K. E. Popov

Body Scheme in the Control of Postural Activity

The changes in the amplitude and direction of vestibulo-motor responses were studied during illusory body tilt and altered perception of the head position. The results obtained under these conditions were similar to those recorded during the real body movement and real head rotation. These results are discussed in the context of the involvement of the body scheme in the control of involuntary spatially oriented movements.

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

Abstract 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 saggital 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.

Is the erect posture in microgravity based on the control of trunk orientation or center of mass position

Abstract In the present experiments carried out in microgravity two questions were addressed. First, when the subject was instructed to adopt a vertical erect posture in microgravity with his feet fixed to the floor of the space cabin, would he control anteroposterior position with respect to the ankle joint axis of the ”vertical projection” of his center of mass (CM) or trunk axis orientation with respect to the ”vertical” (perpendicular to the floor of the space cabin)? Secondly, is CM anteroposterior position regulated during upper trunk movements in microgravity, in the absence of equilibrium constraint? Two subjects were tested in a long-term space flight. Video camera recordings were performed and analyzed off line. The results show that during erect vertical posture in microgravity, the trunk axis with respect to the ”vertical” is inclined some 7° forward. The anteroposterior position of the CM ”vertical” projection is not shifted forward, as might be expected in view of the trunk inclination, but remains close to the ankle joint axis. At the end of the upper trunk forward or backward bending movement, the final position of the vertical CM projection remains close to the ankle joint axis in microgravity. These results are interpreted as indicating that CM anteroposterior position continues to be accurately controlled in microgravity; the forward inclination of the trunk axis observed in microgravity is interpreted as being due to a misevaluation of the ”vertical” axis on the basis of biased information from proprioceptive inputs.

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

Abstract 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° vs 2°; maximum velocity ca. 5°/s vs 1°/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.

Galvanic vestibular stimulation in humans produces online arm movement deviations when reaching towards memorized visual targets.

Using galvanic vestibular stimulation (GVS), we tested whether a change in vestibular input at the onset of goal-directed arm movements induces deviations in arm trajectory. Eight head-fixed standing subjects were instructed to reach for memorized visual targets in complete darkness. In half of the trials, randomly-selected, a 3 mA bipolar binaural galvanic stimulation of randomly alternating polarity was triggered by the movement onset. Results revealed significant GVS-induced directional shifts of reaching movements towards the anode side. The earliest significant deviations of hand path occurred 240 ms after stimulation onset. The likely goal of these online deviations of arm trajectory was to compensate for a vestibular-evoked apparent change in the spatial relationship between the target and the hand.

Evidence for a vestibulo-cardiac reflex in man

Changes in posture demand rapid cardiovascular adjustments to maintain blood pressure and volume distribution. We demonstrated a vestibulo-cardiac reflex in supine individuals by measuring electrocardiogram and arterial blood pressure after small backwards drops of the head triggered at varying intervals after the R-spike. In normal volunteers heart rate was accelerated by drops occurring within 500-600 ms of a beat, but no rapid effect was noted in patients with a vestibular defect. The speed at which the vestibular signal of head drop accelerated heart rate implies a direct reflex. Impairment of the vestibulo-cardiac reflex would help to explain the vaso-vagal consequences of labyrinthitis.

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.

Postural responses to vibration of neck muscles in patients with unilateral vestibular lesions

Postural responses to vibration applied unilaterally to dorsal neck muscles were recorded with a sway platform in nine patients with unilateral vestibular lesions and 19 normal subjects. In normals, the vibration induced a forward postural deviation. In patients, vibration of the neck contralateral to the lesion induced normal forward sway, whereas ipsilateral vibration resulted in sway of lower amplitude than normal and predominantly in the direction of the lesion or backwards. It is suggested that the proprioceptive error signal introduced by the neck vibration combined with an asymmetrical vestibular input due to a unilateral vestibular lesion provoked an erroneous representation of head position in patients resulting in a redirection of their body sway.

Supramodal effects of galvanic vestibular stimulation on the subjective vertical.

This study investigated whether the tilt of the subjective vertical induced by galvanic vestibular stimulation, demonstrated by asking subjects to set a rod to the vertical, was specific to the visual modality or could be found in two tasks relying on proprioceptive and somatosensory cues. In all cases, settings were significantly deviated in the direction of the anode, but errors were smaller in the somatosensory tasks than in the visual task. We propose that the effects observed in the somatosensory modality reflects only a modification of the central representation of gravity, whereas visual effects are also in part the consequence of unregistered ocular torsion.

Postural responses to vibration of neck muscles in patients with uni- and bilateral vestibular loss

Postural responses to vibration applied unilaterally to the dorsal neck muscles were recorded with a sway platform in 11 patients with bilateral vestibular loss (BLD), 13 patients with unilateral vestibular lesions (ULD) and 19 normal subjects. In the normals, the vibration induced a forward postural deviation. Vibration failed to induce postural sway in the BLD patients but induced a backwards head movement in 7 patients. In the ULD patients, vibration contralateral to the lesion induced normal forward sway, whereas ipsilateral vibration resulted in sway directed to the side of the lesion and backwards. The findings demonstrate the importance of concurrent vestibular signals in determining the behavioural response to neck afferent input. We propose that in normal subjects the intact vestibular signal gives no confirmation that a head movement has occurred so it is assumed that the lower body has tilted forwards which provokes a compensatory sway. In the total absence of vestibular function the neck signal may represent a real head movement so the preferential response is a head tilt to restore upright posture. The vestibular imbalance in the ULD patients is roughly equivalent to the asymmetrical signals obtained in a normal subject during head rotation to the intact side. The stretch signal induced by ipsi-lesional vibration confirms possible head rotation, thus provoking a compensatory postural sway. Copyright 1998 Elsevier Science B.V. All rights reserved