Abstract
Little work has been done on the information flow in functional brain imaging and none so far in fNIRS. In this work, alterations in the directionality of net information flow induced by a short-duration, low-current (2 min 40 s; 0.5 mA) and a longer-duration, high-current (8 min; 1 mA) anodal tDCS applied over the Broca's area of the dominant language hemisphere were studied by fNIRS. The tDCS-induced patterns of information flow, quantified by a novel directed phase transfer entropy (dPTE) analysis, were distinct for different hemodynamic frequency bands and were qualitatively similar between low and high-current tDCS. In the endothelial band (0.003-0.02 Hz), the stimulated Broca's area became the strongest hub of outgoing information flow, whereas in the neurogenic band (0.02-0.04 Hz) the contralateral homologous area became the strongest information outflow source. In the myogenic band (0.04-0.15 Hz), only global patterns were seen, independent of tDCS stimulation that were interpreted as Mayer waves. These findings showcase dPTE analysis in fNIRS as a novel, complementary tool for studying cortical activity reorganization after an intervention.
Highlights
Transcranial direct current stimulation is a non-invasive electrical stimulation technique used to modulate cortical activity in the human brain by delivering weak currents through a pair of anode-cathode electrodes [1,2]
A nitric oxide (NO)-based mechanism of endothelial response to stimulation would be consistent with the findings summarized in Table 3, where the flow information patterns during Low Current Transcranial direct current stimulation (tDCS) were similar to the After High Current tDCS session, whereas higher directional Phase Transfer Entropy (PTE) (dPTE) values were observed during the High Current tDCS session
This study demonstrates the feasibility of using resting-state functional near-infrared spectroscopy (fNIRS) to map changes in the direction of information flow induced by tDCS in the language-processing cortical networks of healthy subjects
Summary
Transcranial direct current stimulation (tDCS) is a non-invasive electrical stimulation technique used to modulate cortical activity in the human brain by delivering weak currents through a pair of anode-cathode electrodes (up to 2mA for up to 20 mins) [1,2]. TDCS has been applied to enhance physical performance in healthy subjects [3,4,5] and facilitate neurorehabilitation during stroke recovery [6]. Several studies have suggested that anodal tDCS over either Broca’s area or Wernicke’s area could improve naming accuracy or speed both in stroke-induced aphasia patients [7,8,9] and in healthy subjects [5,10,11]. Little is known about the directionality of cortical interactions in functional language networks when tDCS is applied. To the best of our knowledge, only one study [14] to date has investigated the direction of information flow, which was done by use of Dynamic Causal Modelling (DCM) during a concurrent tDCS-fMRI study of overt picture naming. We used Phase Transfer Entropy (PTE) [15,17], which is a computationally efficient and data-driven method, to estimate changes in the direction of information flow affected by tDCS, quantified by a directional PTE (dPTE) metric
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