Etienne Guillaud

Prediction of the body rotation­induced torques on the arm during reaching movements: Evidence from a proprioceptively deafferented subject

Reaching for a target while rotating the trunk generates substantial Coriolis and centrifugal torques that push the arm in the opposite direction of the rotations. These torques rarely perturb movement accuracy, suggesting that they are compensated for during the movement. Here we tested whether signals generated during body motion (e.g., vestibular) can be used to predict the torques induced by the body rotation and to modify the motor commands accordingly. We asked a deafferented subject to reach for a memorized visual target in darkness. At the onset of the reaching, the patient was rotated 25° or 40° in the clockwise or the counterclockwise directions. During the rotation, the patients head remained either fixed in space (Head-Fixed condition) or fixed on the trunk (Head Rotation condition). At the rotation onset, the deafferented patients hand largely deviated from the mid-sagittal plane in both conditions. The hand deviations were compensated for in the Head Rotation condition only. These results highlight the computational faculty of the brain and show that body rotation-related information can be processed for predicting the consequence of the rotation dynamics on the reaching arm movements.

Prediction in the Vestibular Control of Arm Movements.

The contribution of vestibular signals to motor control has been evidenced in postural, locomotor, and oculomotor studies. Here, we review studies showing that vestibular information also contributes to the control of arm movements during whole-body motion. The data reviewed suggest that vestibular information is used by the arm motor system to maintain the initial hand position or the planned hand trajectory unaltered during body motion. This requires integration of vestibular and cervical inputs to determine the trunk motion dynamics. These studies further suggest that the vestibular control of arm movement relies on rapid and efficient vestibulomotor transformations that cannot be considered automatic. We also reviewed evidence suggesting that the vestibular afferents can be used by the brain to predict and counteract body-rotation-induced torques (e.g., Coriolis) acting on the arm when reaching for a target while turning the trunk.

Is “Circling” Behavior in Humans Related to Postural Asymmetry?

In attempting to walk rectilinearly in the absence of visual landmarks, persons will gradually turn in a circle to eventually become lost. The aim of the present study was to provide insights into the possible underlying mechanisms of this behavior. For each subject (N = 15) six trajectories were monitored during blindfolded walking in a large enclosed area to suppress external cues, and ground irregularities that may elicit unexpected changes in direction. There was a substantial variability from trial to trial for a given subject and between subjects who could either veer very early or relatively late. Of the total number of trials, 50% trajectories terminated on the left side, 39% on the right side and 11% were defined as “straight”. For each subject, we established a “turning score” that reflected his/her preferential side of veering. The turning score was found to be unrelated to any evident biomechanical asymmetry or functional dominance (eye, hand…). Posturographic analysis, used to assess if there was a relationship between functional postural asymmetry and veering revealed that the mean position of the center of foot pressure during balance tests was correlated with the turning score. Finally, we established that the mean position of the center of pressure was correlated with perceived verticality assessed by a subjective verticality test. Together, our results suggest that veering is related to a “sense of straight ahead” that could be shaped by vestibular inputs.

Insights into the control of arm movement during body motion as revealed by EMG analyses

Recent studies have revealed that vestibulomotor transformations contribute to maintain the hand stationary in space during trunk rotation. Here we tested whether these vestibulomotor transformations have the same latencies and whether they are subject to similar cognitive control than the visuomotor transformations during manual tracking of a visual target. We recorded hand displacement and shoulder-muscle activity in two tasks: a stabilization task in which subjects stabilized their hand during passive 30 degrees body rotations, and a tracking task in which subjects tracked with their finger a visual target as it moved 30 degrees around them. The EMG response times recorded in the stabilization task (approximately 165 ms) were twice as short as those observed for the tracking task (approximately 350 ms). Tested with the same paradigm, a deafferented subject showed EMG response times that closely matched those recorded in healthy subjects, thus, suggesting a vestibular origin of the arm movements. Providing advance information about the direction of the required arm movement reduced the response times in the tracking task (by approximately 115 ms) but had no significant effect in the stabilization task. Generally, when providing false information about movement direction in the tracking task, an EMG burst first appeared in the muscle moving the arm in the direction opposite to the actual target motion (i.e., in accord with the precueing). This behavior was rarely observed in the stabilization task. These results show that the sensorimotor transformations that move the arm relative to the trunk have shorter latencies when they originate from vestibular inputs than from visual information and that vestibulomotor transformations are more resistant to cognitive processes than visuomotor transformations.

An examination of camptocormia assessment by dynamic quantification of sagittal posture.

OBJECTIVE Camptocormia is a disabling pathology of the axial system that debilitates patients in their daily life. To date, there have been no studies evaluating the impact of camptocormia on walking performance. This study presents a new method for assessing sagittal posture under walking conditions in patients with camptocormia. DESIGN The severity of camptocormia was evaluated by measuring sagittal inclination, represented indirectly by the horizontal distance between the C7 and S1 markers (C7 sagittal arrow; C7-SAR). Sagittal inclination was measured under various behavioural conditions using clinical, radiological and kinematic approaches. PATIENTS Forty-three patients were included in the study (17 with Parkinsons disease and 26 with idiopathic camptocormia). RESULTS Under static conditions, C7-SAR could be assessed using different methods. During walking, there was a dramatic increase in C7-SAR values. Correlation analysis revealed a relationship between functional impairment and dynamic C7-SAR values, but not with radiological C7-SAR values. PATIENTS with Parkinsons disease behaved differently from idiopathic patients, suggesting the involvement of different underlying physiopathological mechanisms. CONCLUSION Monitoring sagittal inclination during walking is more accurate than radiological measurements to determinine the detrimental effects of camptocormia and its consequences for quality of life.

Effects of underestimating the kinematics of trunk rotation on simultaneous reaching movements: predictions of a biomechanical model.

BackgroundRotation of the torso while reaching produces torques (e.g., Coriolis torque) that deviate the arm from its planned trajectory. To ensure an accurate reaching movement, the brain may take these perturbing torques into account during movement planning or, alternatively, it may correct hand trajectory during movement execution. Irrespective of the process selected, it is expected that an underestimation of trunk rotation would likely induce inaccurate shoulder and elbow torques, resulting in hand deviation. Nonetheless, it is still undetermined to what extent a small error in the perception of trunk rotations, translating into an inappropriate selection of motor commands, would affect reaching accuracy.MethodsTo investigate, we adapted a biomechanical model (J Neurophysiol 89: 276-289, 2003) to predict the consequences of underestimating trunk rotations on right hand reaching movements performed during either clockwise or counter clockwise torso rotations.ResultsThe results revealed that regardless of the degree to which the torso rotation was underestimated, the amplitude of hand deviation was much larger for counter clockwise rotations than for clockwise rotations. This was attributed to the fact that the Coriolis and centripetal joint torques were acting in the same direction during counter clockwise rotation yet in opposite directions during clockwise rotations, effectively cancelling each other out.ConclusionsThese findings suggest that in order to anticipate and compensate for the interaction torques generated during torso rotation while reaching, the brain must have an accurate prediction of torso rotation kinematics. The present study proposes that when designing upper limb prostheses controllers, adding a sensor to monitor trunk kinematics may improve prostheses control and performance.

The influence of scopolamine on motor control and attentional processes.

Background: Motion sickness may be caused by a sensory conflict between the visual and the vestibular systems. Scopolamine, known to be the most effective therapy to control the vegetative symptoms of motion sickness, acts on the vestibular nucleus and potentially the vestibulospinal pathway, which may affect balance and motor tasks requiring both attentional process and motor balance. The aim of this study was to explore the effect of scopolamine on motor control and attentional processes. Methods: Seven subjects were evaluated on four different tasks before and after a subcutaneous injection of scopolamine (0.2 mg): a one-minute balance test, a subjective visual vertical test, a pointing task and a galvanic vestibular stimulation with EMG recordings. Results: The results showed that the reaction time and the movement duration were not modified after the injection of scopolamine. However, there was an increase in the center of pressure displacement during the balance test, a decrease in EMG muscle response after galvanic vestibular stimulation and an alteration in the perception of verticality. Discussion: These results confirm that low doses of scopolamine such as those prescribed to avoid motion sickness have no effect on attentional processes, but that it is essential to consider the responsiveness of each subject. However, scopolamine did affect postural control and the perception of verticality. In conclusion, the use of scopolamine to prevent motion sickness must be considered carefully because it could increase imbalances in situations when individuals are already at risk of falling (e.g., sailing, parabolic flight).

Developmental changes in face visual scanning in autism spectrum disorder as assessed by data-based analysis

Individuals with autism spectrum disorder (ASD) present reduced visual attention to faces. However, contradictory conclusions have been drawn about the strategies involved in visual face scanning due to the various methodologies implemented in the study of facial screening. Here, we used a data-driven approach to compare children and adults with ASD subjected to the same free viewing task and to address developmental aspects of face scanning, including its temporal patterning, in healthy children, and adults. Four groups (54 subjects) were included in the study: typical adults, typically developing children, and adults and children with ASD. Eye tracking was performed on subjects viewing unfamiliar faces. Fixations were analyzed using a data-driven approach that employed spatial statistics to provide an objective, unbiased definition of the areas of interest. Typical adults expressed a spatial and temporal strategy for visual scanning that differed from the three other groups, involving a sequential fixation of the right eye (RE), left eye (LE), and mouth. Typically developing children, adults and children with autism exhibited similar fixation patterns and they always started by looking at the RE. Children (typical or with ASD) subsequently looked at the LE or the mouth. Based on the present results, the patterns of fixation for static faces that mature from childhood to adulthood in typical subjects are not found in adults with ASD. The atypical patterns found after developmental progression and experience in ASD groups appear to remain blocked in an immature state that cannot be differentiated from typical developmental child patterns of fixation.

3-D motion capture for long-term tracking of spontaneous locomotor behaviors and circadian sleep/wake rhythms in mouse

BACKGROUND Locomotor activity provides an index of an animals behavioral state. Here, we report a reliable and cost-effective method that allows long-term (days to months) simultaneous tracking of locomotion in mouse cohorts (here consisting of 24 animals). NEW METHOD The technique is based on a motion capture system used mainly for human movement study. A reflective marker was placed on the head of each mouse using a surgical procedure and labeled animals were returned to their individual home cages. Camera-recorded data of marker displacement resulting from locomotor movements were then analyzed with custom built software. To avoid any data loss, data files were saved every hour and automatically concatenated. Long-term recordings (up to 3 months) with high spatial (<1mm) and temporal (up to 100Hz) resolution of animal movements were obtained. RESULTS The system was validated by analyzing the spontaneous activity of mice from post-natal day 30-90. Daily motor activity increased up to 70days in correspondence with maturational changes in locomotor performance. The recorded actigrams also permitted analysis of circadian and ultradian rhythms in cohort sleep/wake behavior. COMPARISON WITH EXISTING METHOD(S) In contrast to traditional session-based experimental approaches, our technique allows locomotor activity to be recorded with minimal experimenter manipulation, thereby minimizing animal stress. CONCLUSIONS Our method enables the continuous long-term (up to several months) monitoring of tens of animals, generating manageable amounts of data at minimal costs without requiring individual dedicated devices. The actigraphic data collected allows circadian and ultradian analysis of sleep/wake behaviors to be performed.

Globus pallidus internus stimulation in spino-cerebellar ataxia type 3

Spino-cerebellar ataxia type 3 (SCA3) is a polyglutamine repeat neurodegenerative disorder sometimes associated with pyramidal signs and dystonia [3]. Deep brain stimulation (DBS) of the globus pallidus internus (GPi) has been shown to be an effective treatment for reducing symptoms of generalized, focal, or segmental dystonia [4]. We report the case of a patient suffering from a medically refractory generalized dystonia related to SCA3 and improved by GPi-DBS. A 19-year-old man with a positive family history of SCA3 presented with cervical dystonia. SCA3 was confirmed by a heterozygous amplification of CAG (79 ± 3) in ATXN3. Within a year, his clinical state had worsened considerably with generalized dystonia involving the four limbs, the trunk and the face. In addition, cerebellar ataxia and pyramidal signs had also appeared, rapidly leading to major functional limitations including recurrent falls and swallowing difficulties. Despite maximal doses of anticholinergic drugs and botulin toxin injections, medication failed to stabilize the patient who underwent bilateral GPi implantation of a quadripolar electrode (Medtronic Inc. 3389) 1 year later. The leads were implanted stereotactically within the ventro-postero-lateral part of the GPi. The patient was evaluated before and 12 months after surgery by two independent neurologists expert in movement disorders and DBS programming (DG, PB). The best results were obtained with the following settings: (1) right lead, contacts 5 and 6, pulsewidth 210 μs, frequency 130 Hz, amplitude 2 V; (2) left lead, contacts 1 and 2, pulsewidth 210 μs, frequency 130 Hz, and amplitude 2.6 V. Kinematic analysis data are shown in Fig. 1. The dystonic symptoms progressively improved from 3 to 12 month following surgery as attested by Burke–Fahn–Mardsen (BFM) scores (presurgical = 72; 12 months after surgery = 32). The patient was able to stand up without help and his swallowing improved dramatically. However, his cerebellar ataxia did not improve. Kinematic data objectively confirmed these observations. Unfortunately, during the second year after surgery, painful generalized dystonia progressively reoccurred despite changes to the stimulation settings (additional contact more ventrally and pulse duration increase). These results indicate that bilateral stimulation of the GPi is partially and transitorily effective in reducing the severity of generalized dystonia in patients affected by SCA3. GPi-DBS is a well-established treatment for various types of dystonia [4, 7], but is rarely used in dystonia related to SCA [2]. The patient’s clinical improvement was confirmed by the reduction in BFM score obtained after surgery and the decrease in kinematic parameters assessed on the upper and lower limbs in both the standing and sitting positions. Kinematic analysis provides an objective and quantitative link to clinical data (BFM scale). It is frequently used to assess the effectiveness of treatments on gait in parkinsonian patients [6], but only rarely to evaluate the effects of DBS in hyperkinetic syndromes such as dystonia [1, 5]. Some genetic forms of generalized dystonia are improved considerably by GPi stimulation for a longer period (DYT1, DYT6 or DYT11) than in our patient [2, 3], so the response of generalized dystonia to surgery in the context of SCA3 appears to be less effective and more transitory. This difference in the * Dominique Guehl dominique.guehl@chu-bordeaux.fr