Journal of Vestibular Research - Volume 3, issue 3

Authors: Mittelstaedt, H. | Glasauer, S.

Article Type: Research Article

Abstract: This contribution examines the consequences of two remarkable experiences of subjects in weightlessness, 1) the missing of sensations of trunk tilt and of the respective concomitant reflexes when the head is tilted with respect to the trunk, and 2) the persistence of a perception of “up” and “down,” that is, of the polarity of the subjective vertical (SV) in the absence of, as well as in contradiction to, visual cues. The first disproves that the necessary head-to-trunk coordinate transformation be achieved by adding representations of the respective angles gained by utricles and neck receptors, but corroborates an extant model of …cross-multiplication of utricular, saccular, and neck receptor components. The second indicates the existence of force-independent components in the determination of the SV. Although the number of subjects is still small and experimental conditions are not as homogeneous as desired, measurements and/or reports on the ground, in parabolic, and in space flight point to the decisive role of the saccular z-bias, that is, of a difference of the mean resting discharges of saccular units polarized in the rostrad and the caudad (±z-) direction. Show more

Keywords: interaction of otoliths and neck receptors, subjective vertical in weightlessness, saccular bias, force-independent components

DOI: 10.3233/VES-1993-3311

Citation: Journal of Vestibular Research, vol. 3, no. 3, pp. 307-314, 1993

Authors: Oman, C.M. | Balkwill, M.D.

Article Type: Research Article

Abstract: In 1 G, the apparent time constant (Td ) of postrotatory SPV decay with the head tilted face down is 55% of that with head erect (Te ). This phenomenon is called “nystagmus dumping” and has been attributed to G effects on VOR velocity storage. Similarly, postrotatory sensation duration with head tilted (Dd ) is 32% of that when head erect (De ). In parabolic flight, Te and De are 70% of 1-G values, but a pitch back dumping movement produces no further change. Te , Td , and Dd have not previously been measured in orbital …flight. VOR and sensation duration was tested in 4 crewmembers in 4 preflight, 1 inflight (days 4 or 5) and 4 post flight sessions. Bitemporal EOG was recorded with eyes open in darkness. Instructions were to “gaze straight ahead,” and indicate when “rotation sensation disappears or becomes ambiguous”. Subjects were rotated CW and CCW head erect for 1 min at 120∘ /s, stopped, and EOG was recorded for another 1 min. This procedure was then used to study dumping, except that immediately after chair stop, subjects pitched their head forward 90∘ . SPV was calculated using order statistic filtering, and dropouts removed using an iterative model fitting method. Te and Td were determined by logarithmic linear regression of mean SPV for each subject. In orbit 90∘ pitch movement produced rapid subjective dumping, but not nystagmus dumping. Dd was noticeably shorter (“almost instantaneous”) compared in preflight Dd . Te and Td in orbit were similar to preflight Te for 3/4 subjects (rather than to preflight Td as expected). No consistent VOR gain changes were seen in orbit. Although Te is known to decrease acutely in parabolic flight, a longer time constant was measured in 3/4 subjects after 4-5 days adaptation to weightlessness, suggesting a return of angular velocity storage. Show more

Keywords: vestibular, vestibulo-ocular reflex, spatial orientation, space physiology, space shuttle

DOI: 10.3233/VES-1993-3312

Citation: Journal of Vestibular Research, vol. 3, no. 3, pp. 315-330, 1993

Authors: André-Deshays, C. | Israël, I. | Charade, O. | Berthoz, A. | Popov, K. | Lipshits, M.

Article Type: Research Article

Abstract: During the long-duration spaceflight Aragatz on board the Mir station, an experiment exploring the different oculomotor subsystems involved in gaze control during orientation to a fixed target or when tracking a moving target was executed by two cosmonauts. Gaze orientation: with head fixed, the “main sequence” relationships of primary horizontal saccades were modified, peak velocity was higher and saccade duration was shorter in flight than on earth, latency was decreased and saccade accuracy was better in flight. With head free, gaze orientation toward the target was achieved by coordinated eye and head movements, their timing was maintained in the horizontal …plane; when gaze was stabilized on the target, there was a trend of a larger eye than head contribution not seen in preflight tests. Pursuit: Horizontal pursuit at 0.25 and 0.5 Hz frequency remained smooth with a 0.98 gain and minor phase lag, on earth and in flight. In the vertical plane, the eye did not track the target with a pure smooth pursuit eye movement, but the saccadic system contributed to gaze control. Upward tracking was mainly achieved with a succession of saccades, whereas downward tracking was due to combined smooth pursuit and catch-up saccades. This asymmetry was maintained during flight in head fixed and head free situations. On earth head pea velocity was maxima upward, and in flight it was maximal downward. Show more

Keywords: gaze, saccade, pursuit, eye/head, human, microgravity

DOI: 10.3233/VES-1993-3313

Citation: Journal of Vestibular Research, vol. 3, no. 3, pp. 331-343, 1993

Authors: Israël, I. | André-Deshays, C. | Charade, O. | Berthoz, A. | Popov, K. | Lipshits, M.

Article Type: Research Article

Abstract: The reproduction, in complete darkness, of sequences of 5 horizontal saccades towards previously presented visual targets has been investigated in human subjects on the ground (control subjects) and one cosmonaut in microgravity. The incidence of corrective saccades during the execution of the memory-guided saccades in darkness has been examined. It was quite large for the control subjects (more than half of all saccades), and increased during the flight, while the corrective visually guided saccades incidence decreased. Direction errors occurred in about the third of all sequences on the ground, and this parameter also increased in microgravity. Memory-guided sequences were mostly …hypermetric. Whereas the absolute error continuously increased with the target rank, it was not the case with the amplitude ratio, which presented a peak at the third rank, that is, at the middle of the sequence. The accuracy of the reproduction of the sequences did depend on the sequence pattern as much as on the subject. Some learning was observed in repeated reproduction of the same pattern. Although the average error did not change in microgravity, the linear regression coefficient between the visually guided and memory-guided saccades decreased. Show more

Keywords: gaze, saccade, memory, human, microgravity

DOI: 10.3233/VES-1993-3314

Citation: Journal of Vestibular Research, vol. 3, no. 3, pp. 345-360, 1993

Authors: Lackner, James R. | DiZio, Paul

Article Type: Research Article

Abstract: Exposure to weightlessness affects the control and appreciation of body position and orientation. In free fall the perception of one’s own orientation and that of the surroundings is dependent on the presence or absence of contact cues, whether part of the body is visible in relation to the architecturally defined verticals of the space craft, cognitive factors, and exposure history. Sensations of falling are not elicited in free fall when the eyes are closed or the visual field is stabilized. This indicates that visual and cognitive factors as well as vestibular ones must be implicated in the genesis of such …sensations under normal circumstances. Position sense of the limbs is also degraded in free fall. This may be due to alterations in skeletal muscle spindle gain owing to a decreased otolith-spinal activation. We provide evidence that during initial exposure to weightlessness there is a decrease in muscle stiffness which affects movement accuracy. The altered loading of the skeletal muscles due to the head and body being weightless are shown to be significant etiological factors in space motion sickness. Show more

Keywords: orientation, proprioception, muscle spindle, space motion sickness

DOI: 10.3233/VES-1993-3315

Citation: Journal of Vestibular Research, vol. 3, no. 3, pp. 361-372, 1993