Abstract
In the present study, we monitored hemodynamic responses in rat brains during transcranial direct current stimulation (tDCS) using functional near-infrared spectroscopy (fNIRS). Seven rats received transcranial anodal stimulation with 200 μA direct current (DC) on their right barrel cortex for 10 min. The concentration changes of oxygenated hemoglobin (oxy-Hb) were continuously monitored during stimulation (10 min) and after stimulation (20 min). The trend of hemodynamic response changes was modeled using linear regression, and the relationship between incremental and decremental rates of oxy-Hb was investigated by correlation analysis. Our results showed that the oxy-Hb concentration was almost linearly increased and decreased during and after stimulation, respectively. In addition, a significant negative correlation (p < 0.05) was found between the rate of increase of oxy-Hb during stimulation and the rate of decrease of oxy-Hb after stimulation, indicating that the recovery time after tDCS may not depend on the total amount of hemodynamic changes in the stimulated brain area. Our results also demonstrated considerable individual variability in the rate of change of hemodynamic responses even with the same direct current dose to identical brain regions. This suggests that individual differences in tDCS after-effects may originate from intrinsic differences in the speed of DC stimulation "uptake" rather than differences in the total capacity of DC uptake, and thus the stimulation parameters may need to be customized for each individual in order to maximize tDCS after-effects.
Highlights
Transcranial direct current stimulation is a noninvasive brain electrical stimulation technique that modulates cortical excitability with a small direct current (DC) flowing between a pair of scalp electrodes [1]
Oxy-Hb concentration increased during anodal DC stimulation and decreased after transcranial direct current stimulation (tDCS), but deoxyHb did not show any distinct change during and after tDCS, which is consistent with a previous study [17] that reported hemodynamic changes in human prefrontal cortex after tDCS
To the best of our knowledge, no previous studies have reported on continuous hemodynamic response changes during tDCS
Summary
Transcranial direct current stimulation (tDCS) is a noninvasive brain electrical stimulation technique that modulates cortical excitability with a small direct current (DC) flowing between a pair of scalp electrodes [1]. Kwon et al observed hemodynamic response changes during anodal tDCS using interleaved functional magnetic resonance imaging (fMRI) scanning, and found that hemodynamic response changes occurred at least 1 min after DC stimulation onset [16]. Merzagora et al [17] and Wachter et al [18] used functional near-infrared spectroscopy (fNIRS) and laser Doppler flowmetry (LDF), respectively, to observe hemodynamic changes produced by anodal tDCS, but they did not simultaneously record hemodynamic changes during tDCS Both studies observed temporal changes in oxygenated hemoglobin (oxy-Hb) or cerebral blow flow (CBF) after tDCS. To the best of our knowledge, only one study done by Zheng et al [19] recorded regional CBF (rCBF) changes simultaneously during tDCS using Arterial Spin Labeling (ASL) This previous study reported increased rCBF during anodal tDCS and decreased rCBF during cathodal tDCS. They did not show continuous temporal changes of rCBF due to the long acquisition time for ASL scan (only two images could be acquired during the tDCS ‘ON’ period)
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