Alpha rhythm of electroencephalography was modulated differently by three transcranial direct current stimulation protocols in patients with ischemic stroke

Abstract

The heterogeneity of transcranial direct current stimulation (tDCS) protocols and clinical profiles may explain variable results in modulating excitability in the motor cortex after stroke. However, the cortical electrical effects induced by different tDCS protocols remain unclear. Here, we aimed to compare rhythm changes in electroencephalography (EEG) induced by three tDCS position protocols and the association between tDCS effects and clinical factors in stroke. Nineteen patients with chronic ischemic stroke underwent four experimental sessions with three tDCS protocols [anodal (atDCS), cathodal (ctDCS), and bilateral (bi-tDCS)] and a sham protocol, according to a single-blind randomized crossover design. Resting-state EEG was acquired before and after each protocol. First, a paired-sample t-test was used to examine the difference in spectral power between pre- and post-stimulation. Then, linear and quadratic regression models were used separately to describe the association between the clinical factors of stroke and changes in spectral power which was significantly different between pre- and post-tDCS. Finally, repeated measures analysis of variance with lesion hemisphere, stimulation protocol, and the location was performed to investigate the effects of tDCS over time. The induced effect of tDCS was mainly reflected in the alpha rhythms. The alpha power was increased by atDCS, especially low-alpha (8-10 Hz), in localized areas of the central and distant areas of the frontal and parietal lobes. Bi-tDCS also affected alpha power but in a smaller area that mainly focused on high-alpha rhythms (10-13 Hz). However, ctDCS and sham had no significant effects on any EEG rhythm. The clinical factors of time since stroke and motor impairment level were related to the change in high-alpha induced by atDCS and bi-tDCS following quadratic regression models. The above-mentioned modulation effect lasted for 20 min without attenuation. In conclusion, our findings provide evidence that the alpha rhythm of EEG is modulated differently by different tDCS protocols and that high alpha is affected by clinical characteristics such as post-stroke time and motor deficits, which is of great significance for understanding the modulation effect of different tDCS protocols on stroke and the guidance of protocols to promote motor recovery following stroke.

Keywords: alpha rhythm; chronic stroke; quantitative EEG; spectral power; transcranial direct current stimulation.

FIGURE 1

Experimental design of each session. Dark gray blocks represent the eyes-close state; Light gray blocks represent the eyes-open state. atDCS, anodal transcranial direct current stimulation; ctDCS, cathodal transcranial direct current stimulation; btDCS, bilateral transcranial direct current stimulation; Block 1: EEG signals are collected for 12 min which contains six trails with three eyes-closed states and thee eyes-opened states. The EEG signals were collected. Block 2: real (atDCS, ctDCS or btDCS) or sham tDCS is delivered for 20 min. Block 3: Electrodes of EEG acquisition were placed on the scalp, which lasts for 10 min. Block 4: EEG signals are collected for 20 min which contains 10 trials with five eyes closing states and five eyes opening states.

FIGURE 2

Cortical activity changing of alpha (8–13 Hz), alpha1 (8–10 Hz) and alpha2 (10–13 Hz) frequency band induced by anodal transcranial direct current stimulation (atDCS) (A), bilateral (bi-tDCS) (B), cathodal (ctDCS) (C) and sham stimulation (D). For the topographical maps, the Ratio represents the spectral power ratio of post-to pre-stimulation. T-value represents the statistic t-value of the paired-samples T-test. The white dots represent channels that have a significant difference (p < 0.05) between post- and pre-stimulation. The 62 channels were normalized to the lesional hemisphere (left side), contralesional hemisphere (right side) and central.

FIGURE 3

Scatter plots and fitted curves of representative channels with a clinical scale on the abscissa and alpha power’s ratio of post- to pre-stimulation. We applied quadratic fitting to these scatters with co-variables of age, sex and lesion hemisphere. The coefficient of determination R² and p-value of Fisher’s F-test were shown in the plots.

FIGURE 4

Cortical activity changing of alpha (8–13 Hz) frequency band over the 10–30 min after anodal transcranial direct current stimulation (atDCS), cathodal (ctDCS) and bilateral (bi-tDCS) and sham stimulation (sham). T1, T2, T3, T4, and T5 represents the five observation period with the eyes-open state.