A Strategy for Research in Space Biology and Medicine in the New Century (1998) / Chapter Skim
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5 Sensorimotor Integration
5 Sensorimotor Integration
Pages 63-79
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From page 63... ...
In the Goldberg report, the section on sensorimotor integration focused on five areas, including mechanisms of spatial orientation, postural control, vestibulo-ocular reflexes, vestibular processing, and space motion sickness. During the last 10 years, extensive experimental research has been performed in all of these areas.
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From page 64... ...
Studies on the position sense of the limbs have been carried out both in parabolic flights and space flight. 19 Position sense or limb proprioception is derived from afferent signals of the muscle spindles, and possibly also Golgi tendon organs and joint receptors, which are interpreted in relation to ongoing patterns of muscle activity.
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From page 65... ...
Researchers have also made progress in understanding how microgravity affects limb position sense. In the future, crewed space travel, especially interplanetary exploration, will probably involve transitions between different force backgrounds, with the need for adequate sensorimotor performance and orientation control in each force level.
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From page 66... ...
However, later on in flight a more pitched forward posture appears that reflects decreased extensor tonus and increased flexor activity.32 This change is consistent with the gradual shift mentioned above in the interpretation of proprioceptive signals associated with leg muscle vibration.33 In microgravity, deep knee bends or raising and lowering the whole body with the feet anchored can evoke illusions of deck displacement.34 The deck seems to move downward as the body moves toward it and upward as the body moves away from it. The reverse pattern occurs with exposure to high force levels.
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From page 67... ...
Head movements, especially in pitch and roll, can evoke apparent visual motion and disorientation during and after reentry.5~ 52 Alterations in the motor control of the head and its relationship with oculomotor control occur in spaceflight because the head is weightless and adaptive changes in control are necessary in the absence of gravity torques.
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From page 68... ...
After return from spaceflight, persistence of this remapping would mean that tilting movements of the head would initially be interpreted as linear translation for example, a forward head tilt would be interpreted as backward translation of the head, because the same otolith displacement would result from an actual linear acceleration of the head backward. Such reinterpretation of otolith output, although conceptually appealing, is unlikely to be the sole factor responsible for the postflight illusions evoked by head movements, because the magnitude, character, and time course of the perceptual responses are not comoletelv appropriate.
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From page 69... ...
The time constant of nystagmus decay has been studied in weightlessness and at greater than 1-g force levels to evaluate "velocity storage."67 Velocity storage refers to the observation that the nystagmus response to a velocity step outlasts the physical return of the cupula-endolymph system of the semicircular canal to resting levels. Velocity storage is thought to reflect the midbrain integration of a velocity signal originating from the semicircular canals.68 The overall eye movement response is thought to reflect the contribution of a "direct pathway" from the canal as well as velocity storage activity from an "indirect pathway." When the otolith organs are ablated, velocity storage is abolished.69 Interestingly, under microgravity conditions in spaceflight and the microgravity phases of parabolic flights, the time constant of nystagmus decay is significantly shorter than on Earth.70 7~ This suggests that the velocity storage mechanism is sensitive to linear acceleration.
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From page 70... ...
Astronauts and cosmonauts show diminished ocular counterrolling during static head tilts after return from spaceflight.~° 8~ The reduction can persist for 10 days or more following flights lasting more than several weeks. Postflight assessments using a linear sled also show diminished torsional responses.82 Rhesus monkeys show postflight reduction of ocular counterrolling as well.83 Observations of the torsional VOR elicited by voluntary head movements during spaceflight suggest initial decreases in gain followed by increases to levels even greater than preflight values.84 In general, all of the postflight VOR assessments both for angular and linear stimulation are consistent with a decreased contribution of otolith function to the responses.
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From page 71... ...
Recent work is preliminary, with few animals tested so far.95 Two studies performed on Rhesus monkeys have reported a postflight increase in gain of the horizontal semicircular canal afferents after a 14-day mission, but no change in the gain of the horizontal VOR postflight or in velocity storage relative to preflight values. In contrast, experiments performed on two other monkeys from a different mission found a decrease in afferent gain postflight (M.J.
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From page 72... ...
However, in parabolic flight, susceptibility to cross-coupling stimulation changes virtually immediately as a function of g level, with head movements during rotation being much less provocative in 0 g and much more so in 1.8 g than in straight-and-level flight.~07 These force-dependent variations likely reflect factors related both to the altered sensorimotor control of the head and to the changes in otolith activity as a function of force level, given that input to the semicircular canals is kept constant in these studies.~°8 Subjective reports suggest that visual factors can also influence the development of SMS. For example, when astronauts are upside down in relation to the architectural ceiling of the spacecraft, or see other astronauts upside down in relation to themselves, they may develop symptoms.~09 ~0 This is especially true for astronauts who are visually dependent, i.e., down is where the visual architectural floor is, as opposed to astronauts who tend to perceive the "down" direction as corresponding to where their feet are located.
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From page 73... ...
Mal de barquement may also occur in transitions between background force levels and is a potential operational problem for entry or reentry after long-duration missions. · The relationship of motion sickness to altered sensorimotor control of the head and body as a function of altered force background and elective body weight should be assessed.
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From page 74... ...
· The efficacy of different countermeasures against space motion sickness and postilight reentry disturbances should be validated. Those not validated should be discontinued.
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From page 75... ...
1993. Gravitoinertial force level influences arm movement control.
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From page 76... ...
1981. Illusions of postural, visual, and substrate motion elicited by deep knee bends in the increased gravitoinertial force phase of parabolic flight.
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From page 77... ...
1988. The effects of gravitoinertial force level and head movements on post-rotational nystagmus and illusory after-rotation.
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From page 78... ...
1986. Sudden emesis following parabolic flight maneuvers: Implications for space motion sickness.
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From page 79... ...
1991. Motion sickness susceptibility in parabolic flight and velocity storage activity.
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