Estelle Raffin

The moving phantom: motor execution or motor imagery?

Amputees who have a phantom limb often report the ability to move this phantom voluntarily. In the literature, phantom limb movements are generally considered to reflect motor imagery rather than motor execution. The aim of this study was to investigate whether amputees distinguish between executing a movement of the phantom limb and imagining moving the missing limb. We examined the capacity of 19 upper-limb amputees to execute and imagine movements of both their phantom and intact limbs. Their behaviour was compared with that of 18 age-matched normal controls. A global questionnaire-based assessment of imagery ability and timed tests showed that amputees can indeed distinguish between motor execution and motor imagery with the phantom limb, and that the former is associated with activity in stump muscles while the latter is not. Amputation reduced the speed of voluntary movements with the phantom limb but did not change the speed of imagined movements, suggesting that the absence of the limb specifically affects the ability to voluntarily move the phantom but does not change the ability to imagine moving the missing limb. These results suggest that under some conditions, for example amputation, the predicted sensory consequences of a motor command are sufficient to evoke the sensation of voluntary movement. They also suggest that the distinction between imagined and executed movements should be taken into consideration when designing research protocols to investigate the analgesic effects of sensorimotor feedback.

Primary motor cortex changes after amputation correlate with phantom limb pain and the ability to move the phantom limb

A substantial body of evidence documents massive reorganization of primary sensory and motor cortices following hand amputation, the extent of which is correlated with phantom limb pain. Many therapies for phantom limb pain are based upon the idea that plastic changes after amputation are maladaptive and attempt to normalize representations of cortical areas adjacent to the hand area. Recent data suggest, however, that higher levels of phantom pain are associated with stronger local activity and more structural integrity in the missing hand area rather than with reorganization of neighbouring body parts. While these models appear to be mutually exclusive they could co-exist, and one reason for the apparent discrepancy between them might be that no single study has examined the organisation of lip, elbow, and hand movements in the same participants. In this study we thoroughly examined the 3D anatomy of the central sulcus and BOLD responses during movements of the hand, elbow, and lips using MRI techniques in 11 upper-limb amputees and 17 healthy control subjects. We observed different reorganizational patterns for all three body parts as the former hand area showed few signs of reorganization, but the lip and elbow representations reorganized and shifted towards the hand area. We also found that poorer voluntary control and higher levels of pain in the phantom limb were powerful drivers of the lip and elbow topological changes. In addition to providing further support for the maladaptative plasticity model, we demonstrate for the first time that motor capacities of the phantom limb correlate with post-amputation reorganization, and that this reorganization is not limited to the face and hand representations but also includes the proximal upper-limb.

Transcranial brain stimulation to promote functional recovery after stroke

Purpose of review Noninvasive brain stimulation (NIBS) is increasingly used to enhance the recovery of function after stroke. The purpose of this review is to highlight and discuss some unresolved questions that need to be addressed to better understand and exploit the potential of NIBS as a therapeutic tool. Recent findings Recent meta-analyses showed that the treatment effects of NIBS in patients with stroke are rather inconsistent across studies and the evidence for therapeutic efficacy is still uncertain. This raises the question of how NIBS can be developed further to improve its therapeutic efficacy. Summary This review addressed six questions: How does NIBS facilitate the recovery of function after stroke? Which brain regions should be targeted by NIBS? Is there a particularly effective NIBS modality that should be used? Does the location of the stroke influence the therapeutic response? How often should NIBS be repeated? Is the functional state of the brain during or before NIBS relevant to therapeutic efficacy of NIBS? We argue that these questions need to be tackled to obtain sufficient mechanistic understanding of how NIBS facilitates the recovery of function. This knowledge will be critical to fully unfold the therapeutic effects of NIBS and will pave the way towards adaptive NIBS protocols, in which NIBS is tailored to the individual patient.

Concurrent validation of a magnetometer-based step counter in various walking surfaces

BACKGROUND AND AIM Clinicians need a simple method for quantifying gait activity. The aim of this study was to develop and validate the reliability of a quantitative gait assessment based exclusively on one magnetometer located on the shank. METHODS Twenty-five healthy volunteers were simultaneously equipped with a magnetometer (MAG system) on the right shank, and two validated step-counter systems: the StepWatch Activity Monitor (SAM) and three Force-Sensing Resistors (FSRs). Volunteers performed a standard circuit including level walking, up and down stairs and up and down a slope. The three step counting approaches were compared using the Pearson correlation coefficient and the Bland-Altman method for each of the surface-types. RESULTS The step counts measured by the MAG and FSR were highly correlated for all the surfaces (r>.83). Congruently, the Bland-Altman analysis revealed an overall ± 5% limit of agreement. The step counts measured by the MAG and SAM were also well correlated for the level-surface condition (r=.85), with a Bland-Altman ± 5% limit of agreement but comparisons were less satisfying for the other surfaces. CONCLUSIONS These results demonstrate that the use of a single magnetometer is an accurate tool for step counting over varied surfaces. These small sensors are easy to set up and to use and the signal processing is robust, making the MAG method highly applicable for clinical purposes, especially for the analysis of long walking periods in daily life conditions.

Automatized set-up procedure for transcranial magnetic stimulation protocols

Abstract Transcranial Magnetic Stimulation (TMS) established itself as a powerful technique for probing and treating the human brain. Major technological evolutions, such as neuronavigation and robotized systems, have continuously increased the spatial reliability and reproducibility of TMS, by minimizing the influence of human and experimental factors. However, there is still a lack of efficient set‐up procedure, which prevents the automation of TMS protocols. For example, the set‐up procedure for defining the stimulation intensity specific to each subject is classically done manually by experienced practitioners, by assessing the motor cortical excitability level over the motor hotspot (HS) of a targeted muscle. This is time‐consuming and introduces experimental variability. Therefore, we developed a probabilistic Bayesian model (AutoHS) that automatically identifies the HS position. Using virtual and real experiments, we compared the efficacy of the manual and automated procedures. AutoHS appeared to be more reproducible, faster, and at least as reliable as classical manual procedures. By combining AutoHS with robotized TMS and automated motor threshold estimation methods, our approach constitutes the first fully automated set‐up procedure for TMS protocols. The use of this procedure decreases inter‐experimenter variability while facilitating the handling of TMS protocols used for research and clinical routine. HighlightsAutomatized set‐up procedures would facilitate TMS experiments and increase reproducibility.We developed a Bayesian model aiming at automatically finding the motor hotspot.Implementation of this model in a robotized TMS system allows the automatic search for the motor hotspot.Definition of the motor hotspot is significantly improved in terms of speed and reproducibility.

P 8. Bringing TMS-based corticomotor mapping into shape: Shape-based TMS mapping reveals motor finger somatotopy in M1-HAND

Background Movements of the fingers involve overlapping neural representations in the hand region of the primary motor cortex (M1 HAND ). Despite of these overlapping representations, somatotopic gradients of cortical motor representations exist in M1 HAND , but cortical motor somatotopy is difficult to capture with conventional transcranial magnetic stimulation (TMS) mapping techniques. Aim To develop a novel TMS mapping approach, which takes into account the sulcal shape to map motor finger somatotopy in human M1 HAND . Method In nine healthy young subjects, neuronavigated single-pulse TMS was given to the left M1 HAND via a small figure-of-eight coil to map muscle-specific corticomotor representations. We compared three different mapping approaches which differed in terms of target grids and coil orientations: (1) Line-45°condition: Six targets located on a straight medio-to-lateral line which corresponded to the overall orientation of the central sulcus using a fixed coil orientation that induced a posterior-to-anterior current having a direction of 45° relative to the mid-sagittal line, (2) Shape-45°condition: Seven targets which followed the bending of the central sulcus with a fixed coil orientation as in the first condition, and (3) Shape-90°condition: Seven targets which were individually positioned following the bending of the central sulcus as in condition 2 but with the coil orientation adjusted to produce a current direction perpendicular to the central sulcus. We compared cortico-motor representations of the abductor digiti minimi (ADM) and the first dorsal interosseus (FDI) muscle at rest, during isometric contraction of either the FDI or ADM muscles or during co-contraction of both muscles. Based on the amplitudes of the motor evoked potential (MEP) at each target site, we generated muscle excitability profiles, the position of the weighted mean MEP amplitude. Results In contrast to the other mapping conditions, the shape-90°condition demonstrated clear somatotopy of the FDI and ADM muscle representations in M1 HAND and yielded state-dependant shifts in the weighted mean positions of the curves when switching from rest to active conditions. Conclusion Within-hand motor somatotopy in M1 HAND can be readily studied with neuronavigated TMS that follows the sulcal shape and creates a tissue current perpendicular to the central sulcus at all mapping sites.