THE EFFECTS OF TRANSCRANIAL STATIC MAGNETIC STIMULATION OVER THE DORSOLATERAL PREFRONTAL CORTEX ON WORKING MEMORY PERFORMANCE AND BRAIN OSCILLATORY ACTIVITY

T. Watanabe^1,2, X. Chen³, K. Yunoki², T. Matsumoto^2,4, T. Horinouchi², K. Ito², H. Ishida², H. Kirimoto²

¹Aomori University of Health and Welfare, Faculty of Health Sciences, Aomori, Japan, ²Hiroshima University, Graduate School of Biomedical and Health Sciences, Hiroshima, Japan, ³Jinzhou Medical University, College of Physical Education and Sports Rehabilitation, Jinzhou, China, ⁴Japan Society for the Promotion of Science, Tokyo, Japan

Background: Transcranial static magnetic stimulation (tSMS) is a new non-invasive brain stimulation (NIBS) technique in which a strong neodymium (NdFeB) magnet that generates static magnetic field (120-200 mT at a distance of 2-3 cm from the magnet surface) is placed on scalp. It has been shown to reduce cortical excitability. There are a number of studies reporting that the other NIBS techniques, such as repetitive transcranial magnetic stimulation and transcranial direct current stimulation, applied over the dorsolateral prefrontal cortex (DLPFC) are effective in reducing depressive symptoms. However, the effect of tSMS on the DLPFC is currently unclear.

Purpose: The aim of this study was to investigate the effects of tSMS over the left and right DLPFC on working memory performance, which is supported by the DLPFC, and related brain oscillatory activity, in order to obtain the basic data on whether tSMS can modulate activity of the DLPFC for future clinical investigations.

Methods: Fourteen healthy adults participated in this study. We used a cylindrical NdFeB magnet (diameter, 50 mm; height, 30 mm) with a maximum energy density of 49 MGOe, and a strength of 862 N (88 kg) for tSMS and a non-magnetic stainless-steel cylinder of the same size, weight, and appearance for sham stimulation. There were three conditions: tSMS over the left DLPFC, tSMS over the right DLPFC, and sham stimulation. The participants performed a 2-back task, which required them to decide whether a presented stimulus matches the stimulus presented 2 trials previously, before, during (20 min after the start of stimulation), immediately after, and 15 minutes after each stimulation. During the task, electroencephalogram was recorded from the frontal areas (F3, Fz, and F4 based on the international 10-20 system). We examined working memory performance using d-prime, a measure of discriminate sensitivity. Also, we computed event-related spectral perturbations time-locked to visual stimulus.

Results: Working memory performance was impaired by tSMS over the left and right DLPFC. The degree of impairment was similar between them. TSMS over the left DLPFC increased event-related synchronization in beta band while tSMS over the right DLPFC decreased event-related desynchronization in beta band.

Conclusions: TSMS over the left and right DLPFC deteriorated working memory performance by possibly reducing their cortical excitability. Although the impact of tSMS on working memory was comparable between the left and right DLPFC, the underlying brain oscillatory responses could be different.

Implications: A functional interhemispheric imbalance of the DLPFC is a key factor underlying depressive symptoms, with the right DLPFC being hyperactive relative to the left DLPFC. Although further studies to understand the detailed mechanisms and clinical effectiveness are necessary, our findings implicate that tSMS may be a useful therapeutic modality to reduce the hyperactivity of the right DLPFC in patients with depressive symptoms.

Funding acknowledgements: This study was supported by grants from the Japan Society for the Promotion of Science.

Keywords:
Transcranial static magnetic stimulation
Dorsolateral prefrontal cortex
Working memory

Topics:
Neurology
Mental health
Disability & rehabilitation

Did this work require ethics approval? Yes

Institution: Hiroshima Universtiy

Committee: Ethics committee of Graduate School of Biomedical and Health Sciences

Ethics number: C-242