A 4X1 High-Definition Transcranial Direct Current Stimulation Device for Targeting Cerebral Micro Vessels and Functionality using NIRS

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2016 IEEE International Symposium on Nanoelectronic and Information Systems A 4X1 High-Definition Transcranial Direct Current Stimulation Device for Targeting Cerebral Micro Vessels and Functionality

1 2016 IEEE International Symposium on Nanoelectronic and Information Systems A 4X1 High-Definition Transcranial Direct Current Stimulation Device for Targeting Cerebral Micro Vessels and Functionality using NIRS Gaurav Sharma 1, Yashika Arora 2, Shubhajit Roy Chowdhury 3 1,2,3 School of Computing and Electrical Engineering Indian Institute of Technology, Mandi Himachal Pradesh, India 1 g_sharma@students.iitmandi.ac.in, 2 yashika_arora@ students.iitmandi.ac.in, 3 src@iitmandi.ac.in Abstract - The paper presents the development of 4X1 highdefinition transcranial direct current stimulation (tdcs). High- Definition Transcranial Direct Current Stimulation (HD-tDCS) has been proposed as a focal, noninvasive brain stimulation technique. In this technique, cerebral micro vessel and primary motor cortex are targeted with weak intensity direct current (approximately 2 ma) through small array of electrodes placed on the scalp. The primary mechanism of this technique is based on polarity-dependent shift of neuronal membrane potentials. These polarity-dependent shifts are used to up and downregulate cortical activity depending on weak direct current stimulation. The paper focuses on the design of the set up for HDtDCS and conventional tdcs. The power dissipation for highdefinition tdcs has also been compared with conventional tdcs. The performance of the HD-tDCS device has been analyzed and compared with a conventional anodal tdcs device in terms of regulating the cerebral micro vessels functionality, which thereby yields signals with better signal to noise ratio (SNR) using near infrared spectroscopy (NIRS). The average SNR of the NIRS device was found to be db in the OFF-state. In ON-state it is db and db for conventional and high-definition tdcs respectively. Keywords Electroencephalogram, near infrared spectroscopy, transcranial direct current stimulation, high-definition transcranial diect current stimulation. I. INTRODUCTION Transcranial direct current stimulation (tdcs) has been introduced as non-invasive brain stimulation (NIBS) technique during late 90 s [1]. The principal mechanism of action is sub threshold modulation of neuronal membrane potentials, which alters cortical excitability and activity dependent on the current flow direction through the target neurons [2]. There are two types of stimulation with tdcs, namely, anodal and cathodal tdcs stimulation. Anodal tdcs stimulation is positive which increases the neuronal excitability whereas cathodal tdcs stimulation is negative that decreases the neuronal excitability of area being stimulated through electrode placed on the scalp [3]. This technique is used to improve the learning, motor, visual, verbal and memory obtained across various stimulation modalities [4][5]. Large electrode size (typically 9 cm 2 ) and spacing of electrodes are the major limitations of tdcs device. These settings may affect the surroundings of targeted regions [6][7]. In order to overcome these issues, HD-tDCS has been developed as a non-invasive brain electrical stimulation approach that will be used to measure up and down-regulate targeted cerebral micro vessel and primary motor cortex neuronal excitability depending on weak direct current stimulation. Targeting is attained by energizing electrodes placed in fixed configurations. Conventional tdcs uses rectangular pad configuration whereas HD-tDCS uses MXN - ring configuration for the placement of electrodes. MXN - ring configuration utilizes array of small high definition electrodes such that there are M number of return electrodes and N number of main electrodes [8][9]. In our 4X1 configuration, one focus electrode covers the region of primary motor cortex which is bounded by 4 return electrodes, which help limit the area of stimulation. Non-invasive brain electrical stimulation excitability of active neurons in the primary motor cortex can be recorded as voltage differences on the scalp site by using electroencephalogram (EEG) [10]. NIRS has been used to detect functional brain activity in primary motor cortex regions to capture changes in regional cerebral blood flow (rcbf) during and after HD-tDCS [11]. In this paper, NIRS has been used to monitor the neuronal excitability and cerebral blood flow in the primary motor cortex in brain region. The main objectives of this paper are: (a) To design a compatible, low cost and portable non- invasive brain stimulation HDtDCS device which will be used to up and down-regulate cortical activity depending on weak direct current stimulation. (b) To compare the SNRs of conventional and high-definition anodal tdcs (before and after) by using NIRS system. The major factors determining the efficacy of tdcs are: current direction, amplitude, size and positioning of electrodes. These parameters modulate the neural activity as indicated in the literature [12]. The larger sponge electrode in the conventional tdcs covers larger surface area and is less accurate in performance. On the other hand, smaller highdefinition silver chloride (AgCl) electrode has less surface area and gives more precise performance. For HD-tDCS, 4X1 configuration is considered. In this, one central electrode is surrounded by four peripheral electrodes in a ring configuration. The paper is organized into four sections. Section I presents the introduction to the study. Section II describes the various methods and procedures followed up in the study. The SNR ratios and power dissipation results are given in section III. Various issues related to HD-tDCS functionality are discussed in section IV /16 $ IEEE DOI /iNIS

II. METHODS AND PROCEDURES Arduino UNO R3 board model along with NI LabVIEW system design software is used for tdcs setup. The block diagram of tdcs setup is shown in Fig 1.

Then the output of ARDUINO UNO R3 board is filtered, amplified and passed through current control circuit. The simulated VI and waveform graph in LabVIEW for the setup is shown in Fig 2.

2 II. METHODS AND PROCEDURES Arduino UNO R3 board model along with NI LabVIEW system design software is used for tdcs setup. The block diagram of tdcs setup is shown in Fig 1. LabVIEW VI software is used to create a precise waveform configuration. The created waveform is uploaded to ARDUINO UNO R3 board. Then the output of ARDUINO UNO R3 board is filtered, amplified and passed through current control circuit. The simulated VI and waveform graph in LabVIEW for the setup is shown in Fig 2. Transcranial direct current stimulation was applied with current density of 0.526A/m 2 for 15 minutes. (b) Fig. 2. tdcs arrangement. (a) LabVIEW VI (b) simulated waveform A. Subjects Five young healthy volunteers aged 20 to 30 years (3 women, 2 men, mean age 25.8) participated in experiment. B. Conventional transcranial direct current stimulation A pair of sodium chloride (NaCl) solution soaked sponge electrodes of 3X3 cm 2 are used for this technique. Subjects sat in a quiet room with their eyes open and hooked on a point throughout the complete testing. Anodal tdcs was conducted for 15 minutes (current density of 0.526A/m 2 ) with the anode placed at C z and cathode placed at F 3 sites of scalp [13]. Fig. 1. Block diagram of tdcs System (Conventional) Current channel selection block helps in setting the parameters like duration of stimulation and selection of electrodes (polarity). After completing the desired settings, electrodes are placed at the target region of scalp site. (a) Fig. 3. Electrode placement on Cz and F3 scalp site during conventional tdcs 48

3 Fig 3 shows the anode and cathode placement for conventional tdcs. The peak current (2mA) is delivered to C z target scalp sites with the help of 3X3 cm 2 rectangular size electrode. The reference electrode (cathode electrode) is placed F 3 scalp site. In this experimental setup current is delivered 30 seconds of steady stimulation with 10 seconds of ramp-up and rampdown and then rest state of 50 seconds. This experiment was repeated 9 times resulting in 1500 seconds of testing time. C. High definition transcranial direct current stimulation Smaller sized Silver Chloride (AgCl) high definition electrodes are used in this configuration. The electrodes are arranged in 4X1 manner where the middle or active electrode (Cz scalp site) is surrounded by four return (peripheral) electrodes centrifugally separated by 3cms (Fig 4). This 4X1 arrangement is done at Cz scalp site. The peak current in case of central electrode is 2mA, whereas in peripheral electrodes it is reduced to one-fourth (0.5mA). The aim is to design a compatible, low cost and portable non- invasive brain stimulation HD-tDCS device which will be used to target the cerebral micro vessel and primary motor cortex. A controlled 4x1-ring configuration is used to distribute the low intensity direct current through small array of high definition electrodes. This configuration helps to increase or decrease the neuronal excitability of targeted cortical region. In this approach, a focus center electrode (anode or cathode) covering the target cerebral micro vessel and primary motor cortical region is surrounded by 4 returns electrodes, which help to limit the area of stimulation. The setup is similar to conventional tdcs system (Fig 5). Fig. 5. Block diagram of tdcs System (High-Definition) Fig. 4. 4x1-ring electrodes configuration during HD-tDCS D. Near infrared spesctroscopy Near-infrared spectroscopy is a cerebral monitoring method that non-invasively measures regional cerebral blood flow before and after conventional and HD-tDCS. The NIRS setup for the arrangement is shown in the Fig 6. The source photons (wavelength range: nm in the near infrared spectroscopy) are able to penetrate in C z of scalp site. The source is surrounded by four photo detectors centrifugally separated by 3cms. These four photodiodes are used to collect the back scattered near infrared light and converted into electrical signal. Magnitude of the light collected is smaller in magnitude than that emitted at the source photon. The output signal collected from the four photodiodes has smaller signal to noise ratio (SNR). The signal can be filtered and amplified to increase the signal to noise ratio. The NIRS records was collected for 2 minutes pre- and posttdcs (for conventional and HD respectively) at C z sites. The NIRS signal is sampled time-integrated and then re-sampled (at 0.1 Hz). This was done for eliminating high frequency noise. Further, it is passed through high-pass filter having cut- 49

4 off frequency of Hz. The change in the signal to noise ratio (SNR), which is the ratio between signal power and noise power in the whole NIR signal, between the on and off stages of conventional and HD-tDCS was analyzed [13]. Fig.7. value of 5 healthy subjects before and after tdcs and HDtDCS. The peak power dissipation is calculated as product of peak current, peak voltage and power factor. For the conventionaltdcs system it is calculated as 10 mw at C z scalp sites. In case of HD- tdcs, the peak power at the central electrode is same as that of conventional tdcs, whereas for the peripheral electrodes it is reduced to one fourth (2.5 mw). Fig. 6. Top Panel: Block diagram of the NIRS system. Bottom Panel: Design of NIRS probe [13]. III. RESULTS Significant changes have been recorded in SNR in the NIR signal in response to conventional and HD-tDCS on healthy subjects. Table I gives the SNR measurements of all the patients before and after Tdcs and Fig 7 shows the plotted values. As depicted, there is an improvement in SNR with HD-tDCS as compared to the conventional tdcs. The average SNR of the NIRS device was found to be db in the OFF-state. In ON-state it is db and db for conventional and high-definition tdcs respectively. TABLE 1. SNR OF 5 HEALTHY SUBJECTS BEFORE AND AFTER TDCS AND HD-TDCS Patient ID Before tdcs (Off- tdcs) After tdcs (On-tDCS) (Conventional) After HD-tDCS (On-HD-tDCS) IV. DISCUSSION The present study shows that better results in terms of analyzing a given system (in our case NIRS) are obtained by HD-tDCS in comparison to conventional tdcs. This arrangement can be used with other systems like EEG and functional magnetic resonance imaging (fmri). In the conventional tdcs device, the large electrode size and spacing of electrodes are the major limitations. These settings of the large electrodes may affect the surroundings of targeted regions. To overcome this problem, HD-tDCS device has been developed which have major key strength of increased accuracy of weak direct current passing through the brain using arrays of smaller high-definition electrodes. The area covered in terms electrode size is more in case of conventional tdcs setup rather than in HD-tDCS setup. The smaller size of electrodes in HD-tDCS setup provides more focus to a particular region of interest. Additionally, the current intensity on the electrodes of HD-tDCS system can be controlled which affects the amount of power dissipation on the scalp site. The current controllable parameters: intensity, duration and direction in the set-up design will further enhance the efficiency of HD-tDCS. The 4X1 design can be further extended to MXN configuration to analyze the improvements in terms of signal to noise ratio and power dissipation. REFERENCES [1] Nitsche, M.A. & Paulus, Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation, J Physiol, Sep [2] Nitsche MA, Schauenburg A, Lang N, Liebetanz D, Exner C, Paulus W, Tergau F, Facilitation of implicitmotor learning by weak transcranial direct current stimulation of the primary motor cortex in the human, J CognNeurosci,

5 [3] Purpura DP, McMurtry JG, Intracellular activities and evoked potential changes during polarization of motor cortex, Journal of Neurophysiology, 1965 [4] Nitsche, M.A., Cohen, L.G., Wassermann, E.M., Priori, A., Lang, N., Antal, A, Transcranial direct current stimulation: state of the art, Brain Stimul 11, 642, [5] Reynolds, E.O.R., Wyatt, J.S., Azzopardi, D., Delpy, D.T., Cady, E.B., Cope, M., Wray, S. New non-invasive methods for assessing brain oxygenation and hemodynamics, Brit. Med. Bull., [6] B. Molaee-Ardekani, Effects of transcranial Direct Current Stimulation (tdcs) on cortical activity: A computational modeling study, Brain Stimul., vol. 6, no. 1, pp. 2539, Jan [7] PeterchevAV,Wagner TA, Miranda PC, Nitsche MA, Paulus W, Lisanby SH, Fundamentals of transcranial electric and magnetic stimulation dose: definition, selection, and reporting practices, Brain Stimul, [8] Borckardt, J.J., A pilot study of the tolerability and effects of highdefinition transcranial direct current stimulation (HD-tDCS) on pain perception ConfProc IEEE Eng Med BiolSoc, [9] Fregni F, Boggio PS, Nitsche M, Bermpohl F, Antal A, Feredoes E, Anodal transcranial direct current stimulation of prefrontal cortex enhances working memory, Exp Brain Res, Sep [10] A. Dutta and M. A. Nitsche, Neural mass model analysis of online modulation of electroencephalogram with transcranial direct current stimulation, in Proc. 43rd Annu. Meeting Soc. Neurosci., [11] A. Dutta, A. Jacob, S. R. Chowdhury, A. Das, and M. A. Nitsche, EEG- NIRS based assessment of neurovascular coupling during anodal transcranial direct current stimulation a stroke case series, J. Med. Syst, [12] Hannah L. Filmer, Paul E. Dux, and Jason B. Mattingley, Applications of transcranial direct current stimulation for understanding brain function, Trends in Neurosciences, Vol. 37, No. 12, December [13] U. Jindal, M. Sood, A. Dutta, and S. R. Chowdhury, Development ofpoint of Care Testing Device for Neurovascular Coupling FromnSimultaneous Recording of EEG and NIRS During AnodalTranscranialDirect Current Stimulation, IEEE J. Transl. Eng. Health Med., vol. 3, pp.1 12,

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