Fig 1.
Study design: A double-blind, randomized, crossover study.
Twenty-four participants were recruited and randomly assigned to receive either tDCS or sham stimulation during the first trial. After an interval of at least four weeks, the second trial was conducted. The order of the two stimulation conditions (i.e., tDCS or sham) was counterbalanced across participants. After delivering one of the two stimulus conditions, ASSRs were recorded using MEG. We excluded two participants from the statistical analysis because they met the exclusion criteria. ASSR, auditory steady-state response.
Fig 2.
(A) Anodal (F3) and cathodal (F4) electrodes marked on the scalp surface. (B) The computational simulation of brain current flow during the delivery of the tDCS. Major changes in the brain current flow were observed in the dorsal-frontal areas during stimulation [37].
Fig 3.
The grand average of the time-frequency maps of the ERSP in the transverse temporal gyrus under both tDCS and sham conditions.
In each map, the x-axis indicates time (ms), and the y-axis indicates frequency (Hz). The color indicates the ERSP at each time-frequency point (reflected as the percentage change from baseline). The gamma-band (40 Hz) ASSR derived from the 40 Hz auditory stimulation was clearly observed under the tDCS and sham conditions.
Fig 4.
T-maps of the differences in ERSPs (upper row) and ITPCs (lower row) in the transverse temporal gyrus between the tDCS and sham conditions.
Color indicates t-values at each time-frequency point. No significant differences were found in either ERSPs or ITPCs when the statistical threshold was set to p < 0.05 with an FDR correction.
Fig 5.
The grand average of the time-frequency maps for the ITPC in the transverse temporal gyrus under both tDCS and sham conditions.
In each map, the x-axis indicates time (ms), and the y-axis indicates frequency (Hz). The color indicates the ITPCs at each time-frequency point. The ITPC peak in the gamma-band (40 Hz) was clearly observed during the 40 Hz auditory stimulation.
Fig 6.
ERSPs in the left precentral gyrus.
(A) T-maps of the differences in the ERSPs between the tDCS and sham conditions. The threshold of p < 0.05 (FDR corrected) is highlighted with a red rectangle. The color indicates the t-values at each time-frequency point. Significantly higher ERSPs were observed in the beta-band (20 Hz; a red rectangle) after tDCS than after the sham condition. (B) The grand average of the time-frequency maps of ERSPs in the left precentral gyrus under both tDCS (upper row) and sham (lower row) conditions. In each map, the x-axis indicates time (ms), and the y-axis indicates frequency (Hz). The color indicates ERSPs at each time-frequency point (reflected as the percentage change from baseline).