Stimulation Location Determination using a 3D Digitizer with High-Definition Transcranial Direct Current Stimulation

Transcranial direct current stimulation (tDCS) is a noninvasive technique that modulates cortical excitability with weak direct currents over the scalp. It aims to establish causality between neural excitability and behavior in healthy humans^1,2,3. In addition, as a motor neurorehabilitation tool, tDCS is widely used in the treatment of Parkinson's disease, stroke, and cerebral palsy⁴. Existing evidence suggests that traditional pad-based tDCS produces current flow through a relatively larger brain region^5,6,7. High-definition transcranial direct current stimulation (HD-tDCS), with the center ring electrode sitting over a target cortical region surrounded by four return electrodes^8,9, increases focality by circumscribing four ring areas^5,10. In addition, changes in excitability of the brain induced by HD-tDCS have significantly larger magnitudes and longer durations than those generated by traditional tDCS^7,11. Therefore, HD-tDCS is widely used in research^7,11.

Noninvasive brain stimulation (NIBS) requires specialized methods to ensure that a stimulation site is present in the standard MNI and Talairach systems¹². Neuronavigation is a technique that allows for mapping interactions between transcranial stimuli and the human brain. Its visualization and 3D image data are used for precise stimulation. In both tDCS and HD-tDCS, a common assessment of stimulation sites on the scalp is typically the EEG 10-20 system^13,14. This measurement is widely used for placing the tDCS pads and optode holders for functional near infrared spectroscopy (fNIRS) in the initial stage^13,14,15.

Determining the precise stimulation points when using the 10-20 system can be difficult (e.g., in the temporo-parietal junction [TPJ]). The best way to solve this is to obtain structural images from participants using magnetic resonance imaging (MRI), then obtain the exact probe position by matching target points to their structural images using digitizing products¹⁵. MRI provides good spatial resolution but is expensive to use^15,16,17. Moreover, some participants (e.g., those with metal implants, claustrophobic people, pregnant women, etc.) cannot be subjected to MRI scanners. Therefore, there is a strong need for a convenient and efficient way to overcome the abovementioned limitations and increase accuracy in determining stimulation points.

This protocol uses a 3D digitizer to overcome these limitations. Compared to MRI, key advantages of a 3D digitizer are low costs, simple application, and portability. It combines five reference points (i.e., Cz, Fpz, Oz, left preauricular point, and right preauricular point) of individuals with location information of the target stimulation points. Then, it produces a 3D position of electrodes on the subject's head and estimates their cortical positions by fitting with the vast data from the structural image^12,15. This probabilistic registration method enables the presentation of transcranial mapping data in the MNI coordinate system without recording a subject's magnetic resonance images. The approach generates anatomical automatic labels and Brodmann areas¹¹.

The 3D digitizer, used to mark space coordinates based on the data from structural images, was first used to determine the position of optodes in fNIRS research¹⁸. For those who use HD-tDCS, a 3D digitizer breaks the finite stimulation points of the EEG 10-20 system. The distance of the four return electrodes and center electrode is flexible and can be adjusted as needed. When using the 3D digitizer with this protocol, the coordinates of the rTPJ were obtained, which is beyond the 10-20 system. Also shown are the procedures for targeting and stimulating the right temporo-parietal junction (rTPJ) of the human brain.

The protocol meets the guidelines of the Institutional Review Board of Southwest University.

1. Determination of Stimulation Location

1. Review the literature and confirm the stimulation location (here, the rTPJ)^19,20,21.

2. Preparation of Electrode Holding Cap

NOTE: The following steps are shown in Figure 1.

1. Ensure that all necessary materials are readily available: the 3D digitizer (Figure 2), standard measurement tape, a marking pen, the headform, and a swimming cap.
2. Put the cap on the headform and mark the points on the cap.
   1. Localize the Vertex (Cz). To do so, first mark the midpoint of the distance between the nasion and inion using a skin marker^13,14,22. Then, measure the distance between the pre-auricular points and mark the midpoint. The point at which both points intersect is the Cz.
   2. Check the location of the center electrode and the return electrodes. Here, the stimulation was applied on rTPJ. The rTPJ roughly corresponds to the midpoint between CP6 and P6 in the 10-10 EEG system^19,20,21.
   3. Find CP6 and P6^22,23,24,25. According to the proportional requirements of the 10-10 system, locate the approximate location of the rTPJ on the scalp and mark it on the cap.
   4. Adjust the radius of the four return electrodes based on the objectives^11,14,26. After this decision, mark the center electrode and return electrode locations on the cap.

3. 3D Digitizer Measurement

1. Scan with the metal scanner to ensure that the environment for 3D digitizer is metal-free.
2. Placement of the cap on the subject's head
   1. Make sure that the references (Cz, Fpz, Oz, left preauricular point, and right preauricular point) on the cap align with the international 10-10 system for scalp location²². For example, localize the Vertex (Cz) on the scalp and place the cap onto the subject's head, aligning the cap's Cz to the subjects.
3. Arranging the 3D digitizer equipment
   1. Connect the 3D digitizer to the computer using the Universal Serial Bus (USB) interface and make sure that the digitizer software is available and ready²⁷.
   2. Put the source in front of the subject and fasten the elastic rope of the sensor around the head. Importantly, make sure that neither the source nor sensor moves during 3D digitizer measurement.
      NOTE: The source is a magnetic transmitter that emits an electromagnetic dipole field. The sensor is a receiver that detects the field.
   3. Open the digitizer software on the computer and make sure that the 3D digitizer system communicates with the software.
   4. Test the accuracy of the stylus. Find a length of 10 cm on the ruler and record the zero graduation and ten graduation, respectively, using the stylus.
      NOTE: The measurement distance between the two recording points of the 3D digitizer should be captured. Compare the error with the reading from the 3D tracker.
   5. Select the New icon and create a new subject file. Select the Sessions box, then Reference.
      NOTE: Using the 3D digitizer stylus, the reference position data (Cz, inion, nasion, left ear, right ear) of the subject are collected according to the software prompts.
   6. To cater to the requirement of fNIRS experiments, use the Transmitter, Detector, and Channel options. Collect the position data of the center electrode and four return electrodes 3x for the transmitter, detector, and channel, in order to reduce error. Ensure that five electrodes are numbered and localize in turn.
   7. Save the three files that are generated.

4. Data Conversion and Spatial Registration

1. Select the three files into the NIRS-SPM to achieve the real coordinates registration into MNI space²⁸. Affine transform the reference points and five electrode points in participants to the corresponding points in each entry according to the MRI database in MNI space.
2. Register the data to anatomical automatic labels and Brodmann areas and register the spatial information of the five electrode points to both of these.
3. Compare the coordinates of stimulation in previous research with the obtained coordinates^20,29.
4. Make a small cut aligned to the five points marked on the cap, such that the plastic casing is embedded snugly into the cap.

5. Stimulation

1. Make sure that participant have no contraindications (i.e., history of neurological or psychiatric disorders) for HD-tDCS^1,3 and that they provided written informed consent prior to the study (including HD-tDCS stimulation).
2. For device installation, ensure that all the necessary materials are available (Figure 3). Install the device as detailed in published literature¹⁴. A brief description is provided below.
   1. Install the batteries and check that they are charged.
   2. Connect the conventional tDCS and 4x1 Stimulation Adapter.
   3. Connect the cables of five Ag/AgCI sintered ring electrodes to the matching receivers on the 4x1 adapter output cable.
   4. Check that all materials are connected correctly.
3. Measure the head of the participant and place the cap on the head.
   1. Embed the five plastic HD casings in the swimming cap.
   2. Localize the Cz, Fpz, and Oz of the subject^13,14. Adjust the reference on the cap to align with the international 10-10 system for scalp locations²². Once the cap is in position, ensure that it does not move.
   3. Collect the position data of the stimulated brain areas using the 3D digitizer. Make the corresponding adjustments according to the generated data.
4. Cover the scalp surface with electrically conductive gel. First, carefully separate the hair through the opening of the plastic casing using the end of a plastic syringe, until the scalp is exposed. Then, cover the exposed scalp with the electrically conductive gel through the plastic casing opening on the scalp surface.
5. Set the parameters of the tDCS device: quality value, stimulus duration, intensity, and condition setting.
   1. Turn on the 4x1 Multichannel Stimulation Adapter.
   2. Ensure that the default setting is SCAN, which shows the impedance of one electrode at a time in the display window by scanning the electrodes^14,30,31. Here, the impedance is described as "quality value". Values below 1.5 indicate sufficient quality^14,30,31. In this case, the values were lower than 1.
      NOTE: If the impedance value exceeds these required limits, open the cap of the plastic casing with high impedance and adjust the hair and electrode to obtain the desired impedance value.
   3. Press the "MODE SELECT" button and switch from "SCAN" to "PASS", after the impedance values are acceptable.
   4. Select the center-anode or center-cathode by pressing the "POLARITY" button. "CENTRAL ANODE" is the default setting.
   5. Adjust the settings on the conventional tDCS device to include stimulus duration (min), intensity (mA) and sham condition setting. In this case, anodal active stimulation was 1.5 mA, and the stimulus lasted 20 min. Next, push the "RELAX" lever to switch to full current.
   6. Once everything is set, initiate the stimulation. Press the "START" button, and the DC intensity will ramp up until the target current is reached. The timer will then show the remaining time.
      NOTE: Some participants may feel uncomfortable during periods of increased DC intensity. In such cases, the current may be moderately decreased slightly for a few seconds by pulling down the "RELAX" lever. Then, push the dolly bar to full current, gradually, when participants feel comfortable again.

6. Post-stimulation

1. When the stimulation is over, turn the lever slowly to adjust the current to zero before turning off the power. Otherwise, participants may perceive stinging sensation or dizziness when turning off the power directly.
2. After the stimulation, open the plastic cap and remove the Ag/AgCI sintered ring electrodes from the casing.
3. Remove the swimming cap and clean the materials. Provide participants with tools to clean their hair.
4. Ask participants to fill out a questionnaire after each stimulation session, if necessary (e.g., to measure adverse effects of screening following HD-tDCS, participant tolerance to brain stimulation, etc.; see Supplementary File).

Using the methods presented, coordinates of the rTPJ were determined, which requires stimulation points beyond the 10-20 system. First, the circumference of the headform should be similar to the actual head. Here, the length of the nasion to inion of the headform was ~36 cm, and the length between the bilateral preauricular was ~37 cm.

The steps for producing the electrode cap guide the measuring positions of the 10-20 system. Here, Nz, Iz, Cz, Fpz, Oz, Pz, T8, T7, C4, P8, O2, P4, C6, P6, and CP6 were determined. The approximate location of the RTPJ (about the midpoint between CP6 and P6) was found on the scalp. The distance between the central and peripheral electrodes should be adjusted based on experimental objectives. Previous research obtained radius values ranging from 3.5–7.5 cm^11,14,30. With different radius values, DC intensity and stimulation duration may generate different electric field strengths. In this protocol, the distance between all return electrodes and the central active electrode were fixed to 3.5 cm.

Several important reference points on the swimming cap were kept, including Fpz, Cz, Oz, T8, and C4. The Vertex on the scalp was located before the stimulation, and it is critical that the Cz point on the cap exactly aligns with the Vertex. Once the cap is in position, the cap should not move. One .mat file and two .csv files after digitizing were obtained (i.e., sub01_origin.csv, which included the coordinate information of the reference [with subject number 01]), while sub01_others.csv included the coordinate information of the five targeted points [with subject number 01)].

Three .txt files were obtained after data conversion and spatial registration. In digitizer software, there are transmitter, detector (receiver), and channel options for fulfilling the requirements of fNIRS experiments. The coordinate data of the transmitter, detector, or channel should be the same. However, small operating errors may occur, because of laboratory personnel skills, pen holding gesture, etc.

Using the NIRS-SPM stand-alone registration function, the spatial registration function generates MNI coordinates. The numbers in the first line in Table 1 represent the order in the digitizer. In this protocol, the data from number five is the position information about the center electrode. In Brodmann areas (BA), the anatomical label and its number were obtained. The number after each line indicates the percentage of overlap. In anatomical automatic labels (AAL), the anatomical label and percentage of overlap were obtained. To reduce measurement errors, the average value of three data points from the five electrodes' final MNI coordinates were calculated. As for AAL and BA, the value represents a percentage of overlap with the cerebral cortex. All possibilities were combined into final data (Table 1).

According to the data from MNI coordinates, AAL, and BA, if the difference between the value and target value is too large, the swimming cap must be adjusted to the relative position of the actual values of X, Y, Z, and the target value, as explained in sections 2–4^11,14,30,31.

Figure 1: Steps for creating the holding electrode cap. Please click here to view a larger version of this figure.

Figure 2: 3D digitizer. The 3D digitizer is a cost-effective solution for 3D digitizing. It is a dual sensor motion tracker. The source is a magnetic transmitter that emits an electromagnetic dipole field. The sensor is a receiver that detects the field. The stylus allows accurate pinpointing of X, Y, and Z data points. The control box connects to the computer and transfer data. Please click here to view a larger version of this figure.

Figure 3: Necessary materials for stimulation. These materials include a tDCS device, 4x1 Multichannel Stimulation Adapter, four 9 V batteries, five Ag/AgCI sodium ring electrodes, five HD plastic casings and their respective caps, electrically conductive gel, a syringe, a standard tape measure, and a swimming cap. Please click here to view a larger version of this figure.

Table 1: Localization of stimulations in the brain area. Please click here to view this table (Right click to download).

Supplementary File. Please click here to view this file (Right click to download).

Compared to traditional tDCS, HD-tDCS increases the focality of stimulation. Typical sites of stimulation are often based on the 10-20 EEG system. However, determining the precise stimulation points beyond this system can be difficult. This paper combines a 3D digitizer with HD-tDCS to determine stimulation points beyond the 10-20 system. It is important to clearly define the steps and precautions for making and using the electrode cap in such cases.

In general, the position of target stimulation areas is derived from the results of previous brain imaging studies, and the position of the stimulation areas on 10-20 international system or MNI coordinates can be obtained. The steps for creating the electrode cap guide for measuring positions of the 10-20 system are critical. It is key that the reference on the cap aligns with the international 10-20 system for scalp locations when placing the cap on the head. Once the 3D digitizer starts running, the source and sensor should not move, or it will cause data deviation.

In the software, the reference points are on the scalp and not on the cap, unless all the reference points of scalp and cap are matching. If the error between the measured results and target values is out of the acceptable range, the position of the marked points should be slightly adjusted. After adjustment, measurements should then be made again. Once users press the "MODE SELECT" button and switch from "SCAN" to "PASS", the current will start passing from the conventional tDCS device through the electrodes into the 4x1 Multichannel Stimulation Adapter.

The modular electroencephalogram recording cap provides fixed positions of probes. However, determining the precise stimulation points beyond this system can be difficult. The positions of electrodes beyond the 10-20 system can be determined using the protocol described, as well as the coordinates of stimulation points. The radius setting should be based on the experimental objectives. Using the method described here, the radius of the four return electrodes and center electrode can be flexibly adjusted.

There are many digitizer software packages (e.g., the Brainstorm software for an fNIRS task; here, the Vpen software was used)¹⁵. Different data collection software packages emphasize different functions and should be selected according to the research question. Head circumference varies among individuals; hence, using the same cap can produce errors. However, the modular electroencephalogram recording cap also suffers from this problem.