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Covariation of the amplitude and latency of motor evoked potentials elicited by transcranial magnetic stimulation in a resting hand muscle. Exp Brain Res 2023; 241:927-936. [PMID: 36811686 PMCID: PMC9985579 DOI: 10.1007/s00221-023-06575-z] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/02/2022] [Accepted: 02/13/2023] [Indexed: 02/24/2023]
Abstract
Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation technique used to study human neurophysiology. A single TMS pulse delivered to the primary motor cortex can elicit a motor evoked potential (MEP) in a target muscle. MEP amplitude is a measure of corticospinal excitability and MEP latency is a measure of the time taken for intracortical processing, corticofugal conduction, spinal processing, and neuromuscular transmission. Although MEP amplitude is known to vary across trials with constant stimulus intensity, little is known about MEP latency variation. To investigate MEP amplitude and latency variation at the individual level, we scored single-pulse MEP amplitude and latency in a resting hand muscle from two datasets. MEP latency varied from trial to trial in individual participants with a median range of 3.9 ms. Shorter MEP latencies were associated with larger MEP amplitudes for most individuals (median r = - 0.47), showing that latency and amplitude are jointly determined by the excitability of the corticospinal system when TMS is delivered. TMS delivered during heightened excitability could discharge a greater number of cortico-cortical and corticospinal cells, increasing the amplitude and, by recurrent activation of corticospinal cells, the number of descending indirect waves. An increase in the amplitude and number of indirect waves would progressively recruit larger spinal motor neurons with large-diameter fast-conducting fibers, which would shorten MEP onset latency and increase MEP amplitude. In addition to MEP amplitude variability, understanding MEP latency variability is important given that these parameters are used to help characterize pathophysiology of movement disorders.
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2
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Kern K, Vukelić M, Guggenberger R, Gharabaghi A. Oscillatory neurofeedback networks and poststroke rehabilitative potential in severely impaired stroke patients. Neuroimage Clin 2023; 37:103289. [PMID: 36525745 PMCID: PMC9791174 DOI: 10.1016/j.nicl.2022.103289] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2020] [Revised: 12/03/2022] [Accepted: 12/12/2022] [Indexed: 12/15/2022]
Abstract
Motor restoration after severe stroke is often limited. However, some of the severely impaired stroke patients may still have a rehabilitative potential. Biomarkers that identify these patients are sparse. Eighteen severely impaired chronic stroke patients with a lack of volitional finger extension participated in an EEG study. During sixty-six trials of kinesthetic motor imagery, a brain-machine interface turned event-related beta-band desynchronization of the ipsilesional sensorimotor cortex into opening of the paralyzed hand by a robotic orthosis. A subgroup of eight patients participated in a subsequent four-week rehabilitation training. Changes of the movement extent were captured with sensors which objectively quantified even discrete improvements of wrist movement. Albeit with the same motor impairment level, patients could be differentiated into two groups, i.e., with and without task-related increase of bilateral cortico-cortical phase synchronization between frontal/premotor and parietal areas. This fronto-parietal integration (FPI) was associated with a significantly higher volitional beta modulation range in the ipsilesional sensorimotor cortex. Following the four-week training, patients with FPI showed significantly higher improvement in wrist movement than those without FPI. Moreover, only the former group improved significantly in the upper extremity Fugl-Meyer-Assessment score. Neurofeedback-related long-range oscillatory coherence may differentiate severely impaired stroke patients with regard to their rehabilitative potential, a finding that needs to be confirmed in larger patient cohorts.
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Affiliation(s)
- Kevin Kern
- Institute for Neuromodulation and Neurotechnology, University of Tübingen, Germany
| | - Mathias Vukelić
- Institute for Neuromodulation and Neurotechnology, University of Tübingen, Germany
| | - Robert Guggenberger
- Institute for Neuromodulation and Neurotechnology, University of Tübingen, Germany
| | - Alireza Gharabaghi
- Institute for Neuromodulation and Neurotechnology, University of Tübingen, Germany.
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3
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Veldema J, Gharabaghi A. Non-invasive brain stimulation for improving gait, balance, and lower limbs motor function in stroke. J Neuroeng Rehabil 2022; 19:84. [PMID: 35922846 PMCID: PMC9351139 DOI: 10.1186/s12984-022-01062-y] [Citation(s) in RCA: 30] [Impact Index Per Article: 15.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2022] [Accepted: 07/21/2022] [Indexed: 11/27/2022] Open
Abstract
Objectives This systematic review and meta-analysis aim to summarize and analyze the available evidence of non-invasive brain stimulation/spinal cord stimulation on gait, balance and/or lower limb motor recovery in stroke patients. Methods The PubMed database was searched from its inception through to 31/03/2021 for randomized controlled trials investigating repetitive transcranial magnetic stimulation or transcranial/trans-spinal direct current/alternating current stimulation for improving gait, balance and/or lower limb motor function in stroke patients. Results Overall, 25 appropriate studies (including 657 stroke subjects) were found. The data indicates that non-invasive brain stimulation/spinal cord stimulation is effective in supporting recovery. However, the effects are inhomogeneous across studies: (1) transcranial/trans-spinal direct current/alternating current stimulation induce greater effects than repetitive transcranial magnetic stimulation, and (2) bilateral application of non-invasive brain stimulation is superior to unilateral stimulation. Conclusions The current evidence encourages further research and suggests that more individualized approaches are necessary for increasing effect sizes in stroke patients.
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Affiliation(s)
- Jitka Veldema
- Department of Sport Science, Bielefeld University, 33 501, Bielefeld, Germany. .,Institute for Neuromodulation and Neurotechnology, University Hospital and University of Tübingen, Tübingen, Germany.
| | - Alireza Gharabaghi
- Institute for Neuromodulation and Neurotechnology, University Hospital and University of Tübingen, Tübingen, Germany
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4
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Kudo D, Koseki T, Katagiri N, Yoshida K, Takano K, Jin M, Nito M, Tanabe S, Yamaguchi T. Individualized beta-band oscillatory transcranial direct current stimulation over the primary motor cortex enhances corticomuscular coherence and corticospinal excitability in healthy individuals. Brain Stimul 2021; 15:46-52. [PMID: 34742996 DOI: 10.1016/j.brs.2021.11.004] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2021] [Revised: 10/28/2021] [Accepted: 11/01/2021] [Indexed: 12/14/2022] Open
Abstract
BACKGROUND Simultaneously modulating individual neural oscillation and cortical excitability may be important for enhancing communication between the primary motor cortex and spinal motor neurons, which plays a key role in motor control. However, it is unknown whether individualized beta-band oscillatory transcranial direct current stimulation (otDCS) enhances corticospinal oscillation and excitability. OBJECTIVE This study investigated the effects of individualized beta-band otDCS on corticomuscular coherence (CMC) and corticospinal excitability in healthy individuals. METHODS In total, 29 healthy volunteers participated in separate experiments. They received the following stimuli for 10 min on different days: 1) 2-mA otDCS with individualized beta-band frequencies, 2) 2-mA transcranial alternating current stimulation (tACS) with individualized beta-band frequencies, and 3) 2-mA transcranial direct current stimulation (tDCS). The changes in CMC between the vertex and tibialis anterior (TA) muscle and TA muscle motor-evoked potentials (MEPs) were assessed before and after (immediately, 10 min, and 20 min after) stimulation on different days. Additionally, 20-Hz otDCS for 10 min was applied to investigate the effects of a fixed beta-band frequency on CMC. RESULTS otDCS significantly increased CMC and MEPs immediately after stimulation, whereas tACS and tDCS had no effects. There was a significant negative correlation between normalized CMC changes in response to 20-Hz otDCS and the numerical difference between the 20-Hz and individualized CMC peak frequency before the stimulation. CONCLUSIONS These findings suggest that simultaneous modulation of neural oscillation and cortical excitability is critical for enhancing corticospinal communication. Individualized otDCS holds potential as a useful method in the field of neurorehabilitation.
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Affiliation(s)
- Daisuke Kudo
- Department of Physical Therapy, Yamagata Prefectural University of Health Sciences, 260 Kamiyanagi, Yamagata-shi, Yamagata, 990-2212, Japan; Department of Physical Therapy, Graduate School of Health Sciences, Yamagata Prefectural University of Health Sciences, 260 Kamiyanagi, Yamagata-shi, Yamagata, 990-2212, Japan.
| | - Tadaki Koseki
- Department of Physical Therapy, Graduate School of Health Sciences, Yamagata Prefectural University of Health Sciences, 260 Kamiyanagi, Yamagata-shi, Yamagata, 990-2212, Japan.
| | - Natsuki Katagiri
- Department of Physical Therapy, Graduate School of Health Sciences, Yamagata Prefectural University of Health Sciences, 260 Kamiyanagi, Yamagata-shi, Yamagata, 990-2212, Japan.
| | - Kaito Yoshida
- Department of Physical Therapy, Graduate School of Health Sciences, Yamagata Prefectural University of Health Sciences, 260 Kamiyanagi, Yamagata-shi, Yamagata, 990-2212, Japan.
| | - Keita Takano
- Department of Physical Therapy, Graduate School of Health Sciences, Yamagata Prefectural University of Health Sciences, 260 Kamiyanagi, Yamagata-shi, Yamagata, 990-2212, Japan.
| | - Masafumi Jin
- Department of Physical Therapy, Graduate School of Health Sciences, Yamagata Prefectural University of Health Sciences, 260 Kamiyanagi, Yamagata-shi, Yamagata, 990-2212, Japan.
| | - Mitsuhiro Nito
- Department of Anatomy and Structural Science, Yamagata University School of Medicine, 2-2-2 Iida-Nishi, Yamagata, 990-9585, Japan.
| | - Shigeo Tanabe
- Faculty of Rehabilitation, School of Health Sciences, Fujita Health University, 1-98 Dengakugakubo, Kutsukake-cho, Toyoake-shi, Aichi, 470-1192, Japan.
| | - Tomofumi Yamaguchi
- Department of Physical Therapy, Faculty of Health Science, Juntendo University, 2-1-1 Hongo, Bunkyo-ku, Tokyo, 113-8421, Japan.
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5
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Schilberg L, Ten Oever S, Schuhmann T, Sack AT. Phase and power modulations on the amplitude of TMS-induced motor evoked potentials. PLoS One 2021; 16:e0255815. [PMID: 34529682 PMCID: PMC8445484 DOI: 10.1371/journal.pone.0255815] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2021] [Accepted: 07/23/2021] [Indexed: 11/30/2022] Open
Abstract
The evaluation of transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) promises valuable information about fundamental brain related mechanisms and may serve as a diagnostic tool for clinical monitoring of therapeutic progress or surgery procedures. However, reports about spontaneous fluctuations of MEP amplitudes causing high intra-individual variability have led to increased concerns about the reliability of this measure. One possible cause for high variability of MEPs could be neuronal oscillatory activity, which reflects fluctuations of membrane potentials that systematically increase and decrease the excitability of neuronal networks. Here, we investigate the dependence of MEP amplitude on oscillation power and phase by combining the application of single pulse TMS over the primary motor cortex with concurrent recordings of electromyography and electroencephalography. Our results show that MEP amplitude is correlated to alpha phase, alpha power as well as beta phase. These findings may help explain corticospinal excitability fluctuations by highlighting the modulatory effect of alpha and beta phase on MEPs. In the future, controlling for such a causal relationship may allow for the development of new protocols, improve this method as a (diagnostic) tool and increase the specificity and efficacy of general TMS applications.
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Affiliation(s)
- Lukas Schilberg
- Section Brain Stimulation and Cognition, Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - Sanne Ten Oever
- Section Brain Stimulation and Cognition, Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
- Language and Computation in Neural Systems Group, Max Planck Institute for Psycholinguistics, Nijmegen, The Netherlands
- Donders Institute for Neuroimaging, Radboud University, Nijmegen, The Netherlands
| | - Teresa Schuhmann
- Section Brain Stimulation and Cognition, Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
- Maastricht Brain Imaging Centre (MBIC), Maastricht University, Maastricht, The Netherlands
- Faculty of Psychology and Neuroscience, Faculty of Health, Medicine and Life Sciences, Centre for Integrative Neuroscience, Maastricht University, Maastricht, The Netherlands
| | - Alexander T. Sack
- Section Brain Stimulation and Cognition, Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
- Maastricht Brain Imaging Centre (MBIC), Maastricht University, Maastricht, The Netherlands
- Faculty of Psychology and Neuroscience, Faculty of Health, Medicine and Life Sciences, Centre for Integrative Neuroscience, Maastricht University, Maastricht, The Netherlands
- School for Mental Health and Neuroscience (MHeNS), Department of Psychiatry and Neuropsychology, Faculty of Health, Medicine and Life Sciences, Maastricht University Medical Centre, Maastricht, The Netherlands
- * E-mail:
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6
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Rurak BK, Rodrigues JP, Power BD, Drummond PD, Vallence AM. Test Re-test Reliability of Dual-site TMS Measures of SMA-M1 Connectivity Differs Across Inter-stimulus Intervals in Younger and Older Adults. Neuroscience 2021; 472:11-24. [PMID: 34333064 DOI: 10.1016/j.neuroscience.2021.07.023] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2021] [Revised: 07/21/2021] [Accepted: 07/26/2021] [Indexed: 12/24/2022]
Abstract
Dual-site transcranial magnetic stimulation (TMS) is a promising tool to measure supplementary motor area and primary motor cortex (SMA-M1) connectivity in younger and older adults, and could be used to understand the pathophysiology of movement disorders. However, test re-test reliability of dual-site TMS measures of SMA-M1 connectivity has not been established. We examined the reliability of SMA-M1 connectivity using dual-site TMS in two sessions in 30 younger and 30 older adults. For dual-site TMS, a conditioning pulse delivered to SMA (140% of active motor threshold) preceded a test pulse delivered to M1 (intensity that elicited MEPs of ~1 mV) by inter-stimulus intervals (ISI) of 6 ms, 7 ms, and 8 ms. Moderate intraclass correlation coefficients (ICC) were found for SMA-M1 connectivity at an ISI of 7 ms in younger (ICC: 0.69) and older adults (ICC: 0.68). Poor ICCs were found for SMA-M1 connectivity at ISIs of 6 ms and 8 ms in both age groups (ICC range: 0.01-0.40). We report evidence for stable measures of SMA-M1 connectivity at an ISI of 7 ms in both age groups. These findings are foundational for future research developing evidence-based interventions to strengthen SMA-M1 connectivity to improve bilateral motor control in older adults and populations with movement disorders.
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Affiliation(s)
- B K Rurak
- Discipline of Psychology, College of Science, Health, Engineering and Education, Murdoch University, Australia; Centre for Healthy Ageing, Health Futures Institute, Murdoch University, Murdoch 6150, Australia.
| | | | - B D Power
- Hollywood Private Hospital, Australia; School of Medicine Fremantle, University of Notre Dame, Australia
| | - P D Drummond
- Discipline of Psychology, College of Science, Health, Engineering and Education, Murdoch University, Australia; Centre for Healthy Ageing, Health Futures Institute, Murdoch University, Murdoch 6150, Australia
| | - A M Vallence
- Discipline of Psychology, College of Science, Health, Engineering and Education, Murdoch University, Australia; Centre for Healthy Ageing, Health Futures Institute, Murdoch University, Murdoch 6150, Australia; Centre for Molecular Medicine and Innovative Therapeutics, Murdoch 6150, Australia
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7
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Wang X, Zhang Y, Zhang K, Yuan Y. Influence of behavioral state on the neuromodulatory effect of low-intensity transcranial ultrasound stimulation on hippocampal CA1 in mouse. Neuroimage 2021; 241:118441. [PMID: 34339832 DOI: 10.1016/j.neuroimage.2021.118441] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2020] [Revised: 04/25/2021] [Accepted: 07/29/2021] [Indexed: 01/21/2023] Open
Abstract
In process of brain stimulation, the influence of any external stimulus depends on the features of the stimulus and the initial state of the brain. Understanding the state-dependence of brain stimulation is very important. However, it remains unclear whether neural activity induced by ultrasound stimulation is modulated by the behavioral state. We used low-intensity focused ultrasound to stimulate the hippocampal CA1 regions of mice with different behavioral states (anesthesia, awake, and running) and recorded the neural activity in the target area before and after stimulation. We found the following: (1) there were different spike firing rates and response delays computed as the time to reach peak for all behavioral states; (2) the behavioral state significantly modulates the spike firing rate linearly increased with an increase in ultrasound intensity under different behavioral states; (3) the mean power of local field potential induced by TUS significantly increased under anesthesia and awake states; (4) ultrasound stimulation enhanced phase-locking between spike and ripple oscillation under anesthesia state. These results suggest that ultrasound stimulation-induced neural activity is modulated by the behavioral state. Our study has great potential benefits for the application of ultrasound stimulation in neuroscience.
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Affiliation(s)
- Xingran Wang
- School of Electrical Engineering, Yanshan University, No.438, Hebei Street, Qinhuangdao 066004, China; Key Laboratory of Intelligent Rehabilitation and Neuromodulation of Hebei Province, Yanshan University, Qinhuangdao 066004, China
| | - Yiyao Zhang
- Neuroscience Institute, NYU Langone Health, New York 10016, USA
| | - Kaiqing Zhang
- School of Electrical Engineering, Yanshan University, No.438, Hebei Street, Qinhuangdao 066004, China; Key Laboratory of Intelligent Rehabilitation and Neuromodulation of Hebei Province, Yanshan University, Qinhuangdao 066004, China
| | - Yi Yuan
- School of Electrical Engineering, Yanshan University, No.438, Hebei Street, Qinhuangdao 066004, China; Key Laboratory of Intelligent Rehabilitation and Neuromodulation of Hebei Province, Yanshan University, Qinhuangdao 066004, China.
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8
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Nakazono H, Ogata K, Takeda A, Yamada E, Oka S, Tobimatsu S. A specific phase of transcranial alternating current stimulation at the β frequency boosts repetitive paired-pulse TMS-induced plasticity. Sci Rep 2021; 11:13179. [PMID: 34162993 PMCID: PMC8222330 DOI: 10.1038/s41598-021-92768-x] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2020] [Accepted: 06/09/2021] [Indexed: 11/09/2022] Open
Abstract
Transcranial alternating current stimulation (tACS) at 20 Hz (β) has been shown to modulate motor evoked potentials (MEPs) when paired with transcranial magnetic stimulation (TMS) in a phase-dependent manner. Repetitive paired-pulse TMS (rPPS) with I-wave periodicity (1.5 ms) induced short-lived facilitation of MEPs. We hypothesized that tACS would modulate the facilitatory effects of rPPS in a frequency- and phase-dependent manner. To test our hypothesis, we investigated the effects of combined tACS and rPPS. We applied rPPS in combination with peak or trough phase tACS at 10 Hz (α) or β, or sham tACS (rPPS alone). The facilitatory effects of rPPS in the sham condition were temporary and variable among participants. In the β tACS peak condition, significant increases in single-pulse MEPs persisted for over 30 min after the stimulation, and this effect was stable across participants. In contrast, β tACS in the trough condition did not modulate MEPs. Further, α tACS parameters did not affect single-pulse MEPs after the intervention. These results suggest that a rPPS-induced increase in trans-synaptic efficacy could be strengthened depending on the β tACS phase, and that this technique could produce long-lasting plasticity with respect to cortical excitability.
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Affiliation(s)
- Hisato Nakazono
- Department of Clinical Neurophysiology, Neurological Institute, Faculty of Medicine, Graduate School of Medical Sciences, Kyushu University, Fukuoka, 812-8582, Japan. .,Department of Occupational Therapy, Faculty of Medical Science, Fukuoka International University of Health and Welfare, Fukuoka, 814-0001, Japan.
| | - Katsuya Ogata
- Department of Clinical Neurophysiology, Neurological Institute, Faculty of Medicine, Graduate School of Medical Sciences, Kyushu University, Fukuoka, 812-8582, Japan.,Department of Pharmaceutical Sciences, School of Pharmacy at Fukuoka, International University of Health and Welfare, Fukuoka, 831-8501, Japan
| | - Akinori Takeda
- Department of Clinical Neurophysiology, Neurological Institute, Faculty of Medicine, Graduate School of Medical Sciences, Kyushu University, Fukuoka, 812-8582, Japan.,Research Center for Brain Communication, Research Institute, Kochi University of Technology, Kochi, 782-8502, Japan
| | - Emi Yamada
- Department of Clinical Neurophysiology, Neurological Institute, Faculty of Medicine, Graduate School of Medical Sciences, Kyushu University, Fukuoka, 812-8582, Japan.,Department of Linguistics, Faculty of Humanities, Kyushu University, Fukuoka, 819-0395, Japan
| | - Shinichiro Oka
- Department of Physical Therapy, School of Health Sciences at Fukuoka, International University of Health and Welfare, Fukuoka, 831-8501, Japan
| | - Shozo Tobimatsu
- Department of Clinical Neurophysiology, Neurological Institute, Faculty of Medicine, Graduate School of Medical Sciences, Kyushu University, Fukuoka, 812-8582, Japan.,Department of Orthoptics, Faculty of Medical Science, Fukuoka International University of Health and Welfare, Fukuoka, 814-0001, Japan
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9
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Naros G, Lehnertz T, Leão MT, Ziemann U, Gharabaghi A. Brain State-dependent Gain Modulation of Corticospinal Output in the Active Motor System. Cereb Cortex 2021; 30:371-381. [PMID: 31204431 DOI: 10.1093/cercor/bhz093] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2018] [Revised: 03/18/2019] [Accepted: 04/10/2019] [Indexed: 01/17/2023] Open
Abstract
The communication through coherence hypothesis suggests that only coherently oscillating neuronal groups can interact effectively and predicts an intrinsic response modulation along the oscillatory rhythm. For the motor cortex (MC) at rest, the oscillatory cycle has been shown to determine the brain's responsiveness to external stimuli. For the active MC, however, the demonstration of such a phase-specific modulation of corticospinal excitability (CSE) along the rhythm cycle is still missing. Motor evoked potentials in response to transcranial magnetic stimulation (TMS) over the MC were used to probe the effect of cortical oscillations on CSE during several motor conditions. A brain-machine interface (BMI) with a robotic hand orthosis allowed investigating effects of cortical activity on CSE without the confounding effects of voluntary muscle activation. Only this BMI approach (and not active or passive hand opening alone) revealed a frequency- and phase-specific cortical modulation of CSE by sensorimotor beta-band activity that peaked once per oscillatory cycle and was independent of muscle activity. The active MC follows an intrinsic response modulation in accordance with the communication through coherence hypothesis. Furthermore, the BMI approach may facilitate and strengthen effective corticospinal communication in a therapeutic context, for example, when voluntary hand opening is no longer possible after stroke.
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Affiliation(s)
- Georgios Naros
- Division of Functional and Restorative Neurosurgery, and Tuebingen NeuroCampus, Eberhard Karls University Tuebingen, 72076 Tuebingen, Germany
| | - Tobias Lehnertz
- Division of Functional and Restorative Neurosurgery, and Tuebingen NeuroCampus, Eberhard Karls University Tuebingen, 72076 Tuebingen, Germany
| | - Maria Teresa Leão
- Division of Functional and Restorative Neurosurgery, and Tuebingen NeuroCampus, Eberhard Karls University Tuebingen, 72076 Tuebingen, Germany
| | - Ulf Ziemann
- Department of Neurology and Stroke, and Hertie Institute for Clinical Brain Research, Eberhard Karls University Tuebingen, 72076 Tuebingen, Germany
| | - Alireza Gharabaghi
- Division of Functional and Restorative Neurosurgery, and Tuebingen NeuroCampus, Eberhard Karls University Tuebingen, 72076 Tuebingen, Germany
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10
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Vallence AM, Dansie K, Goldsworthy MR, McAllister SM, Yang R, Rothwell JC, Ridding MC. Examining motor evoked potential amplitude and short-interval intracortical inhibition on the up-going and down-going phases of a transcranial alternating current stimulation (tacs) imposed alpha oscillation. Eur J Neurosci 2021; 53:2755-2762. [PMID: 33480046 DOI: 10.1111/ejn.15124] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2020] [Revised: 12/19/2020] [Accepted: 01/17/2021] [Indexed: 01/18/2023]
Abstract
Many brain regions exhibit rhythmical activity thought to reflect the summed behaviour of large populations of neurons. The endogenous alpha rhythm has been associated with phase-dependent modulation of corticospinal excitability. However, whether exogenous alpha rhythm, induced using transcranial alternating current stimulation (tACS) also has a phase-dependent effect on corticospinal excitability remains unknown. Here, we triggered transcranial magnetic stimuli (TMS) on the up- or down-going phase of a tACS-imposed alpha oscillation and measured motor evoked potential (MEP) amplitude and short-interval intracortical inhibition (SICI). There was no significant difference in MEP amplitude or SICI when TMS was triggered on the up- or down-going phase of the tACS-imposed alpha oscillation. The current study provides no evidence of differences in corticospinal excitability or GABAergic inhibition when targeting the up-going (peak) and down-going (trough) phase of the tACS-imposed oscillation.
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Affiliation(s)
- Ann-Maree Vallence
- Discipline of Psychology, College of Science, Health, Engineering, and Education, Murdoch University, Perth, Australia
| | - Kathryn Dansie
- Australia and New Zealand Dialysis and Transplant Registry (ANZDATA), South Australian Health and Medical Research Institute (SAHMIR), Adelaide, South, Australia
| | - Mitchell R Goldsworthy
- Adelaide Medical School, University of Adelaide, Adelaide, Australia.,Hopwood Centre for Neurobiology, Lifelong Health Theme, South Australian Health and Medical Research Institute (SAHMRI), Adelaide, Australia
| | - Suzanne M McAllister
- Formerly of the Discipline of Physiology, School of Medical Science, University of Adelaide, Adelaide, Australia
| | | | - John C Rothwell
- Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, London, UK
| | - Michael C Ridding
- University of South Australia, IIMPACT in Health, Adelaide, Australia
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11
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Milosevic M, Marquez-Chin C, Masani K, Hirata M, Nomura T, Popovic MR, Nakazawa K. Why brain-controlled neuroprosthetics matter: mechanisms underlying electrical stimulation of muscles and nerves in rehabilitation. Biomed Eng Online 2020; 19:81. [PMID: 33148270 PMCID: PMC7641791 DOI: 10.1186/s12938-020-00824-w] [Citation(s) in RCA: 21] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2020] [Accepted: 10/10/2020] [Indexed: 12/11/2022] Open
Abstract
Delivering short trains of electric pulses to the muscles and nerves can elicit action potentials resulting in muscle contractions. When the stimulations are sequenced to generate functional movements, such as grasping or walking, the application is referred to as functional electrical stimulation (FES). Implications of the motor and sensory recruitment of muscles using FES go beyond simple contraction of muscles. Evidence suggests that FES can induce short- and long-term neurophysiological changes in the central nervous system by varying the stimulation parameters and delivery methods. By taking advantage of this, FES has been used to restore voluntary movement in individuals with neurological injuries with a technique called FES therapy (FEST). However, long-lasting cortical re-organization (neuroplasticity) depends on the ability to synchronize the descending (voluntary) commands and the successful execution of the intended task using a FES. Brain-computer interface (BCI) technologies offer a way to synchronize cortical commands and movements generated by FES, which can be advantageous for inducing neuroplasticity. Therefore, the aim of this review paper is to discuss the neurophysiological mechanisms of electrical stimulation of muscles and nerves and how BCI-controlled FES can be used in rehabilitation to improve motor function.
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Affiliation(s)
- Matija Milosevic
- Graduate School of Engineering Science, Department of Mechanical Science and Bioengineering, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka, 560-8531, Japan.
| | - Cesar Marquez-Chin
- Institute of Biomedical Engineering, University of Toronto, 164 College Street, Toronto, ON, M5S 3G9, Canada
- KITE Research Institute, Toronto Rehabilitation Institute - University Health Network, 520 Sutherland Drive, Toronto, ON, M4G 3V9, Canada
- CRANIA, University Health Network & University of Toronto, 550 University Avenue, Toronto, ON, M5G 2A2, Canada
| | - Kei Masani
- Institute of Biomedical Engineering, University of Toronto, 164 College Street, Toronto, ON, M5S 3G9, Canada
- KITE Research Institute, Toronto Rehabilitation Institute - University Health Network, 520 Sutherland Drive, Toronto, ON, M4G 3V9, Canada
- CRANIA, University Health Network & University of Toronto, 550 University Avenue, Toronto, ON, M5G 2A2, Canada
| | - Masayuki Hirata
- Department of Neurological Diagnosis and Restoration, Graduate School of Medicine, Osaka University, 2-2 Yamadaoka, Suita, Osaka, 565-0871, Japan
| | - Taishin Nomura
- Graduate School of Engineering Science, Department of Mechanical Science and Bioengineering, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka, 560-8531, Japan
| | - Milos R Popovic
- Institute of Biomedical Engineering, University of Toronto, 164 College Street, Toronto, ON, M5S 3G9, Canada
- KITE Research Institute, Toronto Rehabilitation Institute - University Health Network, 520 Sutherland Drive, Toronto, ON, M4G 3V9, Canada
- CRANIA, University Health Network & University of Toronto, 550 University Avenue, Toronto, ON, M5G 2A2, Canada
| | - Kimitaka Nakazawa
- Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo, 153-8902, Japan
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12
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Guggenberger R, Raco V, Gharabaghi A. State-Dependent Gain Modulation of Spinal Motor Output. Front Bioeng Biotechnol 2020; 8:523866. [PMID: 33117775 PMCID: PMC7561675 DOI: 10.3389/fbioe.2020.523866] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2019] [Accepted: 09/17/2020] [Indexed: 01/04/2023] Open
Abstract
Afferent somatosensory information plays a crucial role in modulating efferent motor output. A better understanding of this sensorimotor interplay may inform the design of neurorehabilitation interfaces. Current neurotechnological approaches that address motor restoration after trauma or stroke combine motor imagery (MI) and contingent somatosensory feedback, e.g., via peripheral stimulation, to induce corticospinal reorganization. These interventions may, however, change the motor output already at the spinal level dependent on alterations of the afferent input. Neuromuscular electrical stimulation (NMES) was combined with measurements of wrist deflection using a kinematic glove during either MI or rest. We investigated 360 NMES bursts to the right forearm of 12 healthy subjects at two frequencies (30 and 100 Hz) in random order. For each frequency, stimulation was assessed at nine intensities. Measuring the induced wrist deflection across different intensities allowed us to estimate the input-output curve (IOC) of the spinal motor output. MI decreased the slope of the IOC independent of the stimulation frequency. NMES with 100 Hz vs. 30 Hz decreased the threshold of the IOC. Human-machine interfaces for neurorehabilitation that combine MI and NMES need to consider bidirectional communication and may utilize the gain modulation of spinal circuitries by applying low-intensity, high-frequency stimulation.
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Affiliation(s)
- Robert Guggenberger
- Institute for Neuromodulation and Neurotechnology, Department of Neurosurgery and Neurotechnology, University of Tüebingen, Tüebingen, Germany
| | - Valerio Raco
- Institute for Neuromodulation and Neurotechnology, Department of Neurosurgery and Neurotechnology, University of Tüebingen, Tüebingen, Germany
| | - Alireza Gharabaghi
- Institute for Neuromodulation and Neurotechnology, Department of Neurosurgery and Neurotechnology, University of Tüebingen, Tüebingen, Germany
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13
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Javitt DC, Siegel SJ, Spencer KM, Mathalon DH, Hong LE, Martinez A, Ehlers CL, Abbas AI, Teichert T, Lakatos P, Womelsdorf T. A roadmap for development of neuro-oscillations as translational biomarkers for treatment development in neuropsychopharmacology. Neuropsychopharmacology 2020; 45:1411-1422. [PMID: 32375159 PMCID: PMC7360555 DOI: 10.1038/s41386-020-0697-9] [Citation(s) in RCA: 47] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/06/2019] [Revised: 03/16/2020] [Accepted: 04/27/2020] [Indexed: 02/08/2023]
Abstract
New treatment development for psychiatric disorders depends critically upon the development of physiological measures that can accurately translate between preclinical animal models and clinical human studies. Such measures can be used both as stratification biomarkers to define pathophysiologically homogeneous patient populations and as target engagement biomarkers to verify similarity of effects across preclinical and clinical intervention. Traditional "time-domain" event-related potentials (ERP) have been used translationally to date but are limited by the significant differences in timing and distribution across rodent, monkey and human studies. By contrast, neuro-oscillatory responses, analyzed within the "time-frequency" domain, are relatively preserved across species permitting more precise translational comparisons. Moreover, neuro-oscillatory responses are increasingly being mapped to local circuit mechanisms and may be useful for investigating effects of both pharmacological and neuromodulatory interventions on excitatory/inhibitory balance. The present paper provides a roadmap for development of neuro-oscillatory responses as translational biomarkers in neuropsychiatric treatment development.
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Affiliation(s)
- Daniel C Javitt
- Department of Psychiatry, Columbia University Medical Center, New York, NY, 10032, USA.
- Schizophrenia Research Division, Nathan Kline Institute for Psychiatric Research, Orangeburg, NY, 10954, USA.
| | - Steven J Siegel
- Department of Psychiatry and Behavioral Sciences, Keck School of Medicine, University of Southern California, Los Angeles, CA, 90033, USA
| | - Kevin M Spencer
- Research Service, VA Boston Healthcare System, and Dept. of Psychiatry, Harvard Medical School, Boston, MA, 02130, USA
| | - Daniel H Mathalon
- VA San Francisco Healthcare System, University of California, San Francisco, San Francisco, CA, 94121, USA
| | - L Elliot Hong
- Maryland Psychiatric Research Center, Department of Psychiatry, University of Maryland School of Medicine, Baltimore, MD, USA
| | - Antigona Martinez
- Department of Psychiatry, Columbia University Medical Center, New York, NY, 10032, USA
- Schizophrenia Research Division, Nathan Kline Institute for Psychiatric Research, Orangeburg, NY, 10954, USA
| | - Cindy L Ehlers
- Department of Neuroscience, The Scripps Research Institute, 10550 N Torrey Pines Road, La Jolla, CA, 92037, USA
| | - Atheir I Abbas
- VA Portland Health Care System, Portland, OR, 97239, USA
- Department of Behavioral Neuroscience, Oregon Health & Science University, Portland, OR, 97239, USA
- Department of Psychiatry, Oregon Health & Science University, Portland, OR, 97239, USA
| | - Tobias Teichert
- Departments of Psychiatry and Bioengineering, University of Pittsburgh, Pittsburgh, PA, 15213, USA
| | - Peter Lakatos
- Schizophrenia Research Division, Nathan Kline Institute for Psychiatric Research, Orangeburg, NY, 10954, USA
| | - Thilo Womelsdorf
- Department of Psychology, Vanderbilt University, Nashville, TN, 37203, USA
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14
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Guggenberger R, Heringhaus M, Gharabaghi A. Brain-Machine Neurofeedback: Robotics or Electrical Stimulation? Front Bioeng Biotechnol 2020; 8:639. [PMID: 32733860 PMCID: PMC7358603 DOI: 10.3389/fbioe.2020.00639] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2019] [Accepted: 05/26/2020] [Indexed: 12/19/2022] Open
Abstract
Neurotechnology such as brain-machine interfaces (BMI) are currently being investigated as training devices for neurorehabilitation, when active movements are no longer possible. When the hand is paralyzed following a stroke for example, a robotic orthosis, functional electrical stimulation (FES) or their combination may provide movement assistance; i.e., the corresponding sensory and proprioceptive neurofeedback is given contingent to the movement intention or imagination, thereby closing the sensorimotor loop. Controlling these devices may be challenging or even frustrating. Direct comparisons between these two feedback modalities (robotics vs. FES) with regard to the workload they pose for the user are, however, missing. Twenty healthy subjects controlled a BMI by kinesthetic motor imagery of finger extension. Motor imagery-related sensorimotor desynchronization in the EEG beta frequency-band (17–21 Hz) was turned into passive opening of the contralateral hand by a robotic orthosis or FES in a randomized, cross-over block design. Mental demand, physical demand, temporal demand, performance, effort, and frustration level were captured with the NASA Task Load Index (NASA-TLX) questionnaire by comparing these workload components to each other (weights), evaluating them individually (ratings), and estimating the respective combinations (adjusted workload ratings). The findings were compared to the task-related aspects of active hand movement with EMG feedback. Furthermore, both feedback modalities were compared with regard to their BMI performance. Robotic and FES feedback had similar workloads when weighting and rating the different components. For both robotics and FES, mental demand was the most relevant component, and higher than during active movement with EMG feedback. The FES task led to significantly more physical (p = 0.0368) and less temporal demand (p = 0.0403) than the robotic task in the adjusted workload ratings. Notably, the FES task showed a physical demand 2.67 times closer to the EMG task, but a mental demand 6.79 times closer to the robotic task. On average, significantly more onsets were reached during the robotic as compared to the FES task (17.22 onsets, SD = 3.02 vs. 16.46, SD = 2.94 out of 20 opportunities; p = 0.016), even though there were no significant differences between the BMI classification accuracies of the conditions (p = 0.806; CI = −0.027 to −0.034). These findings may inform the design of neurorehabilitation interfaces toward human-centered hardware for a more natural bidirectional interaction and acceptance by the user.
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Affiliation(s)
- Robert Guggenberger
- Institute for Neuromodulation and Neurotechnology, Department of Neurosurgery and Neurotechnology, University of Tübingen, Tübingen, Germany
| | - Monika Heringhaus
- Institute for Neuromodulation and Neurotechnology, Department of Neurosurgery and Neurotechnology, University of Tübingen, Tübingen, Germany
| | - Alireza Gharabaghi
- Institute for Neuromodulation and Neurotechnology, Department of Neurosurgery and Neurotechnology, University of Tübingen, Tübingen, Germany
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15
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Scholten M, Schoellmann A, Ramos-Murguialday A, López-Larraz E, Gharabaghi A, Weiss D. Transitions between repetitive tapping and upper limb freezing show impaired movement-related beta band modulation. Clin Neurophysiol 2020; 131:2499-2507. [PMID: 32684329 DOI: 10.1016/j.clinph.2020.05.037] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2019] [Revised: 04/08/2020] [Accepted: 05/23/2020] [Indexed: 10/23/2022]
Abstract
OBJECTIVE Freezing phenomena in idiopathic Parkinson's disease (PD) constitute an important unaddressed therapeutic need. Changes in cortical neurophysiological signatures may precede a single freezing episode and indicate the evolution of abnormal motor network processes. Here, we hypothesize that the movement-related power modulation in the beta-band observed during regular finger tapping, deteriorates in the transition period before upper limb freezing (ULF). METHODS We analyzed a 36-channel EEG of 13 patients with PD during self-paced repetitive tapping of the right index finger. In offline analysis, we compared the transition period immediately before ULF ('transition') with regular tapping regarding movement-related power modulation and interregional phase synchronization. RESULTS From time-frequency analyses, we observed that the tap cycle related beta-band power modulation over the left sensorimotor area was diminished in the transition period before ULF. Furthermore, increased beta-band power was observed in the transition period compared to regular tapping centered over the left centro-parietal and right frontal areas. Phase synchronization between the left fronto-parietal areas and the left sensorimotor area was elevated during transition compared to regular tapping. CONCLUSION Together, these results indicate that diminished beta band power modulation and increased phase synchronization precede ULF. SIGNIFICANCE We demonstrate that pathological cortical motor processing is present in the transition phase from regular tapping to an ULF episode.
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Affiliation(s)
- Marlieke Scholten
- Department of Neurodegenerative Diseases, and Hertie Institute for Clinical Brain Research (HIH), University of Tuebingen, Tuebingen, Germany; German Centre of Neurodegenerative Diseases (DZNE), University of Tuebingen, Tuebingen, Germany.
| | - Anna Schoellmann
- Department of Neurodegenerative Diseases, and Hertie Institute for Clinical Brain Research (HIH), University of Tuebingen, Tuebingen, Germany; German Centre of Neurodegenerative Diseases (DZNE), University of Tuebingen, Tuebingen, Germany
| | - Ander Ramos-Murguialday
- Institute of Medical Psychology and Behavioural Neurobiology, University of Tuebingen, Tuebingen, Germany; TECNALIA, Health Division, Neurotechnology Laboratory, San Sebastian, Spain
| | - Eduardo López-Larraz
- Institute of Medical Psychology and Behavioural Neurobiology, University of Tuebingen, Tuebingen, Germany
| | - Alireza Gharabaghi
- Division of Functional and Restorative Neurosurgery, Center for Integrative Neuroscience, and Tuebingen NeuroCampus, University of Tuebingen, 72076 Tuebingen, Germany
| | - Daniel Weiss
- Department of Neurodegenerative Diseases, and Hertie Institute for Clinical Brain Research (HIH), University of Tuebingen, Tuebingen, Germany; German Centre of Neurodegenerative Diseases (DZNE), University of Tuebingen, Tuebingen, Germany.
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16
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Carson RG, Buick AR. Neuromuscular electrical stimulation-promoted plasticity of the human brain. J Physiol 2019; 599:2375-2399. [PMID: 31495924 DOI: 10.1113/jp278298] [Citation(s) in RCA: 53] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2019] [Accepted: 08/16/2019] [Indexed: 12/21/2022] Open
Abstract
The application of neuromuscular electrical stimulation (NMES) to paretic limbs has demonstrated utility for motor rehabilitation following brain injury. When NMES is delivered to a mixed peripheral nerve, typically both efferent and afferent fibres are recruited. Muscle contractions brought about by the excitation of motor neurons are often used to compensate for disability by assisting actions such as the formation of hand aperture, or by preventing others including foot drop. In this context, exogenous stimulation provides a direct substitute for endogenous neural drive. The goal of the present narrative review is to describe the means through which NMES may also promote sustained adaptations within central motor pathways, leading ultimately to increases in (intrinsic) functional capacity. There is an obvious practical motivation, in that detailed knowledge concerning the mechanisms of adaptation has the potential to inform neurorehabilitation practice. In addition, responses to NMES provide a means of studying CNS plasticity at a systems level in humans. We summarize the fundamental aspects of NMES, focusing on the forms that are employed most commonly in clinical and experimental practice. Specific attention is devoted to adjuvant techniques that further promote adaptive responses to NMES thereby offering the prospect of increased therapeutic potential. The emergent theme is that an association with centrally initiated neural activity, whether this is generated in the context of NMES triggered by efferent drive or via indirect methods such as mental imagery, may in some circumstances promote the physiological changes that can be induced through peripheral electrical stimulation.
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Affiliation(s)
- Richard G Carson
- Trinity College Institute of Neuroscience and School of Psychology, Trinity College Dublin, Dublin 2, Ireland.,School of Psychology, Queen's University Belfast, Belfast, BT7 1NN, UK.,School of Human Movement and Nutrition Sciences, University of Queensland, Brisbane, QLD 4072, Australia
| | - Alison R Buick
- School of Psychology, Queen's University Belfast, Belfast, BT7 1NN, UK
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Combined endogenous and exogenous disinhibition of intracortical circuits augments plasticity induction in the human motor cortex. Brain Stimul 2019; 12:1027-1040. [PMID: 30894281 DOI: 10.1016/j.brs.2019.03.009] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2018] [Revised: 02/03/2019] [Accepted: 03/08/2019] [Indexed: 12/22/2022] Open
Abstract
BACKGROUND Motor imagery (MI) engages cortical areas in the human brain similar to motor practice. Corticospinal excitability (CSE) is facilitated during but not after MI practice. We hypothesized that lasting CSE changes could be achieved by associatively pairing this endogenous modulation with exogenous stimulation of the same intracortical circuits. METHODS We combined MI with a disinhibition protocol (DIS) targeting intracortical circuits by paired-pulse repetitive transcranial magnetic stimulation in one main and three subsequent experiments. The follow-up experiments were applied to increase effects, e.g., by individualizing inter-stimulus intervals, adding neuromuscular stimulation and expanding the intervention period. CSE was captured during (online) and after (offline) the interventions via input-output changes and cortical maps of motor evoked potentials. A total of 35 healthy subjects (mean age 26.1 ± 2.6 years, 20 females) participated in this study. RESULTS A short intervention (48 stimuli within ∼90s) increased CSE. This plasticity developed rapidly, was associative (with MIon, but not MIoff or REST) and persisted beyond the intervention period. Follow-up experiments revealed the relevance of individualizing inter-stimulus intervals and of consistent inter-burst periods for online and offline effects, respectively. Expanding this combined MI/DIS intervention to 480 stimuli amplified the sustainability of CSE changes. When concurrent neuromuscular electrical stimulation was applied, the plasticity induction was cancelled. CONCLUSIONS This novel associative stimulation protocol augmented plasticity induction in the human motor cortex within a remarkably short period of time and in the absence of active movements. The combination of endogenous and exogenous disinhibition of intracortical circuits may provide a therapeutic backdoor when active movements are no longer possible, e.g., for hand paralysis after stroke.
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