News and controversies regarding essential tremor.

Essential tremor is the most common movement disorder in adults. It is characterized by a postural and kinetic tremor affecting the arms, but it can also affect other body parts. It evolves gradually and can be responsible for a functional impairment in activities of daily living. Its pathophysiology remains poorly understood and effective therapeutic options are limited. There are significant semiological variations between patients, and the term "essential tremor" seems to encompass a wide range of heterogeneous clinical phenotypes. The diagnostic criteria presented in 1998 are now challenged. Furthermore, there is a current debate concerning the etiology of this affection, as to whether essential tremor is a complex degenerative disorder or a functional reversible disorder of neuronal oscillation. In this review, we summarize some aspects of clinical, etiologic and therapeutic news, to better address the questioning on unravelling the clinical presentation and examine the current pathophysiological controversy in this disorder.

Strength of the perception action coupling in human body discrimination tasks.

Perceiving and extracting information from others rely on functional Perception Action Coupling. Because motor experiences influence the strength of the perception action coupling, we explored the duality between vision and proprioceptive information about the self and others in body perception using a sequential same-different visual task from 6 to 7 years old to adults. Using a same-different matching task combined with congruent or incongruent action execution, this study explored in 30 children (M ± SD: 9.2 ± 1.5; 16♀), 33 adolescents (M ± SD: 14.7 ± 1.5; 16♀) and 28 adults (M ± SD: 29.5 ± 6.5; 12♀) how the congruence of the perception action coupling could influence the body representation building in memorizing and discriminating other's postures. The first result revealed better performances to perceive other's body postural changes compared to objects' shapes modifications as early as 6-7 years old. Nevertheless, this ability needs a long time to mature as reflected by the lower performances in children compared to older groups, i.e. adolescents and young adults. In addition, executing a congruent action during the encoding phase increased the RT, reflecting involvement of additional cognitive processes of self-other correspondence, without improving the performance accuracy, due to a slow maturation of multimodal body representations. On the other hand, executing an incongruent action during the encoding phase revealed an interference effect to perceive others' posture, demonstrated by the decrease of performance accuracy. As we initially hypothesized, the strength of perception action coupling appears to be modulated by age. Indeed, the interference effect had a greater impact in adolescents involved in a different judgment of two body postures. Therefore, the assessment of the perception action coupling may predict the body representation maturity in typical development or internal body representation dysfunction in neurodevelopmental disorders.

[Gait disorders: mechanisms and classification]. / Locomotion : physiologie et classification des principaux troubles.

Gait disorders are important because of their prevalence, particularly among the elderly, and the associated risk of falls and injury. The main physiological characteristics of locomotion and neural networks that organize locomotion and maintain balance are briefly reviewed. A simplified classification designed clinical practice and based upon clinical characteristics and more recent data obtained from quantified gait analysis is proposed.

[Preliminary experimental studies of whispered voice: subglottal pressure and postural aspects]. / Etudes expérimentales préiminaires de la voix chuchotée: pression sous-glottique et étude posturale.

OBJECTIVES: Whispered voice is investigated from the energetic point of view. Our hypothese is that whispered voice, and mainly the forced whispers are very similar to vocal effort, for non acoustic vocal parameters, and body movements. DESIGN: Two experiments are combined. The first experiment focused on the laryngeal aspects, mainly the subglottal pressure, and also the electroglottography. The second experiment analyzed the body movements associated with loud whispered voice. RESULTS: No electroglottograhic wave was observed during the two experiments. The vocal intensity was always lower during whispered voice than during comfortable voice (p < 0.001). In experiment 1, subglottal pressure was significantly higher during forced whisper than during the other conditions (p < 0.001). In experiment 2, the amplitude of the trunk movements in the sagittal plane were greater during loud whisper than during comfortable voice (p < 0.001). CONCLUSION: Whispered voice can be close to vocal loading, especially the loud whispered voice.

Subthalamic Deep Brain Stimulation Modulates Proprioceptive Integration in Parkinson's Disease During a Postural Task.

Defective proprioceptive integration may play a role in the pathophysiology of motor symptoms in Parkinson's disease (PD). Dysfunction related to proprioceptively-evoked postural reactions in PD patients is still a controversial issue, with only a limited number of studies to date and mostly discordant results. The aims of the present study were (1) to determine whether or not the proprioceptive defect in PD underlies postural impairment and (2) whether or not deep brain stimulation of the subthalamic nucleus (STN-DBS) affects proprioceptive integration. We examined proprioceptive integration during a postural task in 13 PD patients and 12 age-matched control subjects, using a muscle-tendon vibration paradigm. Analysis of the center of pressure displacement and kinematic data indicates a greater degree of postural destabilization and a reduced ability to maintain a vertical orientation in PD. We found a significant positive effect of STN-DBS on these postural features. Our findings indicate that Parkinson patients, even in the absence of any clinical evidence of instability, falls, or freezing, use proprioceptive information for postural control less efficiently than healthy subjects. Furthermore, STN-DBS was found to improve proprioceptive integration, with positive impacts on postural orientation and balance.

[Postural study on healthy subjects during experimental vocal effort]. / Etude posturale de sujets normaux en situation expérimentale d'effort vocal.

INTRODUCTION: The objective of the study was to assess how posture and vocal acoustics are modified when one needs to communicate despite environmental obstacles (i.e. vocal effort performance). METHOD: 20 healthy young women participated to this study. They had to communicate numbers to a listener. Environmental obstacles were the distance between the subject and the auditor, and the noise. Segmental posture modifications were recorded and analyzed using the numerical infra-red system for movement analysis SMART. Vocal acoustics and electroglottography were also recorded and analyzed using the EVA system. RESULTS: The study showed the presence of objective postural modifications during vocal effort. Their amplitude increased when the environmental obstacles to communications increased, and they anticipated the vocal production. DISCUSSION: The vocal effort concept and its relation with the posture, the voice and the subject's feeling are reviewed on the basis of these results. Laryngeal effort is not isolated, but belongs to a global behavioral strategy which aims to improving the efficiency of the communication.

Postural disorders in Parkinson's disease.

Posture is often affected in Parkinson's disease. Postural abnormalities belong to the motor axial involvement. Generally, postural dysfunction induces clinical impairment at the latest stages of the disease, except in late-onset idiopathic Parkinson's disease and in atypical parkinsonian syndromes. Posture may be affected in its orientation component (stooped posture, camptocormia, Pisa syndrome) or in its balance component (loss of postural reflexes). Overall, postural impairment is poorly improved by levodopa, which implies that it is unlikely due to the nigrostriatal dopaminergic denervation. Several methods of investigation have been proposed but are generally not available in clinical practice. Medical treatment and deep brain stimulation (DBS) of the subthalamic nucleus or globus pallidus pars interna are less efficient on axial than on distal motor signs. The pedonculopontine nucleus seems promising as a new target for DBS in combination with the subthalamic nucleus. Physical therapy is, in most cases, the best way to improve postural dysfunction.

Proprioceptive contribution of postural control as assessed from very slow oscillations of the support in healthy humans.

Maintaining erect human posture depends on graviceptive information. This can come from at least of three origins: vestibular, visual and somaesthetic. We hypothesize here that subject's use proprioception rather than visual or vestibular cues for their control of upright body posture and this even when subjects stand on a tilting body support surface. In order to find experimental evidence for this hypothesis, we exclude in our experiments visual cues (eyes close) and by keeping frequency and amplitude of the tilt stimulus so low that it would be below the detection threshold for vestibular semi-circular canal stimuli. The orientations of body segments were analysed during various phases of the perturbation cycle. Segmental stabilisations were defined in terms of both the global anchoring index calculated during the whole perturbation cycle and an appropriate sequential anchoring index calculated during various phases in the perturbation cycle. We show that subjects tend to align their bodies with the space vertical and do so better for their heads than for their upper bodies and lower bodies. A further finding is that stabilisation is related to the tilt stimulus in the form that it is minimal at the turning points of the tilt, where peak tilt velocity is minimal with the sinusoidal stimulus used. These finding suggest first that proprioceptive cues are predominant in the control of body orientation in quasi-static condition and second that the head and trunk stabilisation strategies used as the basis of postural control depend on the properties of the moving support.

Impaired vertical postural control and proprioceptive integration deficits in Parkinson's disease.

The aim of the present study was to investigate how the orientation and stabilization components of postural control may be affected as the result of the impaired proprioceptive integration possibly occurring in Parkinson's disease. To determine the proprioceptive contribution to postural control, parkinsonian patients and control subjects were asked to maintain vertical stance while very slow sinusoidal oscillations were being applied in the lateral and antero-posterior planes to the platform on which they were standing. The amplitude and frequency of their movements were kept below the semicircular canal perception threshold. Data were collected with the ELITE automatic motion analyzer and the two postural components (orientation and segmental stabilization) were analyzed at head and trunk levels while the subjects were performing the task with their eyes open and closed. The results show that 1) the parkinsonian groups' performances were affected in terms of both the postural orientation and stabilization components in comparison with the control group, 2) the use of vision improved the parkinsonian patients' postural performances, and 3) both parkinsonian patients and control subjects achieved better postural performances when antero-posterior perturbations rather than lateral perturbations were applied to the foot support. These results suggest that Parkinson's disease is associated with proprioceptive impairment, which may be an important factor contributing to these patients' postural deficits. On the basis of these results, the visual dependence observed in parkinsonian patients is re-defined as an adaptive strategy partly compensating for the impaired proprioception.

Postural control and sensory integration in cervical dystonia.

OBJECTIVE: Postural control and sensory integration were assessed in 12 patients with Cervical Dystonia (CD) and 11 healthy control subjects (CS), who were asked to maintain their posture as vertical as possible with their eyes open and closed while standing on a platform tilting laterally at angular accelerations below the vestibular activation threshold. METHODS: Data were collected with a three-dimensional acquisition system. The orientation and stabilization components were studied using specific indexes. We also tested the subjective visual vertical. RESULTS: CD does not affect whole body postural control. CD patients were able to control the orientation of their head. CS stabilized their head in space, especially when their eyes were open. This head stabilization strategy was lost in the CD patients, and the presence of visual inputs made no difference. CD patients seemed to neglect the visual information when controlling their head. The RFT confirmed this visual independence. CONCLUSION: CD patients seem to ignore the sensory information skewed by the disease: their reference segment shifts from head to trunk and they tend to preferentially use proprioceptive information. SIGNIFICANCE: As the proprioceptive chain remains functional in CD patients, it seems likely that the vestibular system might rather be involved in the pathophysiology of CD.

Axial rotation in Parkinson's disease.

AIMS: To investigate the ability of patients with Parkinson's disease to perform a rotation around the longitudinal axis of the body. Three questions were raised. Is body rotation impaired in Parkinson's disease? Is there a level of the kinematic chain from the head to the foot at which the impairment is more severe? Is the deficit related to the general slowness of movement in Parkinson's disease? METHODS: Kinematic data were recorded. The temporal organisation of body rotation during gait initiation was analysed in 10 patients with Parkinson's disease, who were all at an advanced stage of the disease and had all experienced falls and freezing during their daily life, and in five controls. The latency of the onset of the rotation of each segment was measured by taking the onset of the postural phase of step initiation as reference value. Locomotor variables were also analysed. RESULTS: Body rotation was found to be impaired in patients with Parkinson's disease, as the delay in the onset of the rotation of each segment is greater than that in controls. Moreover, a specific uncoupling in the onset of shoulder and pelvis segment rotation was seen in patients. This impairment of rotation is not related only to the general slowness of movements. CONCLUSION: Patients with Parkinson's disease were found to have an impairment of posturo-kinetic coordination and impaired capacity to exert appropriate ground reaction forces to orient the pelvis in space.

[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.

Coordination of axial rotation and step execution: deficits in Parkinson's disease.

To determine why parkinsonian patients (PP) present some difficulties to initiate locomotion, a diagonal step has been investigated in two tasks in five control subjects (CS) and in ten PP. In the first task, the subjects had to perform one diagonal step without change in their orientation (WR); in the second task, they had to perform one diagonal step with a body rotation in the step direction (RO). The defended hypothesis is that the gait initiation deficits in Parkinson disease are a consequence of their difficulties to coordinate al the component of a complex movement. The analysed parameters were the duration of the postural and movement phases, the step length and velocity, and the amplitude of the horizontal ground reaction forces during each phase. Compared to CS, the PP showed a lengthening of the postural phase, a decrease in the step length and velocity and a reduction of the horizontal forces. The comparisons between the performances obtained in the WR versus those obtained the RO show in CS that the performances remained unchanged, whereas in PP the performances were significantly more altered in the RO. It illustrates the specific deficit occurring in PP while performing complex tasks where coordination between several components has to be achieved simultaneously.

Body schema building during childhood and adolescence: a neurosensory approach.

In order to perceive and act in its environment, the individual's body and its interactions with the sensory and social environment are represented in the brain. This internal representation of the moving body segments is labeled the body schema. Throughout life, body schema develops based on the sensory information used by the moving body and by its interactions with the environment including other people. Internal representations including body schema and representations of the outside world develop with learning and actions throughout ontogenesis and are constantly updated based on different sensory inputs. The aim of this review is to present some concepts and experimental data about body schema, internal representations and updating process during childhood and adolescence, as obtained using a neurosensory approach. From our developmental studies, it was possible to explore the slow maturation of the sensorimotor representations by examining the anticipatory control. By manipulating proprioceptive and visual information, which are at the heart of the construction of body schema, we wished to highlight notable differences between adolescents and young adults on both a postural and perceptual level, which confirms the late maturation of multisensory integration for central motor control.

Evaluation of a visual biofeedback on the postural control in Parkinson's disease.

OBJECTIVES: Both stabilization and orientation components of postural control are affected in Parkinson's disease (PD). These deficits are partly due to proprioceptive impairments, which frequently coexist with a visual dependence. This study aimed to evaluate if a visual biofeedback - i.e. real time anteroposterior trunk and head orientations indicated with a simplified avatar and represented in a head-mounted display - could improve the postural control of PD patients in response to a postural disturbance. The influence of focusing on one specific component of the postural control (stabilization or orientation) was also examined. METHODS: Seventeen medicated PD patients performed sequences of pull-tests, either with eyes open, eyes closed or visual biofeedback, crossed with the verbal instruction to focus either on the stabilization or on the vertical body orientation. Kinematics data were collected. RESULTS: Backward trunk tilts consequent to the pulls were unchanged across the different conditions. With eyes open and eyes closed, patients did not recover their initial vertical orientation by adopting a slightly tilted backward position. This bias disappeared with the visual biofeedback. Moreover, falls consecutive to the test were significantly less frequent with the visual biofeedback than in the two other visual conditions. These different orientation and stabilization parameters were not affected by the instruction. CONCLUSION: Unlike a verbal instruction, visualizing in real time their own body's geometry improved both components of postural control of PD patients. This provides evidences in PD about links between impaired vertical orientation, deficits in balance control, and contribution of supplementary sensory cues.

Updating process of internal models of action as assessed from motor and postural strategies in children.

The objective of this study was to investigate the updating process of internal models of action in children and young adults, through the postural and motor strategies adopted in simple tasks, namely sit-to-stand (STS) and back-to-sit (BTS). To this end, 11 healthy children from 7 to 10years (latest stage of childhood) and 12 healthy adults participated in the experiment. The STS and BTS tasks were performed with horizontal support surface and support surface tilted 10° to the right or forward in order to investigate the immediate adaptation of the internal representations of the movement. Movement variables that included the durations of STS and BTS and the amplitudes of the trunk movement were computed. Postural characteristics were also considered during the transition from STS to BTS, including the trunk orientation and the head stabilization strategies. Despite certain similarities with adults, especially in terms of the asymmetry of the performance times for the two tasks (STS vs. BTS) and the partial movement adaptations, the children were less able than adults to adapt both postural and movement controls to the new support conditions. Thus, it appears that the updating of internal models of action is a process that matures slowly throughout ontogenesis.

Extreme short-term environmental constraints do not update internal models of action as assessed from motor imagery in adults.

According to the simulation theory, the internal simulation of a movement (imagined movement, IM) and its execution (actual movement, AM) are based on the same motor representations. The brain uses these representations for controlling action. The specific objective of this study was to investigate the updating process of internal models of action in adults, through massive environmental changes involved by microgravity (0G). 0G has multiple effects on motor control, including short-term adaptations with respect to the planification and performance of actions. However, the effects of 0G on internal representations of action are still largely unknown. To address this issue, thirteen subjects performed first sit-to-stand (STS) and back-to-sit (BTS) tasks, and subsequently had to imagine movement performance in these tasks. The tasks were performed under normogravity (1G) and 0G conditions. Based on durations of actual and IMs, two main results emerged from this study. In 1G, actual and IM's durations were similar. However, in 0G, AM durations were significantly longer than IM durations. Furthermore, IM durations in 0G were similar to the 1G value. These results show that although the planification and execution of action were immediately adapted to the 0G condition, the storage of afferent information was inadequate to recalibrate the predictive model. These results suggest that sudden change in gravity was not considered for updating internal models of action, and that forward model probably required more practice in order to integrate the modification of the sensorial feedback generated by the new environmental constraints.

Development of action representation during adolescence as assessed from anticipatory control in a bimanual load-lifting task.

The aim of this study was to explore, during adolescence, alterations in the use of a sensori-motor representation as unveiled by the measurement of anticipatory postural control in a bimanual load-lifting task. We hypothesised that adolescence constitutes a period of refinement of anticipatory postural control due to on-going updates of the body schema and sensori-motor representations. The anticipatory postural control was assessed using a bimanual load-lifting paradigm in which subjects stabilise their left postural forearm, which is supporting an object, while they use their right hand to lift up the object. Kinematics and electromyographic data were recorded in two groups of adolescents (11-13 and 14-16 years of age) and a group of adults. Age and gender effects were tested. During voluntary unloading, the postural forearm stabilisation in adolescents was still different from the adult one, suggesting that further improvement of the postural forearm stabilisation must take place after the age of 16. No differences occur in the two adolescent groups. Moreover, girls presented a better stabilisation of the postural forearm than boys, indicating an earlier refinement of anticipatory postural control. The decrease of activity over postural flexors, which ensure postural stabilisation, appeared later in adolescents with respect to adults. Delayed timing adjustments and increased variability could reflect intense developmental processes underlain by an intense period of CNS maturation during adolescence. We discuss the role of brain maturation in the refinement of sensori-motor representations and the update of body schema.

Voluntary head stabilisation in space during oscillatory trunk movements in the frontal plane performed before, during and after a prolonged period of weightlessness.

The ability to voluntarily stabilise the head in space exhibited by two subjects during lateral rhythmic oscillations of the trunk has been investigated before, during and after a prolonged period of microgravity (microG) exposure. In flight acquisitions were performed onboard the Core Module of the Russian Space Station MIR as part of the T4 "Human Posture in Microgravity" experiment of the 179-days ESA-RKA mission EUROMIR-95. Data collection and kinematic analysis were performed by means of a space-qualified version of the automatic motion analyser ELITE. Head stabilisation in space strategy was estimated by means of the head anchoring index and cross-correlation analysis. Results show that head orientation may be well stabilised about the roll axis both with and without the presence of visual information. This was true despite the expected reduction in vestibular efficiency and muscular proprioception occurring in-flight. In one subject, however, vision was found to improve head stabilisation in space post-flight, presumably to recover from the postural deficiency induced by the long-term microG exposure. Head stability during trunk movements was achieved with either over-compensatory (out-of-phase), under-compensatory (in-phase) or mixed movement strategies, as was attested by the analysis of cross-correlation functions between head and shoulder movements. In weightlessness, vision occlusion seemed to influence the choice of the strategies to be used as well as the reduction of movement variability. The feedforward nature of compensatory head movements suggests that head stabilisation could be based in weightlessness on the internal postural body scheme, supposed to be adapted to the weightless environment within 5 months of microG exposure.