Context-Specific Adaptation of Gravity-Dependent Vestibular Reflex Responses
Principal Investigator: Mark J. Shelhamer, Sc.D.
As we move about in the environment, we constantly make use of reflexive motor adjustments to maintain posture and balance in reaction to disturbances. Two such motor activities are the movements of the head and the eyes. These movements help keep images of objects stationary on our retinas as we move about the environment. This is a critically important sensorimotor task, since visual acuity can be significantly degraded with retinal slip of only a few degrees per second. Impairment can lead to disorientation and reduced performance in tasks such as piloting of spacecraft.
Adaptive capability in these systems is important to counteract aging and disease. Adaptation should also occur during transitions between different environmental conditions if optimal performance is to be maintained. Transitions between different gravitoinertial force (gif) environments occur in different phases of space flight; examples are: 1) earth orbit and reentry, 2) into and out of artificial gravity, 3) inter-planetary travel and planetary surface exploration. These transitions provide an extreme test of the adaptive neural mechanisms that maintain head and eye stability, and raise important operational questions: If astronauts learn sensorimotor skills such as piloting in the normal gravity of Earth, will they be able to perform them adequately in the weightless or the artificial gravity environment? More generally, can people have two different sets of reflexes, between which they are able to switch rapidly based on a context cue (context-specific adaptation)? Are there procedures that could help to transfer (or to inhibit) training from one situation to another? These are the types of questions that we address in our work. We have recently demonstrated rapid, context specific adaptation of the VOR gain for on-axis earth vertical rotations (Shelhamer et al. 1992) and horizontal linear accelerations (Patel et al. 1998).
Our research team is carrying out an integrated set of experiments to address these issues further. We use the general approach of adapting a particular vestibular or oculomotor behavior in a particular manner (e.g. to change its gain or phase) in one condition of gravitoinertial force, and adapting in a different manner (different gain or phase) in a second gravitoinertial force condition. Then we see if the gravitoinertial force itself the context cue can recall the previously learned adapted responses. Some of the responses that we examine are the following:
The Linear Vestibulo-Ocular Reflex (LVOR): the eye movements that are made in response to lateral translation of the head, to maintain the line of sight on a fixed location.
The Angular Vestibulo-Ocular Reflex (AVOR): the eye movements that are made in response to rotation of the head, to maintain the line of sight on a fixed location.
Ocular Counter-Rolling (OCR): the eye movements that are made in response to static tilt of the head, to help maintain the retinal meridian in a vertical orientation.
The Vestibulo-Collic Reflex (VCR): neck muscle response induced by rotations and translations of the body, to reduce head oscillations and maintain head orientation.
Saccadic eye movements: rapid movements (as made when reading) that are used to examine an environment and are crucial when, for example, scanning an instrument panel.
The knowledge gained from these studies will help us to design potential pre-adaptation strategies to assist flight crews in making transitions between different gravitoinertial force situations, and can provide design data for spacecraft facilities (artificial gravity, exercise centrifuge) by delineating the limits of human adaptive capabilities. The experiments consist of a core set of studies using the Johns Hopkins human linear sled and animal facilities, and NASA's KC-135 parabolic flight aircraft, to examine the AVOR, the LVOR, and saccades, using gif (magnitude or direction) as a context cue. The overall goal of the animal studies is to understand the role of the cerebellum (a part of the brain involved with coordination and control of movement) in these vestibular responses.
Human linear acceleration sled at Johns Hopkins (PI: Shelhamer & Zee). Used to test oculomotor reflexes to controlled translations of the head and body, the sled travels on air bearings, and is guided by aluminum rails. Linear induction motors provide the motive force. Sled motion, visual stimuli, and target lights are controlled by computer from an adjacent room. Ultimate capability of the sled will be sustained oscillation at greater than 1 Hz, 0.5 G, for 30 minutes. Click on this picture, or any of the ones below, to view an enlarged version.
Subject seated in sled, in preparation for LVOR (linear vestibulo-ocular reflex) adaptation experiment. A bit bar (not shown) and ear pads are used to stabilize the head during sled motion. Eye movements are measured with a scleral search coil system. The large aluminum frame surrounding the subject generates orthogonal low-level magnetic fields, which induced current in a contact-lens mounted coil on the subject's eye (see next photo); the amplitude of the current in the coil from each field is related to the angle of the eye (lens and coil) with repect to the field, allowing the instantaneous measurement of eye position. Click on this picture, or any of the ones below, to view an enlarged version.
NASA's KC-135 aircraft in parabolic flight pattern. Used to provide short (approx 25 sec) periods of "weightlessness" for conducting training and experiments. In our project, we will use this aircraft to produce alternating periods of low-g and high-g. Human sensorimotor responses will be adaptively altered via a learning procedure to give one type of behavior during low-g, and a different type of behavior during high-g. Afterward, responses will be tested in high and low g, to see if the learned behavior can be associated with the instantaneous g-level (context-specific behavior). Click on this picture, or any of the ones below, to view an enlarged version.
A related study at MIT investigates the ability of humans to maintain adaptations simultaneously to a rotating (Coriolis) and to a stationary environment. Another, at Baylor College of Medicine, involves adaptation of the VCR and extends the VOR work to the situation of head/neck stabilization (http://www.bctm.tmc.edu/cfbd/NSBRI/).
Consideration of a rotating device to produce artificial gravity as a countermeasure dates back to 1865 when Jules Verne published Around the Moon. The case for artificial gravity as a countermeasure has recently received renewed attention because of the failure of existing treatments for dealing with the debilitating effects of long duration spaceflight. Large rotating structures are unlikely a present day solution to the physiological deconditioning because of their complexity and exorbitant costs. Short arm centrifuges however, such as the MIT Artificial Gravity Simulator (AGS), offer both a cost and space effective alternative. The AGS, pictured below, is a 2-meter rotating bed that produces a 100% gravity gradient along the subject's longitudinal axis. Rotation rate, subject position, and instrumentation can be adjusted as desired.
The purpose of this investigation is to demonstrate
the ability to learn and maintain dual adaptation to a rotating
and a non-rotating environment. Only by the development of dual
adaptation or an equivalent elevation of the threshold to motion
sickness can we be confident that the countermeasure will be
both effective and acceptable. Our hypothesis states that repeated
exposure to a program of all-axis head movements on a 23 rpm
rotating bed and in a non-rotating environment will yield successively
shorter periods of adaptation to each environment. It is our
hope that finally the transitions can be made with no measurable
transient autonomic disturbances, inappropriate eye movements,
spatial disorientation or motion sickness. Based on the present
modeling of the vestibular system, we anticipate that the current
experiment will elicit, initially, a transient component of eye
movements about an axis orthogonal to the head movement (yaw)
and the vertical spin axis. We fully anticipate adaptation to
occur, however, we are primarily interested in whether or not
the adaptation will be retained during subsequent exposures following
intervening periods of activity in a non-rotating environment.
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