Elbow arthroscopy: An alternative to anteromedial portals.

BACKGROUND: Elbow arthroscopy is considered to be a difficult procedure with a high complication rate. These two disadvantages are due to the proximity of neurovascular structures. HYPOTHESIS: The aim of our study was to evaluate the efficacy and complication rate of a new elbow arthroscopy technique without anteromedial portals. This approach was taken because of the high rate of ulnar nerve damage using the medial portal, and the difficulty of performing triangulation of opposite portals in a patient in the lateral decubitus position. MATERIAL AND METHODS: Fifteen patients were operated on by the same surgeon between 2010 and 2012. Range of motion and the "MEPS" elbow score were calculated preoperatively and at the final postoperative follow-up. The average age of patients was 38.3 years. The follow-up was 11.1 months. Personal portals (high anterolateral and intermediate anterolateral portals) were used instead of the anteromedial portals. RESULTS: Elbow flexion increased from 113° preoperatively to 129° at the final follow-up (P=0.009). Extension increased from -33° to -10° (P<0.0001). The preoperative and final postoperative "MEPS" scores were 56.3 and 94 respectively (P<0.0001). Two patients (13.3%) had radial nerve palsy with complete recovery 6 and 9 months after surgery. DISCUSSION: The rate of nerve complications following elbow arthroscopy varies from 0 to 14%. The rate in our series (13.3%) is comparable to the results of the literature. This rate should be placed in perspective (since one patient had multiple open surgery elbow operations before arthroscopy). All complications were transient. Improved elbow range of motion in our study is consistent with the results in literature.

Validation of the newborn larynx modeling with aerodynamical experimental data.

BACKGROUND: Many authors have studied adult's larynx modelization, but the mechanisms of newborn's voice production have very rarely been investigated. After validating a numerical model with acoustic data, studies were performed on larynges of human fetuses in order to validate this model with aerodynamical experiments. MATERIAL AND METHODS: Anatomical measurements were performed and a simplified numerical model was built using Fluent((R)) with the vocal folds in phonatory position. The results obtained are in good agreement with those obtained by laser Doppler velocimetry (LDV) and high-frame rate particle image velocimetry (HFR-PIV), on an experimental bench with excised human fetus larynges. RESULTS: It appears that computing with first cry physiological parameters leads to a model which is close to those obtained in experiments with real organs.

[Study on the modeling of the glottic vibration: towards a nonlinear model of type stick and slip]. / Etude sur la modélisation de la vibration glottique: vers un modèle non linéaire de type stick and slip.

BACKGROUND: Mass springs models of phonation are used to describe the main behaviours observed in human voicing but their principle of functioning, based on harmonic oscillations, can appear complex. OBJECTIVES: This study has been conceived to demonstrate that a simple one mass model governed by the principle of relaxation oscillations, as already mentioned by Cornut and Lafon, can also describe, and at a reduced cost, the main behaviours of the vocal folds dynamics. METHODS: The theory of relaxation oscillations rests on the existence of phases of accumulation and release of energy. The oscillation between the two phases is self-sustained as soon as the energy source is sufficient to pass over the priming threshold. A numeric simulation allowed the testing of the model behaviour in several situations by using realistic values of the mechanic parameters of the vocal folds, the aerodynamic regimens of the subglottis pressure and of the subglottis flow. RESULTS: As expected, the increasing of the mass leads to a fall of the fundamental frequency and to a rising of the vibratory amplitude of the vocal folds. The phonation threshold pressure is close to the one routinely proposed, 0.1 to 1 kPa, and increases with the fundamental frequency. CONCLUSION: The relaxation oscillations suggest a simple and realistic model of the physical principle of the vocal cord vibration.

[The role of laryngeal kinesthetic feedback in the control of pitch in speech production]. / Mise en évidence du rôle de la proprioception laryngée dans le contrôle de la courbe mélodique lors de la production de parole.

OBJECTIVES: Pitch change during voice production is under the control of auditive and kinesthetic feedback phenomenona. The aim of the study was to determine the role of larynx kinesthetic feedback in speech production control. MATERIALS AND METHODS: To validate our laryngeal model, five porcine larynges were excised and placed on a test bench consisting of a blower and a laser photoglottometer for measurement of vocal cord vibration. We applied vibratory stimuli to the cricothyroid muscles to determine whether vibrators supplied by a generator alter the biomechanics of the larynx. Fundamental frequency (F0) was recorded under three conditions--larynx on vibration--larynx on vibration with vibrators activation--larynx on rest and vibrators activation. Once validated, 7 male and 7 female, mean age 29 years, were enrolled. Vibrators were positioned on cricothyroid muscles, cricoid and thyroid cartilages. Masking noise was used. Fundamental frequency, intensity and speech time production was measured during speech production under three conditions--normal speech, vibrators activation, and masking activation. RESULTS: The preliminary study demonstrated that larynx on vibration with vibrators activation doesn't alter the vocal fold F0. For the human trial, between normal speech and vibrators activation conditions, we found a significant difference (p= 0.045 for male, p= 0.010 for female) in intonation, and no significant difference for intensity and speech time. Between normal speech and masking activation conditions, no significant differences were measured for any condition. CONCLUSION: Use of the vibrators is a reliable method to allow the evaluation of larynx kinesthetic feedback in speech production control.

[Vocal forcing and posture: experimental studies on normal subject]. / Forçage vocal et posture : études expérimentales chez le sujet sain.

UNLABELLED: One of the well-known characteristics of vocal forcing is postural with an increase in the antero-posterior movements of the trunk and head during phonation. OBJECTIVE: we conceived different physiological experiments on normal subjects to explore in an objective way these movements. MATERIALS AND RESULTS: A series of experiments using a platform of posturography confirmed that there is an increase in the tensions in the muscles implied in the posture when the subject forces his voice because of an ambient noise. This increase is characterized by the index VCOP rms (variance of the displacement of the center of pressure in upright position) which passes from 13.19 in normal voice to 18.63 in forced voice. A complementary study was carried out with an equipment of analysis of the movements (ELITE). CONCLUSION: We could, thus, confirm the existence of the contemporary antero-posterior movements of vocal forcing. The discussion concerns the application perspectives of these experimental knowledge in the clinical field of the dysfunctional dysphonia.

[Normal mouth opening in the adult French population]. / L'ouverture buccale normale dans la population française adulte.

INTRODUCTION: Limitation of mouth opening is a frequent symptom in the pathologies of the temporomandibular joint. The aim of this study is to establish normative basis for this criterion in the French population. MATERIALS AND METHODS: Maximal mouth opening was measured by an electronic goniometric device. Measurements were taken in 228 people (110 men and 126 women) aged between 18 to 84 years, representative of the French population. RESULTS: The average mouth opening is 50.7 7 mm, but it was greater in the male population. It was also greater in tall patients and in younger patients (under 50 years). DISCUSSION: Our study is original because it is based on normal subjects and because our population sample age is the same as that of the French population. Men have a greater mouth opening than women, but this can be due to the fact that they are taller. Mouth opening is wider in young subjects, under 50, because they are younger and because they are taller. Mouth opening is wider in tall patients, whatever their sex or their age.

[Twenty nine shoulder reconstructions after resection of the proximal humerus for neoplasm with mean 7-year follow-up]. / Les reconstructions après résection tumorale de l'extrémité supérieure de l'humérus.

PURPOSE OF THE STUDY: Techniques available for shoulder reconstruction after resection of a tumor of the proximal humerus include scapulohumeral arthrodesis, humerus prosthesis with or without an allograft, inverted prostheses, and massive allografts. The purpose of this study was to review clinical and radiological outcomes in a series of 29 patients (20 men and 9 women) who underwent resection-reconstruction of the proximal humerus and to establish from these cases a decision making algorithm for therapeutic indications as a function of tumor invasion. MATERIAL AND METHODS: The tumors were 20 chondrosarcomas, five osteosarcomas, two Ewing sarcomas and one malignant hemangiopericytoma. In 17 patients epiphyso-metaphyseal or epiphyso-metaphyso-diaphyseal resection was performed with preservation of the abductor muscles (type S34A or S345A according to the Musculoskeletal Tumor Society classification). For 12 patients epiphyso-metaphyseal or epiphyso-metaphyso-diaphyseal resection was performed without preservation of the abductor muscles (type S34B or S345B). Reconstruction was achieved using a centromedullary cemented nail in one patient, scapulohumeral arthrodesis in three, a massive humerus prosthesis in 15, and composite humerus prosthesis in three and an inverted prosthesis in seven. The functional score of the Musculoskeletal Tumor Society (MSTS) and standard x-rays were used to assess outcome. RESULTS: Mean follow-up was 85 months (range 16-300). The mean MSTS score was 88% for inverted prostheses, 76% for composite prostheses, 72.6% for massive prostheses, 75% for scapulohumeral arthrodeses, 67% for massive prostheses, and 80% for cemented centromendullary nail. Five patients died from their malignant disease and one from another cause. Four patients are alive but with active disease after a mean follow-up of 108 months and 19 patients (65.5%) are alive and free of locoregional recurrence or metastasis after a mean 83.5 months. We had 28 complications. Glenohumeral instability was the most frequent (11 cases). DISCUSSION: Resection of the upper portion of the humerus should be performed to achieve cancerologically satisfactory tumor resection and enable shoulder resection, if possible, with preservation of a viable and functional abductor system. The functional outcome after such reconstruction depends on the type of bony resection, but also on the sacrifice of the rotator cuff and the deltoid muscle. In light of our experience and results in the literature, we advocate, despite the small number of cases for the different reconstructions, the following decision-making algorithm after resection of the proximal humerus without joint invasion: when the resection removes the rotator cuff and the deltoid (or the axillary nerve), there are two options: scapulohumeral arthrodesis or massive humerus prosthesis for patients who do not desire a complex therapy with a long postoperative period; when the resection preserves the rotator cuff and/or the deltoid muscle, reconstruction can be achieved with a composite (inverted or not) prosthesis with suture of the cuff tendons. We prefer the inverted composite prosthesis; if the deltoid muscle can be preserved but not the rotator cuff, the composite inverted prosthesis appears to be the most logical solution, but scapulohumeral arthrodesis can be proposed in selected cases.

[Increase in the abdominal respiratory movements during the sudden passage from a conversational voice to a loud voice]. / Augmentation des mouvements respiratoires abdominaux lors du passage soudain de la voix conversationnelle à la voix forte.

UNLABELLED: The passage of the simple conversational voice to a loud voice is accompanied by an increase in the muscular tensions as well as increase in the mobilized volumes of air. OBJECTIVES: We sought to know if these additional volumes were mobilized rather by a thoracic breathing or an abdominal breathing and if this effort were accompanied by a reduction in the variability of the movements. MATERIAL AND METHOD: The study presented is a prospective experimental study undertaken on healthy subjects (10 subjects) of which we analyzed the spontaneous vocal behavior (conversation with the experimenter) in the medium of vocal spots lures (production of vowels) in a calm environment allowing a conversational voice (condition 1) and in a noisy environment obliging the subjects abruptly to use a strong voice (condition 2). Thoracic and abdominal volumes were recorded using belts provided with pressure pick-ups, while the voice was synchronized with the recordings. RESULTS: The results show a reduction in the number of rheses realized with abdominal volumes and a reduction in the abdominal volumes mobilized in condition 2, testifying to a preferential use of thoracic breathing in loud voice. They also show a reduction in the number of movements of the inspiratory type (increase in the thoracic diameter or reduction in the abdominal diameter) during the phonatory phase in condition 2, which testifies to a reduction in the variability of the strategies used by the subjects. CONCLUSION: The sudden passage to a loud voice because of the appearance of an ambient noise leads to an increase of the use of thoracic breathing, which is in agreement with the concept of voice of distress or of emergency suggested by Le Huche.

[Improvement in postural orientation and stability after stereotactic gamma irradiation of acoustic neurinomas]. / Effets posturaux du traitement par gamma knife des neurinomes de l'acoustique (schwannomes vestibulaires).

Non-surgical stereotactic gamma ionizing irradiation was developed in order to produce a lethal effect on unilateral acoustic neurinomas (vestibular schvannomas). We present a study of postural vestibular dependent performances in 218 patients before and after curative irradiation. Subjects were asked to stand at ease on a static dynamometric foot-plate, gazing at a fixed point in front of them (EO condition) or to stand eyes closed (EC condition). Statokinesigrams were registered during two consecutive sessions of 51.2 sec, under EO and EC conditions. These sessions were first managed the days before (d-1), and after (d+1) irradiation; others were performed later (d+1 to 5 Years; n=37). Center of pressure (CofP) mediolateral (X) and sagittal (Y) positions were quantified every 100 msec. Mean Xm session value (with SD) was taken as a personal parameter for left or right body inclination. Area S for 90% of the XY successive placements of the CofP observed during a session was taken as an index for 2D postural way. Before irradiation the overall average of the 218 personal Xm mean values demonstrated a statistically significant body inclination toward the affected side, however under EC condition only. The day after irradiation, a significant reduction of the overall mean value of body inclination was observed. Paired Xm statistics of confirmed this trend toward usual symmetry. The day after stereo-irradiation, averaging areas S of ellipses indicated a shift of instability toward normality. For the two parameters, the pseudo-Romberg ratios (performance EC/performance EO) indicated that a special visual contribution to balance control is present in patients with a neurinoma tumor (here I and II grades). The relative importance of this visual support declined shortly after ionizing treatment. Because the radiation is neither noxious nor excitatory we think such a rapid recovery is due to some recovery of vestibular nerve afferent conduction, and a rapid neural reprogramming of the balance control. These attractive results call for deeper investigations of both vestibular and auditory functions.

[Trauma-induced tears of the quadriceps tendon: 47 cases]. / Ruptures traumatiques du tendon quadricipital. A propos de 47 cas.

PURPOSE OF THE STUDY: A torn quadriceps tendon is an exceptional finding generally observed after high-energy knee trauma in patients over 40 who present a sprain-like syndrome or after low-energy trauma in elderly subjects who experience knee instability. We reviewed a series of 47 cases of traumatic quadriceps tendon tears treated between 1976 and 1996 in order to evaluate outcome after surgical repair. MATERIAL AND METHODS: Clinical diagnosis was the rule. Forty-two patients, mean age 55 years (range 17-92) were treated for tears of one or both quadriceps tendons subsequent to low-energy trauma (40 tears) or high-energy trauma in younger subjects. The diagnosis was established early in all cases except eight (diagnosis at three weeks to one year). Surgical repair was performed in all cases except one. After surgery, the knee was either immobilized with a plaster cast or held in a removable splint to allow early mobilization. RESULTS: Average time to recovery compatible with daily life or occupational activities was four months. Recovery was not complete at this time. Long-term follow-up revealed that complete recovery with very good or good subjective results was achieved in 90% of the cases. Complete joint motion and normal quadriceps force was achieved in 80% of the cases. Patients who started rehabilitation exercises early generally achieved less satisfactory results although no significant correlation was identified with objective clinical variables. DISCUSSION: Quadriceps tendon tear is a clinical diagnosis which does not require complementary exploration for confirmation. Plain x-rays may be useful to identify associated bony lesions and specific signs of tendon tears. Early surgical repair followed by complete immobilization appears to be preferable for functional recovery allowing better recovery of muscle force without compromising flexion.

[Study of the supra-glottic pressure during partial constriction of the vocal tract]. / Etude du comportement de la pression supra-glottique lors de la constriction partielle du tractus vocal.

OBJECTIVES: Phonation in a small plastic tube 22 cm length and 5 mms diameter (basic exercise of the method of Dr Amy de la Brèteque), is current practice in vocal rehabilitation in France. This work aims to show the effects of this method on the glottic vibration. The hypothesis was that at the time of phonation in the tube with a strong flow as recommended in the method, the vocal cords vibrate without contact. This limits the mechanical trauma at this level. MATERIAL AND METHODS: We have analyzed the sound production in a tube in 11 trained and not trained subjects. We simultaneously collected the intra-oral air pressure and the vocal signal which was subjected to a spectral analysis. RESULTS: Spectral analysis confirmed that the signal was produced correctly i.e. with a strong flow and without interruption of the sound less rich in harmonics. We interpreted these results in the light of our preceding works on the glottic vibration and we show that this vocal production was of the sinusoidal type; this implies the absence of physical contact between the vocal cords, which validates our hypothesis. CONCLUSION: Further works are necessary to better understand the physical relations between the supra-glottic aerodynamic phenomena and the vibratory functioning of the vocal cords and also to analyze the therapeutic potential ofthe method within speech therapy rehabilitation.

Separate detection of vocal fold vibration by optoreflectometry: a study of biphonation on excised porcine larynges.

This study describes a novel system allowing the noncontact measurement of vibration of each vocal fold. The system is based on reflectometry, i.e. measurement of light bouncing off a vibrating object. The system works in natural light and is sensitive, compact, and inexpensive. We have used this system for separate and simultaneous detection of the vibration of each vocal fold in excised porcine larynges. Normal conditions of vibration and asymmetry of tension leading to biphonations were studied. Analysis of each vibrating cycle reveals interaction between the vocal folds allowing synchronous vibration despite their physical asymmetry or episodes of period doubling.

[Function of the laryngeal muscles in the control of the fundamental frequency of voice]. / Rôle des muscles laryngés dans le contrôle de la fréquence fondamentale de la voix.

The control of the laryngeal fundamental frequency by the laryngeal muscles is not yet clear and the subject of many controversies. We report here a review of the literature of many experimental studies on human and animals, to explain different theories about the function of these muscles.

[Non-linear model of glottic vibration. Potential clinical implications]. / Un modèle non-linéaire de la vibration glottique. Implications cliniques potentielles.

Glottic vibration is governed by a certain number of physical laws which in general correspond, as far as myo-elastic theory is concerned, with the laws of passive vibration of an elastic body under the influence of the expired air. Some of these laws are non-linear, especially those that deal with the exchange of energy between the expired air and the vocal folds. This non-linearity is specially important when it comes to the key concepts of the phonatory threshold and the passive synchronisation of the vocal cords. Based on their experimental work, which has been presented previously, the authors propose a mixed model, which is both linear and non-linear, like the oscillators of Van der Pool. This model is known as Slip-Stick.

[Comparative study of respiratory mechanisms between professional lyrical singers and beginners]. / Etude comparative des stratégies respiratoires entre les chanteurs lyriques professionnels et débutants.

The apprenticeship for classical singing requires years of training due to its complexity. This training is directed at the different levels of the vocal apparatus: the source of energy or air flow (lungs), the vibrator (vocal cords) and the resonators (bucco-pharyngeal cavity). For this study we have concentrated on the first level, that is, respiration. When a greater demand of vocal activity is required respiration becomes more sustained and more complex; this is when difficulties may be observed. Singers overcome these difficulties through different strategies according to their level of vocal training. Our study compared respiratory strategies used by professional singers and singing students (first year conservatoire students) during singing tasks; none of the subjects had any history of vocal pathology. Electromyographic analysis of muscle activity was registered for the rectus abdominals muscle, external oblique muscle and transverse muscle with surface electrodes. Kinetic analysis was used to measure thoracic cage displacement: pressure sensitive belts were placed around the pubis, the epigastric region and thorax. The subjects were asked to perform two vocal tasks: vocalization of a sustained "i" and singing of an extract of a vaccaï. Results were compared between the 7 professional singers and the 6 singing students. They allowed us to confirm the existence of distinct respiratory strategies according to the level of vocal training, as described in literature. Professional singers inhibited the activity of their rectus abdominals muscles during singing and used mostly their external oblique and transverse muscles, thus, sustaining an expanded ribcage and a longer expiratory breath. Singing students, on the other hand, worked intensively all the abdominal muscles leading to a collapse of the ribcage. Management of air was more difficult and breath was shorter. Future studies will consider a larger population sample in order to define quantitative parameters that might allow a significant differentiation of respiratory strategies between professional singers and singing students.

Phonatory threshold pressure in a healthy population before and after aerosol treatment, a preliminary study.

The viscosity of the surface mucus of the vocal cords is one of the important elements for good laryngeal functioning. It has been demonstrated that inhalation of hydrated air increases the phonatory threshold pressure by decreasing viscosity of the mucus (1) leading to a more regular vibration that can be appreciated by jitter (2). In an attempt to correlate the concepts of tissue viscosity and surface mucus considering the theoretical model of vibration we measured the phonatory threshold pressure in 6 healthy female subjects before and after aerosol treatment. We were able to demonstrate that the pressure threshold is lower (3.15 hPa) after aerosol treatment than before (3.79 hPa) and this was statistically significant (p: 0.041). The discussion is based on this decrease of mucus viscosity applied to the physiological concepts necessary to understand glottic vibration.

Objective voice analysis for dysphonic patients: a multiparametric protocol including acoustic and aerodynamic measurements.

The purpose was to determine the clinical value of a multiparametric objective voice evaluation protocol including acoustic and aerodynamic parameters measured mainly on a sustained /a/. This was done by comparison with perceptual analysis of continuous speech by a jury composed of 6 experienced listeners. Voice samples (continuous speech) from 63 male patients with dysphonia and 21 control subjects with normal voices were recorded and assesed by a jury of listeners. The jury was instructed to classify voice samples according to the G (overall dysphonia) component of the GRBAS score on a 4-point scale ranging from 0 for normal to 3 for severe dysphonia. Objective parameters were recorded on an EVA workstation. As usual with this type of system, parameters were measured mainly on a sustained /a/. Measured parameters included fundamental frequency (F0), intensity, jitter, shimmer, signal-to-noise ratio, Lyapunov coefficient (LC), oral airflow (OAF), maximum phonatory time (MPT), and vocal range (range). Estimated subglottic pressure (ESGP) was determined on a series of /pa/. Discriminant analysis was performed to detect correlation between jury classification and combinations of parameters. Results showed that a nonlinear combination of only six parameters (range, LC, ESGP, MPT, signal-to-noise ratio, and F0) allowed 86% concordance with jury classification. Discussion deals with the relative importance of the different objective parameters for discriminant analysis. Special emphasis is placed on two measurements rarely made in routine clinical workup, i.e., estimated subglottic pressure and Lyapunov coefficient.

[Sacral chordoma: retrospective review of 11 surgically treated cases]. / Chordome du sacrum: une étude rétrospective de onze cas traités chirurgicalement.

PURPOSE OF THE STUDY: Chordoma is a malignant neoplasm believed to arise from notochord remnants. It accounts for approximately 3 to 4 p. 100 of primary bone tumors and is localized along the axial skeleton, 50 p. 100 being sacrococcygeal. Clinical, radiographical and histological findings have been well established since the first description by Ribbert in 1894. Sacral chordomas are however difficult to manage and remain a challenge for surgeons and radiotherapists alike. The purpose of this study was to evaluate the long-term results of surgical treatment and patterns of failure in patients treated for chordoma of the sacrum in our department. MATERIALS AND METHODS: This retrospective study included 11 cases of sacral chordomas treated from 1973 to 1998. Patient age ranged from 36 to 77 years (mean 59 years). Six patients were female and five male. The initial treatment was surgery in all cases including intralesional removal in two cases, marginal resection in seven and complete en bloc resection in two. RESULTS: Median follow-up was 6 years (1 month to 14 years). Tumoral recurrences were observed in nine cases 5 months to 8 years after treatment. In two cases, recurrence was observed 8 years after radical sacrectomy. Treatment of recurrences was partial surgical removal with radiotherapy (40 to 70 Grays). Three patients developed metastases in lungs, liver and bone, respectively. Seven patients died, two from metastatic disease. The 5-year overall survival was 64 p. 100 but only 18 p. 100 of the patients survived 10 years. Average disease-free survival was 18 p. 100 at 5 years and 0 p. 100 at 10 years. DISCUSSION: Chordoma is a slow-growing tumor allowing survival for several years despite recurrent disease. However, only 10 to 20 p. 100 of the patients survive free of disease at 5 years. Recurrences are frequent (45 to 80 p. 100) and often multiple. Chordoma inevitably recurs and eventually leads to death after intralesional removal or marginal resection. Radical surgery should be attempted whenever technically feasible. When performed early, particularly for smaller lesions, it offers the best chance for cure. However, tumoral recurrence can occur postoperatively despite a macroscopically complete resection. Because radiation therapy seems to be more successful in controlling microscopic disease, it should be considered as a pre- or postoperative adjuvant to a macroscopically complete resection.

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.