Limitation
While this study has several strengths, such as a well-powered
within-subjects design and the measurement of various parasympathetic
indicators, it is important to acknowledge and address some notable
limitations. First, a measure of the sympathetic activity was not
evaluated. Therefore, we were not able to investigate how tDCS, via
increasing parasympathetic control, can affect the sympathetic nervous
system. Second, the neuronavigation method used here was based on a
predetermined MRI coordinate. Perhaps, applying individualized DLPFC
location based on functional MRI targeting
(Cash et al., 2021) may be a
further improvement for future studies. Third, due to the within-subject
design, blinding was not assessed as it could increase participants’
awareness applying individualized DLPFC location regarding the
intervention protocol in the subsequent session.