Countermeasures to Reduce Sensorimotor Impairment and Space Motion Sickness Resulting from Altered Gravity Levels
Laurence R. Young, Sc.D.
Massachusetts Institute of Technology
Human adaptation to altered net gravito-inertial force has been of concern from the earliest reports of space motion sickness, through the Apollo exploration era, and into current planning of exploration missions. For example, the ability to perceive motion correctly is necessary for safe piloting of spacecraft. While the ability to perceive motion is initially disrupted when gravity changes, experience in new gravity levels causes errors to be reduced. Likewise, motion sickness is common when astronauts transition between gravity levels, but symptoms often subside, likely due to adaptation. Ideally, adaptation would occur more quickly so that errors cause fewer problems. Towards this goal, it is important to know if some people are faster adaptors than others. Likewise, it would be very helpful to have a method to train people to adapt faster.
The proposed research program, to be conducted by a collaboration of highly experienced space flight investigators from MIT and the Massachusetts Eye and Ear Infirmary at Harvard, takes a new approach which could lead to an effective, practical and acceptable protocol for pre-adapting astronauts to space flight. Using a centrifuge, we will quantify an individual???s sensory adaptation capability and use it to predict and to minimize the consequences of movement in any other gravity environment ??? eventually including weightlessness. In combination with appropriate use of a drug (promethazine) we anticipate the development of a new pre-flight adaptation protocol to minimize disorientation and motion sickness and to overcome disturbances in manual control. An important step in the development will be the determination of the benefit and risks associated with the use of promethazine in conjunction with adaptation training.
NASA Taskbook Entry
We propose a new approach herein, combining pre-training in an altered gravity environment with a pharmacological countermeasure to reduce motion sickness associated with gravitational transitions. As previously discussed, adaptation to an altered gravity environment requires an update to the cerebellar internal model which provides cancelling efferent copy information to the utricular otolith VO neurons. Specifically, in altered gravity the internal model must relearn the relationship between head tilt and utricular otolith cues which are modified in different gravity environments. We hypothesize that an individual’s speed and effectiveness in making this internal model adaptation is a personal trait. As seen in microgravity exposure, some individuals excel at this reinterpretation and have minimal symptoms, while others struggle. Further, we propose that through pre-training this personal trait of internal model adaptability can be temporarily enhanced. While other pre-training protocols have had limit success, many of these have aimed simply at increasing an individual’s emetic threshold for motion sickness through exposure to different non-gravity related sensory-rearrangement environments. Now, we propose pre-training directly in an altered gravity environment. Specifically, we hypothesize that with practice the subject’s internal model can learn to adapt efference copy outflow to a new gravitational environment more rapidly, thereby reducing sensory conflict. During first exposure, prior to adaptation, sensory conflict is inevitable and motion sickness may ensue. To reduce motion sickness symptoms we further propose using promethazine, hypothesizing that its application will minimize the symptoms but not impair sensorimotor adaptation.
We will use measures of orientation perception and manual control to characterize sensorimotor impairment during g-adaptation and subjective reports for motion sickness intensity. We have previously validated these measures in related preflight/postflight Shuttle/Spacelab mission testing (Merfeld 1996) or ground-based studies (Brown, Hecht et al. 2002). We will use hyper-G, as provided by a centrifuge, as our altered gravity test-bed.
We will use these measures to test the following Hypotheses:
(1) Individual differences exist in the ability to adapt to altered gravity environments and these differences can be predicted by measuring adaptability in one altered gravity environment.
(2) Pre-adaptation training in one altered gravity environment will improve sensorimotor adaptation in another altered gravity environment.
(3) Promethazine will reduce motion sickness, but will have no influence on either basic vestibular perceptual function or sensorimotor adaptation to altered gravity environments.
The perception measurement task aims at quantifying a subject’s tilt perception of a time-varying motion stimulus. Subjects use a two-handed somatosensory indicator (e.g. horizontal bar), which they attempt to align with the gravito-inertial horizontal in the dark. The somatosensory indicator is a lightweight 30 cm metal tube that pivots about its center point as seen in Figure 2 and is installed at waist level. Subjects hold the ends of the bar on either side with their fingertips and are not allowed to move their hands along the length of the bar. Subjects will perform the task in a discrete manner, aligning the bar to their perceived horizontal and upon alignment pressing a push-button located at the end of the bar to mark their setting. Multiple measures will be taken, where subjects are asked to offset the bar back-and-forth at least twice (by ≥20°) before re-aligning the bar. Offsetting the bar is required to help minimize the influence of previous settings. Subjects will be asked to make as many settings as possible while maintaining high accuracy. In our previous experiments, subjects performed this task at 2-second intervals, on average (Park, Gianna-Poulin et al. 2006). During experimental testing, subjects will be placed in a novel gravity environment (either -1.5 Gz or +1.5 Gz). Subjects will then passively experience a roll tilt profile in the dark. In each profile, the subject will initially be sitting “upright”, aligned with the centripetal acceleration vector. Then subjects will be passively rolled about an approximately head-centered axis. The motion profile will be a pseudo-random sum-of-sines with ten frequencies ranging from 0.05-0.2 Hz with a maximum roll tilt of 20 degrees. Each trial will last 30 seconds and will end with the subjects back to “upright”. After returning to upright, subjects will remain upright for approximately 20 seconds prior the next roll tilt in the series.
Because of its relevance to spacecraft stabilization and control, a manual control task will be utilized to study sensorimotor adaptation. Subjects will use a control wheel to attempt to keep the centrifuge chair aligned with the centripetal acceleration while it is perturbed by a pseudorandom roll disturbance. It resembles manual control of a vehicle in an altered gravity environment, without regard to instruments or external visual guidance. The manual control experiment will follow the same procedure used in our previous pre-post space flight studies (Merfeld 1996). The manual control task requires the subject to use a control wheel to maintain the roll device level. The wheel is mounted in front of the subject and provides a command signal by which the subject directly commands roll position. The control wheel will be circular (8.5 inch diameter, 1/2 inch thick), and will have a featureless surface that provides neither visual nor tactile cues as to center (position of zero command). Motion disturbance commands are a computer-generated, pseudo-random, zero-mean sum-of-sines signal.
Perceptual thresholds will be used to measure basic vestibular function and investigate the potential influence of promethazine. Vestibular threshold testing is analogous to an audiogram. The smallest motion amplitude that can be correctly distinguished is determined by moving subjects in random directions (e.g. yaw left vs. yaw right, or translate left vs. translate right) at adaptively changing amplitudes. After each trial subjects indicate the direction of their perceived motion. We use a standard “two alternative categorical forced choice” technique (Treutwein 1995; Leek 2001). We will measure thresholds for three types of stimuli:
(1) Upright yaw rotation;
(2) Upright inter-aural translation; and
(3) Upright roll tilt.
The project provides knowledge relating to motion sensation and perception that directly impacts human health. For example, an estimated one-third of US adults aged 40 years and older (69 million Americans) have dysfunction of the vestibular organs, located in the inner ear, which sense motion. A large fraction (20% to 40%) of the population suffer from vertigo, dizziness, or vestibular dysfunction and about 30% of these are not diagnosed via current clinical testing. Thus, this project will likely further knowledge on motion perception such that diagnostics can be improved.
Many people get motion sickness, for example, on boats. What we learn about motion sickness in altered gravity will directly transfer to people who get sick in other situations.