Mark Lipshits

Effect of gravity on human spontaneous 10-Hz electroencephalographic oscillations during the arrest reaction.

Electroencephalographic oscillations at 10 Hz (alpha and mu rhythms) are the most prominent rhythms observed in awake, relaxed (eye-closed) subjects. These oscillations may be considered as a marker of cortical inactivity or an index of the active inhibition of the sensory information. Different cortical sources may participate in the 10-Hz oscillation and appear to be modulated by the sensory context and functional demands. In microgravity, the marked reduction in multimodal graviceptive inputs to cortical networks participating in the representation of space could be expected to affect the 10-Hz activity. The effect of microgravity on this basic oscillation has heretofore not been studied quantitatively. Because the alpha rhythm has a functional role in the regulation of network properties of the visual areas, we hypothesised that the absence of gravity would affect its strength. Here, we report the results of an experiment conducted over the course of 3 space flights, in which we quantified the power of the 10-Hz activity in relation to the arrest reaction (i.e., in 2 distinct physiological states: eyes open and eyes closed). We observed that the power of the spontaneous 10-Hz oscillation recorded in the eyes-closed state in the parieto-occipital (alpha rhythm) and sensorimotor areas (mu rhythm) increased in the absence of gravity. The suppression coefficient during the arrest reaction and the related spectral perturbations produced by eye-opening/closure state transition also increased in on orbit. These results are discussed in terms of current theories on the source and the importance of the alpha rhythm for cognitive function.

Gravity affects the preferred vertical and horizontal in visual perception of orientation

The aim of this study was to evaluate the influence of gravity on the representation and storage of visual orientation information. On earth, measurements of response time and variability for a task of aligning remembered visual stimuli showed a distinct preference for horizontally and vertically oriented stimuli when the body and gravitational axes were aligned. This preference was markedly decreased or disappeared when the body axis was tilted with respect to gravity but was maintained for tests performed in microgravity. We conclude that subjects acquire and store visual orientation in a multi-modal reference frame that combines proprioceptive and gravitational information when both are available.

Is there an effect of weightlessness on mental rotation of three-dimensional objects?

We studied the performance of eight cosmonauts in a mental rotation paradigm with simultaneously presented perspective views of three-dimensional objects. The cosmonauts were tested successively on earth, in microgravity aboard the Russian MIR station and again on earth. Their performance was compared to performance of a control group of five subjects tested on earth on the same dates. We particularly tried to disambiguate the effect of microgravity, procedural bias and practice. Our results show that the microgravity did not alter the mental rotation process. The performance of cosmonauts increased with practice, similarly to the performance of control groups subjects suggesting that the weightlessness did not impair implicit learning as well. Finally, we propose an explanation of previous contradictory results.

Two reference frames for visual perception in two gravity conditions

The processing and storage of visual information concerning the orientation of objects in space is carried out in anisotropic reference frames in which all orientations are not treated equally. The perceptual anisotropies, and the implicit reference frames that they define, are evidenced by the observation of ‘oblique effects’ in which performance on a given perceptual task is better for horizontally and vertically oriented stimuli. The question remains how the preferred horizontal and vertical reference frames are defined. In these experiments cosmonaut subjects reproduced the remembered orientation of a visual stimulus in 1g (on the ground) and in 0g, both attached to a chair and while free-floating within the International Space Station. Results show that while the remembered orientation of a visual stimulus may be stored in a multimodal reference frame that includes gravity, an egocentric reference is sufficient to elicit the oblique effect when all gravitational and haptic cues are absent.

Temporal control and motor control: two functional modules which may be influenced differently under microgravity.

Three subjects performed sequences of periodic movements by synchronizing their movements (button pressing with the thumb) to a series of visual stimuli (induction phase), and by continuing to produce the movements with the same rhythm after the metronome had been switched off (continuation phase). The required inter-response intervals (IRIs) were 450, 550 or 650 ms. Two subjects were members of the EUROMIR 94 spaceflight mission. The inter-response intervals of the continuation phase were analyzed in terms of mean and variability. The mean inter-response intervals did not differ systematically during spaceflight from the pre- and post-flight values. The variability of the inter-response intervals significantly increased during the flight with both experimental subjects. The total variance of the inter-response intervals was partitioned into variance due to the internal timekeeper and variance due to the motor implementation processes, following the method proposed by Wing, A.M., Kristofferson, A.B., 1973. Response delays in the timing of discrete motor responses. Perception and Psychophysics 14, 5-12. The variance attributed to the timekeeper showed a significant increase with both subjects, whereas the variance attributed to the motor processes showed inconsistent trends during the spaceflight. It is concluded that during spaceflight, the functioning of the internal timing module may undergo some changes, as the result of which the regularity of the motor timing is slightly impaired.

Weightlessness alters up/down asymmetries in the perception of self-motion

In the present study, we investigated the effect of weightlessness on the ability to perceive and remember self-motion when passing through virtual 3D tunnels that curve in different direction (up, down, left, right). We asked cosmonaut subjects to perform the experiment before, during and after long-duration space flight aboard the International Space Station (ISS), and we manipulated vestibular versus haptic cues by having subjects perform the task either in a rigidly fixed posture with respect to the space station or during free-floating, in weightlessness. Subjects were driven passively at constant speed through the virtual 3D tunnels containing a single turn in the middle of a linear segment, either in pitch or in yaw, in increments of 12.5°. After exiting each tunnel, subjects were asked to report their perception of the turn’s angular magnitude by adjusting, with a trackball, the angular bend in a rod symbolizing the outside view of the tunnel. We demonstrate that the strong asymmetry between downward and upward pitch turns observed on Earth showed an immediate and significant reduction when free-floating in weightlessness and a delayed reduction when the cosmonauts were firmly in contact with the floor of the station. These effects of weightlessness on the early processing stages (vestibular and optokinetics) that underlie the perception of self-motion did not stem from a change in alertness or any other uncontrolled factor in the ISS, as evidenced by the fact that weightlessness had no effect on the perception of yaw turns. That the effects on the perception of pitch may be partially overcome by haptic cues reflects the fusion of multisensory cues and top-down influences on visual perception.

Influence of graviceptives cues at different level of visual information processing: The effect of prolonged weightlessness

We evaluated the influence of prolonged weightlessness on the performance of visual tasks in the course of the Russian-French missions ANTARES, Post-ANTARES and ALTAIR aboard the MIR station. Eight cosmonauts were subjects in two experiments executed pre-flight, in-flight and post-flight sessions. In the first experiment, cosmonauts performed a task of symmetry detection in 2-D polygons. The results indicate that this detection is locked in a head retinal reference frame rather than in an environmentally defined one as meridional orientations of symmetry axis (vertical and horizontal) elicited faster response times than oblique ones. However, in weightlessness the saliency of a retinally vertical axis of symmetry is no longer significantly different from an horizontal axis. In the second experiment, cosmonauts performed a mental rotation task in which they judged whether two 3-D objects presented in different orientations were identical. Performance on this task is basically identical in weightlessness and normal gravity.

Central Processes Amplify and Transform Anisotropies of the Visual System in a Test of Visual–Haptic Coordination

The CNS may use multimodal reference frames to combine proprioceptive, visual, and gravitational information. Indeed, spatial information could be encoded simultaneously with respect to egocentric and allocentric references such as the body axis and gravity, respectively. It has further been proposed that gravity might serve to align reference frames between different sensory modalities. We performed a series of experiments in which human subjects matched the orientation of a visual stimulus to a visual reference (visual–visual), a haptic stimulus to a haptic reference (haptic–haptic), or a visual stimulus to a haptic reference (visual–haptic). These tests were performed in a normal upright posture, with the body tilted with respect to gravity, and in the weightless environment of Earth orbit. We found systematic patterns of errors in the matching of stimulus orientations. For an upright posture on Earth, a classic oblique effect appeared in the visual–visual comparison, which was then amplified in the haptic–visual task. Leftward or rightward whole-body tilt on Earth abolished both of these effects, yet each persisted in the absence of gravity. Leftward and rightward tilt also produced asymmetric biases in the visual–haptic but not in the visual–visual or haptic–haptic responses. These results illustrate how spatial anisotropy can be molded by sensorimotor transformations in the CNS. Furthermore, the results indicate that gravity plays a significant, but nonessential role in defining the reference frames for these tasks. These results provide insight into how the nervous system processes spatial information between different sensory modalities.

MOTOR TIMING UNDER MICROGRAVITY

Five participants were tested on their ability to produce accurate and regular inter-response intervals in the 350 to 530 ms time range. Three of them were members of the French-Russian CASSIOPEE 96 spaceflight mission, and the other two were control subjects tested on the ground. During spaceflight, the target inter-response intervals were increasingly undershot and the timing became more variable (less regular). The increase in the timing variability was mostly attributable to the internal timekeeping processes rather than those involved in motor execution. The results are discussed with reference to the physiological mechanisms possibly underlying the timing of fast serial movements.

Does gravity play an essential role in the asymmetrical visual perception of vertical and horizontal line length

The eye perceives the length of vertical and horizontal lines with an inherent asymmetry. A vertical line having the same length as a horizontal one is usually perceived to be longer. In this experimental investigation we tested the hypothesis that gravity has a direct role in producing the observed perceptual asymmetry. To this end we performed experiments in weightlessness during long-orbital space flights onboard the MIR station. Subjects performed a psychophysical task in which the length of a visually-presented vertical line was adjusted to match the length of a horizontal reference. On Earth, almost all subjects produce errors in adjusting the length of the vertical line, consistently under-estimating the length of the horizontal reference. The asymmetry of perception of the line lengths persisted in weightlessness. From these results we conclude that the phenomena of asymmetry of perception of the lengths of vertical and horizontal lines is not dependent on gravity, but is instead defined by properties of the system of internal representation. Grant numbers: 99-04-48450.