Conference Paper

Transcranial direct current stimulation induced modulation of cortical haemodynamics: A comparison between time-domain and continuous-wave functional near-infrared spectroscopy

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Transcranial direct current stimulation (tDCS) is a non-invasive electrical brain stimulation technique that modulates cortical neuronal excitability and activity. An indirect marker of increases in cortical neuronal activity is the subsequent increase in the regional cortical blood flow (i.e., neurovascular coupling), which can be assessed using functional near-infrared spectroscopy (fNIRS). Continuous-wave (CW) fNIRS measures light attenuation through both the cortical and superficial (e.g., skin) tissues, while time- domain (TD) fNIRS directly measures the photon time-of flight that contains information about the depth probed by photons in the tissue, and in turn provides a possibility to separate the superficial from the cortical layer haemodynamic signals. Therefore, the aim of this study was to compare the changes in resting cortical haemo- dynamics measured by a CW and TD- fNIRS system during anodal tDCS. Methods: A Startstim tDCS system (Neuroelectrics) was used to deliver a constant direct current (2mA) to the left sensorimotor cortex (SMC) via an anodal 4x1 high definition (HD)-tDCS electrode montage. During 10min of anodal HD-tDCS, changes in resting oxygenated (O2Hb) and deoxygenated (HHb) haemoglobin con- centrations from the left and right SMC were monitored by a CW- fNIRS system (Oxymon MK III, AMS) and in a subsequent session with a TD-fNIRS system (Politecnico di Milano). Results/Discussion: CW- and TD- fNIRS showed a similar time course (i.e. increase in O2Hb from baseline with no or only minimal decrease in HHb) in the stimulated left SMC (no or only minimal changes in right SMC) over the 10min of anodal HD-tDCS (Fig. a, b). The TD-fNIRS confirmed this time course in the cortical layer of the stimulated left SMC (Fig. d), but the changes were much smaller than the superficial layer changes (Fig. b). Conclusion: This preliminary study has shown that 10min of anodal HD-tDCS induces an increase in activation in the cortical layer of the stimulated left SMC.

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... This research work has been positioned on the premise that a protocol coupling motor task and cerebral stimulation by anodal tDCS is more advantageous than other traditional protocols. Regarding the preliminary study conducted , the effects of tDCS-task coupling were (Muthalib et al., 2015). Changes in HHb are considered less affected by skin blood flow changes (Kirilina et al., 2012) and we found less variability in HHb responses during the five trials of the SFO task than with O 2 Hb responses. ...
... Therefore, in order to optimize potential applications of the tDCS to enhance motor and cognitive performance, there is a critical need to identify a neurophysiological correlate of the electric field spatial distribution from the scalp-applied current. Within this context, neuroimaging methods can be used to provide information about the brain-tissue effects of the tDCS electric fields when measured in a resting-state during (Muthalib et al., 2015;Zheng et al., 2011) and/or after (Amadi et al., 2014;Sood et al., 2016) neurostimulation. Since the strength of the electric field diminishes exponentially with distance from the electrode (Sood et al., 2016), the larger fNIRS O 2 Hb values found within the spatial boundary of the 4x1 HD-tDCS electrodes compared to outside would suggest that there was a stronger electric field distribution in the stimulated hemisphere, more specifically inside the square formed by the electrodes. ...
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Historically, humans have sought various ways to improve their daily lives. With the current technological advances, this quest is facilitated, especially in the desire to increase their cognitive and / or motor skills. Neuro imagery now makes it possible to inform the areas activated during different functional tasks. Today, it is now possible to modulate brain activity by stimulating the brain locally with weak electrical currents. One of the most common techniques for this purpose is called tDCS for transcranial direct current stimulation. The polarity of the induced current (anodal or cathodal stimulation) allows to modulate upward or downward cortico-spinal excitability by depolarizing or hyperpolarizing the membrane of the neurons, respectively. Despite a growing interest of neuromodulation techniques via tDCS, the results reported by the scientific community are relatively heterogeneous. The work initiated at the beginning of the 2000s is called into question by current results showing a rather large inter and intra variability. This stumbling block requires the development of new protocols for the application of anodal tDCS (atDCS). In this thesis, we were interested in optimizing atDCS protocols in order to increase the persistence of the induced-neuroplastic effects and to increase the behavioral performances. Two studies were carried out in order to first reveal the impact from the motor task/atDCS coupling and then to highlight the cumulative effects of multiple motor-tDCS task sessions with priming atDCS on motor performance. The first study through the use of near infrared spectroscopy allowed to report various hemodynamic changes subsequent to the motor task/atDCS coupling with respect to independent and controlled stimulation protocols. The primacy of the concomitant use of tDCS with the motor task was revealed by the slightest activation of the sensorimotor cortex during stimulation and by an increased delayed cerebral activation which could represent a neuroplastic reorganization. The second study examined the effects of repeated atDCS sessions with anoadal or cathodal tDCS priming in order to improve the learning and retention gains of the sensorimotor system. TDCS priming was more favorable for repeated atDCS sessions to generate higher motor performances contrary to sham. The cathodal polarity produced prolonged persistence. The major findings of this work allow to support the concomitant use of atDCS with the motor task. Future research is needed to study the transfer of these results into the fields of coaching and rehabilitation.
... In particular, it allows for continuous noninvasive measurements, does not require the immobility of the subject, and is not necessary to change the environment preferable for the stimulation to integrate CW-fNIRS in the protocol. 18 It has indeed already been used as a monitor during DC-tCS, both in animals 19 and on humans at rest [20][21][22] and during tasks. [22][23][24] It has also been used together with EEG to monitor DC-tCS 25 and a device integrating the two monitors has been developed and characterized. ...
... Moreover, these results are in accordance with a previous single subject experiment, where TR-fNIRS was used during anodal stimulation. 21 In this work, TR-fNIRS measurements were analyzed with a gated analysis. 66 This method exploits the depth information encoded in time in the broadened pulse collected after propagation into the tissue and allows to decouple changes in the extracerebral and cerebral layers. ...
Transcranial direct current stimulation (tDCS) is currently being used for research and treatment of some neurological and neuropsychiatric disorders, as well as for improvement of cognitive functions. In order to better understand cerebral response to the stimulation and to redefine protocols and dosage, its effects must be monitored. To this end, we have used functional diffuse correlation spectroscopy (fDCS) and time-resolved functional near-infrared spectroscopy (TR-fNIRS) together with electroencephalography (EEG) during and after stimulation of the frontal cortex. Twenty subjects participated in two sessions of stimulation with two different polarity montages and twelve also underwent a sham session. Cerebral blood flow and oxyhemoglobin concentration increased during and after active stimulation in the region under the stimulation electrode while deoxyhemoglobin concentration decreased. The EEG spectrum displayed statistically significant power changes across different stimulation sessions in delta (2 to 4 Hz), theta (4 to 8 Hz), and beta (12 to 18 Hz) bands. Results suggest that fDCS and TR-fNIRS can be employed as neuromonitors of the effects of transcranial electrical stimulation and can be used together with EEG.
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Objective: High-definition transcranial direct current stimulation (HD-tDCS) using a 4 × 1 electrode montage has been previously shown using modeling and physiological studies to constrain the electric field within the spatial extent of the electrodes. The aim of this proof-of-concept study was to determine if functional near-infrared spectroscopy (fNIRS) neuroimaging can be used to determine a hemodynamic correlate of this 4 × 1 HD-tDCS electric field on the brain. Materials and methods: In a three session cross-over study design, 13 healthy males received one sham (2 mA, 30 sec) and two real (HD-tDCS-1 and HD-tDCS-2, 2 mA, 10 min) anodal HD-tDCS targeting the left M1 via a 4 × 1 electrode montage (anode on C3 and 4 return electrodes 3.5 cm from anode). The two real HD-tDCS sessions afforded a within-subject replication of the findings. fNIRS was used to measure changes in brain hemodynamics (oxygenated hemoglobin integral-O2 Hbint ) during each 10 min session from two regions of interest (ROIs) in the stimulated left hemisphere that corresponded to "within" (Lin ) and "outside" (Lout ) the spatial extent of the 4 × 1 electrode montage, and two corresponding ROIs (Rin and Rout ) in the right hemisphere. Results: The ANOVA showed that both real anodal HD-tDCS compared to sham induced a significantly greater O2 Hbint in the Lin than Lout ROIs of the stimulated left hemisphere; while there were no significant differences between the real and sham sessions for the right hemisphere ROIs. Intra-class correlation coefficients showed "fair-to-good" reproducibility for the left stimulated hemisphere ROIs. Conclusions: The greater O2 Hbint "within" than "outside" the spatial extent of the 4 × 1 electrode montage represents a hemodynamic correlate of the electrical field distribution, and thus provides a prospective reliable method to determine the dose of stimulation that is necessary to optimize HD-tDCS parameters in various applications.
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