Orientation of the body response to galvanic stimulation as a function of the inter-vestibular imbalance.

We proposed to study and quantify the anteroposterior component, on top of the lateral one, of the body sway induced by different configurations of galvanic vestibular stimulation (GVS) in order to advance the understanding of the orientation of the response. Four stimulation configurations were used in two separate experiments: monaural, binaural, and opposite double monaural in the first experiment (11 subjects); monaural and double monaural in the second (13 subjects). The postural response of the subjects, standing with their eyes closed, to the stimulus (0.6 mA, 4 s) was assessed by measuring the displacement of the center of pressure (CoP) using a force platform. As usual, binaural GVS induced a strictly lateral deviation of the center of pressure. The opposite double monaural condition induced a similar lateral sway to that obtained in the binaural mode, although with a very different stimulation configuration. Monaural GVS induced an oblique, stereotyped deviation in each subject. The anteroposterior component comprised a forward deviation when the anode was on the forehead and a backward deviation when the anode was on the mastoid. The lateral component, directed towards the anode as in the binaural design, was twice as large in the binaural than in the monaural mode. The second experiment showed that double monaural stimulation elicited an anteroposterior deviation (backwards when the anode was on the mastoids and forwards when it was on the forehead) that was equivalent to the addition of two complementary monaural configurations. The present results show that monaural stimulation activates one side of the vestibular apparatus and induces reproducible, stereotyped deviations of the CoP in both the anteroposterior and lateral plane. Secondly, binaural GVS appears to result from the addition of two complementary monaural stimulations. Lateral components of the response to each stimulation, being in the same direction, are summed, whilst anteroposterior components, being in opposite directions, cancel each other out. The opposite happens when both labyrinths are polarized in the same way, as in the double monaural configuration. We suggest that the orientation of the response to GVS is a function of the imbalance between right and left vestibular polarization, rather than a function of the actual position of the electrodes.

Effect of galvanic vestibular stimulation on perception of subjective vertical in standing humans.

The present work reinvestigated the influence of bimastoidal galvanic vestibular stimulation (0.4 mA during 10 sec.) on subjective vertical. We tested the hypothesis that deviations are directed towards the anode side, like postural tilt evoked by galvanic vestibular stimulation. 15 subjects were instructed to orient vertically in darkness a light-rod during 3 experimental conditions of control, anode right, and anode left. The statistical analysis showed that the perception of the vertical was modified according to the experimental conditions and the subjects. Angular deviations occurred toward the anode side. The results are interpreted as a consequence of a modification of the central representation of the vertical or of ocular torsion directed towards the anode side and likely to induce an apparent displacement of the rod towards the cathode side.

Alteration of human cutaneous afferent discharges as the result of long-lasting vibration.

The unitary activities of slowly (15 SAI) and fast-adapting type I (12 FAI) skin mechanoreceptive afferent units innervating the anterior part of the human leg and foot were recorded by using the microneurographic method. The recordings were performed both at rest and on application of cutaneous stimuli of various intensities before and after exposure of the corresponding receptive fields to vibration (0.5 mm peak to peak, 100 pulses/s, 10 min). The results show that 11% of the units tested, which were previously silent, developed a bursting pattern of postvibratory activity, which lasted 12 min on average. This induced resting activity may account for the tingling sensations usually perceived after exposure to vibration. Furthermore, application of vibration to the cutaneous receptive fields impaired the response properties of the corresponding cutaneous fibers much more markedly in the case of the SAI than in the FAI units. More specifically, less than one-half of the FAI fibers tested showed a postvibratory depressed sensitivity to skin stroking applied at various velocities that persisted for only a few minutes, whereas the responses of all the SAI units to suprathreshold maintained skin indentations applied with increasing amplitudes decreased significantly for 20 min. These fairly durable changes in the transductive properties of the mechanoreceptive afferent units probably lead to an impairment of perceptual and sensorimotor processes and consequently may at least partly account for the alterations in sensorimotor performance that have been reported to occur in humans after exposure to vibration.

Substance P-like immunoreactivity and analgesic effects of vibratory stimulation on patients suffering from chronic pain.

By applying vibratory stimulation to patients suffering from pain, it is possible to set up an inhibitory control on the pain pathways which is based on the activation of large-sized afferent fibres. The exact mechanisms responsible for these analgesic effects still remain to be determined, however. For this purpose, we investigated in the present study whether or not the analgesic effects were accompanied by a decrease in the CSF substance P-like immunoreactivity levels (SPLI) of seven patients suffering from chronic pain, who were fitted with a ventriculo-peritoneal drain. The SPLI levels were determined before and after 30-min vibratory stimulation sessions. The results show that the SPLI levels decreased as the result of the vibration, but this decrease seems to be too slight to account for the pain relief obtained.

[Transcutaneous mechanical vibrations: analgesic effect and antinociceptive mechanisms]. / Les vibrations mécaniques transcutanées: effets antalgiques et mécanismes antinociceptifs.

Although vibration has been used empirically for a long time in medicine, it was only about ten years ago that it began to be investigated scientifically. In the first part of this paper, we review the main data showing that vibratory stimulations can relieve both acute and chronic pain. In the second part, we consider the mechanisms responsible for the observed analgesic effects and discuss the nature of the sensory receptors involved and the effects of their activation on the triggering of central pain control systems.

[Met-enkephalin and substance P. Comparison of CSF levels in patients with chronic pain based on a sampling procedure]. / Met-enképhaline et substance P. Comparaison des taux dans le LCR chez des patients souffrant de douleurs chroniques en fonction du mode de prélèvement.

The purpose of this study was to evaluate the Met-enkephalin and the SP-like immunoreactivity levels in the CSF of 16 patients suffering from chronic sciatalgia and to compare them with those of 8 control subjects. Eight of the patients had a ventriculo-peritoneal shunt which made it possible to collect samples of CSF painlessly. For the others patients and controls, CSF samples were collected by lumbar puncture. The results showed that there was no difference in the Met-enkephalin and SP levels whatever the method of sample collection. On the other hand, SP-like immunoreactivity was lower in patients suffering from chronic pain.

Analgesic effects of vibration and transcutaneous electrical nerve stimulation applied separately and simultaneously to patients with chronic pain.

The analgesic effects of transcutaneous electrical nerve stimulation (TENS) and vibratory stimulation (VS), used both separately and simultaneously, were compared in 24 patients suffering from chronic pain. We tested the hypothesis that these combined procedures might improve the pain reducing effects obtained with a single type of stimulation, since they make it possible to recruit a larger number of large diameter afferents and/or to increase the discharge frequencies. Four 35-minute treatment sessions (VS, TENS, VS + TENS, Sham stimulation) were run with each patient. The vibrations (100 Hz) and TENS (100 Hz) were applied to the surface of the painful region. The sham stimulation treatment consisted of positioning the TENS electrodes without actually delivering any current. The short form of the McGill pain questionnaire was used to assess the subjects' pain levels. The assessments took place immediately after any treatment (0h.), and again 4 hours and 24 hours later. The results showed that dual stimulation not only alleviated pain in more cases than either VS or TENS alone, but also had stronger and more long-lasting analgesic effects. On the other hand, all three types of stimulation used produced stronger analgesic effects than those obtained with the sham stimulation.

Post-contraction changes in human muscle spindle resting discharge and stretch sensitivity.

The activities of human muscle spindle primary endings were recorded in the lateral peroneal nerve using the microneurographic method. The aim of the study was to test whether voluntary isometric contraction causes any after-effects, first in the resting discharge of muscle spindle primary endings and secondly in their responses to a slow ramp stretch. To investigate the latter point, the initial angular position of the ankle was passively adjusted until the unit fell silent, in order to introduce a delay in the responses to muscle stretch. The results were as follows: (1) most of the units did not exhibit the "post-contraction sensory discharge" reported to occur in numerous animal experiments; this means that the muscle spindle resting discharge was essentially the same before and after isometric voluntary contraction. (2) Isometric voluntary contraction led to changes in muscle spindle stretch sensitivity which resulted in a reduction in the stretch threshold and a decrease in the muscle spindle dynamic sensitivity. These data suggest that the after-effects observed may have been triggered by static fusimotor neurones. The results are discussed with reference to the theory according to which the processing by the CNS of muscular proprioceptive messages deals mainly with signals arising from muscles stretched during movement, and it is concluded that the coactivation of alpha and gamma motoneurones during the contraction facilitates the coding of the parameters of forthcoming stretching movements, by the muscle spindles.

Pain relief achieved by transcutaneous electrical nerve stimulation and/or vibratory stimulation in a case of painful legs and moving toes.

A patient is described with painful legs and moving toes. The pain had been occurring for more than 15 years, and a variety of therapies had been attempted with only partial, if any, success. Only morphine had succeeded in relieving the pain, but it had to be discontinued to avoid tolerance and dependence. We devised a treatment consisting of transcutaneous electrical nerve stimulation (TENS), vibratory stimulation (VS), and a combination of the two methods (TENS + VS). TENS brought about partial pain relief, but was less effective than VS; dual stimulation (TENS + VS) led to complete alleviation of the pain. Four months later, the patient was applying dual stimulation himself at home and was thus able to maintain complete relief with 3 or 4 weekly sessions. We suggest that dual stimulation results in a large-scale recruitment of large-diameter afferent fibres and may thus set up a powerful inhibitory control of nociception in our patient.

[Tendon vibrations as a tool for clinical investigation of the myotatic loop]. / Les vibrations tendineuses comme moyen clinique d'exploration de la boucle myotatique.

The degree of alteration of the myotatic loop was evaluated in 6 patients with polyradiculoneuropathy by studying deep reflexes and vibratory motor responses. The results showed that the reflex motor responses induced by vibration reappeared 2 to 4 weeks before the deep reflexes. Vibrations, therefore, could be a useful tool for the clinical follow-up of polyradiculoneuropathy.

Induction of illusory limb movement as a means of studying sensorimotor interactions in the eye-arm system.

In the present study, illusory movements were used as a means of investigating certain sensorimotor interactions: it was proposed to describe the effects produced on the oculomotor system when muscular proprioceptive afferents in the arm were artificially activated by tendon vibration. The gaze displacements induced by these vibratory stimuli were analyzed in terms of the instructions to subjects (i.e., 'simple fixation', allowing the possibility of ocular tracking, or 'obligatory fixation', not allowing any eye movements) and the points in space on which the subject's gaze was required to be fixed (on the hand or elsewhere). The results show that, whatever the experimental conditions applied, the oculomotor system always reacted to vibratory stimulation of proprioceptive afferents, even though no corresponding visual stimulus was present.

Forearm vs whole-body sensations of self-motion: some results on the role of the sensorimotor context.

Making a subject's visual surroundings move can give rise to sensations of self-motion, which can either be restricted to the arm or involve the whole body. The aim of the present study was to investigate the role played by the sensorimotor context in eliciting one or the other of these two types of illusory movement. Whether the type of sensation experienced by the subjects depended on their adoption of an actively maintained or relaxed posture was examined. Analysis showed subjects' posture was certainly one of the factors involved: a rigidly held position favoured the occurrence of whole body sensations of movement, whereas a relaxed attitude favoured occurrence of arm-restricted sensations. This postural factor alone does not, however, account for the variations recorded in our experiment which seem to be related to the stimulus parameters as well as to individual factors.

Interactions between visual and muscular information in illusions of limb movement.

The ability of visual and muscle-proprioceptive information to elicit simultaneous sensory illusions and motor responses has been utilized to study some interactions between these two types of kinesthetic information. The effects of double stimulation (visual and muscular) have been analyzed in terms of the sensations and EMG activities induced and compared to the effects of the same stimuli employed singly. The parameters manipulated concerned chiefly the conflicting or non-conflicting character of the double stimulations as well as their organization in time. The results obtained demonstrate an absence of dominance of one or the other of the two kinesthetic inputs as well as a diversity of types of interaction.

Perceptual and motor effects of agonist-antagonist muscle vibration in man.

Perceptual and motor effects of vibration applied simultaneously to the distal tendons of the Biceps and Triceps muscles, in isometric conditions and without sight of the stimulated arm, have been studied in human volunteers. Motor effects, measured by surface EMG, are inexistent when the flexor and extensor muscles are simultaneously vibrated at the same frequency. However, EMG activity appears in the muscle being vibrated at the lower frequency when simultaneous vibration is applied at different frequencies. The sensations felt by the subjects were reproduced by the nonvibrated arm and recorded by a goniometer. The studies show that the velocity and the amplitude of the ilusory movement is related to the difference in vibration frequency applied to the two muscles. The direction of movement felt (flexion or extension) is that produced by shortening of the muscle being vibrated at the lower frequency. When the two vibration frequencies are the same, there is either no sensation of movement, or a sensation of very slow movement. These results support the notion that the sensation of movement at a joint may be derived from a central processing of the proprioceptive inflow data obtained from flexor and extensor muscles. This interpretation may also be valid for the results obtained earlier by vibration of a single muscle. Furthermore, it is coherent with data on spindle afferent fibres obtained by microneurography in man during passive or active movements.

Perceptual and motor effects elicited by a moving visual stimulus below the forearm: an example of segmentary vection.

It has been shown in man that movement of a visual stimulus under the forearm may induce illusory sensations of movement (segmentary vection) and motor activity in the same direction, such that, for example, a sensation of elbow flexion is accompanied by EMG activity in biceps brachialis. The characteristics of these two types of activity, perceptual and motor, are analyzed and compared with analogous phenomena described for the whole body. These comparisons bring out two essential features: the effectiveness of stimuli of small surface area applied to central regions of the retina, and the non-compensatory character of the motor phenomena. The results are discussed with respect to the participation of vision in the control and regulation of limb movements.

Demonstration of an illusory limb movement and associated motor activities induced by a moving visual stimulus in man. A descriptive study.

We show that, in man, a moving visual stimulus can induce sensations of movement of the forearm when it is exhibited beneath the hand and part of the forearm. These sensations in the limb are analagous to vection which has been described in the whole body. They are accompanied by involuntary motor activities coherent with the direction of illusory movement. Thus, a sensation of flexion is accompanied by EMG activity in the biceps and a sensation of extension by EMG activity in the triceps. There results make it possible to extend the kinesthetic role of visual information to limb segments entering the visual field, and suggest that vision works within an assistance loop for limb movement.

[Effects of vision on tonic vibration response of a muscle or its antagonists in normal man (author's transl)]. / Effets de la vision sur la réponse tonique vibratoire d'un muscle ou de ses antagonistes chez l'homme normal.

In normal man, a vibratory stimulus applied to tendon of arm muscles can induce either a tonic motor response in the vibrated muscle or in its antagonists depending upon presence or absence of visual feedback from arm (perception of position vrs. perception of an illusive motion). Spinal motor effects of inputs from muscle spindle afferents elicited by vibration can be modified by the perceptive events experienced by the subject.