Léa Pillette
email: lea.pillette@ensc.fr

Fabien Lotte
email: fabien.lotte@inria.fr

Bernard N'kaoua
email: bernard.nkaoua@u-bordeaux.fr

Pierre-Alain Joseph
email: pierre-alain.joseph@chu-bordeaux.fr

Camille Jeunet
email: camille.jeunet@univ-tlse2.fr

Bertrand Glize
email: bertrand.glize@chu-bordeaux.fr

Why we should systematically assess, control and report somatosensory impairments in BCI-based motor rehabilitation after stroke studies

Keywords:

The neuronal loss resulting from stroke forces 80% of the patients to undergo motor rehabilitation, for which Brain-Computer Interfaces (BCIs) and NeuroFeedback (NF) can be used. During the rehabilitation, when patients attempt or imagine performing a movement, BCIs/NF provide them with a synchronized sensory (e.g., tactile) feedback based on their sensorimotor-related brain activity that aims at fostering brain plasticity and motor recovery. The co-activation of ascending (i.e., somatosensory) and descending (i.e., motor) networks indeed enables significant functional motor improvement, together with significant sensorimotor-related neurophysiological changes. Somatosensory abilities are essential for patients to perceive the feedback provided by the BCI system. Thus, somatosensory impairments may significantly alter the efficiency of BCI-based motor rehabilitation. In order to precisely understand and assess the impact of somatosensory impairments, we first review the literature on post-stroke BCI-based motor rehabilitation (14 randomized clinical trials). We show that despite the central role that somatosensory abilities play on BCI-based motor rehabilitation post-stroke, the latter are rarely reported and used as inclusion/exclusion criteria in the literature on the matter. We then argue that somatosensory abilities have repeatedly been shown to influence the motor rehabilitation outcome, in general. This stresses the importance of also considering them and reporting them in the literature in BCI-based rehabilitation after stroke, especially since half of post-stroke patients suffer from somatosensory impairments. We argue that somatosensory abilities should systematically be assessed, controlled and reported if we want to precisely assess the influence they have on BCI efficiency. Not doing so could result in the misinterpretation of reported results, while doing so could improve (1) our understanding of the mechanisms underlying motor recovery (2) our ability to adapt the therapy to the patients' impairments and (3) our comprehension of the between-subject and between-study variability of therapeutic outcomes mentioned in the literature.

Introduction

Upper-limb paresis is a frequent consequence of stroke [START_REF] Rathore | Characterization of incident stroke signs and symptoms: findings from the atherosclerosis risk in communities study[END_REF]. Despite spontaneous improvement of motor function, this impairment lingers at the chronic phase ( ~3 months post-stroke onset), resulting in disabilities for around 40% of patients [START_REF] Duncan | Defining post-stroke recovery: implications for design and interpretation of drug trials[END_REF].

Neuroplasticity, i.e., the ability of the brain to structurally adapt at the cellular, molecular and system levels in order to foster functional abilities, encompasses several mechanisms [START_REF] Murphy | Plasticity during stroke recovery: from synapse to behaviour[END_REF]. It leads to a very plastic functional cortical representation which can favor the improvement of functional outcomes. Underlying mechanisms include the functional use of pre-existing synaptic networks as well as structural changes, with the creation of new networks [START_REF] Murphy | Plasticity during stroke recovery: from synapse to behaviour[END_REF]. Hence, a crucial question for rehabilitation is how these mechanisms could be enhanced.

Post-stroke rehabilitation training procedures aim to improve recovery of functional abilities or to establish adaptive strategies in order to compensate for impaired body functions [START_REF] Murphy | Plasticity during stroke recovery: from synapse to behaviour[END_REF]. Among the different rehabilitation procedures of the upper-limb, the ones providing patients with sensory feedback (e.g., visual feedback based on mirror therapy) or somatosensory stimulation (e.g., transcutaneous electrical stimulation) appear to be promising. On the one https://doi.org/10.1016/j.nicl.2020.102417 Received 7 May 2020; Received in revised form 22 August 2020; Accepted 3 September 2020 hand, mirror visual feedback1 induces changes from molecular to anatomical and physiological levels associated with functional recovery. Indeed, it is known to increase neurons' excitability [START_REF] Thieme | Mirror therapy for improving motor function after stroke[END_REF], cortical reorganization in the primary motor cortex (M1) and to induce functional changes in somatosensory, premotor or higherorder visual areas [START_REF] Fritzsch | Different effects of the mirror illusion on motor and somatosensory processing[END_REF]. On the other hand, somatosensory stimulation can enhance excitability of the motor cortex and motor function of post-stroke patients [START_REF] Edwards | Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke[END_REF][START_REF] Conforto | Repetitive Peripheral Sensory Stimulation and Upper Limb Performance in Stroke: A Systematic Review and Meta-analysis[END_REF].

These therapies provide sensory feedback through afferent networks regardless of the voluntary activation of efferent sensorimotor networks. However, a co-occurrence of these synergistic networks seems to improve the outcome of the therapies [START_REF] Biasiucci | Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke[END_REF][START_REF] Frolov | Post-stroke rehabilitation training with a motor-imagery-based brain-computer interface (BCI)-controlled hand exoskeleton: a randomized controlled multicenter trial[END_REF][START_REF] Mihara | Near-infrared spectroscopy-mediated neurofeedback enhances efficacy of motor imagery-based training in poststroke victims: a pilot study[END_REF][START_REF] Ramos-Murguialday | Brain-machine interface in chronic stroke rehabilitation: a controlled study[END_REF].

Such co-occurrence is possible using Brain-Computer Interfaces (BCIs) [START_REF] Clerc | Brain-computer interfaces[END_REF]. These technologies record (e.g., using electroencephalography), process (using machine learning techniques) and translate patterns of brain activity into commands for different types of digital technologies. For example, BCIs enable paralyzed patients to drive a wheelchair by imagining right or left hand movements [START_REF] Carlson | Brain-controlled wheelchairs: a robotic architecture[END_REF]. BCIs share multiple characteristics with neurofeedback (NF), which protocol aims to train people to self-regulate specific functional biomarkers, often associated with mental disorders [START_REF] Batail | EEG neurofeedback research: A fertile ground for psychiatry?[END_REF][START_REF] Sitaram | Closed-loop brain training: the science of neurofeedback[END_REF]. Actually, when used to provide feedback on sensorimotor activity for motor-rehabilitation, some authors even argue that BCIs and NF are two different names for the same concept [START_REF] Perronnet | Brain training with neurofeedback[END_REF].

BCIs are used to provide a time-matched sensory feedback depending on the sensorimotor cortex activity for post-stroke motor rehabilitation [START_REF] Grosse-Wentrup | Using brain-computer interfaces to induce neural plasticity and restore function[END_REF]. Motor imagery-based BCI therapies seem to be more efficient in improving motor functions than motor imagery alone [START_REF] Pichiorri | Brain-computer interface boosts motor imagery practice during stroke recovery[END_REF] or proprioceptive stimulation alone [START_REF] Biasiucci | Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke[END_REF]. BCIs enable the online detection of the neuronal activity associated either with a motor imagery or attempted movement task (i.e., top-down processes) and then reward the patient by providing a sensory feedback (i.e., bottom-up processes) [START_REF] Grosse-Wentrup | Using brain-computer interfaces to induce neural plasticity and restore function[END_REF]. BCI-based motor rehabilitation post-stroke can thus be seen as a type of clinical neurofeedback [START_REF] Sitaram | Closed-loop brain training: the science of neurofeedback[END_REF]. BCI-based training promotes the co-activation of sensorimotor neural networks associated with movements and induces Hebbian plasticity, which underlies functional improvement [START_REF] Grosse-Wentrup | Using brain-computer interfaces to induce neural plasticity and restore function[END_REF].

The efficiency of therapies in general and of BCI-based therapies in particular greatly depends on patients' ability to sense the feedback provided during therapy [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. Thereby, it depends on the integrity of bottom-up afferent processes, i.e., somatosensory-related network. Somatosensory sensations encompass two types of information: exteroception, which represents the information arising from the skin, and proprioception, which encompasses information arising from the muscles and joint receptors [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. Both may be impaired after a stroke [START_REF] Carey | Review on Somatosensory Loss after Stroke[END_REF][START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. More than half of the patients experience somatosensory loss, which crucially interferes with post-stroke motor recovery [START_REF] Carey | Review on Somatosensory Loss after Stroke[END_REF][START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF][START_REF] Pumpa | Somatosensory assessment and treatment after stroke: an evidence-practice gap[END_REF]. Indeed, somatosensory loss is known to have a negative effect on motor rehabilitation and daily use of the paretic arm [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. Also, the prevalence of extremity paresis is significantly higher for patients with abnormal somatosensations [START_REF] Andersen | Incidence of central post-stroke pain[END_REF].

Given the essential role of somatosensory afferences in motor rehabilitation in general, it would seem logical that somatosensory impairments have an impact on BCI-based motor rehabilitation post-stroke. Thus, it raises the following questions: (1) Are somatosensory impairments assessed, controlled and reported in clinical studies carried out with BCI-based motor therapies? (2) What impact do these impairments have on motor rehabilitation? (3) How are somatosensory abilities impaired post-stroke? (4) Do patients recover from these impairments and how? (5) How to assess somatosensory impairments for post-stroke patients? (6) What could be the possible repercussion of assessing, controlling and reporting somatosensory-related measures on BCI-based therapies' efficiency in future studies? This paper offers a theoretical analysis, based on the literature, to answer these questions. Because BCIs have proven promising for upper limb rehabilitation [START_REF] Cervera | Brain-computer interfaces for post-stroke motor rehabilitation: a metaanalysis[END_REF][START_REF] Bai | Immediate and long-term effects of BCI-based rehabilitation of the upper extremity after stroke: a systematic review and meta-analysis[END_REF], we chose to focus on strokes affecting motor abilities of the upper limb, e.g., hemiplegia or hemiparesis.

This paper is organized as follows. In Section 2 -BCI therapies for motor rehabilitation poststroke, we start by determining the role that somatosensory abilities play in BCI-based motor therapy after stroke. A literature-based study of the randomized control trials on the matter is presented afterward to weigh the importance that has been given to somatosensory abilities in these previous studies. Then, in Section 3 -The interconnection of somatosensory and motor abilities during recovery-, to further support our hypothesis that somatosensory abilities have an incidence on BCI-based motor rehabilitation post-stroke outcome, we introduce the results of previous research in other fields, indicating that somatosensory impairments influence neuroplasticity and motor recovery. This literature further support our hypothesis that somatosensory abilities have an incidence on BCI-based motor rehabilitation post-stroke outcome. Finally, in Section 4 -Somatosensory impairments, recovery and assessment post-stroke-, to evaluate to which extent somatosensory impairments could influence BCI-based motor therapies, we focus on the prevalence and specificity of these impairments post-stroke. In this same Section, we also provide information regarding the tools to assess these somatosensory impairments to promote their assessment by researchers and clinicians in future experiments. Throughout this article we discuss the main benefits that could arise from assessing, controlling and reporting patients' somatosensory impairments and present some leads for future BCI research.

BCI therapies for motor rehabilitation post-stroke

Evidence of BCIs' effectiveness for improving plasticity and motor rehabilitation post-stroke has only recently started to arise from the different research that have been led on the topic [START_REF] Cervera | Brain-computer interfaces for post-stroke motor rehabilitation: a metaanalysis[END_REF][START_REF] Bai | Immediate and long-term effects of BCI-based rehabilitation of the upper extremity after stroke: a systematic review and meta-analysis[END_REF]. The literature indicates that using a BCI to provide visual feedback (e.g., a virtual representation of the patient's hands movements) when motor imagery is detected, enables a significantly higher improvement of motor functions than motor imagery alone [START_REF] Pichiorri | Brain-computer interface boosts motor imagery practice during stroke recovery[END_REF]. When providing somatosensory feedback (e.g., using an exoskeleton [START_REF] Frolov | Post-stroke rehabilitation training with a motor-imagery-based brain-computer interface (BCI)-controlled hand exoskeleton: a randomized controlled multicenter trial[END_REF][START_REF] Ramos-Murguialday | Brain-machine interface in chronic stroke rehabilitation: a controlled study[END_REF][START_REF] Ang | A clinical study of motor imagery-based brain-computer interface for upper limb robotic rehabilitation[END_REF] or functional electrical stimulation [START_REF] Biasiucci | Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke[END_REF]) BCIs have proven more effective than proprioceptive stimulation alone [START_REF] Biasiucci | Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke[END_REF][START_REF] Frolov | Post-stroke rehabilitation training with a motor-imagery-based brain-computer interface (BCI)-controlled hand exoskeleton: a randomized controlled multicenter trial[END_REF][START_REF] Ramos-Murguialday | Brain-machine interface in chronic stroke rehabilitation: a controlled study[END_REF][START_REF] Ang | A clinical study of motor imagery-based brain-computer interface for upper limb robotic rehabilitation[END_REF]. Compared to traditional therapies, such as motor imagery or muscle and proprioceptive stimulation alone, BCIs allow the co-activation of both top-down processes (i.e., motor imagination or attempt) and bottom-up processes (i.e., coherent somatosensory afferences from visual or somatosensory stimulation of the affected limb). BCI efficiency is assumed to be the result of the timely somatosensory feedback in regards to motor imagination or attempt [START_REF] Grosse-Wentrup | Using brain-computer interfaces to induce neural plasticity and restore function[END_REF].

Somatosensory impairments might impede the activation of somatosensory-related networks and thereby negatively affect the outcome of the therapy. Furthermore, somatosensory loss might interfere with motor imagery, which is the basis of various BCI studies. Indeed, severe somatosensory impairment affects the temporal aspects of motor imagery, i.e., the ability to estimate the time needed to perform a motor imagery task, even though spatial aspects such as the ability to visualize a 3D object do not seem to be altered [START_REF] Liepert | Mental chronometry and mental rotation abilities in stroke patients with different degrees of sensory deficit[END_REF]. Thus, somatosensory impairments most probably interfere with the use of BCI tools and the efficiency of BCI rehabilitation post-stroke. Hence, describing either inclusion/exclusion criteria that refer to somatosensory impairments or assessing these impairments a priori and taking them into account when analysing the results seems important.

Therefore, we investigated how somatosensory abilities were taken into account and reported in the literature on BCI-based motor rehabilitation post-stroke. We assessed the papers that were cited in reviews focusing on BCI-based motor rehabilitation of the upper-limb [START_REF] Remsik | A review of the progression and future implications of brain-computer interface therapies for restoration of distal upper extremity motor function after stroke[END_REF][START_REF] Monge-Pereira | Use of electroencephalography brain-computer interface systems as a rehabilitative approach for upper limb function after a stroke: a systematic review[END_REF][START_REF] Cervera | Brain-computer interfaces for post-stroke motor rehabilitation: a metaanalysis[END_REF][START_REF] Carvalho | Brain-machine interface of upper limb recovery in stroke patients rehabilitation: a systematic review[END_REF][START_REF] Bai | Immediate and long-term effects of BCI-based rehabilitation of the upper extremity after stroke: a systematic review and meta-analysis[END_REF]. From the different papers cited we selected the 14 papers focusing on Randomized Clinical Trials (RCTs) of BCIs based on sensorimotor rhythms for post-stroke motor rehabilitation of the upper-limbs with different clinical trial registration number [START_REF] Ang | A clinical study of motor imagery-based brain-computer interface for upper limb robotic rehabilitation[END_REF][START_REF] Ang | Clinical study of neurorehabilitation in stroke using EEG-based motor imagery brain-computer interface with robotic feedback[END_REF][START_REF] Ang | Brain-computer interface-based robotic end effector system for wrist and hand rehabilitation: results of a three-armed randomized controlled trial for chronic stroke[END_REF][START_REF] Ang | A randomized controlled trial of EEG-based motor imagery brain-computer interface robotic rehabilitation for stroke[END_REF][START_REF] Biasiucci | Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke[END_REF][START_REF] Frolov | Post-stroke rehabilitation training with a motor-imagery-based brain-computer interface (BCI)-controlled hand exoskeleton: a randomized controlled multicenter trial[END_REF][START_REF] Li | Neurophysiological substrates of stroke patients with motor imagery-based brain-computer interface training[END_REF][START_REF] Mihara | Near-infrared spectroscopy-mediated neurofeedback enhances efficacy of motor imagery-based training in poststroke victims: a pilot study[END_REF][START_REF] Pichiorri | Brain-computer interface boosts motor imagery practice during stroke recovery[END_REF][START_REF] Ramos-Murguialday | Brain-machine interface in chronic stroke rehabilitation: a controlled study[END_REF][START_REF] Rayegani | Effect of neurofeedback and electromyographicbiofeedback therapy on improving hand function in stroke patients[END_REF][START_REF] Várkuti | Resting state changes in functional connectivity correlate with movement recovery for BCI and robot-assisted upper-extremity training after stroke[END_REF][START_REF] Wada | Development of a Brain-machine Interface for Stroke Rehabilitation Using Event-related Desynchronization and Proprioceptive Feedback[END_REF][START_REF] Young | Brain-computer interface training after stroke affects patterns of brain-behavior relationships in corticospinal motor fibers[END_REF].

The results of our review are summarized in two tables. Table 1 provides information regarding the procedure, the results and the interpretation of these studies. This table also provides insights on which underlying mechanisms the authors rely on to explain their results. Three articles explicitly state that the therapeutic outcome results from the coactivation of efferent and afferent sensorimotor networks [START_REF] Biasiucci | Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke[END_REF][START_REF] Ramos-Murguialday | Brain-machine interface in chronic stroke rehabilitation: a controlled study[END_REF][START_REF] Várkuti | Resting state changes in functional connectivity correlate with movement recovery for BCI and robot-assisted upper-extremity training after stroke[END_REF]. For instance, Biasiucci et al. state that "BCI[…] therapy can drive significant functional recovery and purposeful plasticity thanks to contingent activation of body natural efferent and afferent pathways" through "somatosensory input, in the form of peripheral nerve stimulation" [START_REF] Biasiucci | Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke[END_REF]. Others authors argue that an increase in sensorimotor cortex (SMC) activation, premotor cortex activation, or sensorimotor rhythm contributes to the functional improvements observed [START_REF] Biasiucci | Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke[END_REF][START_REF] Li | Neurophysiological substrates of stroke patients with motor imagery-based brain-computer interface training[END_REF][START_REF] Mihara | Near-infrared spectroscopy-mediated neurofeedback enhances efficacy of motor imagery-based training in poststroke victims: a pilot study[END_REF][START_REF] Pichiorri | Brain-computer interface boosts motor imagery practice during stroke recovery[END_REF][START_REF] Ramos-Murguialday | Brain-machine interface in chronic stroke rehabilitation: a controlled study[END_REF][START_REF] Rayegani | Effect of neurofeedback and electromyographicbiofeedback therapy on improving hand function in stroke patients[END_REF][START_REF] Várkuti | Resting state changes in functional connectivity correlate with movement recovery for BCI and robot-assisted upper-extremity training after stroke[END_REF]. For instance, Li et al. state that "BCI training may enhance the activation of the affected SMC to prime the motor functional reorganization" [START_REF] Li | Neurophysiological substrates of stroke patients with motor imagery-based brain-computer interface training[END_REF]. Based on these statements, sensorimotor-related inclusion/exclusion criteria might be expected to be used as sensorimotor-related neurophysiological activation, including somatosensory ones, mediate the therapeutic outcome.

Thus, in Table 2, we report the sensorimotor-related inclusion/exclusion criteria that were used in these studies. All studies report using inclusion/exclusion criteria known to potentially influence motor rehabilitation outcomes (see Table 2). For example, the time since the stroke onset, that correlates with recovery [START_REF] Kwakkel | Impact of time on improvement of outcome after stroke[END_REF], has been used as an inclusion criterion by 71% of the studies included in this review. These criteria limit the bias that could arise from comparing subjects that do not have the same potential for recovery. Table 2 summarizes the information on somatosensory-related inclusion/exclusion criteria used in previous studies. Surprisingly, only 14% of these studies report checking the somatosensory impairments of the included patients. Rayegani et al. [START_REF] Rayegani | Effect of neurofeedback and electromyographicbiofeedback therapy on improving hand function in stroke patients[END_REF] and [START_REF] Mihara | Near-infrared spectroscopy-mediated neurofeedback enhances efficacy of motor imagery-based training in poststroke victims: a pilot study[END_REF] respectively used sensory impairments as exclusion criteria. Mihara et al. [START_REF] Mihara | Near-infrared spectroscopy-mediated neurofeedback enhances efficacy of motor imagery-based training in poststroke victims: a pilot study[END_REF] were the only ones to provide information regarding the somatosensory impairments of their patients. Though, they did not report how they assessed these impairments and provided only subjective scales, i.e., 'None', 'Mild', 'Moderate', which limits the reliability and reproductibility between studies. Another exclusion criteria that involve somatosensory impairments is pain. [START_REF] Ang | Brain-computer interface-based robotic end effector system for wrist and hand rehabilitation: results of a three-armed randomized controlled trial for chronic stroke[END_REF][START_REF] Ang | A randomized controlled trial of EEG-based motor imagery brain-computer interface robotic rehabilitation for stroke[END_REF] and [START_REF] Ramos-Murguialday | Brain-machine interface in chronic stroke rehabilitation: a controlled study[END_REF] did not include patients that were respectively in pain or in severe pain.

In Table 1, based on the central role that somatosensory abilities play in the perception of somatosensory feedback and the inclusion/ exclusion criteria that the authors used, we hypothesize which potential somatosensory-related biases could arise. For instance, the presence of non-responders in several studies might have been related to abnormal somatosensory abilities [START_REF] Ang | A randomized controlled trial of EEG-based motor imagery brain-computer interface robotic rehabilitation for stroke[END_REF][START_REF] Young | Brain-computer interface training after stroke affects patterns of brain-behavior relationships in corticospinal motor fibers[END_REF]. Also, a potential randomization bias might be present in studies that did not report using somatosensory-related inclusion/exclusion criteria.

To summarize, our review of the literature indicates that somatosensory abilities may be assessed but are rarely reported in the literature. Though, we have reasons to believe that somatosensory impairments could impact BCI-based rehabilitation outcome. In future research we argue that it would be relevant to: (1) assess somatosensory impairments, (2) use the resulting somatosensory-related measures for inclusion/exclusion criteria and randomization and (3) report these measures. We argue that doing so might provide information regarding the mechanisms underlying the therapy, explain part of the between-subject and/or between-study variability, contribute to the improvement and adaptation of the therapy and offer leads for somatosensory rehabilitation. Each of these statements is developed in the following sections.

The interconnection of somatosensory and motor abilities during recovery

In the previous section, we focused on the specific influence of somatosensory abilities on BCI-based motor rehabilitation post-stroke. However, somatosensory abilities are interrelated with motor recovery in general. The influence of somatosensory abilities on motor rehabilitation therapies post-stroke was already demonstrated in other fields [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF][START_REF] Edwards | Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke[END_REF]. To study the literature on the matter, we used the keywords "somatosensory", "proprioceptive", "kinesthetic", "kinaesthetic", "motor" and "stroke" (mostly in Medline and Scopus) and focused our research on human studies and models. We also analysed the references quoted in the relevant articles found and the articles quoting those articles.

Motor function is the main focus of sensorimotor assessment and rehabilitation, considering both clinical management and research [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF][START_REF] Edwards | Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke[END_REF]. It is now acknowledged that spontaneous motor recovery reaches a plateau 3 months after stroke onset for most of the patients, due to the spontaneous cortical reorganization of the motor system which mostly occurs during this period of time [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF][START_REF] Kwakkel | Impact of time on improvement of outcome after stroke[END_REF]. However, sensory and motor improvements are not specific but interrelated. An influence of motor rehabilitation on somatosensory network is to be expected in view of the role that somatosensory inputs play in motor rehabilitation [START_REF] Edwards | Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke[END_REF]. Somatosensory impairment due to cortical lesion is almost always associated with motor impairment [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF][START_REF] Sullivan | Sensory dysfunction following stroke: incidence, significance, examination, and intervention[END_REF]. Interestingly, somatosensory therapy seems to have an impact on motor function and vice versa [START_REF] Byl | Effectiveness of sensory and motor rehabilitation of the upper limb following the principles of neuroplasticity: patients stable poststroke[END_REF]. For instance, therapies based on repetitive electrical peripheral nerve stimulation have proven efficient to enhance excitability of the motor cortex and improve motor functions in daily activities [START_REF] Conforto | Repetitive Peripheral Sensory Stimulation and Upper Limb Performance in Stroke: A Systematic Review and Meta-analysis[END_REF].

Motor skill learning is crucial for motor recovery and somatosensory inputs are involved in this learning [START_REF] Edwards | Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke[END_REF][START_REF] Krakauer | Motor learning: its relevance to stroke recovery and neurorehabilitation[END_REF]. Motor learning involves neuroplasticity, notably in the primary motor cortex [START_REF] Stefan | Induction of plasticity in the human motor cortex by paired associative stimulation[END_REF] which has dense connections with the primary somatosensory cortex. The conjoint activation of somatosensory afferences and motor cortical circuits affects the neural mechanisms of plasticity associated with skills learning [START_REF] Edwards | Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke[END_REF][START_REF] Stefan | Induction of plasticity in the human motor cortex by paired associative stimulation[END_REF]. Hence, the primary somatosensory cortex is crucial in motor skill learning [START_REF] Edwards | Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke[END_REF]. Ablation of the area dedicated to the hand in the primary sensory cortex of monkeys does not interfere with motor tasks learned before but impedes new learning [START_REF] Pavlides | Projection from the sensory to the motor cortex is important in learning motor skills in the monkey[END_REF]. This influence of the somatosensory afferences on motor skill learning is also supported by post-stroke upper extremity motor rehabilitation studies.

First, somatosensory loss is associated with more paresis of the distal parts of the limbs, greater motor and functional impairments, as well as less independence in daily living in chronic stage [START_REF] Edwards | Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke[END_REF][START_REF] Andersen | Incidence of central post-stroke pain[END_REF][START_REF] Carey | Effects of Somatosensory Impairment on Participation After Stroke[END_REF]. Also, somatosensory loss and especially proprioceptive loss, has a negative influence on the rehabilitation's efficiency assessed through functional outcome, but also on the length of the rehabilitative treatment and on the participation in daily activities [START_REF] Carey | Review on Somatosensory Loss after Stroke[END_REF][START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. Abnormal SomatoSensory Evoked Potentials 2 (SSEPs) are biomarkers of

Table 1

Characteristics of the studies selected for this review. Main elements of the studies (columns 1 to 8), then mechanisms quoted by the authors to explain their results (column 9) and finally our analysis of the potential factors (intrinsic to the patients, e.g., somatosensory loss, or extrinsic to the patients, e.g., design issues) that could arise from the inclusion/exclusion criteria presented in Table 2 (column 10) are introduced. During the studies patients were either asked to perform motor-imagery (MI) or motor-attempt (MA) tasks. If they were reported in the article for both the experimental (exp) and control (contr) groups the number of patients included, chronicity and motor impairment of are reported in such order and separated by a slash. If not the average is provided. Chronicity is defined by the time between the stroke and the inclusion in the study such as: Acute= < 1 month, Subacute= < 3 months and Chronic= > 3 months. Mean and standard deviation of time from stroke for each group distinctly or globally are provided either in days or months. When provided in the article minimal and maximal values are also reported, i.e., (min-max). Mean and standard deviation of motor impairment at inclusion is provided using the Fugl- poor motor recovery. [START_REF] Zeman | Functional prognosis in stroke: use of somatosensory evoked potentials[END_REF] found that the pattern of the SSEPs, i.e., the amplitude of the negative and positive peaks, correlates with the functional rehabilitation outcome measured using the length of the stay at the rehabilitation center and the daily living abilities, e.g., the ability to dress. The authors also hypothesized that the correlation between SSEPs and motor recovery could be influenced by the location of the lesion. Abnormal SSEPs due to cortical lesions resulted in poorer motor outcomes than abnormal SSEPs due to subcortical lesions. The negative influence of somatosensory impairments on post-stroke motor rehabilitation could also originate from the non-use mechanism, which is the rarefied use of the plegic limb occurring in the absence of relevant proprioceptive and exteroceptive feedback [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. Second, the use of a constant sensory stimulation (mechanical vibration on the wrist) during motor rehabilitation has proven efficient in enhancing the motor function both at short and long terms and in increasing motor related brain activity [START_REF] Fleming | The effect of combined somatosensory stimulation and task-specific training on upper limb function in chronic stroke: a double-blind randomized controlled trial[END_REF]. Also, exteroceptive and proprioceptive stimulations seem to have a beneficial influence on motor function, even though further high quality studies are required [START_REF] Yilmazer | Effectiveness of somatosensory interventions on somatosensory, motor and functional outcomes in the upper limb post-stroke: A systematic review and meta-analysis[END_REF].

Results

Mechanisms discussed supposedly underlying the results

Potential bias due to inclusion/exclusion criteria (detailed in

Finally, the rehabilitation of somatosensory perception requires taking into account motor abilities. For instance, somatosensory therapies could increase the daily use of the impacted limb, but only if the motor abilities are not too damaged [START_REF] Turville | Change in Functional Arm Use Is Associated With Somatosensory Skills After Sensory Retraining Poststroke[END_REF]. Motor therapies might have variable effects depending on the somatosensory impairments of the patients (Van der [START_REF] Van Der Lee | Forced use of the upper extremity in chronic stroke patients: results from a single-blind randomized clinical trial[END_REF]. The feedback might also need to be adapted with regards to the somatosensory impairments of patients. For instance, post-stroke patients with pure somatosensory impairment can perform complex motor tasks with but not without visual feedback [START_REF] Jeannerod | The control of hand movements in a case of hemianaesthesia following a parietal lesion[END_REF]. Also, somatosensory impairments sometimes include deficits in the integration of multimodal sensory information, e.g., visual and tactile, which could further impact feedback perception [START_REF] Carey | Review on Somatosensory Loss after Stroke[END_REF]. Therefore, future research should provide more information about which therapies are the most beneficial depending not only on the motor impairments but also on the type of somatosensory loss [START_REF] Sullivan | Sensory dysfunction following stroke: incidence, significance, examination, and intervention[END_REF].

Based on these results, assessing, controlling and reporting somatosensory impairments could also provide some insight on the intersubject and inter-study variability of BCI-based motor rehabilitation post-stroke outcome found in the literature (see Table 1). The somatosensory impairments might be relevant predictors of how patients would respond to BCI-based therapies. Taking into account these impairments might enable to better screen the patients that would most likely benefit from the therapy. Main elements of the therapy, such as the instructions, the feedback or the tasks to perform, might benefit from being adapted to the somatosensory impairments of the patients. For instance, the type of feedback, i.e., extrinsic (information originating from an external source, e.g., a screen or a person) or intrinsic (somatosensory sensations felt by the person during the training), could be adapted depending on the type and amount of somatosensory impairments. While both extrinsic and intrinsic feedback have proven efficient [START_REF] Subramanian | Does provision of extrinsic feedback result in improved motor learning in the upper limb poststroke? A systematic review of the evidence[END_REF][START_REF] Biasiucci | Brain-actuated functional electrical stimulation elicits lasting arm motor recovery after stroke[END_REF], an irrelevant or inefficient proprioceptive feedback, e.g., altered due to somatosensory impairments, might impede the efficiency of the BCI therapy.

Also, the influence of the BCI therapy on the somatosensory rehabilitation should be explored. Interestingly enough, BCI-based motor rehabilitation might improve somatosensory abilities, along with motor ones. Sun et al. [START_REF] Sun | neurological rehabilitation of stroke patientsviamotor imaginary-based brain-computer interface technology[END_REF] mentioned the improvement of somatosensory abilities post BCI therapy in a non randomized clinical trial. This improvement could be related to the instruction given to the patients to perform kinesthetic motor imagery, i.e., focus on somatosensory sensations associated with the imagined movement. Asking the participants to perform kinesthetic motor imagery might participate to somatosensory rehabilitation. The vast majority of motor imagery BCIbased RCTs (73%) do report providing such instructions (see Table 1). Though, attempted movements BCI-based RCTs do not report asking their patients to focus on their sensations while trying to perform the movement. Mihara et al. reported the activation of the somatosensory associative cortex and the somatosensory primary cortex that could underly such somatosensory improvement [START_REF] Mihara | Near-infrared spectroscopy-mediated neurofeedback enhances efficacy of motor imagery-based training in poststroke victims: a pilot study[END_REF]. Thus, the central role that sensory feedback plays in BCIs might be harnessed and beneficial for somatosensory rehabilitation. Such positive results were observed in previous experiments [START_REF] Sun | neurological rehabilitation of stroke patientsviamotor imaginary-based brain-computer interface technology[END_REF] and should be further investigated in future studies, for instance by recording somatosensory-related EEG activity using parietal electrodes.

Somatosensory impairments, recovery and assessment poststroke

In the previous sections, we argued that somatosensory abilities play a central role in BCI-based motor rehabilitation post-stroke and for motor rehabilitation post-stroke in general. The following section provides more insights regarding the prevalence and characteristics of somatosensory impairments and recovery post-stroke. The aim is to better understand the extent of the influence that somatosensory abilities might have on BCI-based rehabilitation. We also present different methods to assess the somatosensory impairments post-stroke for experimenters and/or clinicians that wish to take into account and/or report their patients' somatosensory impairments in their experimental protocol.

Impairments of somatosensory abilities post-stroke

Information regarding how stroke impacts somatosensory abilities is important to better understand to which extent somatosensory impairments could disrupt BCI-based motor therapy post-stroke. The prevalence of somatosensory impairments in stroke is still difficult to estimate because the studied population is heterogeneous and it remains subjective because it involves patient's participation. Also, the assessment outcome depends on the time between the evaluation and the stroke onset, as well as on the spontaneous somatosensory recovery occurring in the first three months [START_REF] Kwakkel | Impact of time on improvement of outcome after stroke[END_REF]. Finally, the prevalence is probably under-estimated because of the lack of standardized psychometric tools available to assess somatosensory impairments (see Section 4.3).

Despite these drawbacks, it has been estimated that more than half of strokes lead to somatosensory impairments [START_REF] Carey | Review on Somatosensory Loss after Stroke[END_REF][START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF][START_REF] Pumpa | Somatosensory assessment and treatment after stroke: an evidence-practice gap[END_REF]. Most often, i.e., 75% of the case, the sensory loss impacts the upper limbs [START_REF] Rathore | Characterization of incident stroke signs and symptoms: findings from the atherosclerosis risk in communities study[END_REF]. Among the different types of somatosensory loss, exteroceptive impairments seem to be the most frequent. Indeed, most of the literature suggests that tactile impairments are for instance twice more frequent than proprioceptive impairments [START_REF] Tyson | Sensory loss in hospital-admitted people with stroke: characteristics, associated factors, and relationship with function[END_REF] despite opposite findings [START_REF] Connell | Somatosensory impairment after stroke: frequency of different deficits and their recovery[END_REF]. Impairments in proprioception and elementary sensory modalities, such as touch, pressure, pain, vibration and temperature, are equally reported for 53 to 64% of patients [START_REF] Tyson | Sensory loss in hospital-admitted people with stroke: characteristics, associated factors, and relationship with function[END_REF][START_REF] Connell | Somatosensory impairment after stroke: frequency of different deficits and their recovery[END_REF]. Moreover, discriminative sensations, such as stereognosis (i.e., ability to recognize objects using tactile sensations only), texture discrimination, position sense or two-point discrimination seem to be particularly affected [START_REF] Klingner | Sensory syndromes[END_REF].

The following paragraphs present information regarding the neurophysiological correlates of these somatosensory impairments. Such anatomical aspects might be worth taking into account when selecting somatosensory-related inclusion/exclusion and randomization criteria for BCI-based motor rehabilitation studies.

The amount of sensory loss is correlated to the severity of the stroke as well as to the extent of the lesion [START_REF] Tyson | Sensory loss in hospital-admitted people with stroke: characteristics, associated factors, and relationship with function[END_REF][START_REF] Connell | Somatosensory impairment after stroke: frequency of different deficits and their recovery[END_REF]. Somatosensory submodalities can be differently affected in a given body part. For instance, at the level of the wrist, the light touch ability might not be as impacted as the proprioceptive one (Connell et al., 2 Somatosensory evoked potentials are spontaneous electrical potentials from the nervous system following a tactile stimulation.

L. Pillette, et al. NeuroImage: Clinical 28 (2020) 1024172008). Nonetheless, adjacent body parts are most likely to have similar amount of loss for a given somatosensory submodality, e.g., touch ability between wrist and hand are likely to be similar [START_REF] Connell | Somatosensory impairment after stroke: frequency of different deficits and their recovery[END_REF].

Stroke lesions can also result in somatosensory loss (notably impairments in tactile discrimination and position senses) to the ipsilesional hand, even though the impairment seems less important than for the contralesional hand [START_REF] Carey | Frequency of discriminative sensory loss in the hand after stroke in a rehabilitation setting[END_REF]. This phenomenon might be the consequence of damages in ipsilateral somatosensory pathways and bilateral networks processing somatosensory information [START_REF] Connell | Somatosensory impairment after stroke: frequency of different deficits and their recovery[END_REF]. This result is of the utmost importance as it implies that the ipsilesional limb, i.e., the 'unimpacted limb', cannot always be considered as a reference to evaluate the somatosensory impairments of the contralesional limb, i.e., the 'impacted limb'. This contributes to the difficulty of assessing somatosensory abilities. Different types of strokes have been associated with different somatosensory losses [START_REF] Carey | Review on Somatosensory Loss after Stroke[END_REF][START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. Ischemic strokes are more likely to lead to sensory impairments than hemorrhagic strokes [START_REF] Rathore | Characterization of incident stroke signs and symptoms: findings from the atherosclerosis risk in communities study[END_REF]. Also, right hemispheric strokes are more likely to be associated with somatosensory loss than left hemispheric strokes [START_REF] Sullivan | Sensory dysfunction following stroke: incidence, significance, examination, and intervention[END_REF]. Though, spatial neglect 3 is common for patients with right hemispheric strokes and could also explain this difference of somatosensory loss observed between right and left hemispheric strokes. Lesions affecting the thalamus, brainstem, lenticulocapsular or parietal regions are known to induce somatosensory symptoms [START_REF] Klingner | Sensory syndromes[END_REF]. The impairment of one somatosensory submodality or another (e.g., touch, pressure, pain, vibration and temperature) might be different depending on the lesion location [START_REF] Carey | Review on Somatosensory Loss after Stroke[END_REF][START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF].

To summarize, somatosensory impairments are frequent and diverse post-stroke. Their influence on BCI-based motor rehabilitation poststroke outcome is thus worth assessing.

Recovery of somatosensory abilities post-stroke

Beyond knowledge regarding somatosensory impairments poststroke, the characteristics of the somatosensory recovery are important to describe. Indeed, a difference of somatosensory recovery between patients might lead to disparate somatosensory abilities among them and thereby to distinct BCI-based therapy outcomes.

While the time course of somatosensory recovery has not been as much studied as the motor one, this function also spontaneously improves after a stroke [START_REF] Klingner | Sensory syndromes[END_REF][START_REF] Zeman | Functional prognosis in stroke: use of somatosensory evoked potentials[END_REF]. Somatosensory recovery occurs for a majority of patients within the first 3 months following the stroke [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF][START_REF] Julkunen | Recovery of somatosensory deficits in acute stroke[END_REF]. Results indicate a functional and structural plasticity occurring in the primary and secondary somatosensory cortices after stroke regardless of the sensorimotor therapy followed [START_REF] Schaechter | Structural and functional plasticity in the somatosensory cortex of chronic stroke patients[END_REF]. The recovery is highly variable between individuals and somatosensory functions can sometimes decrease and fluctuate over time [START_REF] Schaechter | Structural and functional plasticity in the somatosensory cortex of chronic stroke patients[END_REF][START_REF] Julkunen | Recovery of somatosensory deficits in acute stroke[END_REF]. The somatosensory assessment on admission is a main predictor of recovery after 6 months [START_REF] Connell | Somatosensory impairment after stroke: frequency of different deficits and their recovery[END_REF]. The amount of somatosensory recovery correlates positively with the severity of the stroke [START_REF] Connell | Somatosensory impairment after stroke: frequency of different deficits and their recovery[END_REF]. Lesion location has an influence on the recovery from somatosensory impairment. One could hypothesize that cortical redundancy would lead to greater recovery of cortical lesions compared to subcortical ones [START_REF] Sullivan | Sensory dysfunction following stroke: incidence, significance, examination, and intervention[END_REF]. Nonetheless, recent studies on proprioception have shown that persistent proprioceptive loss was associated with both subcortical and cortical lesions [START_REF] Findlater | Lesion locations associated with persistent proprioceptive impairment in the upper limbs after stroke[END_REF].

Some research has been led to foster the recovery of somatosensory abilities. They focused on somatosensory discrimination tasks or on sensory stimulation involving tactile, electrical, thermal and magnetic stimulation. For an overview of the different somatosensory feedback investigated, see [START_REF] Sullivan | Sensory dysfunction following stroke: incidence, significance, examination, and intervention[END_REF].

Influence of peripheral somatosensory stimulation has however been questioned by [START_REF] Grant | Somatosensory stimulation to improve hand and upper limb function after stroke-a systematic review with meta-analyses[END_REF]. Recent reviews [START_REF] Conforto | Repetitive Peripheral Sensory Stimulation and Upper Limb Performance in Stroke: A Systematic Review and Meta-analysis[END_REF][START_REF] Grant | Somatosensory stimulation to improve hand and upper limb function after stroke-a systematic review with meta-analyses[END_REF] concur on the need for further investigation with qualitative randomized controlled trials.

It is worth noting that it has been shown that Hebbian-like reinforcement occurs in the context of robot-assisted somatosensory rehabilitation [START_REF] Ingemanson | Proprioception and motor learning after stroke-insights from neuroimaging studies[END_REF]. Hebbian plasticity, i.e., the reinforcement of the synaptic connection induced by the conjoint activation of pre and post synaptic neurons, is also currently used to explain how BCIs foster plasticity and improve motor functions. BCI therapy often used robotic tools to improve motor control without assessing the impact on somatosensory abilities. Several authors have suggested that motor improvement observed using BCI therapy with robotic proprioceptive feedback might be due to the involvement of timely somatosensory afferences (see Table 1). However, these studies have not explored the possible biases that could arise from somatosensory abilities (see Table 2). We argue that assessing the influence of somatosensory impairments on BCI-based post-stroke motor rehabilitation outcome could lead to a better understanding of the underlying mechanisms of the therapy. The characteristics of the therapy should also be taken into account to assess the influence of somatosensory impairments. For instance, testing different types of modalities of feedback might be relevant, e.g., visual and somatosensory feedback. Indeed, depending on the somatosensory impairments, patients might benefit from one feedback more than another. If such results are found, they could be leveraged to then adapt the therapy to the patients' profile.

Another approach might be to focus BCI-based rehabilitation on somatosensory abilities. Previous results of Yao et al. indicate that somatosensory imagination tasks can be recognized using EEG in healthy people [START_REF] Yao | A Multiclass BCI based on Somatosensory Imagery[END_REF]. They have shown that imagining a tactile sensation of the right or left hand could be discriminated from one another with an accuracy comparable to the one of a motor imagery task, i.e., with 75.7% of online classification accuracy (percentage of mental tasks accurately recognized by the BCI). Using a BCI to provide somatosensory feedback when patients are imagining a somatosensory feeling might lead to improvements of somatosensory abilities and maybe motor ones as well (see Section 3) -The interconnection of somatosensory and motor abilities during recovery-.

Assessment of somatosensory abilities post-stroke

With the goal of fostering clinical trials assessing, controlling for and reporting somatosensory impairment, in the following paragraph we provide some information regarding somatosensory assessments tools.

Frequently, routine tests of patients after stroke consist in clinical tests and do not precisely assess all somatosensory submodalities [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. They mostly focus on light touch and proprioception assessment but often fail to assess other submodalities, e.g., twopoint discrimination or point localization [START_REF] Pumpa | Somatosensory assessment and treatment after stroke: an evidence-practice gap[END_REF]. This limited scope in clinical somatosensory examination also contributes to the underestimation of the somatosensory loss [START_REF] Sullivan | Sensory dysfunction following stroke: incidence, significance, examination, and intervention[END_REF]. Indeed, using standardized assessment of discriminative sensations, Kim and Choi-Kwon found that around 90% of patients who were thought to suffer from pure motor stroke had somatosensory impairments [START_REF] Kim | Discriminative sensory dysfunction after unilateral stroke[END_REF]. Clinical assessment can also be in contradiction with the results from standardized tests and patients identified as unimpaired using standardized tests can be identified as impaired using clinical assessment and vice versa [START_REF] Carey | Evaluation of impaired fingertip texture discrimination and wrist position sense in patients affected by stroke: comparison of clinical and new quantitative measures[END_REF].

Several standardized test protocols dedicated to the assessment of somatosensory loss have been identified in the literature (see [START_REF] Carey | Review on Somatosensory Loss after Stroke[END_REF] for a review of the different tests assessing specific somatosensory abilities). Kessner et al. have summarized the different tools that assess different somatosensory modalities [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. In their review, the authors recommended to use the "Erasmus-modified Nottingham Sensory Assessment" (EmNSA) [START_REF] Stolk-Hornsveld | The Erasmus MC modifications to the (revised) Nottingham Sensory Assessment: a reliable somatosensory assessment measure for patients with intracranial disorders[END_REF] for clinical purpose because of its fair compromise between robustness and usability.

3 Spatial neglect corresponds to a deficit of attention dedicated to sensory information arising from one side of the body.

L. Pillette, et al. NeuroImage: Clinical 28 (2020) 102417

For research purposes, they recommended the "Rivermead Assessment of Somatosensory Performance" [START_REF] Winward | The Rivermead Assessment of Somatosensory Performance (RASP): standardization and reliability data[END_REF] (RASP) because it is highly standardized and provides measures related to interval scales which are easier for statistical use. Though, the RASP is not produced by any company at the moment. Promising research was recently led using robotic technology to create more reliable proprioceptive, kinesthesic and motor assessments [START_REF] Semrau | Examining differences in patterns of sensory and motor recovery after stroke with robotics[END_REF]. It is worth noting that cognitive impairments, e.g., aphasia or spatial neglect, might interfere with somatosensory assessment when assessed using clinical scales. For example, spatial neglect 3 could lead to an overestimation of somatosensory impairments of the left sided limbs and to a greater difference of somatosensory impairments between left and right sided limbs. Such cognitive impairments being frequent after a stroke, their influence on somatosensory tests should be assessed [START_REF] Kessner | Somatosensory deficits after stroke: a scoping review[END_REF]. Inclusion criteria should take into account such impairments when assessing somatosensory impairments using non-physiological measures.

In order to avoid potential bias arising from cognitive impairment, specific biomarkers, such as somatosensory evoked potentials2 (SSEP), could be used in addition to standardized tools. Indeed, SSEP correlates with somatosensory impairments [START_REF] Giblin | Somatosensory evoked potentials in healthy subjects and in patients with lesions of the nervous system[END_REF]. However, the relevance of such a biomarker remains unclear. Indeed, previous results found that two thirds of patients with abnormal SSEPs had somatosensory loss and four out of five patients with normal SSEP had normal sensations [START_REF] Zeman | Functional prognosis in stroke: use of somatosensory evoked potentials[END_REF]. Finally, other biomarkers, such as diffusion tensor imaging measures of fractional anisotropy seem to be well correlated with clinical symptoms [START_REF] Yamada | Fiber-tracking method reveals sensorimotor pathway involvement in stroke patients[END_REF], but are still difficult to include in a clinical routine.

We have argued previously that the somatosensory impairments should be considered when selecting inclusion/exclusion and randomization criteria for BCI-based motor rehabilitation post-stroke studies. To summarize, from our review of the literature, we would suggest to use a dedicated standardized somatosensory test to assess, control and report the patients' somatosensory impairments. The RASP was described as the most relevant test for research purpose. Though, at the moment, the test is not developed by any company. Therefore, the EmNSA, which can be passed fairly quickly (around 10 to 20 min) without specific equipment, might be used. Moreover, several factors that impact on somatosensory impairment should be taken into account. This includes the presence of cognitive impairments (e.g., aphasia, spatial neglect) and anatomical aspects (e.g., ischaemia, right hemispheric strokes, lesions to the thalamus, cortical lesions, etc., see Section 4.1 and 4.2 for details). Assessing the somatosensory impairments using physiological measures such as SSEP or a diffusion tensor imaging measures of fractional anisotropy could be further informative even though it might be more difficult to include in a clinical routine.

Conclusion

BCI therapies have proven efficient to enhance motor functions post-stroke particularly post-intervention [START_REF] Bai | Immediate and long-term effects of BCI-based rehabilitation of the upper extremity after stroke: a systematic review and meta-analysis[END_REF] and with effect sizes that are medium to large [START_REF] Cervera | Brain-computer interfaces for post-stroke motor rehabilitation: a metaanalysis[END_REF]. Further studies with larger sample sizes are necessary to validate these promising results. The therapy is based on the co-activation of top-down pathways, resulting from either motor imagery or attempted movements, and bottom-up pathways, resulting from visual and/or somatosensory feedback provided by the BCI. Based on the Hebbian theory, this co-activation should foster plasticity and improve motor abilities [START_REF] Grosse-Wentrup | Using brain-computer interfaces to induce neural plasticity and restore function[END_REF]. Hence, the integrity of the ascending sensory pathways, such as somatosensory ones, should be assessed. Our literature-based study of RCTs on BCI-based motor rehabilitation poststroke indicates that somatosensory impairments may be assessed but usually are not used as inclusion/exclusion criteria and are not reported in papers. Somatosensory abilities have repeatedly been shown to influence the motor rehabilitation outcome. Yet, more than half of poststroke patients suffer from somatosensory impairments. Based on these elements, we argue that somatosensory abilities should be systematically and rigorously reported in future experiments. of clinical and cognitive-behavioural neurofeedback studies (CRED-nf) checklist [START_REF] Ros | Consensus on the reporting and experimental design of clinical and cognitive-behavioural neurofeedback studies (CRED-nf checklist)[END_REF].

The theoretical and experimental results reviewed in the previous sections can be combined and synthesized into a global model of BCIbased post-stroke rehabilitation that explicitly takes into account somatosensory abilities. This model that we propose can be seen in Fig. 1. In short, as can be seen on this model and as discussed previously, a deficit in somatosensory abilities can negatively influence BCI-based stroke rehabilitation at two main levels: (1) at the feedback level, as it should lead to an impaired perception of the BCI feedback, that generally includes (and benefits from) somatosensory stimuli (e.g., tactile or proprioceptive feedback). Such impaired feedback perception was notably observed with various rehabilitation procedures other than BCI, as discussed previously; (2) at the BCI motor command production level, as it should lead to impaired motor imagery abilities, whereas motor imagery is a key paradigm used in BCI for stroke rehabilitation.

From this model and from the studies reviewed above, we can also derive a number of guidelines to take into account somatosensory abilities during BCI-based stroke rehabilitation, as well as to improve our understanding of their influence. These guidelines are summarized in Table 3. Such guidelines notably include testing, measuring and reporting patients' somatosensory abilities, as well as considering them in inclusion/exclusion criteria and for assessing the effectiveness of a given BCI feedback type.

Similarly, from the model, we can identify a number of promising research directions, that would enable us to refine our understanding of BCI-based stroke rehabilitation, to hopefully improve its effectiveness as well as to derive more precise and specific guidelines. These new research directions are also summarized in Table 3. Such research directions include formally studying how patients' somatosensory abilities and BCI-based motor rehabilitation post-stroke influence each other. Another promising lead is to explore somatosensory rehabilitation specifically, possibly using a BCI. Finally, future research should aim at finding how to adapt BCI-based stroke rehabilitation to the patients' somatosensory abilities, notably by adapting the BCI design accordingly, e.g., the instructions, MI tasks, feedback or signal processing.

Finally, studies on healthy subjects have shown that the BCI ability to recognize the activation of top-down processes through brain activity patterns is modulated by numerous factors, such as the type of algorithm used to process the data [START_REF] Lotte | A review of classification algorithms for EEG-based brain-computer interfaces: a 10 year update[END_REF], the psychological profile of the subjects [START_REF] Jeunet | Advances in user-training for mental-imagerybased BCI control: Psychological and cognitive factors and their neural correlates[END_REF] or the characteristics of the feedback (e.g., modality of presentation, accuracy or latency) [START_REF] Grosse-Wentrup | Using brain-computer interfaces to induce neural plasticity and restore function[END_REF][START_REF] Lotte | Flaws in current human training protocols for spontaneous brain-computer interfaces: lessons learned from instructional design[END_REF][START_REF] Pillette | Redefining and Adapting Feedback for Mental-Imagery based Brain-Computer Interface User Training to the Learners' Traits and States[END_REF]. Though, the impact of these factors on BCIbased motor rehabilitation post-stroke outcome remains to be evaluated.

  Fig. 1. Schematic representation of BCI-based motor rehabilitation post-stroke taking into account the somatosensory abilities. The elements that might be impacted by the somatosensory impairments of the patients are colored in green. The photo of the person receiving sensory feedback during BCI-based motor rehabilitation is provided courtesy of ©EPFL/Alain Herzog.

time to stroke) Motor impairment at inclusion BCI intervention Control group Outcome measures

	Design of the study,	Patients number	(exp/contr),	Chronicity (
	Aim
	Study

Meyer Assessment of Upper Extremity (FMA-UE), Jebsen Hand Function Test (JHFT), Action Research Arm Test (ARAT) and Brunnstrom recovery stage. Indication regarding the motor capacity of the subjects are provided depending on the FMA-UE scores, i.e., no: 0-22, poor: 23-31, limited: 32-47, notable: 48-52, full:53-66, or on the ARAT scores, i.e., no: 0-10, poor: 11-21, limited: 22-42, notable: 43-54, full:55-57. Following is a list of the abbreviations used in the table and their signification: Classification Accuracy (CA), Blood Oxygen Level Dependent effect (BOLD), Diffusion Tensor Imaging (DTI), ElectroEncephaloGraphy (EEG), Electromyogram (EMG), European Stroke Scale (ESS), Event Related Desynchronization (ERD), functional Magnetic Resonance Imaging (fMRI), Goal Attainment Scale (GAS), Hand Grip Strength (HGS), Lateralization Index (LI), Medical Research Council (MRC), Modified Ashworth Scale (MAS), Motor Activity Log (MAL), Motor Evoked Potential (MEP), NASA Task Load Index (NASA-TLX), National Institute of Health Stroke Scale (NIHSS), Near InfraRed Spectroscopy (NIRS), Nine Hole Peg test (9-HPT), Power Spectral Density (PSD), Resting State Connectivity (RSC), revised Brain Symmetry Index (rBSI), SensoriMotor Cortex (SMC), Stroke Impact Scale (SIS), Transcranial Magnetic Stimulation (TMS).

Table 2

 2

	) for	mechanisms	supposedly underlying	the results

(continued on next page) Table 1 (continued) (continued on next page) Table 1 (continued) (continued on next page) Table 1 (continued) (continued on next page) Table 1 (continued)

Table 2

 2 Inclusion and exclusion criteria related to somatosensory impairments of the studies selected for this review. When stated by the authors the test or questionnaire associated with the criteria are stated in parentheses after the later. Following is a list of the abbreviations used in the table and their signification: Abbreviated Mental Test (AMT), Fugl-Meyer Assessment of Upper Extremity (FMA-UE), Intelligence quotient (IQ), Medical Research Council (MRC), Mini-Mental State Examination (MMSE), Modified Ashworth Scale (MAS), Montreal Cognitive Assessment (MoCA), Upper Extremity (UE), Visual Analogue Scale (VAS).

	Inclusion/
	Instructions to	use kinesthetic	MI
	Study

Exclusion criteria related to somatosensory assessment Time to stroke onset Spasticity, dystonia, movement disorders Visual abilities Pain Somato-sensory abilities Cognitive Abilities

	Unilateral	spatial neglect		No hemispatial	neglect			No hemispatial	neglect			No hemispatial	neglect													No spatial	neglect	No severe	hemispatial	neglect
	Language			No severe aphasia				No severe aphasia											No sensory aphasia, No	severe motor aphasia	No speech disorders					No moderate to severe	aphasia	No severe aphasia			No severe aphasia			No cognitive disorders	making	communication	difficult
				Able to understand simple	instructions, No inattention, No severe	depression, No psychiatric disorder		Able to understand simple instructions	(AMT > 6), No cognitive deficits, No	severe depression		Able to understand simple	instructions, No cognitive deficits	preventing to perform the	rehabilitation task (Raven's Test), No	patients under heavy medication	affecting the central nervous system	(including vigilance)	No severe cognitive impairment	(MoCA > 10)	No cognitive impairment	(MMSE > 27), Able to perform MI	tasks evidenced by KVIQ, Able to	understand the experimental	commands	No cognitive impairment (MMSE 23),	No depression	No cognitive impairment	(MMSE > 24)		Able to follow and understand	instruction, No psychiatric or	neurological condition other than	stroke, No depression, IQ above 80	No other known neurologic,	psychiatric or developmental	disabilities
																										No sensory loss								No sensory	impairment in	the upper limbs
				No pain	(VAS 4)			No pain	(VAS 4)																						No severe	pain
				No capacity to notable capacity No severe visual	(FMA-UE 10 and 50), Motor impairement	power controlled (MRC), No	severe spasticity (MAS 2)	No capacity to limited capacity No severe visual	(FMA-UE 45), No severe impairement	spasticity (MAS 2), No fixed	joint contractures	No capacity to limited capacity Good or	(FMA-UE 40), No severe corrected	dystonia/involontary eyesight	movements, No other	neurological disorders (e.g.,	Parkinson's disease)		Hand paresis (MRC, mild to No severe vision	plegia), No spasticity (MAS < 4) impairment	Affected UE (Brunnstrom period	level between I and III)				Motor hemiparesis (FMA-UE 50) No hemianopia		Hemiplegia or hemiparesis, No	spasticity (MAS < 4), No	apraxia	Paresis of one hand, No active	finger extension, No cerebellar	lesion or bilateral motor deficit	Good trunk balance, Good motor	recovery (stage 4 to 5 of	Brunnstrom's stage of motor	recovery), Partial ability to grasp	and release	No capacity to limited capacity	(FMAleq45)	Persistent UE motor impairment
				> 4 months				> 3 months				10 months							1 month		1 to 6 months					12 weeks		6 weeks to	6 months		> 10 months			3 to 12 months	1 month
				Yes				Yes											Yes		Yes (with KVIQ	assessment)				Yes (with KVIQ	assessment)	Yes						Yes	Yes
		(Ang et al., 2009)	(Ang et al., 2010)	(Ang et al., 2014)				(Ang et al., 2015)				(Biasiucci et al., 2018)							(Frolov et al., 2017)		(Li et al., 2014)					(Mihara et al., 2013)		(Pichiorri et al., 2015)			(Ramos-Murguialday	et al., 2013)		Rayegani et al., 2013	(Rayegani et al.,	2014)	Varkuti et al., 2013	(Várkuti et al., 2013)	(Wada et al., 2019) (No	criteria mentioned)	(Wada et al., 2019)	(Young et al., 2016)	(Young et al., 2016)

Mirror visual feedback consists in positioning, with respect to a mirror, the patients' arms in order for them to perceive their unimpaired limb in the position of the impaired limb, therefore providing the patients with the impression of two unimpaired arms.

NeuroImage: Clinical 28 (2020) 102417

Acknowledgements

This work was supported by the French National Research Agency (project REBEL, grant ANR-15-CE23-0013-01) and the European Research Council with the Brain-Conquest project (grant ERC-2016-STG-714567).

Funding information

This work was supported by the French National Research Agency (project REBEL, grant ANR-15-CE23-0013-01) and the European Research Council with the Brain-Conquest project (grant ERC-2016-STG-714567).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Table 3

Summary of the recommendations and ideas for future research for each of the main elements of BCI-based motor rehabilitation post-stroke presented in Fig. 1.