Astronauts’ sensory systems slowly adapt to the environment on arrival in space and readapt to conditions upon return to Earth. Dr. Mark J. Shelhamer is exploring ways to speed up the process by pre-adapting astronauts so they have some of the sensory rearrangement needed for optimal function in space before they arrive. Similarly, this research seeks to keep astronauts “Earth-adapted” while in space, so they can return to Earth more safely and function more effectively. Earth benefits come in part from a better knowledge of human adaptive capabilities, leading to countermeasures for people whose balance systems are affected by age or disease.
Overview
Context-Specificity and Other Approaches to Neurovestibular Adaptation
Principal Investigator:
Mark J. Shelhamer, Sc.D.
Organization:
Johns Hopkins University School of Medicine
Technical Summary
There are several operational issues involved with altered human performance during and immediately after space flight. These issues have implications for human safety and effectiveness. Our planned experiments are designed to give us the information needed to develop and assess appropriate countermeasures (pre-flight or in-flight activities) for the vestibular deconditioning that occurs during flight (and often persists upon return to a planetary environment). Whenever g-transitions occur, there is a very real possibility of disruptions in perceptual and sensorimotor processing and reflex calibrations. These can have serious consequences in a dynamic environment such as shuttle re-entry or Mars landing.
We propose context-specific adaptation (CSA) as a countermeasure to some of the deleterious neurovestibular effects of space flight. By CSA we mean the ability of an organism to 1) maintain two different adapted states for a response (such as two different saccade gains), 2) have each state associated with a specific context (such as g level), and 3) switch between the adapted states immediately upon a change in context (i.e., without de-adaptation and re-adaptation upon each transition). This phenomenon can be useful during phases of space flight that require transitions between different g environments (e.g., in and out of artificial gravity, from orbital flight to planetary landing). A related theme is the determination of effective adaptation procedures and effective context cues. The role of the cerebellum, and its possible disruption during flight, is another central issue, as is transfer of adaptation between motor systems.
Outline of sub-projects in this proposal
Our project consists of an integrated set of experiments that have as their overall goal the design of a spaceflight countermeasure based on forms of vestibular adaptation. Briefly, the experiments include three main investigations at Johns Hopkins: 1) studies on the effects of torsional misalignment, and the use of saccade adaptation and cyclovergence adaptation as countermeasures (Shelhamer/Zee, aims 1-3), 2) studies on the relationship between the LVOR and smooth pursuit and the role of the cerebellum on adaptation of these responses (Zee/Minor/Shelhamer, aims 4-6), and 3) a study on context cues in the human LVOR (Shelhamer, aim 9). Another set of experiments will be conducted at Washington University (St. Louis) to study how CSA might transfer between eye movements and limb movements (Angelaki/Snyder, aim 7), and experiments at the University of Mississippi Medical Center will investigate adaptation of the LVOR with transient accelerations (Zhou, aim 8).
General outline of the progress report
The research progress described in this report represents something of a major redirection of effort from the previous reporting period, as some projects wind down (Aims 7 and 8) and others focus on new findings (e.g., Aims 1-3 focusing on skew). We also take advantage of related ongoing work and its applicability to some of the neurovestibular problems of space flight (the LVOR stimulated by small rapid translations, vertical saccade asymmetries).
Specific Aims (as originally planned)
1. To determine if static torsional eye position (induced by a visual display or by parabolic flight) can be used as a context cue for the adaptation of saccade metrics. Previous work implies that torsional changes in flight may affect saccades and other spatially-oriented behaviors. We will attempt to demonstrate that saccades can be made veridical in two different torsional states.
2. To see if CSA can be more readily acquired by allowing consolidation of adaptation to take place before changing contexts. We will allow for consolidation of each adapted state to occur by inserting a rest interval between the two context states during the CSA procedure.
3. To develop cyclovergence adaptation as a countermeasure to torsional offsets during changes in gravity. A visual stimulus can be used to induce torsional misalignment (cyclovergence). We will design an effective cyclotorsion adaptation stimulus in lab experiments, and use it to maintain the usual (1g-based) torsional alignment during parabolic flight, and see if otherwise inappropriate responses (saccades) in flight are evoked correctly if torsion is "corrected" to its normal (1g) state.
4. To compare horizontal and vertical pursuit and LVOR deficits over a wide range of frequencies, in cerebellar patients and in monkeys with vestibulocerebellar lesions.
5. To study in normal humans, and in monkeys before and after vestibulocerebellar lesions, pursuit and LVOR adaptation and their transfer over a wide range of frequencies.
6. To study in normal humans, and in monkeys before and after vestibulocerebellar lesions, CSA of the LVOR and in particular the ability to use pursuit stimuli with different g cues as a stimulus for learning multiple LVOR gains as a function of the g state.
7. To determine if CSA learned in one behavior (eye movements) will transfer to a different behavior (arm movements) in rhesus monkeys. We will use static head tilt as a context cue to adapt either the horizontal AVOR or horizontal saccades. T hen we will investigate whether this context-specific adaptation is also present in memory-guided saccades and reaching. Experiments will be performed in intact animals and in animals with cerebellar lesions.
8. To use the transient linear vestibulo-ocular reflex (LVOR) to study context-specific otolith-ocular adaptation in human subjects. Our goal is to find the most effective procedure for adaptation of the transient LVOR, in anticipation of its possible use as part of a space flight countermeasure. (a) Systematically characterize task-specific LVOR adaptation in human subjects. (b) Identify the most effective training protocols to induce context-specific adaptation in human subjects. (c) Test for the ability of visual cues to substitute for vestibular cues in context-specific LVOR adaptation in human subjects.
9. To study CSA in the naso-occipital LVOR as for the inter-aural LVOR, and to determine what context cues are effective in each case.
Key findings and their impact
1. Two negative findings are of interest. First, we found that there is not a noticeable vertical error in horizontal saccades in parabolic flight. We thought that there might be, based on separate findings of disconjugate torsion in altered g levels, and saccade errors with the eyes deviated torsionally. The fact that we did not find such an error in the altered g levels of parabolic flight suggests that we can rule out this aspect of saccade accuracy as a confounding sensorimotor issue in space flight. The other negative finding is that asymmetries in vertical saccades (gain and latency) do not appear to be solely, or even predominantly, gravity-related. Again, this suggests that vertical saccades are likely not adversely affected during flight, at least as a direct consequence of altered g level.
2. A very significant and unexpected finding is the presence of vertical ocular misalignment (skew) during the altered g levels of parabolic flight. This was found in the course of the investigations on saccade error described above. It was first noticed when subjects reported that they saw a single small target light split into two (diplopia), and that they could not fuse the two images, especially in the 1.8 g phase of flight. This phenomenon may be another consequence (as disconjugate torsion likely is) of otolith asymmetry, and has clear implications for piloting and other tasks during g transitions.
3. Our various findings on the properties and adaptability of the linear VOR (LVOR) have increased our base of knowledge of this fundamental response. We hope to use this paradigm as a test of otolith and cerebellar function in the future. In particular we feel that it can be an important part of a standard pre-flight/post-flight vestibular test battery for flight crews.
This project's funding ended in 2004