Proprioceptive information processing in weightlessness.

The "illusions" experiment carried out on five astronauts during the last two French-Russian flights (Antarès in 1992 and Altaïr in 1993) and in the Russian Post-Antarès mission (1993) was designed to investigate the adaptive changes in human proprioceptive functions occurring in weightlessness at both the sensorimotor and cognitive levels, focusing on two kinds of responses: (1) whole-body postural reflexes, and (2) whole-body movement perception. These kinesthetic and motor responses were induced using the tendon-vibration method, which is known to selectively activate the proprioceptive muscular sensory channel and to elicit either motor reactions or illusory movement sensations. Vibration (70 Hz) was therefore applied to ankle (soleus or tibialis) and neck (splenii) muscles. The subject's whole-body motor responses were analyzed from EMG and goniometric recordings. The perceived vibration-induced kinesthetic sensations were mimicked by the subjects with a joystick. The main results show that a parallel in-flight attenuation of the vibration-induced postural responses and kinesthetic illusions occurred, which seems to indicate that the proprioceptive system adapts to the microgravity context, where standing posture and conscious coding of anteroposterior body movements are no longer relevant. The same sensory messages are used at the same time in different sensory motor loops and in the coding of newly developed behavioral movements under microgravity. These results suggest that the human proprioceptive system has a high degree of adaptive functional plasticity, at least as far as the perceptual and motor aspects are concerned.

Gaze strategies during linear motion in head-free humans.

1. Eye-head coordination strategies during horizontal displacements along the y (interaural) axis were investigated in human subjects seated on a sled (linear accelerator device) and tested in head-free conditions. They were instructed to stabilize their gaze, while in motion, on an earth-fixed memorized target and then, after cart immobilization, to look again at the real target. The last part of the test required a corrective saccade, which enabled us to evaluate the error of the subject's displacement estimation. Eye and head compensatory reflexes were tested within the 0.001-0.2 g acceleration range with a sinusoidal motion amplitude of 0.8 m peak to peak. 2. Fixation stabilization on a memorized target was achieved by different eye-head coordination strategies. According to the relative contribution of eye and head motion, a continuum among individual strategies was observed, covering a range of head contributions varying from 0 to almost 100%. All these strategies were well adapted because they contributed to the counteraction of the displacement and led to an optimal gaze accuracy. 3. The use of various gaze strategies during linear motion to achieve the same movement differed according to the subject, but also depended upon motion kinematics. As a rule, head contribution increased as the magnitude of linear acceleration was enhanced. 4. Different eye-head coordination strategies implicated either a linear vestibulo-ocular reflex (LVOR) or ocular responses composed of a combination of antagonistic angular and linear vestibulo-ocular reflexes (AVOR-LVOR). The slow phase direction of these two oculomotor responses for fixation stabilization on the target were compensatory and anticompensatory, respectively. 5. One of the major points of this study was the contribution of the saccadic system to gaze strategies, even in our experimental conditions where the head was free to move. We concluded that vestibular-saccadic cooperation appears to be a common feature in the elaboration of adequate fixation stabilization in daily life situations. 6. The functional coupling of these various subsystems involved in fixation stabilization depended on the range of motion: while the acceleration increased, the saccadic eye movements were replaced by vestibulo-ocular responses whose slow phase direction was opposite that of head motion and, therefore, directed away from the target. 7. Fast components of the nystagmic pattern of eye movements were able to improve gaze position accuracy, bringing the eyes toward the memorized target.(ABSTRACT TRUNCATED AT 400 WORDS)

Estimation of passive horizontal linear whole-body displacement in humans.

1. Passive linear self-motion estimation along the X and Y axes was investigated in human subjects. 2. A target was viewed from a distance of 0.8 or 2.4 m from the starting point. Subjects were then blindfolded and transported toward the target on a distance of 3.2 m and back to the start. Acceleration was constant: +/- 0.2 or +/- 1 m/s2. The subjects pushed a button on both outward and return paths, when they passed the previously seen target. 3. The results showed anticipation of the target on the outward path, and not on the return. This was identical for both axes and both accelerations. 4. The data are in accord with a model of double integration of the otolith signal, suggesting that linear path integration is a basic sensory mechanism.

Gaze control in microgravity. 1. Saccades, pursuit, eye-head coordination.

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 peak velocity was maximal upward, and in flight it was maximal downward.

Gaze control in microgravity. 2. Sequences of saccades toward memorized visual targets.

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.