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Anterior temporal artery tap to identify systemic interference using short-separation NIRS measurements: A NIRS/EEG-tDCS study

Authors:

Abstract

Transcranial direct current stimulation (tDCS) has been shown to modulate neural activity. Neural activity has been shown to be closely related, spatially and temporally, to cerebral blood flow (CBF) that supplies glucose via neurovascular coupling. Therefore, noninvasive and continuous monitoring of neural activity is possible with a measure of cerebral hemoglobin oxygenation using near-infrared spectroscopy (NIRS). In principal accordance, NIRS can capture the hemodynamic response to tDCS but the challenge remains in removing the systemic interference occurring in the superficial layers of the head that are also affected by tDCS. An approach may be to use short optode separations to measure systemic hemodynamic fluctuations occurring in the superficial layers which can then be used as regressors to remove the systemic contamination. Here, we demonstrate that temporal artery tap may be used to better identify systemic interference using this short-separation NIRS. Moreover, NIRS-EEG joint-imaging during anodal tDCS was used to measure changes in mean cerebral haemoglobin oxygen saturation (rSO2) along with changes in the log-transformed mean-power of EEG within 0.5 Hz-11.25 Hz. We found that percent change in the mean rSO2 better correlated with the corresponding percent change in log-transformed mean-power of EEG within 0.5 Hz-11.25 Hz frequency band after removing the systemic contamination using the temporal artery tap method. Based on our findings, we propose that anterior temporal artery tap technique presented in this paper may be able to classify carotid stenosis, external carotid artery stenosis, and internal carotid artery stenosis patients using the laterality in the hemodynamic response evoked by anodal tDCS both at the brain as well as at the superficial layers. These findings may have important implications for both prognosis and rehabilitation of patients with intracranial stenosis.
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Abstract—Transcranial direct current stimulation (tDCS)
has been shown to modulate neural activity. Neural activity has
been shown to be closely related, spatially and temporally, to
cerebral blood flow (CBF) that supplies glucose via
neurovascular coupling. Therefore, noninvasive and continuous
monitoring of neural activity is possible with a measure of
cerebral hemoglobin oxygenation using near-infrared
spectroscopy (NIRS). In principal accordance, NIRS can
capture the hemodynamic response to tDCS but the challenge
remains in removing the systemic interference occurring in the
superficial layers of the head that are also affected by tDCS. An
approach may be to use short optode separations to measure
systemic hemodynamic fluctuations occurring in the superficial
layers which can then be used as regressors to remove the
systemic contamination. Here, we demonstrate that temporal
artery tap may be used to better identify systemic interference
using this short-separation NIRS. Moreover, NIRS-EEG joint-
imaging during anodal tDCS was used to measure changes in
mean cerebral haemoglobin oxygen saturation (rSO2) along
with changes in the log-transformed mean-power of EEG
within 0.5 Hz-11.25 Hz. We found that percent change in the
mean rSO2 better correlated with the corresponding percent
change in log-transformed mean-power of EEG within 0.5 Hz-
11.25 Hz frequency band after removing the systemic
contamination using the temporal artery tap method. Based on
our findings, we propose that anterior temporal artery tap
technique presented in this paper may be able to classify
carotid stenosis, external carotid artery stenosis, and internal
carotid artery stenosis patients using the laterality in the
hemodynamic response evoked by anodal tDCS both at the
brain as well as at the superficial layers. These findings may
have important implications for both prognosis and
rehabilitation of patients with intracranial stenosis.
I. I
NTRODUCTION
Transcranial direct current stimulation (tDCS) - an
electrically based intervention directed at the central nervous
*
Research supported by INRIA-DST Associate Team 2014-2017.
M. Sood is with the International Institute of Information Technology,
Hyderabad, India.
U. Jindal is with the International Institute of Information Technology,
Hyderabad, India.
S. Roy Chowdhury is with the International Institute of Information
Technology, Hyderabad, India.
A. Das is with the Institute of Neurosciences Kolkata, India.
D. Kondziella is with the Department of Neurology, Rigshospitalet,
Blegdamsvej 9, København Ø, Denmark 2100 and Institute of
Neuroscience, Norwegian University of Science and Technology,
Trondheim, Norway.
A. Dutta is with the Institut national de recherche en informatique et en
automatique (INRIA), Montpellier, France (e-mail: adutta@ieee.org).
system level - has been shown to modulate cortical neural
activity and is a promising tool to facilitate neuroplasticity
[1]. During neural activity, the electric currents from
excitable membranes of brain tissue superimpose at a given
location in the extracellular medium and generate a
potential, which is referred to as the electroencephalogram
(EEG) when recorded from the scalp [2]. Respective neural
activity has been shown to be closely related, spatially and
temporally, to cerebral blood flow (CBF) that supplies
glucose via neurovascular coupling [3]. The hemodynamic
response to neural activity can be captured by near-infrared
spectroscopy (NIRS), which enables continuous monitoring
of cerebral oxygenation and blood volume [4]. CBF is
increased in brain regions with enhanced neural activity via
metabolic coupling mechanisms [5]. We developed a
method for electroencephalography (EEG) - near-infrared
spectroscopy (NIRS) based assessment of neurovascular
coupling (NVC) during anodal tDCS [6]. Our preliminary
studies showed the feasibility of identifying the lesional
hemisphere in subacute stroke with a low-cost NIRS-tDCS
hardware [7]. A significant change in oxy-haemoglobin
(HbO2) post-tDCS from pre-tDCS baseline was found for
the contralesional (3.43 ± 0.86), but not the lesional side
(0.26 ± 0.28), p<0.01. Moreover, the results of the stroke
case series [6] showed that anodal tDCS induces a local
neurovascular response which may be used for assessing
regional NVC functionality. However, the challenge
remained in removing the systemic interference occurring in
the superficial layers of the head that are also affected by
tDCS where the importance of integrating short-separation
measurements both at the source and at the detector optode
has been shown [9].
In this paper, we present a temporal artery tap technique
to identify systemic interference using short-separation
NIRS measurements. We use NIRS-EEG joint-imaging to
measure the changes due to anodal tDCS in the mean
cerebral haemoglobin oxygen saturation (rSO2) along with
the changes in log-transformed mean-power of EEG within
0.5 Hz-11.25 Hz frequency band. We show that the percent
change in the mean rSO2 better correlated with the
corresponding percent change in log-transformed mean-
power of EEG within 0.5 Hz-11.25 Hz frequency band after
removing the systemic contamination using the proposed
temporal artery tap method.
Anterior temporal artery tap to identify systemic interference using
short-separation NIRS measurements: a NIRS/EEG-tDCS study
Mehak Sood, Utkarsh Jindal, Shubhajit Roy Chowdhury, Abhijit Das, Daniel Kondziella, Anirban
Dutta, Member, IEEE
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II. M
ETHODS
A. NIRS-EEG/tDCS joint-imaging
Simultaneous recording of NIRS and EEG [6] for
evaluation of NVC (see Figure 1a) was conducted on five
chronic (>6 months, see Table 1) ischemic stroke survivors
after obtaining informed consent. The patients had no
contraindications to non-invasive brain stimulation. All
experiments were conducted in accordance with the
Declaration of Helsinki. The NIRS-EEG/tDCS joint-imaging
protocol was similar to that of our prior NIRS-tDCS study
[7]. Participants were seated in a quiet room and their eyes
were open and fixed on a point on the wall in front of them
during the entire experiment. PISTIM (Neuroelectrics,
Spain) electrodes were placed over F3 (corresponding to left
hemisphere) and F4 (right hemisphere) of the international
10-20 EEG system, and SPONSTIM-25 (Neuroelectrics,
Spain) electrodes were placed over Cz. This F3 (F4 when
monitoring right hemisphere) anodal and Cz cathodal tDCS
montage was selected based on computational modeling
(using StimViewer, Neuroelectrics, Spain)[6] in order to
target primarily the outer convex brain territory (superficial
divisions) of the middle cerebral artery (MCA). MCA is one
of the three major paired arteries that supply blood to the
cerebrum and is the most commonly occluded vessel in
ischemic stroke. The tDCS at a current density of 0.526
A/m
2
was turned ON for 30 sec with 10 sec ramp-up and
ramp-down (see Figure 1b), which was repeated 15 times in
random order with 30 sec OFF periods in between for both
the lesional and contralesional hemispheres (ischemic stroke
was restricted to a single hemisphere). Eyes-open block-
averaged resting-state NIRS oximeter measurements were
conducted using our own custom-built hardware [7] just
above each eyebrow at the F3 and F4 sites using a custom-
built NIRS sensor (see Figure c) that was similar in design to
SomaSensor (SAFB-SM, INVOS, USA). The custom-built
NIRS sensor consisted of two LED sources (770 nm and 850
nm) and two photodiode detectors at a distance of 3 cm and
4 cm so that the short separation NIRS signal can be
regressed out from the longer separation NIRS signal in
order to diminish the systemic interference [10]. Also, eyes-
open resting state EEG (StarStim, Neuroelectrics, Spain)
was recorded at 500 Hz from the nearby electrodes F1, FC3,
F5, F2, FC4, F6 (international 10-20 system).
B. Temporal artery tap technique to identify systemic
interference using short separation NIRS measurements
The frontal branch of the superficial temporal artery
(anterior temporal) runs forward to the forehead and can be
manually tapped lightly to create fluctuations in the blood
supply to the forehead (see Figure 1c). These fluctuations
should be registered primarily by the short-separation NIRS
signal where the long-separation NIRS signal was
considered contaminated [10] with a filtered version of the
short-separation NIRS signal. This filter was estimated in the
resting-state (without tDCS) by least-square fitting the
recorded fluctuations in the short-separation NIRS signal
during temporal artery tap to the long-separation NIRS
signal. The temporal artery tap produced large (compared to
baseline NIRS signals) impulse-like fluctuations which
made the least-square fitting easier for the impulse response.
The systemic interference was then found by convolving
short-separation NIRS signal with the impulse response, and
then subtracting that from the respective long-separation
NIRS signal.
C. NIRS-EEG/tDCS joint-imaging data analysis with and
without temporal artery tap technique
EEG artifacts related to tDCS are possible due to issues
with electrical (e.g. unknown electrode impedance changes
during stimulation) and mechanical compatibility (saline
from sponges shunting neighboring electrodes) where
concurrent recording is possible with an optimized device
(Starstim; Neuroelectrics, Spain) and rational experimental
design (using PISTIM electrodes; Neuroelectrics, Spain)
[11]. The raw EEG was pre-processed using EEGLAB
functions (specifically, Artifact Subspace Reconstruction
method) [12] where artefactual epochs were also removed
following subsequent visual inspection of the data. Then, log-
transformed mean-power of EEG within the 0.5 Hz-11.25 Hz
range (selected based on our prior work [6]) from F3, F1,
FC3, F5 sites were averaged for left hemisphere and from the
F4, F2, FC4, F6 sites were averaged for the right hemisphere.
The percent change in log-transformed mean-power of EEG
within 0.5 Hz-11.25 Hz frequency band was computed for
the first 10 sec of ON periods (called "initial dip" in our prior
work [6]) relative to the first 10 sec of OFF periods. The
percent change in log-transformed mean-power of EEG
within 0.5 Hz-11.25 Hz frequency band was analyzed for
both the lesional and contralesional hemispheres. Eyes-open
block-averaged resting-state percent change in the mean
a)
b)
F3 anode or
F4 anode
OFF
10s 30s 10s 30s
30 times
tDCS at 0.526A/m
2
30s
OFF
F3 anode or
F4 anode
OFF
10s 30s 10s 30s
30 times
tDCS at 0.526A/m
2
30s
OFF
c)
Fig. 1. a) NIRS-EEG/tDCS joint-imaging montage, b) tDCS protocol,
c) digital tapping of anterior temporal artery.
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regional cerebral haemoglobin oxygen saturation (rSO2) in
the first 10 sec of ON periods relative to the first 10 sec of
OFF periods was measured from the NIRS signals where the
Baseline was set at the start of the experiment. In one case
(rSO2
subtract
), the short separation NIRS signal was subtracted
from the longer separation NIRS signal while in the other
case (rSO2
filter
), the short separation NIRS signal was
convolved with the impulse response and then subtracted
from the respective longer separation NIRS signal.
Table 1: Subject summary (M: male, F: female, MCA: middle cerebral
artery)
III. R
ESULTS
There was an increase in log-transformed mean-power of
EEG within 0.5 Hz-11.25 Hz frequency band in the first 10
sec of ON periods relative to the first 10 sec of OFF periods
for the contralesional hemisphere stimulation/recording.
This increase was primarily in the Theta (4–8 Hz) frequency
band in agreement to our prior results [6]. Also, there was a
corresponding decrease in the mean rSO2
subtract
in the first 10
sec of ON periods (called "initial dip" in our prior work [6])
relative to the first 10 sec of OFF periods where the
individual Baseline for rSO2
subtract
measurements was set at
the beginning of the session. The "initial dip" was greater for
the lesional hemisphere stimulation/recording where the
percent change in the mean rSO2
subtract
mostly correlated
with the corresponding percent change in the log-
transformed mean-power of EEG within 0.5 Hz-11.25 Hz
frequency band. Here, the percent change in the mean
rSO2
filter
for the lesional hemisphere better correlated with
the log-transformed mean-power of EEG within 0.5 Hz-
11.25 Hz frequency band as shown in Table 2.
Table 2: Summary of the results from the case series.
IV. D
ISCUSSION
In this study, we found that the percent change in the
mean rSO2 typically correlated with the corresponding
percent change in log-transformed mean-power of EEG
within 0.5 Hz-11.25 Hz frequency band. We also found in the
contralesional hemisphere of the stroke survivors that there
was an immediate increase in the 0.5Hz-11.25Hz frequency
band during anodal tDCS [6]; however, there was also
significant inter-subject variability after tDCS. Here, the
percent change in the mean rSO2
filter
for the lesional
hemisphere better correlated with the log-transformed mean-
power of EEG within 0.5 Hz-11.25 Hz frequency band than
the percent change in the mean rSO2
subtract
.
Of note, we found in some stroke subjects inter-
hemispheric laterality in the systemic interference as well as
rSO2
filter
evoked by anodal tDCS. Indeed, Large Artery
Atherothrombosis leads commonly to stenosis at the
bifurcation of the carotid artery (carotid stenosis). At the
bifurcation of the carotid artery arises the internal carotid
artery (ICA) that supplies blood to the brain as well as the
external carotid artery (ECA) that supplies blood to the head
and neck, such as face, scalp, etc. Inter-hemispheric laterality
of carotid stenosis may lead to laterality in the hemodynamic
response to tDCS both at the brain (due to ICA) as well as the
superficial layers (due to ECA). Since ischemic stroke
secondary to carotid stenosis is common in older people so
our preliminary studies showed the feasibility of identifying
the lesional hemisphere in subacute ischemic stroke with a
low-cost NIRS-tDCS hardware [7]. Nevertheless, it cannot be
excluded that in certain stroke survivors the lesioned
hemisphere may not respond to anodal tDCS with an increase
in neural activity. This may result in an absent change in the
cerebral metabolic rate of oxygen, CMRO2 (i.e., oxygen
consumption [13] that is defined as the difference of oxygen
flow into and out of the tissue) in the lesioned hemisphere as
compared to the non-lesioned hemisphere. To avoid this
potential problem, the effects of tDCS on neural activity can
be elucidated with simultaneous EEG, which provides an
independent measure to supplement NIRS recordings. Based
on prior work [14] that found an association of individual
resting state EEG alpha frequency and cerebral blood flow,
we hypothesize that if the changes in EEG during tDCS are
correlated with the changes in NIRS, then this may enable us
to measure the state of neurovascular coupling.
Indeed, prior cross-sectional studies suggested that
impaired cerebral haemodynamics precedes stroke where
cerebrovascular reactivity (CVR) reflects the capacity of
blood vessels to dilate, and is an important marker for brain
vascular reserve [15]. CVR provides an useful addition to the
traditional baseline blood flow measurement where severely
reduced CVR predicts the risk of ipsilateral ischemic stroke
and TIA [15] as well as mild cognitive impairment [3] and
vascular dementia [16]. A meta-analysis summarizing the
association between CVR impairment with stroke risk
demonstrated an ≈4-fold higher stroke risk in asymptomatic
patients with impaired cerebral blood flow [17]. However,
the cost, access, and availability of trained technicians and
physicians to perform the Transcranial Doppler (TCD) blood
flow velocity measurements of CVR in a community setting
is one of the main limitations [18]. Therefore, a low-cost
NIRS-tDCS hardware [7] for point-of-care-testing of CVR is
relevant. Moreover, with the anterior temporal artery tap
Case
Age/gender
Diagnosis Year of stroke
1 68/M Right MCA stroke 2010
2 74/F Left MCA stroke 2011
3 76/M Left MCA stroke 2011
4 72/M Right MCA stroke 2012
5 75/M Right MCA stroke 2012
Case
Post-tDCS
(mean±1 std. dev.)
Correlation
Coefficient with
rSO2
subtract
Correlation
Coefficient with
rSO2
filter
1 0.42±0.18 0.54±0.12
2 0.49±0.14 0.58±0.14
3 0.54±0.17 0.61±0.12
4 0.48±0.14 0.57±0.15
5 0.53±0.18 0.59±0.13
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technique presented in this paper, we may be able to classify
carotid stenosis, ECA stenosis, and ICA stenosis patients
based on the laterality in the hemodynamic response evoked
by anodal tDCS both at the brain (due to ICA) as well as at
the superficial layers (due to ECA). For example, ICA
stenosis vs. ECA stenosis may be classified based on the
laterality in the rSO2
filter
vs. laterality in the systemic
interference. If verified by future prospective multicenter
trials, these findings may have important implications for
both prognosis and rehabilitation of patients with intracranial
stenosis.
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... Increasing the source-detector (SD) separation past 2 cm monotonically increases sensitivity to brain tissue; diminishing returns appear to begin at around 4?5 cm (Strangman et al., 2013). In order to identify systemic interference using short-separation NIRS measurements (Sood et al., 2015a), a short SD separation was incorporated in the probe design to explicitly sample extracerebral tissues. The optimum short SD separation is 8.4 mm with the Colin27 head model (Brigadoi and Cooper, 2015). ...
... However, the ad hoc probe placement was limited to the forehead so that the hair follicles do not affect the readings. Also, NIRS imaging during tDCS requires identification of systemic interference using short-separation NIRS measurements (Sood et al., 2015a) to explicitly sample the extra-cerebral tissues response. We found interhemispheric laterality in the systemic interference as well as mean cerebral hemoglobin oxygen saturation evoked by anodal tDCS in some stroke subjects (Sood et al., 2015a). ...
... Also, NIRS imaging during tDCS requires identification of systemic interference using short-separation NIRS measurements (Sood et al., 2015a) to explicitly sample the extra-cerebral tissues response. We found interhemispheric laterality in the systemic interference as well as mean cerebral hemoglobin oxygen saturation evoked by anodal tDCS in some stroke subjects (Sood et al., 2015a). In those subjects, primarily with Large Artery Atherothrombosis (LAA), we hypothesized that?since LAA leads commonly to stenosis at the bifurcation of the carotid artery (carotid stenosis) the internal carotid artery (ICA) that supplies blood to the brain and the external carotid artery (ECA) that supplies blood to the head and neck (such as face, scalp, etc.)?interhemispheric laterality of a carotid stenosis may lead to laterality in the hemodynamic response to tDCS both at the brain tissue (due to ICA) as well as at the extracerebral tissues (due to ECA). ...
Article
Full-text available
Transcranial direct current stimulation (tDCS) modulates cortical neural activity and hemodynamics. Electrophysiological methods (electroencephalography-EEG) measure neural activity while optical methods (near-infrared spectroscopy-NIRS) measure hemodynamics coupled through neurovascular coupling (NVC). Assessment of NVC requires development of NIRS-EEG joint-imaging sensor montages that are sensitive to the tDCS affected brain areas. In this methods paper, we present a software pipeline incorporating freely available software tools that can be used to target vascular territories with tDCS and develop a NIRS-EEG probe for joint imaging of tDCS-evoked responses. We apply this software pipeline to target primarily the outer convexity of the brain territory (superficial divisions) of the middle cerebral artery (MCA). We then present a computational method based on Empirical Mode Decomposition of NIRS and EEG time series into a set of intrinsic mode functions (IMFs), and then perform a cross-correlation analysis on those IMFs from NIRS and EEG signals to model NVC at the lesional and contralesional hemispheres of an ischemic stroke patient. For the contralesional hemisphere, a strong positive correlation between IMFs of regional cerebral haemoglobin oxygen saturation and the log-transformed mean-power time-series of IMFs for EEG with a lag of about -15sec was found after a cumulative 550 sec stimulation of anodal tDCS. It is postulated that system identification, for example using a continuous-time autoregressive model, of this coupling relation under tDCS perturbation may provide spatiotemporal discriminatory features for the identification of ischemia. Furthermore, portable NIRS-EEG joint imaging can be incorporated into brain computer interfaces to monitor tDCS-facilitated neurointervention as well as cortical reorganization.
... Nevertheless, CW fNIRS offers a relatively inexpen- sive, non-invasive, safe, and portable method of monitoring microvascular hemodynamics in parallel to tDCS in a neurorehabilitation setting. However, CW fNIRS imaging during tDCS requires identification of systemic interference to avoid measuring fNIRS hemodynamic responses that are not due to neurovascular coupling (Tachtsidis and Scholkmann 2016), e.g., by the means of a regression analysis ( Kirilina et al. 2012) using short-separation NIRS measurements ( Sood et al. 2015) to explicitly sample the extra-cerebral tissue response. ...
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.
... Here, simultaneous EEG-NIRS imaging during perturbation with tDCS may help us in understanding the NVC underlying neural and hemodynamic responses post-stroke [17]. Furthermore, EEG-NIRS joint imaging may improve the specificity by estimating NVC as a biomarker for post-stroke impaired cerebral microvessels functionality [17] where non-neuronal systemic physiological fluctuations often contaminate fNIRS signals [18]. Therefore it is important to develop and integrate an EEG hardware with the NIRS hardware so that the joint neuroimaging technology can be leveraged to objectively detect and monitor the brain status. ...
... Since 210 no real-world network has exact symmetries so with intelligent placement of EEG-NIRS sensors (e.g. 211 to avoid systemic interference (Sood et al., 2015)) along with system identification and parameter 212 estimation techniques, it may be possible to track the spatiotemporal change of the states of the NVU. ...
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Transcranial direct current stimulation (tDCS) has been shown to modulate cortical neural activity. During neural activity, the electric currents from excitable membranes of brain tissue superimpose in the extracellular medium and generate a potential at scalp, which is referred as the electroencephalogram (EEG). Respective neural activity (energy demand) has been shown to be closely related, spatially and temporally, to cerebral blood flow (CBF) that supplies glucose (energy supply) via neurovascular coupling. The hemodynamic response can be captured by near-infrared spectroscopy (NIRS), which enables continuous monitoring of cerebral oxygenation and blood volume. This neurovascular coupling phenomenon led to the concept of neurovascular unit (NVU) that consists of the endothelium, glia, neurons, pericytes, and the basal lamina. Here, recent works suggest NVU as an integrated system working in concert using feedback mechanisms to enable proper brain homeostasis and function where the challenge remains in capturing these mostly nonlinear spatiotemporal interactions within NVU for brain-state dependent tDCS. In principal accordance, we propose EEG-NIRS-based whole-head monitoring of tDCS-induced neuronal and hemodynamic alterations during tDCS.
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The paper presents a point of care testing device for neurovascular coupling (NVC) from simultaneous recording of electroencephalogram (EEG) and near infra red spectroscopy (NIRS) during anodal transcranial direct current stimulation (tDCS). Here, anodal tDCS modulated cortical neural activity leading to hemodynamic response can be used to identify impaired cerebral microvessels functionality. The impairments in the cerebral microvessels functionality may lead to impairments in the cerebrovascular reactivity (CVR) where severely reduced CVR predicts the chances of transient ischemic attack (TIA) and ipsilateral stroke. The neural and hemodynamic responses to anodal tDCS were studied through joint imaging with EEG and NIRS where NIRS provided optical measurement of changes in tissue oxy-( ) and deoxy-( ) haemoglobin concentration and EEG captured alterations in the underlying neuronal current generators. Then, a cross-correlation method for the assessment of neurovascular coupling (NVC) underlying the site of anodal tDCS is presented. The feasibility studies on healthy subjects and stroke survivors showed detectable changes in the EEG and NIRS responses to a 0.526A/m2 of anodal tDCS. The NIRS system was bench tested on 15 healthy subjects that showed a statistically significant (p<0.01) difference in the signal to noise ratio (SNR) between the on and off states of anodal tDCS where the mean SNR of the NIRS device was found to be 42.33±1.33dB in the on state and 40.67±1.23dB in the off state. Moreover, the clinical study conducted on 14 stroke survivors revealed that the lesioned hemisphere with impaired circulation showed significantly (p<0.01) less change in than the non-lesioned side in response to anodal tDCS. The EEG study on healthy subjects showed a statistically significant (p<0.05) decrease around "individual alpha frequency" in the Alpha band (8-13Hz) following anodal tDCS. Moreover, the joint EEG-NIRS imaging on 4 stroke survivors showed an immediate increase in the Theta band (4Hz-8Hz) EEG activity after the start of anodal tDCS at the non-lesioned hemisphere. Furthermore, cross-correlation function revealed a significant (95 percent confidence interval) negative cross-correlation only at the non-lesioned hemisphere during anodal tDCS where the log-transformed mean-power of EEG within 0.5Hz-11.25Hz lagged response in one of the stroke survivors with white matter lesions. Therefore, it was concluded that anodal tDCS can perturb local neural and vascular activity (via NVC) which can be used for assessing regional NVC functionality where confirmatory clinical studies are required.
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Objective: A method for electroencephalography (EEG) - near-infrared spectroscopy (NIRS) based assessment of neurovascular coupling (NVC) during anodal transcranial direct current stimulation (tDCS) is presented. Methods: Anodal tDCS modulates cortical neural activity leading to a hemodynamic response, which was used to identify impaired NVC functionality. In this study, the hemodynamic response was estimated with NIRS. NIRS recorded changes in oxy-hemoglobin ( ) and deoxy-hemoglobin ( ) concentrations during anodal tDCS-induced activation of the cortical region located under the electrode and in-between the light sources and detectors. Anodal tDCS-induced alterations in the underlying neuronal current generators were also captured with EEG. Then, a method for the assessment of NVC underlying the site of anodal tDCS was proposed that leverages the Hilbert-Huang Transform. Results: The case series including four chronic (>6 months) ischemic stroke survivors (3 males, 1 female from age 31 to 76) showed non-stationary effects of anodal tDCS on EEG that correlated with the response. Here, the initial dip in at the beginning of anodal tDCS corresponded with an increase in the log-transformed mean-power of EEG within 0.5Hz-11.25Hz frequency band. The cross-correlation coefficient changed signs but was comparable across subjects during and after anodal tDCS. The log-transformed mean-power of EEG lagged response during tDCS but then led post-tDCS. Conclusion: This case series demonstrates changes in the degree of neurovascular coupling to a 0.526A/m2 square-pulse (0-30sec) of anodal tDCS. The initial dip in needs to be carefully investigated in a larger cohort, for example in patients with small vessel disease.
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A method for electroencephalography (EEG) - near-infrared spectroscopy (NIRS) based assessment of neurovascular coupling (NVC) during anodal transcranial direct current stimulation (tDCS). Anodal tDCS modulates cortical neural activity leading to a hemodynamic response, which was used to identify impaired NVC functionality. In this study, the hemodynamic response was estimated with NIRS. NIRS recorded changes in oxy-hemoglobin (HbO2) and deoxy-hemoglobin (Hb) concentrations during anodal tDCS-induced activation of the cortical region located under the electrode and in-between the light sources and detectors. Anodal tDCS-induced alterations in the underlying neuronal current generators were also captured with EEG. Then, a method for the assessment of NVC underlying the site of anodal tDCS was proposed that leverages the Hilbert-Huang Transform. The case series including four chronic (>6 months) ischemic stroke survivors (3 males, 1 female from age 31 to 76) showed non-stationary effects of anodal tDCS on EEG that correlated with the HbO2 response. Here, the initial dip in HbO2 at the beginning of anodal tDCS corresponded with an increase in the log-transformed mean-power of EEG within 0.5Hz-11.25Hz frequency band. The cross-correlation coefficient changed signs but was comparable across subjects during and after anodal tDCS. The log-transformed mean-power of EEG lagged HbO2 response during tDCS but then led post-tDCS. This case series demonstrated changes in the degree of neurovascular coupling to a 0.526 A/m(2) square-pulse (0-30 s) of anodal tDCS. The initial dip in HbO2 needs to be carefully investigated in a larger cohort, for example in patients with small vessel disease.
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The paper presents a phenomological model to capture cerebrovascular reactivity (CVR) that represented the capacity of blood vessels to dilate during anodal transcranial direct current stimulation (tDCS). Anodal tDCS modulated cortical neural activity leading to CVR, where it can identify impaired cerebral microvessels functionality leading to impairments of cerebral blood flow that may cause impairments in the cerebral functions. In this study, CVR was probed with near infra-red spectroscopy (NIRS) where NIRS recorded changes in oxy-haemoglobin and deoxy-haemoglobin concentrations during anodal tDCS-induced activation of the cortical region located under the electrode and in-between the light sources and detectors. The regional CVR during anodal tDCS was captured by adapting an arteriolar compliance model of the cerebral blood flow response to neural stimuli, where a fourth-order discrete-time model represented the haemodynamic response to anodal tDCS. A case study showed detectable CVR response (0-60sec) to a 0.526A/m 2 square-pulse (0-30sec) of anodal tDCS where these alterations in the vascular system may result in secondary changes in the cortical excitability. This needs to be carefully studied in the future with multi-modal imaging in a larger patient group, for example, in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy.
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Transcranial direct current stimulation (tDCS) is a technique that delivers weak electric currents through the scalp. This constant electric current induces shifts in neuronal membrane excitability, resulting in secondary changes in cortical activity. Although tDCS has most of its neuromodulatory effects on the underlying cortex, tDCS effects can also be observed in distant neural networks. Therefore, concomitant EEG monitoring of the effects of tDCS can provide valuable information on the mechanisms of tDCS. In addition, EEG findings can be an important surrogate marker for the effects of tDCS and thus can be used to optimize its parameters. This combined EEG-tDCS system can also be used for preventive treatment of neurological conditions characterized by abnormal peaks of cortical excitability, such as seizures. Such a system would be the basis of a non-invasive closed-loop device. In this article, we present a novel device that is capable of utilizing tDCS and EEG simultaneously. For that, we describe in a step-by-step fashion the main procedures of the application of this device using schematic figures, tables and video demonstrations. Additionally, we provide a literature review on clinical uses of tDCS and its cortical effects measured by EEG techniques.
Conference Paper
Near-infrared spectroscopy (NIRS) is a cerebral monitoring method that noninvasively and continuously measures cerebral hemoglobin oxygenation which is widely used for measurement of cerebral vascular status under various clinical condition. This paper describes the development of a 4-channel functional near infrared spectroscopy (fNIRS) based hardware that captures the hemodynamic changes in the frontal cortex of the brain, as a measure of cerebrovascular reserve (CVR), before and after anodal transcranial direct current stimulation. Impairments in CVR have been associated with increased risk of ischemic events and may stratify stroke risk in patients with high-grade internal carotid artery stenosis or occlusion. Transcranial direct current stimulation (tDCS) can up- and down-regulate cortical excitability depending on current direction and anodal stimulation can increase regional cerebral blood flow (rCBF) during stimulation. Thus combining NIRS with tDCS can be an easy and economical setup for use in clinical population at risk for ischemic stroke.
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