V. S. Gurfinkel

Kinesthetic reference for human orthograde posture

Humans with occluded vision were subjected to superslow tilts of the supporting platform, producing the inclination of the subjects body in the sagittal plane, but subthreshold for the most vestibular and proprioceptive phasic reactions. Two types of perturbation were used: sinusoidal tilts (frequency 0.007 Hz, amplitude 1.5 degrees) and ramps (amplitude 1.0 and 0.25 degrees, angular velocity 0.04 degrees/s). During slow sinusoidal tilts of the platform, the ankle angle and body position undergo periodical changes, but these changes have significant phase lead relative to the platform movement: 119 +/- 26 for ankle angle and 55 +/- 19 degrees for body sway. Gains were about 0.9 for both parameters. Large phase shift (tens of seconds) indicated a long delay in compensation of body inclination by ankle joint. The ramp tilt produced an initial body deviation followed by a slow (seconds or tens of seconds) approach of body position to a new steady level after the termination of ramp. Large slow body movements were superimposed with small irregular oscillations (about 10% of the amplitude of large displacements) of higher frequency. These oscillations resembled normal stabilograms on a stationary support. Thus, the usual process of stabilization of body gravity center was continued, though not around a fixed set-point but relative to a slowly changing position. Data obtained support the hypothesis that, besides operative control assigned to compensate deviations from a reference position, the system of postural control includes at least one additional level, which elaborates this reference using information about mutual position of body links, muscular torques and interaction with the support on the basis of criteria taking into account the energy cost of standing and demands for stability and security.

Locomotor‐like movements evoked by leg muscle vibration in humans

We attempted to elicit automatic stepping in healthy humans using appropriate afferent stimulation. It was found that continuous leg muscle vibration produced rhythmic locomotor‐like stepping movements of the suspended leg, persisting up to the end of stimulation and sometimes outlasting it by a few cycles. Air‐stepping elicited by vibration did not differ from the intentional stepping under the same conditions, and involved movements in hip and knee joints with reciprocal electromyogram (EMG) bursts in corresponding flexor and extensor muscles. The phase shift between evoked hip and knee movements could be positive or negative, corresponding to ‘backward’ or ‘forward’ locomotion. Such an essential feature of natural human locomotion as alternating movements of two legs, was also present in vibratory‐evoked leg movements under appropriate conditions. It is suggested that vibration evokes locomotor‐like movements because vibratory‐induced afferent input sets into active state the central structures responsible for stepping generation.

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.

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.

Spatial perception and vestibulomotor responses in man

A study was made on normal human subjects, using a stabilograph to investigate changes in posture produced in response to transcutaneous galvanic stimulation of the right labyrinth. Results were obtained for different head positions and under the illusion of head and trunk rotation produced by stimulating (vibrating) the gulteus maximus muscle. In the absence of illusion of movement, the direction of the vestibulomotor response was determined by the position of the head in relation to the feed: with the normal head position, the body swayed on a frontal plane, and on a sagittal plane when the heat turned through 90°. Vestibulomotor responses were sagittally oriented, as with real head turning, when illusory head and trunk turning through 90° was produced by vibration. When the illusion of head rotation (in relation to the feet) was not produced by this stimulus, the direction of the postural response was not produced by this stimulus, the direction of the postural response was determined by the real orientation of the head. It is concluded that the spatial perception system plays a major part in controlling spatially oriented vestibulomotor responses.

Effect of movement and illusion of movement on human vestibulomotor response

Lateral stabilographic response to galvanic labyrinth stimulation was investigated in healthy subjects in the standing position. Vestibulomotor response increased during forwards volitional body tilt as well as involuntary tilt occurring in response to stimulating (by vibration) the proprioceptors of the anterior tibial muscles. An illusion of the forward body tilt induced by stimulating (vibrating) the proprioceptors of the triceps surae muscles with the trunk fastened in a fixed position was accompanied by practically the same intensification of vestibulomotor response as during actual body movement. It was concluded that reinforcement of vestibulomotor response during volitional movements is brought about by the spatial perception system.

Immediate and remote postactivation effects in the human motor system

Postactivation effects consisting of protracted involuntary muscular contraction after 30–60 sec sustained voluntary effort were investigated. It was found that postactivation effects may be observed at the proximal muscles (uninvolved in the voluntary activity) following distal muscle contraction. Testing the state of muscles by the vibration activity of the muscle receptors showed that concealed changes persisting for 15–20 min occur apart from the direct postactivation effects already known. The point is made that postactivation phenomena reflecting the operation of certain central tonogenic structures activated by a voluntary effort or an increased afferent inflow may successfully be used in the study of postural control mechanisms.

Changes in the direction of vestibulomotor response in the course of adaptation to protracted static head turning in man

Vestibulomotor response during the course of adaptation to prolonged (10 min) static head turning to the furthest limit was investigated in healthy subjects standing upright with the eyes closed. The head was either actively or passively maintained in this position. The sensation of a decline in the angle of head turning was experienced during adaptation to the position by five of the 12 subjects tested. Error in appreciating this angle ranged up to 70–80°. Matching changes occurred in the direction of vestibulomotor response to electrical stimulation of the vestibular apparatus. When true and perceived head position conflict, direction of vestibulomotor response thus matches spatial perception rather than actual orientation of the head.

The suppression of cervico-ocular response by the haptokinetic information about the contact with a rigid, immobile object.

Horizontal eye movements were recorded in eight healthy subjects during super-slow trunk rotation with respect to the space-stationary head. In some trials, subjects simultaneously indicated their perception of selfmotion by means of a joystick. Over the frequency range employed (0.007–0.05 cycles per second, ±20°), all subjects perceived the relative motion of head and trunk as a head rotation with respect to the stationary trunk. Eye movements were observed which were in phase with imaginary head rotation; their amplitude exceeded the amplitude of actual body rotation. The grasping of a rigid ground-based handle (1) produced a sensation of trunk rotation in space, (2) suppressed the sensation of imaginary head rotation in space and (3) gave rise to a significant decrease in amplitude of eye movements. The grasping of a stiff rod with non-zero compliance did not produce these effects. It is concluded that eye movements in response to body rotation with respect to the fixed head are not purely reflex reactions, but are influenced by the internal representation of body motion.

The effects of moving sound images on postural responses and the head rotation illusion in humans.

The results of pilot studies on the effects of sound images moving in the horizontal plane on poststimulus responses and the head rotation illusion are presented. These phenomena are demonstrated to occur.