THz Smith-Purcell and grating transition radiation from metasurface: experiment and theory.

We report the results of experimental and theoretical studies of monochromatic coherent terahertz radiation generated by a short relativistic electron bunch interacting with a metasurface. The metasurface consists of subwavelength metal elements arranged on a dielectric substrate. The constructed theory explains the experimental spectra of Smith-Purcell radiation and grating transition radiation with very high precision. The orientational distribution of transition radiation shows a fine structure, which, as we suppose, may be due to contribution of coupling between the metasurface's elements.

[Errors in pointing under various body orientation relative to the gravity vector]. / Oshibki tselevykh dvizhenii ruki pri raznoi orientatsii tela otnositel'no gravitatsionnoi vertikali.

Errors in pointing to remembered target locations were measured in healthy subjects in upright, supine and prone positions, i.e. in three positions differing in body and arm movement orientation relative to the gravity vector. The X-error was unaffected by the body position, whereas the Y-error was dependent on subject's orientation relative to the gravity vector. In two lying positions, the subjects pointed below and in the vertical posture above the remembered target locations. On the contrary, the variable error was differentially affected by orientation of the movement trajectory relative to the gravity vector. In the supine position, the variable error was larger and in the prone position, smaller than in the upright posture. The gravity bias in pointing errors suggests that the motor program generated in the lying position is not modified to adapt to altered orientation relative to the gravity vector.

Postural responses to combined vestibular and hip proprioceptive stimulation in man.

Vestibular-proprioceptive interaction in human postural control in the frontal plane was studied by analysing the lateral body sway evoked in a standing subject by a weak, near-threshold galvanic vestibular stimulation combined with a balanced, bilateral vibration of the medial gluteus muscles. The intensities of the stimuli were adjusted so that none of them produced a consistent postural response when delivered alone. The pattern of the lateral body sway evoked by the combined stimulation was compared with postural responses to suprathreshold vestibular stimulation and asymmetric (unilateral) vibration of the hip abductors. During the vestibular stimulation alone the head movement started earlier and was larger than movement of the hip. During unilateral vibration the head movement was delayed with respect to the hip movement and the amplitude of head deviation was less than that of the hip. The pattern of postural response to combined vestibular stimulation and balanced vibration resembled that observed under unbalanced, unilateral vibration in terms of both the latencies and amplitudes of deviation of the body segments from their respective baseline positions. It is suggested that the asymmetric vestibular signal provided by galvanic stimulation of the labyrinth introduces a bias into the reference frame for central interpretation of proprioceptive signals so that a symmetric proprioceptive input gives rise to a lateral body sway when referenced to an asymmetric vestibular input.

Visual and oculomotor responses induced by neck vibration in normal subjects and labyrinthine-defective patients.

Three-dimensional scleral search coil eye movement recordings were obtained in five normal subjects and four patients with absent vestibular function, during unilateral vibration of the neck in the supine position. The purpose of the experiments was to investigate any role played by eye movements in the illusion that a small fixation target, viewed in an otherwise dark room, moves when vibration is applied to the neck (propriogyral illusion). Vibration was applied to the right dorsal neck muscles in three visual conditions: total darkness, fixating a light-emitting diode (LED) in an otherwise totally dark room and LED fixation in the normally lit room. Normal subjects reported that during vibration, with LED fixation in an otherwise dark room, the target appeared to move predominantly leftwards and patients reported a predominantly downward movement. Eye movements were consistently elicited in all subjects. In normal subjects there was a slow-phase eye movement predominantly to the right, interrupted by nystagmic quick phases in the opposite direction, whereas in the patients slow phases were predominantly upward with quick phases downward. Eye movements were larger in the dark but the velocity of the initial slow-phase component (<200 ms) did not change with visual conditions. Mean latencies of the eye movements were typically 80 ms but in individual trials could be as short as 40- 60 ms. The eye movements were considerably larger in the patients (e.g. mean cumulative slow-phase displacement in the dark 12 degrees vs 2 degrees; maximum velocity ca. 5 degrees /s vs 1 degrees /s). These results indicate that the propriogyral illusion is secondary to vibration-induced eye movements, presumably mediated by the cervico-ocular reflex (COR). The difference in direction of the illusion and eye movements in the patients may be related to a predominant enhancement of the vertical COR, secondary to the prominent exposure to vertical retinal slippage experienced by these patients during daily activities such as locomotion.

[Errors in targeted movements of the hand under the conditions of orbital space flight]. / Oshibki tselevykh dvizhenii ruki v usloviiakh orbila'nogo poleta.

Three crew members of the Russian-French MIR mission were tested to determine errors in pointing to memorized visual targets. In the laboratory, all test-subjects consistently used to point to the spot below the actual target presentation. The mean Y-error (the vertical error) made up -31.6 +/- 21.8 mm. In microgravity, the Y-error moved "upward" so that the mean Y-error was -16.8 +/- 37.0 mm. The data demonstrate adaptation of the central program of aiming arm movement to the microgravity conditions.

Hand trajectories of vertical arm movements in one-G and zero-G environments. Evidence for a central representation of gravitational force.

The purpose of the present experiment was to study the way in which the central nervous system (CNS), represents gravitational force during vertical arm pointing movements. Movements in upward (against gravity) and downward (with gravity) directions, with two different mass loads (hand empty and with a hand-held 0.5-kg weight) were executed by eight subjects in a normal gravitational environment. Movements by two cosmonauts, in the two directions, were also tested in a state of weightlessness. Analyses focused upon finger trajectories in the sagittal plane. Subjects in a normal gravitational environment showed curved paths for both directions and weight conditions. In addition, downward movements showed significantly smaller curvatures than upward movements. Movement times were approximately the same for all the experimental conditions. Curvature differences between upward and downward movements persisted during space flight and immediately postflight. Movement times from both cosmonauts increased slightly during flight, but returned to normal immediately on reentry in a one-G environment. Results from the present study provide evidence that gravity is centrally represented in an anticipatory fashion as a driving force during vertical arm movement planning.

Effect of body orientation with respect to gravity on directional accuracy of human pointing movements.

Errors in pointing to remembered target locations were measured in normal subjects in the upright posture and in a supine or prone lying position, three conditions that differed in the orientation of the moving arm with respect to gravity. Vertical errors, or, more strictly, errors along the body axis, were significantly larger in both supine and prone lying positions as compared with the vertical posture. Subjects' orientation with respect to gravity produced a consistent error pattern, i.e. subjects pointed below the remembered target locations in the vertical posture and above them under the two lying conditions, in the body-related co-ordinates. On the contrary, variable error (the confidence ellipse) was differentially affected by the orientation of the movement trajectory with respect to gravity. In the supine body position the variable error was larger, and in the prone position it was smaller than in the upright posture. The finding of gravity-related bias in pointing errors suggests that the motor programme generated in the lying position is not modified to adapt to an altered orientation with respect to gravity.

Motor unit discharge during muscular after-contraction.

Discharges of single motor units (MUs) in human triceps brachii and deltoid muscle were recorded using needle electromyography during after-contraction and voluntary contraction performed either against a small elastic load or under isometry. The steady-state firing rate of the MUs was lower under after-contraction than during voluntary movement of comparable amplitude and time course (or isometric force level), whereas variability of interspike intervals was similar under the two conditions. In the tibialis anterior muscle (where after-contraction was lacking), a weak voluntary contraction preceded by sustained strong voluntary effort also showed lower firing rate of MUs as compared to similar voluntary movement performed after a rest period. We concluded that sustained contraction gave rise to peripheral potentiation of contractile properties of the muscle, irrespective of whether it was proximal or distal, whereas after-contraction was due to a central tonic drive that differed for proximal and distal muscles.

Egocentric references and human spatial orientation in microgravity. I. Perception of complex tactile stimuli.

This paper is devoted to the results of the "tactile matrix" experiment performed during the second French-Soviet spaceflight (project Aragatz). The perception of the orientation of complex tactile stimuli (letters and digits) applied to different skin areas under varied conditions was studied. The task of interpretation of complex tactile stimuli was not affected by the absence of the gravitational vertical, although this task is closely associated with mechanisms for the perception of body configuration, as well as the spatial orientation of different body parts. The number of errors made under conditions of weightlessness was often even less than on Earth. The results confirm the high stability of the egocentric reference system and provide evidence that this system is based on a body scheme which cannot be easily modified by changing external conditions.

Egocentric references and human spatial orientation in microgravity. II. Body-centred coordinates in the task of drawing ellipses with prescribed orientation.

This article describes the results of the "ellipses" experiment conducted during the second French-Soviet spaceflight (project Aragatz). The realization of oriented motor tasks, on the basis of internal body representation and without visual feedback, was chosen as a paradigm for studying the determinants of spatial orientation under weightlessness. The process of drawing ellipses in the air, using arm movements with axes parallel or perpendicular to the longitudinal body axis, was studied under normal gravity and in weightlessness, and recorded using a video computer motion-analyzing system (Kinesigraph). On Earth, the experiments were performed in standing and lying positions, and in flight, in the erect position with the feet fixed to the floor. In general, performance of the task in microgravity was not disturbed. Under conditions of spaceflight, the longitudinal ellipse was inclined forward in accordance with the inclination of the whole body relative to the fixed feet. On Earth, the angle between the long axes of longitudinal and transverse ellipses deviated from 90 degrees by 20-30 degrees. The same deviation persisted under microgravity conditions. The distinctive features of ellipses traced by individual subjects were also preserved. It is concluded that an egocentric reference system ensures normal performance of sensorimotor tasks in the absence of a gravitational reference.

Effects of prolonged weightlessness on horizontal and vertical optokinetic nystagmus and optokinetic after-nystagmus in humans.

Horizontal and vertical optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) provided by a partial-field, binocular optokinetic stimulator were recorded in one astronaut before, during, and after a 25-day space flight. A ground-based study was performed on six control subjects. During the flight experiment, performed on flight days 5, 18, 19, and 21, the subject either had their feet attached to the deck or was free-floating. Vertical OKN gain only slightly increased in weightlessness compared with ground data, but the center of interest (CI) during vertical OKN, evaluated by the eye position in the saggital plane at the end of the fast phases relative to the straight-ahead direction, was found to be significantly changed during long-term exposure to weightlessness. The horizontal CI showed very little change in-flight, but the gain was increased. The time constant for the astronaut was small for vertical OKAN, but there was an increase in slow-phase velocity (SPV) by the end of the flight, which returned to normal postflight. These results partly confirm the data obtained during head-tilt studies on the ground and are in accordance with the hypothesis of a gravity-dependent control of vertical gaze direction during orientation reflexes.

[Visual illusions during the electrical stimulation of the labyrinth]. / Zritel'nye illiuzii pri élektricheskoi stimuliatsii labirinta.

Transcutaneous electrical stimulation of the labyrinth induced apparent motion of a stationary light source in darkness in normal subjects. This effect is similar to the oculogyric illusion induced by rotatory vestibular stimulation. Monaural anodal stimulation of the right labyrinth evoked apparent movement of the target to the left, whereas cathodal stimulation induced opposite illusion. The threshold current was 0.35-0.6 mA. Binaural bipolar stimuli induced illusory target motion directed to the side of the cathode, the threshold decreased 1.5-2.5 times. Binaural monopolar stimulation induced vertical apparent displacement of a target, the threshold being 1.4-3.0 mA. The amplitude and velocity of illusory target motion increased with current. The subject eyes began to move with much higher currents than those necessary for illusory sensation. It is therefore suggested that the visual illusion is related not to vestibulo-visual interaction but to vestibular effects on the spatial perception system.

[The dependence of the changes in vestibular postural reactions on the information content of visual feedback]. / Zavisimost' izmenenii vestibuliarnykh poznykh reaktsii ot informatsionnogo soderzhaniia zritel'noi obratnoi sviazi.

Electrical stimulation of the labyrinth in standing subjects induced the body sway predominantly in the frontal plane. Stabilographic response included both early (latency 120-200 ms) and late (200-500 ms) components. Their magnitudes depended on the visual control condition. Maximal responses were recorded in the eyes-closed condition. The response decreased when subjects maintained erect posture with their eyes closed, when fixing a stable visual target, and when tracking the frontal stabilogram displayed on an oscilloscope screen (visual feedback). In all the conditions the early component decreased by 10-20 percent whereas the late one decreased by 50-70 percent. Visual fixation of a small light stationary relative to the head did not influence the response. On the other hand, information on the direction of the expected body sway given in the visual fixation condition resulted in a considerable and approximately equal decrease of the two components (by 70-80 percent). It is concluded that the early and late components of the vestibulo-motor response are mediated via different mechanisms with specific temporal and functional characteristics.

[Changes in the direction of vestibulomotor responses in the process of adaptation to prolonged static head turning in man]. / Izmeneniia napravleniia vestibulomotornykh reaktsii v protsesse privykaniia k dlitel'nomu staticheskomu povorotu golovy u cheloveka.

Vestibulomotor responses were studied in normal blindfold subjects in the upright posture during adaptation to maintaining the head turned to the left up to the extreme position for 10 min. The head was maintained in that position either passively or actively. In 5 out of 12 subjects tested, adaptation to the unusual head position resulted in a gradual decrease of the appreciated angle of the head position. The error in appreciating the head position reached as much as 70-80 degrees. The direction of the vestibulomotor response was found to change in parallel with the appreciated head orientation. Thus, under mismatch between the perceived and actual head positions the direction of the vestibulomotor response corresponded to the spatial perception rather than to the actual head orientation.

[Effect of real and illusory movements on the human vestibulomotor reaction]. / Vliianie real'nykh i illiuzornykh dvizhenii na vestibulomotornuiu reaktsiiu u cheloveka.

Lateral stabilographic responses to galvanic labyrinth stimulation were studied in healthy subjects in the standing posture. The responses increased during the body tilt forward performed either voluntarily or involuntarily as a result of vibration of the tibialis anterior muscles. The illusory body tilt forward induced by the vibration of the triceps surae muscles under mechanical fixation of the trunk resulted in similar increase of the vestibulomotor response. It is concluded that the enhancement of the vestibulomotor responses during voluntary movements is mediated by the spatial perception system.

[Spatial perception and vestibulomotor reactions in man]. / Prostranstvennoe vospriiatie i vestibulomotornye reaktsii u cheloveka.

Postural responses to transcutaneous galvanic stimulation of the right labyrinth were recorded by means of a stabilograph in normal human subjects in various head positions as well as under the illusion of head and trunk rotation induced by vibration of the gluteus maximum muscle. The direction of the vestibulo-motor response was determined by the position of the head: in a normal head position the body swayed in the frontal plane, whereas with the head turned 90 degrees it moved in the sagittal plane. During the illusory head and trunk 90 degrees rotation the responses were sagittally directed like those recorded in the real head-turned-sideways position. When the vibration did not induce the illusion of the head rotation with respect to the feet, the direction of the postural response was determined by the real head orientation. It is concluded that the spatial perception system is involved in the control of spatially oriented vestibulo-motor responses.

Changes of posture during transient perturbations in microgravity.

The control of goal-directed arm movement and of body stability before, during, and 3 d after a 7-d spaceflight has been investigated. The findings show that the anticipatory and compensatory activities of the postural muscles were highly reproducible during the first days of the space mission. The sequence of these activities, studied in two situations--in which the platform either was fixed or could rotate about near the rotation axis of the ankle--was similar to a ground-based situation. The trajectory of various body segments demonstrates that a 7-d exposure to microgravity did not result in major changes in posture. Furthermore, vision seemed to play an important role in the control of standing posture at the beginning of the flight. Postural perturbations, elicited by unexpected displacements of the foot support, involved leg muscle reflexes whose amplitudes were greatly reduced compared to those on earth.

Adaptation of postural control to weightlessness.

Adaptation of motor control to weightlessness was studied during a 7-day spaceflight. The maintenance of control of upright posture was examined during a voluntary raising movement of the arm and during the voluntary raising on tiptoe. In order to evaluate the contribution of visual cues, three types of visual situations were examined: normal vision, central vision, and without vision. On the basis of cinematographic and mechanographic data, the postural perturbations consecutive to the movement of a body part in conditions of weightlessness were found to be similar to those observed on earth. However, in weightlessness, in contrast to the ground-based situation, erectness of posture was maintained primarily due to the predominant contraction of the ankle flexor muscles. The sequences of postural leg muscle activity associated with the arm or foot movement were well structured and varied slightly in the course of the flight. In addition, the initial posture, that is the erect posture before the movement was executed, changed throughout the flight from an exaggerated oblique position to a terrestrial standing position. Visual information was preponderant at the beginning of the space mission for the recalibration of other sensory cues affected by weightlessness. The findings are indicative of two types of adaptation of the central program of posture regulation to weightlessness: fast, short-term adaptation, characterized by a quasi-instantaneous redistribution of motor commands between ankle flexors and extensors (an "operative process") and slow, long-term adaptation, exemplified by the loss of anticipatory activation of certain muscles by the end of the flight (a "conservative process").