  Transcranial direct current stimulation (tDCS) is known to have a modulatory
effect on neural tissue and that it is polarity specific. It is also shown that
tDCS demonstrated the lasting effect in therapeutic applications. The main aim
of the study was to examine the effects of tDCS on cortical dynamics by
analyzing EEG recordings. We applied here measures taken from Recurrence
Quantification Analysis, Mean State Shift (MSS) and State Variance (SV) which
were previously used to detect changes in brain-state dynamics after TMS. The
studied cohort comprised of 16 healthy subjects; all subjects received anodal
and cathodal tDCS, which were given in two separate sessions on the same day.
The EEG was recorded from 10 electrodes, positioned over left motor cortex and
mirroring right cortex corresponding to 10/20 standard. From three traces of
recordings (pre, post1 and post2/before the stimulation, immediately after and
30min after tDCS) we extracted five different intervals (T1-T5) comprising of
500 samples. After calculating MSS and SV on those epochs and statistical
testing for a significant difference, we applied Principal Component Analysis
(PCA) on the same time series to check whether the data are separable. The
results show that tDCS exert polarity specific effects on the MSS as shown by
significantly lower MSS values after cathodal stimulation compared to anodal
stimulation. Cathodal stimulation affected the SV, as compared to anodal
stimulation, which did not lead to detectable changes. We are offering here for
the first time an informative PCA visualization of a time-effect of a tDCS
stimulation on brain state shift. Further research is needed to elucidate for
how long that change can be detected and what neurobiological changes are
introduced by that phenomena.
