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Magnetic Impulses Help Create Muscle Activity Maps to Diagnose Motor Disorders

Magnetic Impulses Help Create Muscle Activity Maps to Diagnose Motor Disorders

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Using transcranial magnetic stimulation, Russians scientists were able to precisely track inter-muscle interactions between cortical representations of arm muscles. In the future, this method will help track brain changes in patients with motor disorders. The study was published in Human Brain Mapping. The project was supported by the Presidential Programme of the Russian Science Foundation (RSF).

Today, transcranial magnetic stimulation is actively used in psychiatry and neurology to treat depression, pain, and other conditions. But the method is still underused for the assessment of the motor cortex and musculoskeletal conditions in different disorders, exercise, or rehabilitation.

Russian researchers examined the reliability of motor mapping with the use of this method. The scholars managed to precisely track inter-muscle interactions between cortical representations of arm muscles. In the future, this will help track brain changes in patients with motor disorders.

Transcranial magnetic stimulation (TMS) helps doctors and researchers activate the human cortex with short magnetic impulses. Today, this method is used in psychiatry and neurology to treat, for example, such conditions as depression, pain, Parkinson’s disease and many disorders. In addition, TMS looks quite promising in terms of brain research and its functional mapping — the creation of brain maps. Combining TMS with MRI navigation is particularly effective. This method is called navigated TMS or nTMS. For some tasks, it is more precise than other brain mapping methods, such as functional MRI. In nTMS, the motor cortex is stimulated. This leads to muscle contractions, which are assessed by researchers who register muscle electric activity. The spatial accuracy of nTMS mapping may be as small as two millimeters, and its results are called muscle cortical representation (MCR) or a TMS motor map. This approach may be used to assess motor cortex changes in different disorders, exercise, or rehabilitation.

Arm muscle cortical representation in volunteers after two days of research
Source: Maria Nazarova et al. / Human brain mapping, 2021

Despite the advantages of nTMS mapping, in practice this method is rarely used. Further confirmation of data received with this method is needed. This issue was tackled by Russian researchers of the Centre for Cognition & Decision Making of HSE University’s Institute for Cognitive Neuroscience, the Research Centre of Neurology, and the Federal Centre of Brain Research and Neurotechnologies. They carried out a study of the absolute and comparative reliability of mapping data (muscle cortical representations) of arm muscles. For this purpose, they invited healthy male volunteers aged 19 to 33. None of them had any neurological or mental disorders; the scholars also excluded athletes, musicians, and surgeons, since they are likely to have highly precise motor function due to their work.

The volunteers participated in two nTMS mapping sessions separated by 5-10 days. The researchers registered contractions of three muscles that control the movement of hand and fingers. This way, they received TMS muscle cortical representations. Reliability analysis showed that the commonly used metrics, such as areas, volumes, and centres of gravity had a high relative and low absolute reliability for the muscles. The former assesses the results of repeated measurements, while the latter tracks the change of data in one participant. Overlaps between different muscle MCRs were highly reliable, which allowed the researchers to track the interactions between these maps.

Maria Nazarova

‘Our study is important not only for fundamental science. It also opens new opportunities for the use of nTMS motor mapping to evaluate cortical changes in healthy people and patients with neurological conditions, such as those who are undergoing rehabilitation after a stroke,’ said Maria Nazarova, head of the RSF grant project, Candidate of Science (Medicine), and researcher of the Centre for Cognition & Decision Making (Institute for Cognitive Neuroscience, HSE University) and the Federal Centre of Brain Research and Neurotechnologies.

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