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Effects of Anodal High-Definition Transcranial Direct Current Stimulation on Bilateral Sensorimotor Cortex Activation During Sequential Finger Movements: An fNIRS Study.

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Transcranial direct current stimulation (tDCS) is a non-invasive electrical brain stimulation technique that can modulate cortical neuronal excitability and activity. This study utilized functional near infrared spectroscopy (fNIRS) neuroimaging to determine the effects of anodal high-definition (HD)-tDCS on bilateral sensorimotor cortex (SMC) activation. Before (Pre), during (Online), and after (Offline) anodal HD-tDCS (2 mA, 20 min) targeting the left SMC, eight healthy subjects performed a simple finger sequence (SFS) task with their right or left hand in an alternating blocked design (30-s rest and 30-s SFS task, repeated five times). In order to determine the level of bilateral SMC activation during the SFS task, an Oxymon MkIII fNIRS system was used to measure from the left and right SMC, changes in oxygenated (O2Hb) and deoxygenated (HHb) haemoglobin concentration values. The fNIRS data suggests a finding that compared to the Pre condition both the "Online" and "Offline" anodal HD-tDCS conditions induced a significant reduction in bilateral SMC activation (i.e., smaller decrease in HHb) for a similar motor output (i.e., SFS tap rate). These findings could be related to anodal HD-tDCS inducing a greater efficiency of neuronal transmission in the bilateral SMC to perform the same SFS task.
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Chapter 44
Effects of Anodal High-Definition
Transcranial Direct Current Stimulation
on Bilateral Sensorimotor Cortex Activation
During Sequential Finger Movements: An
fNIRS Study
Makii Muthalib, Pierre Besson, John Rothwell, Tomas Ward,
and Stephane Perrey
Abstract Transcranial direct current stimulation (tDCS) is a non-invasive electri-
cal brain stimulation technique that can modulate cortical neuronal excitability and
activity. This study utilized functional near infrared spectroscopy (fNIRS) neuro-
imaging to determine the effects of anodal high-definition (HD)-tDCS on bilateral
sensorimotor cortex (SMC) activation. Before (Pre), during (Online), and after
(Offline) anodal HD-tDCS (2 mA, 20 min) targeting the left SMC, eight healthy
subjects performed a simple finger sequence (SFS) task with their right or left hand
in an alternating blocked design (30-s rest and 30-s SFS task, repeated five times).
In order to determine the level of bilateral SMC activation during the SFS task, an
Oxymon MkIII fNIRS system was used to measure from the left and right SMC,
changes in oxygenated (O
2
Hb) and deoxygenated (HHb) haemoglobin concentra-
tion values. The fNIRS data suggests a finding that compared to the Pre condition
both the “Online” and “Offline” anodal HD-tDCS conditions induced a significant
reduction in bilateral SMC activation (i.e., smaller decrease in HHb) for a similar
motor output (i.e., SFS tap rate). These findings could be related to anodal
HD-tDCS inducing a greater efficiency of neuronal transmission in the bilateral
SMC to perform the same SFS task.
Keywords Functional near-infrared spectroscopy • tDCS • Neuroplasticity •
Neuromodulation • Sensorimotor cortex
M. Muthalib (*) • P. Besson • S. Perrey
Movement to Health (M2H) Laboratory, EuroMov, University of Montpellier, Montpellier,
France
e-mail: makii.muthalib@gmail.com;makii.muthalib@univ-montp1.fr
J. Rothwell
Institute of Neurology, University College London, London, UK
T. Ward
Department of Electronic Engineering, National University of Ireland, Maynooth, Ireland
©Springer Science+Business Media, New York 2016
C.E. Elwell et al. (eds.), Oxygen Transport to Tissue XXXVII, Advances in
Experimental Medicine and Biology 876, DOI 10.1007/978-1-4939-3023-4_44
351
1 Introduction
Transcranial direct current stimulation (tDCS) is a non-invasive electrical brain
stimulation technique that applies mild (1–2 mA) direct currents over time (10–
20 min) via the scalp to increase (anodal tDCS) or decrease (cathodal tDCS)
cortical neuronal excitability [1]. The subsequent increase in spontaneous neuronal
firing rates (during “Online” tDCS), coupled with synaptic neuroplasticity
(“Online” and after “Offline” tDCS), contribute to anodal tDCS effects of increas-
ing cortical excitability [1].
In order to increase sensorimotor cortex (SMC) excitability, tDCS is conven-
tionally applied using two large (~35 cm
2
) rubber-sponge electrodes with the anode
electrode placed on a target region (i.e., SMC) and return electrode on the contra-
lateral supraorbital region or non-cephalic region [2]. High-definition (HD)-tDCS is
a recent approach that uses arrays of small EEG size (~3 cm
2
) electrodes whose
configuration can be optimized for more focal targeting of cortical regions deter-
mined using computational modeling of current flows between the electrodes
[3]. Recently, anodal HD-tDCS (2 mA, 20 min) targeting the SMC (via a 4 !1
electrode montage) was shown to induce Offline increases in resting corticospinal
excitability assessed using transcranial magnetic stimulation [3]. However, it is not
clear how Online and Offline anodal HD-tDCS modulates SMC activation during
performance of a motor task.
An indirect marker of motor task-related SMC activation is the subsequent
increase in the regional cortical blood flow and oxygenation (i.e., neurovascular
coupling), which can be assessed using functional near infrared spectroscopy
(fNIRS) neuroimaging [4]. fNIRS measures several physiological parameters
related to cortical blood flow and oxygenation including measurements of changes
in oxygenated (O
2
Hb) and deoxygenated (HHb) haemoglobin concentration values
[5]. Therefore, the aim of this study was to utilize fNIRS neuroimaging to measure
bilateral SMC activation during a simple finger sequence (SFS) task in order to
determine the Online and Offline effects of anodal HD-tDCS targeting the
left SMC.
2 Methods
2.1 Subjects
Eight healthy subjects 30.4 "10.6 years (mean "SD) participated in the study. All
subjects were right handed as determined by the Edinburgh handedness question-
naire [6]. All subjects had no known health problems (e.g. metabolic or neuromus-
cular disorders) or any upper extremity muscle or joint injuries. The study
conformed to the recommendations of the local Human Research Ethics Committee
in accordance with the Declaration of Helsinki.
352 M. Muthalib et al.
2.2 Protocol
Before (Pre), at 10 min during (Online), and 3 min after (Offline) anodal HD-tDCS
(2 mA, 20 min) targeting the left SMC, subjects performed a self-paced SFS task
(i.e., sequential tapping of the index, middle, ring and fourth finger against the
thumb) with their right or left hand in an alternating blocked design (30-s rest and
30-s SFS task, repeated five times for each hand). Prior to the start of the experi-
ment, subjects were familiarised with the SFS task in order to maintain a consistent
rate of finger sequence taps (between 2 and 3 Hz), which was confirmed prior to the
start of the Pre condition. The number of finger sequence taps was counted by the
experimenter during each of the experimental SFS task blocks.
2.3 Experimental Setup
tDCS A Startim
®
tDCS system (Neuroelectrics, Spain) was used to deliver con-
stant direct currents to the left SMC via a 4 !1 anodal HD-tDCS electrode montage
(active anode electrode at the centre surrounded by four return electrodes each at a
distance of ~3.5 cm from the active electrode) [3]. The five electrodes (3.14 cm
2
AgCl electrodes) were secured on the scalp in the adjacent 10-10 EEG electrode
system positions (anode: C3, and 4 return electrodes: FC1, FC5, CP1, CP5) using
conductive paste (Ten20
®
, Weaver and Company, USA) and held in place using a
specially designed synthetic cap to hold the HD-tDCS electrodes and fNIRS probes
on the head (see Fig. 44.1 for layout).
fNIRS A continuous wave multi-channel Oxymon MkIII fNIRS system (Artinis
Medical Systems, The Netherlands) was used to measure changes in bilateral SMC
O
2
Hb and HHb concentration values during the SFS task. Four receiver (avalanche
photodiode) and 12 transmitter (pulsed laser diode) probes were placed in the
synthetic cap to obtain 16 channels (each channel represented by a receiver-
transmitter combination separated by ~3 cm) primarily covering the left (eight
channels) and right (eight channels) SMC regions (see Fig. 44.1 for locations of the
16 channels). Two wavelengths (856 and 781 nm) per channel were used at a
sampling rate of 10 Hz.
The changes in O
2
Hb and HHb concentration values (expressed in μM), calculated
according to a modified Beer-Lambert Law and including an age-dependent con-
stant differential pathlength factor (4.99 + 0.067*Age
0.814
)[7], were transferred
from the fNIRS system to a personal computer. During the data collection proce-
dure, the time course of changes in O
2
Hb and HHb concentration values were
displayed in real time, and the signal quality and absence of movement artefacts
were verified.
44 Effects of Anodal High-Definition Transcranial Direct Current Stimulation on... 353
2.4 Data Analysis
The time course of changes in O
2
Hb and HHb concentration values for each of the
16 channels were first low-pass filtered at 0.1 Hz to attenuate cardiac signal,
respiration, and Mayer-wave systemic oscillations [5]. The time course of changes
in O
2
Hb and HHb concentration values for each SFS task block (30-s duration)
were then normalized using the mean of the O
2
Hb and HHb values measured during
the last 5 s of the 30-s rest period preceding each SFS task block. These were then
sample-to-sample averaged (i.e., 10 samples/s) the O
2
Hb and HHb time course
values over the 5 SFS task blocks, yielding one average O
2
Hb and HHb time course
for each subject.
In order to locate the channel to represent the level of SMC activation during the
SFS task period for each subject, we first selected in the Pre condition one channel
Fig. 44.1 Locations of the HD-tDCS electrodes and fNIRS probes on a 10-10 EEG electrode
system layout. Each of the 16 fNIRS channels are represented by a receiver-transmitter
combination
354 M. Muthalib et al.
on the left and right SMC (i.e., the channel corresponding to one of the four
channels located adjacent to the C3 and C4 electrode positions; see Fig. 44.1)
showing peak and consistent SFS task-related haemodynamic responses (i.e.,
increase in O
2
Hb and decrease in HHb), and then used the same channels for
analysis in the Online and Offline conditions. Following the selection of the left
and right SMC channel, we computed first the individual subject O
2
Hb maximum
(O
2
Hb
max
) and the HHb minimum (HHb
min
) values from the left and right SMC and
then group averaged these values for the Pre, Online and Offline conditions.
2.5 Statistical Analysis
For statistical analysis of the fNIRS dependent variables (O
2
Hb
max
and HHb
min
), a
Condition (Pre, Online, Offline) x Hemisphere (Left SMC, Right SMC) x Hand
(Left, Right) repeated measures ANOVA was used. If a significant main or inter-
action effect was evident, then post-hoc Tukeys HSD (honestly significant differ-
ence) tests were performed. For statistical analysis of the behavioural dependent
variable (SFS tap rate), a Condition (Pre, Online, Offline) !Hand (Left and Right)
repeated measures ANOVA was used. Significance was set at P #0.05. Data are
presented as mean "SD.
3 Results
The behavioural results indicated that subjects were able to perform the SFS task at
a consistent rate (2.51 "0.32 Hz) with their right and left hand with no significant
difference over the three conditions.
Figure 44.2 shows a typical time course of changes in O
2
Hb and HHb from the
left and right SMC during the right hand SFS task. Before anodal HD-tDCS (i.e.,
Pre condition), the right and left hand SFS task induced a cortical haemodynamic
response (i.e., increase in O
2
Hb and decrease in HHb) in the bilateral SMC, with
a greater response in the contralateral hemisphere to the hand performing the task
(see Fig. 44.2 for right hand SFS task).
Table 44.1 shows the group average O
2
Hb
max
and HHb
min
values from the left
and right SMC during the SFS task for the Pre, Online and Offline conditions. The
ANOVA showed no significant Condition x Hemisphere x Hand interaction, but a
significant (p <0.001) Condition x Hemisphere interaction effect for both O
2
Hb
max
and HHb
min
. The post-hoc showed that for the right SMC (i.e., unstimulated
hemisphere), although there was no significant difference in O
2
Hb
max
over the
three conditions, there was a significantly (p <0.001) smaller HHb
min
in both the
Online and Offline conditions compared to Pre. For the left SMC (i.e., stimulated
hemisphere), O
2
Hb
max
significantly (p <0.001) increased in the Online condition
compared to Pre, but returned to Pre levels in the Offline condition. In contrast,
44 Effects of Anodal High-Definition Transcranial Direct Current Stimulation on... 355
there was a significantly (p <0.001) smaller HHb
min
during both the Online and
Offline conditions compared to Pre.
4 Discussion
The main new finding of this study was of a significant reduction in bilateral SMC
activation (based on smaller HHb
min
) for a similar motor behaviour (i.e., SFS tap
rate) in the Online and Offline conditions compared to the Pre condition.
Although O
2
Hb
max
increased significantly only in the Online condition for the
stimulated left SMC, we consider that changes in O
2
Hb were likely contaminated
by anodal HD-tDCS induced local skin blood flow changes in the vicinity of the
HD-tDCS electrodes. In contrast, changes in HHb are considered less affected by
skin blood flow changes [8] and we found less variability in HHb responses during
the five blocks of the SFS task than with O
2
Hb responses. Therefore, we suggest
that HHb may be a more reliable marker of HD-tDCS induced effects on task-
related cortical activation.
The present study findings of smaller bilateral SMC HHb
min
values during the
SFS task in the Online and Offline conditions compared to Pre could be related to a
greater efficiency of neuronal transmission [9] in the bilateral SMC (i.e., less
synaptic input for the same neuronal output) that reduced SFS task-induced
regional blood flow and thus produced smaller changes in fNIRS-derived HHb in
the bilateral SMC. Furthermore, since the effect of anodal HD-tDCS on SFS task-
related SMC activation was similar for both the Online and Offline conditions, it
seems that synaptic neuroplastic modifications are necessary to induce these motor
task-related reductions in SMC activation.
Fig. 44.2 Typical task-related changes in oxygenated (O
2
Hb) and deoxygenated (HHb)
haemoglobin concentrations in the left (Left SMC) and right (Right SMC) sensorimotor cortex
during the right hand simple finger sequence task in the Pre condition. Dashed vertical lines denote
the start and end of the 30-s SFS task period
356 M. Muthalib et al.
Table 44.1 Group mean ("SD) oxygenated haemoglobin maximum (O
2
Hb
max
) and deoxygenated haemoglobin minimum (HHb
min
) concentration values
from the left (Left SMC) and right (Right SMC) sensorimotor cortex during the simple finger sequence task performed before (Pre), during (Online) and after
(Offline) anodal HD-tDCS
Left SMC Right SMC
Pre Online Offline Pre Online Offline
O
2
Hb
max
(ΔμM) 0.83 "0.28 0.99 "0.29* 0.83 "0.26 0.78 "0.38 0.76 "0.49 0.77 "0.40
HHb
min
(ΔμM) $0.38 "0.14 $0.33 "0.11* $0.27 "0.09* $0.34 "0.14 $0.29 "0.17* $0.28 "0.11*
*:p <0.001; significantly different from Pre
44 Effects of Anodal High-Definition Transcranial Direct Current Stimulation on... 357
In the present study, despite the attempt at focal stimulation to the left SMC
by anodal 4 !1 HD-tDCS, the effects on motor task-related cortical activation
were bilateral, probably because intervening in one part of a distributed neural
network system has effects on many nodes in the system [10]. It should also be
noted that we found the same effect on bilateral SMC activation during SFS
movements with the left and right hand. Although it would have been more
expected to have observed a difference in ipsilateral and contralateral SMC acti-
vation between the left and right hand, evidence exists from recent studies which
demonstrate that unilateral tDCS of the SMC can have bilateral effects [11,12]. For
example, Hendy et al. [12] have found bilateral changes in activation and muscle
strength after anodal unilateral tDCS, and Roy et al. [12] found wide bihemispheric
effects on EEG of unilateral HD-tDCS.
5 Conclusion
This preliminary study has shown for the first time that both Online and Offline
anodal HD-tDCS reduced bilateral SMC activation to perform a sequential finger
movement task. These positive initial results justify further research efforts to
optimize the effects and enhance our understanding of the neurophysiological
mechanisms of HD-tDCS-induced neuroplastic modifications.
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44 Effects of Anodal High-Definition Transcranial Direct Current Stimulation on... 359
... atDCS using a high-definition (HD-atDCS) electrode montage (4x1) has been shown to increase the focality and intensity of stimulation at the primary motor cortex target (Datta et al., 2009). Our preliminary fNIRS study (Muthalib et al., 2016) using HD-atDCS (2 mA, 20 min) during a sequential finger opposition (SFO) task found a decrease in task-related activation in the targeted left SMC compared to prestimulation. However, since the after effects of HD-atDCS show peak changes in cortical excitability after a delay of 30 minutes from the cessation of the stimulation (Kuo et al., 2013), it is not known whether task-related SMC activation would also show greater 6 neuromodulatory effects up to 30 min. ...
... Patterns of O 2 Hb and HHb changes are well correlated with the fMRI BOLD signal (Leff et al., 2011) and can be used to identify the level of cortical activation (Leff et al., 2011). Due to the greater influence of superficial blood vessels on O 2 Hb signals (Kirilina et al., 2012), HHb changes (Muthalib et al., 2016) and an integrated measure combining O 2 Hb and HHb (i.e., Hb diff = O 2 Hb -HHb) (Lu et al., 2015) is the most suitable metric for accurately detecting task-related changes in SMC activation. Indeed we found much larger variability in the O 2 Hb integral values between subjects, which could account for the non-significant ANOVA effects. ...
... Direct current was generated by a current stimulator (Startim®, Neuroelectrics NE, Spain) and delivered to the left SMC of the subject through a 4x1 anodal HD-tDCS montage (active anode electrode on C3 surrounded by four return electrodes on FC1, FC5, CP5 and CP1; each at a distance of ~4 cm from the active electrode (Muthalib et al., 2016). The five electrodes (3.14 cm² AgCl electrodes) were secured on the scalp according to the 10-10 EEG electrode system positions using conductive paste (Ten20®, Weaver and Company, USA) and held in place using a specially designed plastic headgear to arrange the HD-tDCS electrodes and fNIRS probes on the head (see Fig. 3 for layout). ...
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... Recent advancements in software-based signal processing techniques have enabled rapid or real-time analyses of functional activation of the brain [71][72][73][74][75][76]. Integration of these techniques into the tDCS system may improve the efficacy in a real-clinical setting. ...
... An increase in the resting-state inter-hemispheric connectivity with increased flexion speed was measured after bi-hemispheric tDCS over the primary motor cortex [76]. tDCS over the sensorimotor cortex resulted in a significant reduction in the local brain activities required for the same sequential finger movement, representing a greater efficiency of neural transmission after tDCS [75]. With respect to simultaneous measurement with tDCS, fNIRS may be a better option than EEG, considering that its optical measurement system has no interference with the electrical current induced by tDCS. ...
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... Direct electrical current was generated by a tDCS stimulator (Starstim ® , Neuroelectrics NE, Spain) and delivered to the left M1 representation of the dominant right ECR of the subject though a 4Â1 ring HD a-tDCS montage with the active anode on the TMS "hot-spot" (see TMS measurement section) surrounded by four return electrodes, each at a distance of 35 mm from the active electrode (Muthalib, Besson, Rothwell, Ward, & Perrey, 2016a). The five electrodes (3.14 cm 2 AgCl electrodes) were secured on the scalp using conductive paste (Ten20 ® , Weaver and Company, USA). ...
... The left M1 aHD-tDCS intervention produced a surprising increase in excitability in the opposite right M1. Pioneer tDCS study claimed the impossibility to modulate the cortical excitability of the opposite M1 via transcallosal pathways (Lang, Nitsche, Paulus, Rothwell, & Lemon, 2004). However, the increase of current intensities and protocol durations may bring this possibility as recent research reported contralateral effects of tDCS (Davidson, Bolic, & Tremblay, 2016;Muthalib et al., 2016a;Tazoe, Endoh, Kitamura, & Ogata, 2014;Teo et al., 2015). These studies show the direct impact of tDCS of one M1 on the opposite M1 most likely via transcallosal pathways. ...
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... The experiment was divided into three sessions: 10 min before (pre), at 10 min during ("online"), and 3 min after ("offline") anode centered HD-tDCS of the SMC (2 mA: 20 min), the subject performed a self initi- ated simple finger sequence (SFS) task with their right and left hand in an alternat- ing block design (30-s task and 30-s rest, repeated 5 times). The fNIRS results showed that anodal HD-tDCS induced a significant reduction in bilateral SMC acti- vation (i.e., smaller decrease in HHb) for a similar SFS frequency (i.e., motor out- put) ( Muthalib et al. 2016) that is shown for NIRS channels 4 and 12 (left and right SMC respectively) in Fig. 11.2a. Muthalib and colleagues (2016) postulated that anodal HD-tDCS induced a "greater efficiency" of neuronal transmission in the bilateral SMC to perform the same SFS task where "greater efficiency" can be related to anodal HD-tDCS "priming" the NVU with evoked hemodynamic response (Guhathakurta and Dutta 2016). ...
... Muthalib and colleagues (2016) postulated that anodal HD-tDCS induced a "greater efficiency" of neuronal transmission in the bilateral SMC to perform the same SFS task where "greater efficiency" can be related to anodal HD-tDCS "priming" the NVU with evoked hemodynamic response (Guhathakurta and Dutta 2016). Indeed, the resting state fNIRS data showed focal hemodynamic responses as a correlate of the electrical field distribution (see Fig. 11.1c) in the stimulated hemisphere during HD-tDCS ( Muthalib et al. 2016). Figure 11.2b shows that online HD-tDCS at rest induced a gradual increase in the concentration of O2Hb (red line) at the left hemisphere peaking after 5 min at the fNIRS channels located adjacent to the 4 × 1 HD-tDCS electrode montage (e.g., channels 3, 4, 5, 6). ...
Chapter
Transcranial direct current stimulation provides researchers and clinicians with the ability to non-invasively modulate the firing rate of neurons. However, the focality and overall consequences of tDCS for neural systems is often unclear based on tDCS alone. When tDCS is paired with state-of-the-art neurophysiology, neuroimaging and spectroscopic techniques, researchers and clinicians can gain important insight into the neural underpinnings of tDCS effects, as well as gain novel insight into brain-behaviour relationships. In this chapter, we will consider approaches for integration of tDCS with magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), near infrared spectroscopy (NIRS) imaging, and electroencephalography (EEG). We will discuss technical considerations, benefits, limitations, and optimal application strategies for the integration of each methodology with transcranial direct current stimulation. This chapter will provide an important foundation for understanding “how” to integrate these technologies, as well as “when” integration can be of benefit for researchers and clinicians.
... Changes in microvasculature cerebral blood flow and blood oxygenation during and after HD-tDCS were described in healthy subjects [38][39][40] and patients after traumatic brain injury [41]. Indeed, different methods such as functional magnetic resonance imaging, functional near-infrared spectroscopy, and computerized tomography can also investigate cerebral hemodynamic features [3,4]. ...
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Since neuronal activity is coupled with neurovascular activity, we aimed to analyze the cerebral blood flow hemodynamics during and following high-definition transcranial direct current stimulation (HD-tDCS). We assessed the mean middle cerebral artery blood flow velocity (MCA-BFv) bilaterally using transcranial doppler ultrasound, during and after HD-tDCS, in eleven right-handed healthy adult participants (6 women, 5 men; mean age 31±5.6 years old), with no evidence of brain or cardiovascular dysfunction. The HD-tDCS electrode montage was centered over the right temporo-parietal junction. The stimulation protocol comprised 3 blocks of 2 minutes at each current intensity (1, 2, and 3 mA) and an inter-stimulus interval of 5 minutes between blocks. Participants received three electrical stimulation conditions (anode center, cathode center, and sham) on three different days, with an interval of at least 24 hours. Stimulation was well tolerated across HD-tDCS conditions tested, and the volunteers reported no significant discomfort related to stimulation. There was no significant difference in the right or the left MCA-BFv during or after the stimulation protocol across all stimulation conditions. We conclude that at a range of intensities, vascular reaction assessed using middle cerebral artery blood flow is not significantly altered during or after HD-tDCS both locally and remotely, which provides further evidence for the safety of HD-tDCS.
... The data was sampled with a frequency of 7.81 Hz. After the tDCS application, the concentration changes in O 2 Hb and HHb were recorded during the performance of the three-state Markov decision task for approximately 50 min, including 2 min baseline measurement before and after the task execution to control for time-related drift rates in the data (Choe et al., 2016;Muthalib et al., 2016). ...
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The ability to learn sequential contingencies of actions for predicting future outcomes is indispensable for flexible behavior in many daily decision-making contexts. It remains open whether such ability may be enhanced by transcranial direct current stimulation (tDCS). The present study combined tDCS with functional near-infrared spectroscopy (fNIRS) to investigate potential tDCS-induced effects on sequential decision-making and the neural mechanisms underlying such modulations. Offline tDCS and sham stimulation were applied over the left and right dorsolateral prefrontal cortex (dlPFC) in young male adults ( N = 29, mean age = 23.4 years, SD = 3.2) in a double-blind between-subject design using a three-state Markov decision task. The results showed (i) an enhanced dlPFC hemodynamic response during the acquisition of sequential state transitions that is consistent with the findings from a previous functional magnetic resonance imaging (fMRI) study; (ii) a tDCS-induced increase of the hemodynamic response in the dlPFC, but without accompanying performance-enhancing effects at the behavioral level; and (iii) a greater tDCS-induced upregulation of hemodynamic responses in the delayed reward condition that seems to be associated with faster decision speed. Taken together, these findings provide empirical evidence for fNIRS as a suitable method for investigating hemodynamic correlates of sequential decision-making as well as functional brain correlates underlying tDCS-induced modulation. Future research with larger sample sizes for carrying out subgroup analysis is necessary in order to decipher interindividual differences in tDCS-induced effects on sequential decision-making process at the behavioral and brain levels.
... [45,46] Muthalib et al., were the first to study the effect of HD-tDCS on sequential finger movements by stimulating the human sensorimotor cortex. [47] HD-tDCS was recently introduced and research is being conducted in modelling smaller electrodes with different configurations. Currently, the areas being concentrated on are psychiatry, visual cortex, and auditory cortex. ...
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Transcranial electrical stimulation (TES) uses direct or alternating current to non-invasively stimulate the brain. Neuronal activity in the brain is modulated by the electrical field according to the polarity of the current being applied. TES includes transcranial direct current stimulation (tDCS), transcranial random noise stimulation, and transcranial alternating current stimulation (tACS). tDCS and tACS are the two non-invasive brain stimulation techniques that have been used alone or in combination with other rehabilitative therapies for the improvement of motor control in hemiparesis. Increasing research in these methods is being carried out to improvise on the existing technology because they have proven to exhibit a lasting effect, thereby contributing to brain plasticity and motor re-learning. Artificial stimulation of the lesioned or non-lesioned hemisphere induces participation of its cells when a movement is being performed. The devices are portable, stimulation is easy to deliver, and they are not known to cause any major side effects which are the foremost reasons for their trials in stroke rehabilitation. Recent research is focused on maximizing the outcome of stroke rehabilitation by combining them with other modalities. This review focuses on stimulation protocols, parameters, and the results obtained by these techniques and their combinations.
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FNIRS pre-processing and processing methodologies are very important—how a researcher chooses to process their data can change the outcome of an experiment. The purpose of this review is to provide a guide on fNIRS pre-processing and processing techniques pertinent to the field of human motor control research. One hundred and twenty-three articles were selected from the motor control field and were examined on the basis of their fNIRS pre-processing and processing methodologies. Information was gathered about the most frequently used techniques in the field, which included frequency cutoff filters, wavelet filters, smoothing filters, and the general linear model (GLM). We discuss the methodologies of and considerations for these frequently used techniques, as well as those for some alternative techniques. Additionally, general considerations for processing are discussed.
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HD-tDCS (High-definition transcranial direct current stimulation) is a novel non-invasive brain stimulation (NIBS) technique based on the principle that when weak intensity electric currents are targeted on specific areas of the scalp, they cause underlying cortical stimulation. HD-tDCS shares its technical methodology with conventional tDCS (montage comprising of one anode and one cathode) except for a few modifications that are believed to have focal and longer-lasting neuromodulation effects. Although HD-tDCS is a recently available NIBS technique, impactful studies, case reports, and few controlled trials have been conducted in this context, facilitating an understanding of its neurobiological effects and the clinical translation of the same in health care set-up. The current article narratively reviews the mechanism of action of HD-tDCS, and it systematically examines the cognitive, clinical, and neurobiological effects of HD-tDCS in healthy volunteers as well as patients with neuropsychiatric conditions. Thus, this review attempts to explore the role of HD-tDCS in present-day practice and the future in the context of various neurological and psychiatric disorders.
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Evidence suggests that the cross-transfer of strength following unilateral training may be modulated by increased corticospinal excitability of the ipsilateral primary motor cortex, due to cross-activation. Anodal-tDCS (a-tDCS) has been shown to acutely increase corticospinal excitability and motor performance, which may enhance this process. Therefore, we sought to examine changes in neural activation and strength of the untrained limb following the application of a-tDCS during a single unilateral strength training session. Ten participants underwent three conditions in a randomized, double-blinded crossover design: (1) strength training + a-tDCS, (2) strength training + sham-tDCS and (3) a-tDCS alone. a-tDCS was applied for 20 min at 2 mA over the right motor cortex. Unilateral strength training of the right wrist involved 4 × 6 wrist extensions at 70 % of maximum. Outcome measures included maximal voluntary strength, corticospinal excitability, short-interval intracortical inhibition, and cross-activation. We observed a significant increase in strength of the untrained wrist (5.27 %), a decrease in short-interval intracortical inhibition (-13.49 %), and an increase in cross-activation (15.71 %) when strength training was performed with a-tDCS, but not following strength training with sham-tDCS, or tDCS alone. Corticospinal excitability of the untrained wrist increased significantly following both strength training with a-tDCS (17.29 %), and a-tDCS alone (15.15 %), but not following strength training with sham-tDCS. These findings suggest that a single session of a-tDCS combined with unilateral strength training of the right limb increases maximal strength and cross-activation to the contralateral untrained limb.
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This study investigated whether manipulation of motor cortex excitability by transcranial direct current stimulation (tDCS) modulates neuromuscular fatigue and functional near-infrared spectroscopy (fNIRS)-derived prefrontal cortex (PFC) activation. Fifteen healthy men (27.7 ± 8.4 years) underwent anodal (2 mA, 10 min) and sham (2 mA, first 30 s only) tDCS delivered to the scalp over the right motor cortex. Subjects initially performed a baseline sustained submaximal (30 % maximal voluntary isometric contraction, MVC) isometric contraction task (SSIT) of the left elbow flexors until task failure, which was followed 50 min later by either an anodal or sham treatment condition, then a subsequent posttreatment SSIT. Endurance time (ET), torque integral (TI), and fNIRS-derived contralateral PFC oxygenated (O2Hb) and deoxygenated (HHb) hemoglobin concentration changes were determined at task failure. Results indicated that during the baseline and posttreatment SSIT, there were no significant differences in TI and ET, and increases in fNIRS-derived PFC activation at task failure were observed similarly regardless of the tDCS conditions. This suggests that the PFC neuronal activation to maintain muscle force production was not modulated by anodal tDCS.
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A major methodological challenge of functional near-infrared spectroscopy (fNIRS) is its high sensitivity to haemodynamic fluctuations in the scalp. Superficial fluctuations contribute on the one hand to the physiological noise of fNIRS, impairing the signal-to-noise ratio, and may on the other hand be erroneously attributed to cerebral changes, leading to false positives in fNIRS experiments. Here we explore the localisation, time course and physiological origin of task-evoked superficial signals in fNIRS and present a method to separate them from cortical signals. We used complementary fNIRS, fMRI, MR-angiography and peripheral physiological measurements (blood pressure, heart rate, skin conductance and skin blood flow) to study activation in the frontal lobe during a continuous performance task. The General Linear Model (GLM) was applied to analyse the fNIRS data, which included an additional predictor to account for systemic changes in the skin. We found that skin blood volume strongly depends on the cognitive state and that sources of task-evoked systemic signals in fNIRS are co-localized with veins draining the scalp. Task-evoked superficial artefacts were mainly observed in concentration changes of oxygenated haemoglobin and could be effectively separated from cerebral signals by GLM analysis. Based on temporal correlation of fNIRS and fMRI signals with peripheral physiological measurements we conclude that the physiological origin of the systemic artefact is a task-evoked sympathetic arterial vasoconstriction followed by a decrease in venous volume. Since changes in sympathetic outflow accompany almost any cognitive and emotional process, we expect scalp vessel artefacts to be present in a wide range of fNIRS settings used in neurocognitive research. Therefore a careful separation of fNIRS signals originating from activated brain and from scalp is a necessary precondition for unbiased fNIRS brain activation maps.
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Electrophysiological studies in humans and animals suggest that noninvasive neurostimulation methods such as transcranial direct current stimulation (tDCS) can elicit long-lasting [1], polarity-dependent [2] changes in neocortical excitability. Application of tDCS can have significant and selective behavioral consequences that are associated with the cortical location of the stimulation electrodes and the task engaged during stimulation [3-8]. However, the mechanism by which tDCS affects human behavior is unclear. Recently, functional magnetic resonance imaging (fMRI) has been used to determine the spatial topography of tDCS effects [9-13], but no behavioral data were collected during stimulation. The present study is unique in this regard, in that both neural and behavioral responses were recorded using a novel combination of left frontal anodal tDCS during an overt picture-naming fMRI study. We found that tDCS had significant behavioral and regionally specific neural facilitation effects. Furthermore, faster naming responses correlated with decreased blood oxygen level-dependent (BOLD) signal in Broca's area. Our data support the importance of Broca's area within the normal naming network and as such indicate that Broca's area may be a suitable candidate site for tDCS in neurorehabilitation of anomic patients, whose brain damage spares this region.
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Since the rediscovery of transcranial direct current stimulation (tDCS) about 10 years ago, interest in tDCS has grown exponentially. A noninvasive stimulation technique that induces robust excitability changes within the stimulated cortex, tDCS is increasingly being used in proof-of-principle and stage IIa clinical trials in a wide range of neurological and psychiatric disorders. Alongside these clinical studies, detailed work has been performed to elucidate the mechanisms underlying the observed effects. In this review, the authors bring together the results from these pharmacological, neurophysiological, and imaging studies to describe their current knowledge of the physiological effects of tDCS. In addition, the theoretical framework for how tDCS affects motor learning is proposed.
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The goal of this study was to develop methods for simultaneously acquiring electrophysiological data during high-definition transcranial direct current stimulation (tDCS) using high-resolution electroencephalography (EEG). Previous studies have pointed to the after-effects of tDCS on both motor and cognitive performance, and there appears to be potential for using tDCS in a variety of clinical applications. However, little is known about the real-time effects of tDCS on rhythmic cortical activity in humans due to the technical challenges of simultaneously obtaining electrophysiological data during ongoing stimulation. Furthermore, the mechanisms of action of tDCS in humans are not well understood. We have conducted a simultaneous tDCS-EEG study in a group of healthy human subjects. Significant acute and persistent changes in spontaneous neural activity and event-related synchronization (ERS) were observed during and after the application of high-definition tDCS over the left sensorimotor cortex. Both anodal and cathodal stimulation resulted in acute global changes in broadband cortical activity which were significantly different than the changes observed in response to sham stimulation. For the group of eight subjects studied, broadband individual changes in spontaneous activity during stimulation were apparent both locally and globally. In addition, we found that high-definition tDCS of the left sensorimotor cortex can induce significant ipsilateral and contralateral changes in event-related desynchronization and ERS during motor imagination following the end of the stimulation period. Overall, our results demonstrate the feasibility of acquiring high-resolution EEG during high-definition tDCS and provide evidence that tDCS in humans directly modulates rhythmic cortical synchronization during and after its administration.
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