NATIONAL SPACE BIOMEDICAL
RESEARCH INSTITUTE
RESEARCH INSTITUTE
| Principal Investigator: | Mark J. Shelhamer, Sc.D. |
| Organization: | Johns Hopkins University School of Medicine |
| Project Ended: | 2000 |
| Principal Investigator: | Mark J. Shelhamer, Sc.D. |
| Organization: | Johns Hopkins University School of Medicine |
| Project Ended: | 2000 |
In the absence of a normal earth gravity field, the dynamics of head stabilization, and the interpretation of vestibular signals that sense gravity and linear acceleration, are subject to change. Transitions between different GIF environments - as during different phases of space flight - provide an extreme test of the adaptive mechanisms that maintain these reflexes. During extended space flight, crew members may live in artificial gravity and make transitions to weightlessness, planetary exploration, and return to Earth. If they 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 vestibular reflexes, which they are able to switch between rapidly? Are there procedures that could help to transfer (or to inhibit) training from one situation to another? These are the main types of questions addressed by our work. The overall goal of this project is the study of context-specific vestibular and oculomotor reflexes. Special emphasis is placed on the use of GIF as a context cue for switching between adapted reflexes. The general approach is to adapt a specific motor response (saccades, VOR, VCR) in one way (e.g. increase in gain) in one GIF condition, and another way (e.g. decrease in gain) in another GIF condition, and then see if the GIF condition itself (the context cue) can recall the previously learned adapted responses.
The knowledge gained from our studies will help us to design adaptation strategies (pre-flight and in-flight) 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.
A prerequisite to the use of context-specific adaptation procedures as a countermeasure is to identify those responses that need to change in a context-specific manner during space flight. There are many physiological changes that occur during flight, but not all of them are adaptive in the sense of bringing performance back to the normal pre-flight level. One must also think in terms of possible detrimental effects during long-duration flight. If reflexes become inappropriately calibrated during extended flight, then this "incorrect" calibration may generalize to the planetary gravity phase. There is certainly evidence that this occurs, as evidenced by difficulties with posture and locomotion immediately after shuttle flight.
Some reflex responses that develop in flight are inappropriate for planetary gravitational fields, while perfectly acceptable for 0-G. An example is the putative reinterpretation, in space, of all otolith stimulation as translation (rather than tilt). As there is no true tilt (change in orientation with respect to gravito-inertial vector) in space, the absence of tilt responses is acceptable there. However, when this response configuration generalizes to a planetary gravity environment (whether Earth 1-G or Mars 0.38-G), it is inappropriate. Thus we might tailor context-specific adaptation to maintain acceptable planetary responses while in flight, in association with artificial gravity or another stimulus arrangement that simulates one requiring tilt responses.
As an example, two responses which may be amenable to context-specific adaptation follow:
Outline of Individual Sub-Projects
Various experiments investigate the behavioral properties, neurophysiological bases, and anatomical substrate of context-specific learning mechanisms. We use otolith (gravity) signals as the contextual cue for switching between adapted states of the saccadic system, the angular and linear vestibulo-ocular reflexes, and the VCR. (By LVOR we mean the oculomotor response - horizontal, vertical, and torsional - to linear translation of the head and body.)
Context-specific saccade adaptation - We have evidence for context-specificity in human saccades. Two sets of parabolic flight experiments examined the use of instantaneous gravity level (alternating 0-G and 1.8-G) as a context cue for adapted saccadic eye movements. Saccades (rapid eye motions that move the eyes between targets) can be adaptively altered by presenting a target, then moving that target to a new location before the eyes can get to its first location. After several trials, an adaptive sensorimotor mapping takes place, so that the eyes move directly to the new target location when presented with the original target. Ground experiments at Johns Hopkins successfully used vertical eye position, horizontal eye position, head roll tilt, and upright/supine posture as context cues, so that saccades are increased in size in one context (when subjects look upward, or tilt their heads to the right, or are seated upright), and decreased in size in the other context (when subjects look down, or tilt their heads to the left, or are supine). Data from parabolic flight indicate that g-level also can serve as an effective context cue.
Context-specific LVOR adaptation - We demonstrated the ability to use a gravity cue (head orientation) as a context for switching between two different adapted versions of the linear VOR. The gain of the LVOR can be adaptively changed by having the subject view a visual field that moves with him or her on the sled (driving the gain down, since no eye movements in response to head/body translation are required to stabilize the visual field) or view a visual field that moves opposite to sled motion (driving the gain up). We have been able to induce changes in gain that are associated with head roll tilts (context cues) in different directions.
Properties of AVOR and LVOR in squirrel monkey - In the squirrel monkey, we completed baseline investigations of the dynamics of the AVOR with high frequencies and accelerations, revealing interesting nonlinearities which must be understood before adaptive effects can be investigated. Monkey LVOR adaptation studies were also performed, demonstrating adaptive increases and decreases. Torsional eye movement responses to the linear translations did not change significantly after adaptation, suggesting that the translational and tilt components of the LVOR (horizontal and torsional eye movements, respectively) may not be closely coupled. This has implications for paradigms designed to adaptively change tilt-translation interpretation.
Pursuit and the LVOR in humans and in rhesus money - Results in animals indicate that the LVOR is abolished after flocculectomy, and it is greatly impaired in humans with vestibular deficits as well. Pursuit deficits mirror these changes in the LVOR. This suggests that pursuit and the translational LVOR are tightly linked. A separate set of experiments has demonstrated context-specific adaptation of pursuit gain in humans and monkeys. These two results together may form the basis for a powerful strategy to adapt the otolith-mediated translational LVOR.
Properties and adaptation of head-neck reflexes - Experiments at Baylor College of Medicine on adaptation of the VCR also show evidence of context-specificity. These experiments have established baseline properties of the response along different axes, in terms of mathematical models. Adaptation to an artificial increase in inertia of the head has been demonstrated, as manifest by a decrease in head oscillation during body perturbations. The appropriate adapted response was stored by the head-neck control system even after subsequent re-adaptation back to normal inertia: the system responded appropriately to each inertial load to keep head oscillations at the same level. This capacity to switch between two sets of system parameters persists for at least 35 days after the initial adaptation: the appropriate head damping occurred immediately for both normal and increased inertia loads, showing that two sets of damping parameters can exist simultaneously and be switched in and out as needed.
Adaptation to a rotating environment - Short-radius centrifugation (a form of artificial gravity) is a promising potential countermeasure to long-term weightlessness. Unfortunately, it has a number of side effects related to the unexpected effects of head movements in the rotating environment. Transitions between the artificial gravity (rotating) and weightless (non-rotating) environments will likely cause additional problems. Experiments at MIT are investigating the extent to which these side-effects can be overcome through adaptation. Head movements during centrifugation induce discomfort, non-compensatory vestibulo-ocular reflexes, and illusions of body tilt. Significant adaptation occurred following a series of experimental sessions of head turns during rotation in the light, such that these detrimental effects were reduced.
Key Findings and Implications
Additional Implications, Relationship to NASA Critical Path Issues
Neurovestibular problems have been identified and listed on the NASA "Critical Path Roadmap" for serious problems that could affect a mission to Mars. Some indication of the range of problems and their severity is found in the May 1997 "Final Report of the NASA Task Force on Countermeasures," which states: "Based on the experience of both the cosmonauts and the astronauts, it is apparent that the ability to egress suddenly will be limited unless effective countermeasures for the loss of neuromuscular performance are identified and adhered to rigidly during prolonged spaceflights." Specific problems listed in the report include changes in eye-head coordination, decrements in postural control, sensory illusions such as otolith tilt-translation reinterpretation, and "flashbacks" between 1-G and 0-G states with associated motor dysfunction. Concerns were raised for the effects of these problems on vehicle control and unassisted egress. The issue of "flashbacks" is especially interesting relative to our work, as it indicates the simultaneous existence of two adapted states (one for 0-G and another for 1-G). Knowledge about how to avoid such inadvertent flashbacks, as well as how to make use of contextually-gated dual-state adaptation, is the central aim of all of our studies in this project.
An especially useful aspect of our parabolic flight experiments is that we fly in consecutive years. With the same subject tested each year, we can assess how much the 0-G responses have been maintained throughout the intervening period of 1-G exposure. This is particularly germane to Mars missions, when gravity-based responses which may have been trained before flight may have to be recalled in the Martian gravity environment many months later.
Not only do our studies provide valuable information for the development of countermeasures, they will also provide basic information on adaptive neurovestibular processes. This is especially true of experiments dealing with the role of the cerebellum in motor control, and signal processing of otolith information for the generation of reflex responses in different environments.
One specific clinical implication of these studies is in the area of vestibular rehabilitation (and physical rehabilitation in general). Rehabilitation exercises are generally learned and carried out under supervision in a clinical setting. There is the possibility that inadvertent contextual cues in this setting will be associated with improved performance while in the clinical setting, which will not transfer completely to settings of normal daily living. In this respect, it is useful to know what context cues are most effective, what types of responses can be made context-specific, and how to avoid such context-specificity when it is detrimental (i.e. when generalization is desired).
| Principal Investigator: | Mark J. Shelhamer, Sc.D. |
| Organization: | Johns Hopkins University School of Medicine |
| Project Ended: | 2000 |
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