VOR Gain Experiment.
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Figure3. Visual StimulatorExperiment.
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Figure 4. Proprioception (Relaxed) Experiment.
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Figure 5. Proprioception (Active) Experiment.
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KNEE BENDS
ARM BENDS
Figure 6. Proprioception (Illusions) Experiment.
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vFigure 7. Tactile Acuity Experiment.
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Space adaptation syndrome experiments (8-IML-1)

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