Misconceptions about mirror-induced motor cortex activation

Improving motor imagery through a mirror box for BCI users

The aim of this study was to evaluate mirror visual feedback (MVF) as a training tool for brain-computer interface (BCI) users. This is because approximately 20-30% of subjects require more training to operate a BCI system using motor imagery. Electroencephalograms (EEGs) were recorded from 18 healthy subjects, using event-related desynchronization (ERD) to observe the responses during the movement or movement intention of the hand for the conditions of control, imagination, and the MVF with the mirror box. We constituted two groups: group 1: control, imagination, and MVF; group 2: control, MVF, and imagination. There were significant differences in imagination conditions between groups using MVF before or after imagination (right-hand, P = 0.0403; left-hand, P = 0.00939). The illusion of movement through MVF is not possible in all subjects, but even in those cases, we found an increase in imagination when the subject used the MVF previously. The increase in the r2s of imagination in the right and left hands suggests cross-learning. The increase in motor imagery recorded with EEG after MVF suggests that the mirror box made it easier to imagine movements. Our results provide evidence that the MVF could be used as a training tool to improve motor imagery.NEW & NOTEWORTHY The increase in motor imagery recorded with EEG after MVF (mirror visual feedback) suggests that the mirror box made it easier to imagine movements. Our results demonstrate that MVF could be used as a training tool to improve motor imagery.

Mirror visual feedback during unilateral finger movements is related to the desynchronization of cortical electroencephalographic somatomotor alpha rhythms

Using a mirror adequately oriented, the motion of just one hand induces the illusion of the movement with the other hand. Here, we tested the hypothesis that such a mirror phenomenon may be underpinned by an electroencephalographic (EEG) event-related desynchronization/synchronization (ERD/ERS) of central alpha rhythms (around 10 Hz) as a neurophysiological measure of the interactions among cerebral cortex, basal ganglia, and thalamus during movement preparation and execution. Eighteen healthy right-handed male participants performed standard auditory-triggered unilateral (right) or bilateral finger movements in the No Mirror (M-) conditions. In the Mirror (M+) condition, the unilateral right finger movements were performed in front of a mirror oriented to induce the illusion of simultaneous left finger movements. EEG activity was recorded from 64 scalp electrodes, and the artifact-free event-related EEG epochs were used to compute alpha ERD. In the M- conditions, a bilateral prominent central alpha ERD was observed during the bilateral movements, while left central alpha ERD and right alpha ERS were seen during unilateral right movements. In contrast, the M+ condition showed significant bilateral and widespread alpha ERD during the unilateral right movements. These results suggest that the above illusion of the left movements may be related to alpha ERD measures reflecting excitatory desynchronizing signals in right lateral premotor and primary somatomotor areas possibly in relation to basal ganglia-thalamic loops.

Role of kinaesthetic motor imagery in mirror-induced visual illusion as intervention in post-stroke rehabilitation

Mirror-induced visual illusion obtained through mirror therapy is widely used to facilitate motor recovery after stroke. Activation of primary motor cortex (M1) ipsilateral to the moving limb has been reported during mirror-induced visual illusion. However, the mechanism through which the mirror illusion elicits motor execution processes without movements observed in the mirrored limb remains unclear. This study aims to review evidence based on brain imaging studies for testing the hypothesis that neural processes associated with kinaesthetic motor imagery are attributed to ipsilateral M1 activation. Four electronic databases were searched. Studies on functional brain imaging, investigating the instant effects of mirror-induced visual illusion among stroke survivors and healthy participants were included. Thirty-five studies engaging 78 stroke survivors and 396 healthy participants were reviewed. Results of functional brain scans (n = 20) indicated that half of the studies (n = 10, 50%) reported significant changes in the activation of ipsilateral M1, which mediates motor preparation and execution. Other common neural substrates included primary somatosensory cortex (45%, kinaesthesia), precuneus (40%, image generation and self-processing operations) and cerebellum (20%, motor control). Similar patterns of ipsilateral M1 activations were observed in the two groups. These neural substrates mediated the generation, maintenance, and manipulation of motor-related images, which were the key processes in kinaesthetic motor imagery. Relationships in terms of shared neural substrates and mental processes between mirror-induced visual illusion and kinaesthetic motor imagery generate new evidence on the role of the latter in mirror therapy. Future studies should investigate the imagery processes in illusion training for post-stroke patients.

EEG Based Analysis of Cortical Activity during Mirror Visual Feedback Target-Directed Movement

In mirror visual feedback (MVF) based rehabilitation, the illusion of mirror reflection of volitional movement of non-paretic limb tends to have a modulatory effect on visuomotor and sensorimotor brain activations. This paper presents EEG based analysis of hemispheric activation asymmetry within the beta band (15-28 Hz) when MVF is combined with a target-directed hand motor task in a block design versus a similar task without any target requirements. MVF coupled with target-directed movement was associated with a decrease in hemispheric asymmetry in both preparation and execution phases of movement. These results emphasize the potential importance of incorporating visuomotor goals into the task to maximize the rehabilitation outcomes of MVF-based training activities.

Effects of virtual reality therapy on upper limb function after stroke and the role of neuroimaging as a predictor of a better response

OBJECTIVE

Virtual reality therapy (VRT) is an interactive intervention that induces neuroplasticity. The aim was to evaluate the effects of VRT associated with conventional rehabilitation for an upper limb after stroke, and the neuroimaging predictors of a better response to VRT.

METHODS

Patients with stroke were selected, and clinical neurological, upper limb function, and quality of life were evaluated. Statistical analysis was performed using a linear model comparing pre- and post-VRT. Lesions were segmented in the post-stroke computed tomography. A voxel-based lesion-symptom mapping approach was used to investigate the relationship between the lesion and upper limb function.

RESULTS

Eighteen patients were studied (55.5 ± 13.9 years of age). Quality of life, functional independence, and dexterity of the upper limb showed improvement after VRT (p < 0.001). Neuroimaging analysis showed negative correlations between the internal capsule lesion and functional recovery.

CONCLUSION

VRT showed benefits for patients with stroke, but when there was an internal capsule lesion, a worse response was observed.

Cortical Functional Domains Show Distinctive Oscillatory Dynamic in Bimanual and Mirror Visual Feedback Tasks

It is believed that Mirror Visual Feedback (MVF) increases the interlimb transfer but the exact mechanism is still a matter of debate. The aim of this study was to compare between a bimanual task (BM) and a MVF task, within functionally rather than geometrically defined cortical domains. Measure Projection Analysis (MPA) approach was applied to compare the dynamic oscillatory activity (event-related synchronization/desynchronization ERS/ERD) between and within domains. EEG was recorded in 14 healthy participants performing a BM and an MVF task with the right hand. The MPA was applied on fitted equivalent current dipoles based on independent components to define domains containing functionally similar areas. The measure of intradomain similarity was a "signed mutual information," a parameter based on the coherence. Domain analysis was performed for joint tasks (BM and MVF) and for each task separately. MVF created 9 functional domains while MB task had only 4 functionally distinctive domains, two over the left hemispheres and two bilateraly. For all domains identified for BM task alone, similar domains could be identified in MVF and joint tasks analysis. In addition MVF had domains related to motor planning on the right hemisphere and to self-recognition of action. For joint tasks analysis, seven domains were identified, with similar functions for the left and the right hand with exception of a domain covering BA32 (self-recognition of action) of the left hand only. In joint task domain analysis, the ERD/ERS showed a larger difference between domains than between tasks. All domains which involved the sensory cortex had a visible beta ERS at the onset of movement, and post movement beta ERS. The frequency of ERD varied between domains. Largest difference between tasks existed in domains responsible for the awareness of action. In conclusion, functionally distinctive domains have different ERD/ERS patterns, similar for both tasks. MVF activates contralateral hemisphere in similar manner to BM movements, while at the same time also activating the ipsilateral hemisphere. Significance: Following stroke cortical activation and interhemispheric inhibition from the contralesional side is reduced. MVF creates stronger ipsilateral activity than BM, which is highly relevant of neurorehabilitation of movements.

Supplementary motor area plays a causal role in automatic inhibition of motor responses

BACKGROUND

The masked-priming paradigm is used to test unconscious inhibitory processes of the brain. A tendency towards responses that are incompatible with the prime, designated as negative compatibility effect (NCE), emerges when the perception of a priming visual stimulus is "masked" afterwards. This effect presumably stems from a subliminal inhibitory process against the masked-prime. Prior lesions as well as activation studies suggest a key role of SMA in this effect.

OBJECTIVE

This study was conducted to elucidate a causal role of SMA in the subliminal response inhibition represented by the NCE.

METHODS

Using a repeated-measures pre-post design with a group of healthy people, physiological measures (resting and active motor thresholds and motor evoked potential (MEP) amplitude) and behavioral ones (choice reaction time (CRT), positive compatibility effect (PCE) and NCE) were obtained before and after three quadripulse stimulation (QPS), namely sham, M1-QPS, and SMA-QPS, on different days. CRT and PCE served as indices for different aspects of motor execution.

RESULTS

Motor thresholds were not altered after any QPS, although the M1-QPS increased MEP amplitude. Neither CRT nor PCE was altered significantly after QPS protocols. NCE was abolished after the SMA-QPS.

CONCLUSIONS

Abolished NCE after the SMA-QPS in the absence of MEP changes suggests that (1) SMA plays a cardinal role in the NCE, and (2) the network involved in NCE is different from that of MEP generation.

Neurophysiological effects of mirror visual feedback in stroke patients with unilateral hemispheric damage

BACKGROUND AND OBJECTIVE

Mirror visual feedback (MVF; the illusory perception of movement in one hand upon viewing the moving opposite hand in a midsagittal mirror) is thought to facilitate restoration of mal-adaptive neurophysiological processes underlying conditions like complex regional pain syndrome and phantom limb pain, and to have a positive effect on brain plasticity processes underlying motor recovery after stroke. However, its exact mode of action remains unclear. The aim of the current study was to explore the immediate neurophysiological effects of MVF in patients with stroke-related hemiparesis. We also investigated how these effects relate to lesion location and extent.

METHOD

EEG and EMG data were obtained from 14 first-event sub-acute stroke patients (8 with right-, 6 with left-hemiparesis) during repeated wrist extension movements of the Non-paretic Upper-Limb (NUL), without (NUL/M-) and with (NUL/M+) a midsagittal mirror, as well as during bilateral movements with a mirror (Bil/M+). EEG data was correlated with normalized lesion data obtained from follow-up CT scans.

RESULTS

NUL movement was accompanied by an asymmetric event-related de-synchronization (ERD) of low-beta EEG oscillations, with a more conspicuous ERD in the non-affected hemisphere. In the mirror condition, ERD magnitude was attenuated in both hemispheres. Stronger attenuation in the non-affected hemisphere abolished the hemispheric asymmetry. ERD attenuation by the mirror was affected by lesion side, the severity of hemiparesis and by lesion location and extent.

CONCLUSION

Following hemispheric stroke, the magnitude of low-beta ERD accompanying unilateral movement of the non-involved upper limb, and its hemispheric asymmetry, are both reduced by MVF. Low-beta ERD dynamics may serve as a marker of neurophysiological response to MVF in research aimed to elucidate the factors influencing patients' clinical gain from this treatment.

The Effects of Mirror Feedback during Target Directed Movements on Ipsilateral Corticospinal Excitability

Mirror visual feedback (MVF) training is a promising technique to promote activation in the lesioned hemisphere following stroke, and aid recovery. However, current outcomes of MVF training are mixed, in part, due to variability in the task undertaken during MVF. The present study investigated the hypothesis that movements directed toward visual targets may enhance MVF modulation of motor cortex (M1) excitability ipsilateral to the trained hand compared to movements without visual targets. Ten healthy subjects participated in a 2 × 2 factorial design in which feedback (veridical, mirror) and presence of a visual target (target present, target absent) for a right index-finger flexion task were systematically manipulated in a virtual environment. To measure M1 excitability, transcranial magnetic stimulation (TMS) was applied to the hemisphere ipsilateral to the trained hand to elicit motor evoked potentials (MEPs) in the untrained first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles at rest prior to and following each of four 2-min blocks of 30 movements (B1–B4). Targeted movement kinematics without visual feedback was measured before and after training to assess learning and transfer. FDI MEPs were decreased in B1 and B2 when movements were made with veridical feedback and visual targets were absent. FDI MEPs were decreased in B2 and B3 when movements were made with mirror feedback and visual targets were absent. FDI MEPs were increased in B3 when movements were made with mirror feedback and visual targets were present. Significant MEP changes were not present for the uninvolved ADM, suggesting a task-specific effect. Analysis of kinematics revealed learning occurred in visual target-directed conditions, but transfer was not sensitive to mirror feedback. Results are discussed with respect to current theoretical mechanisms underlying MVF-induced changes in ipsilateral excitability.

Potential determinants of efficacy of mirror therapy in stroke patients--A pilot study

BACKGROUND

Mirror therapy (MT) was found to improve motor function after stroke. However, there is high variability between patients regarding motor recovery.

OBJECTIVES

The following pilot study was designed to identify potential factors determining this variability between patients with severe upper limb paresis, receiving MT.

METHODS

Eleven sub-acute stroke patients with severe upper limb paresis participated, receiving in-patient rehabilitation. After a set of pre-assessments (including measurement of brain activity at the primary motor cortex and precuneus during the mirror illusion, using near-infrared spectroscopy as described previously), four weeks of MT were applied, followed by a set of post-assessments. Discriminant group analysis for MT responders and non-responders was performed.

RESULTS

Six out of eleven patients were defined as responders and five as non-responders on the basis of their functional motor improvement. The initial motor function and the activity shift in both precunei (mirror index) were found to discriminate significantly between responders and non-responders.

CONCLUSIONS

In line with earlier results, initial motor function was confirmed as crucial determinant of motor recovery. Additionally, activity response to the mirror illusion in both precunei was found to be a candidate for determination of the efficacy of MT.

Reflections on Mirror Therapy

Background. Mirror visual feedback (MVF), a phenomenon where movement of one limb is perceived as movement of the other limb, has the capacity to alleviate phantom limb pain or promote motor recovery of the upper limbs after stroke. The tool has received great interest from health professionals; however, a clear understanding of the mechanisms underlying the neural recovery owing to MVF is lacking. Objective. We performed a systematic review to assess the effect of MVF on brain activation during a motor task. Methods. We searched PubMed, CINAHL, and EMBASE databases for neuroimaging studies investigating the effect of MVF on the brain. Key details for each study regarding participants, imaging methods, and results were extracted. Results. The database search yielded 347 article, of which we identified 33 suitable for inclusion. Compared with a control condition, MVF increases neural activity in areas involved with allocation of attention and cognitive control (dorsolateral prefrontal cortex, posterior cingulate cortex, S1 and S2, precuneus). Apart from activation in the superior temporal gyrus and premotor cortex, there is little evidence that MVF activates the mirror neuron system. MVF increases the excitability of the ipsilateral primary motor cortex (M1) that projects to the “untrained” hand/arm. There is also evidence for ipsilateral projections from the contralateral M1 to the untrained/affected hand as a consequence of training with MVF. Conclusion. MVF can exert a strong influence on the motor network, mainly through increased cognitive penetration in action control, though the variance in methodology and the lack of studies that shed light on the functional connectivity between areas still limit insight into the actual underlying mechanisms.

Bimanual coordination with three hands: is the mirror hand of any help?

The mirror paradigm has been used extensively both as a research tool for studying kinesthesia in healthy individuals and as a therapeutic tool for improving recovery and/or alleviating symptoms in patients. The present study of healthy participants assessed the contribution of the mirror paradigm to motor control in a bimanual coordination task performed under sensorimotor disturbance conditions. In Experiment 1, the participants were required to produce symmetrical circles with both hands/arms at the same time. In Experiment 2, the task consisted of synchronous extension-flexion movements of both arms in the sagittal plane. These tasks were performed under four different visual conditions: (i) mirror vision (i.e. with the non-dominant arm reflected in a mirror--the third hand--and the dominant arm hidden), (ii) full vision (i.e. both arms visible), (iii) with only the non-dominant arm visible and (iv) with the eyes closed. In Experiments 1 and 2, sensorimotor disturbance was applied to the participant's dominant arm by co-vibrating antagonistic muscles (the biceps and the triceps). In the complex circle drawing task, bimanual performance was better in the mirror condition than when participants saw their non-dominant arm only. However, motor performance in the mirror vision condition was little better than in the eyes closed condition, regardless of whether or not sensorimotor disturbance was applied. In Experiment 2, there were no differences between the "eyes closed" and "mirror vision" conditions. Although mirror reflection of one arm has been shown to induce consistent, vivid, perceptual illusions (kinesthetic illusion), our results suggest that it is less effective in modulating motor behavior.