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

Presented here is a protocol to achieve higher accuracy in determination of stimulation location combining a 3D digitizer with high-definition transcranial direct current stimulation.

This method uses a 10-10 system to overcome the limitations of transcranial direct current stimulation electrode placement, improving the accuracy and reproducibility of the method. The main advantages of this technique are its low cost, simple application and portability. This method is suitable for use with other techniques, for example, functional near-infrared spectroscopy, to verify the location of specific brain regions of interest.

This technique is very simple if you are familiar with the proportional requirement of the 10-10 system. Demonstrating the procedure will be Chenyu Lv, a graduate student. To prepare an electrode holding cap, place a swimming cap on headform and measure the distance between the nasion and inion.

To localize the vertex, use a skin marker to mark the midpoint of the distance between the nasion and the inion, and measure the distance between the preauricular points. Mark the midpoint of the preauricular points. The point at which both midpoints intersect is the vertex.

According to the 10-10 system, find the CP6 and P6, and mark the appropriate midpoint at which these points intersect, called the right temporoparietal junction on the scalp. Then adjust the radius of the four return electrodes, based on the objectives. And mark the center and return electrode locations on the cap.

To obtain a 3D digitizer measurement, use a metal scanner to confirm that the environment for 3D digitization is metal-free. Place the swim cap on the subject's head, and using the reference marks on the cap to align the cap with the international 10-10 system for scalp location. When the cap is in place, use a USB interface to connect the 3D digitizer to the computer, and confirm the digitizer software is available and ready.

Place the source in front of the subject and fasten the elastic rope of the sensor around the subject's head. In the digitizer software, confirm that the 3D digitizer system is communicating with the software, and use the stylus, and a 10 centimeter ruler length, to record the zero and ten graduations to confirm the accuracy of the stylus. In the software, display the distance between the two recording points.

Then collect the position data of the reference points, center electrodes, and the four return electrodes. To cater to the requirement of the functional near-infrared spectroscopy experiments, select and save the transmitter detector and channel options. To initiate the stimulation, install fully charged batteries into the device, and connect the conventional transcranial direct current stimulation device and the four by one stimulation adapter.

Connect the cables of the five silver-silver chloride centered ring electrodes to the matching receivers on the four by one adapter output cable. And measure the head of the subject. Place the plastic device cap onto the subject's head, and embed the five plastic high-definition casings in the swimming cap.

Locate the vertex FPZ and OZ of the subject, and adjust the reference on the cap to align with the international 10-10 system for scalp locations. When the cap is in place, use the 3D digitizer to collect the position data of the stimulated brain areas. Next, use the end of a plastic syringe to carefully separate the hair through the opening of the plastic casing, until the scalp is exposed, and cover the exposed skin with the electrically conductive gel.

On the device, turn on the four by one multi-channel stimulation adapter and set the quality value. Confirm that the default setting is set to scan, and press the mode select button. Switch from the scan to pass and press the polarity button to select, either center anode or cathode.

Adjust the settings on the conventional transcranial direct current stimulation device to include the stimulus duration, intensity and sham condition setting and push the relax lever to switch to full current. Once everything has been set, press the start button to initiate the stimulation. The D.C.intensity will ramp up until the target current is reached, and the timer will then show the remaining time.

At the end of the stimulation, turn the lever slowly to adjust the current to zero before turning off the power. Open the plastic cap, remove the silver or silver chloride sintered ring electrodes from the casing and remove the swimming cap. Then clean the instruments and provide the subject with materials with which to clean their hair.

Using the near infrared spectroscopy statistical parametric mapping standalone registration function, the spatial registration function generates MNI coordinates. To reduce measurement errors, the average value of three data points from the final MNI coordinates of the five electrodes were calculated. In Broadman areas, the anatomical label and its number are obtained.

The number after each line indicates the percentage of overlap. In anatomical automatic labels, the anatomical label and percentage of overlap are obtained. For the Broadman areas and anatomical automatic labels, the value represents a percentage of overlap with the cerebral cortex.

It is important to make sure that neither the source nor the sensor moves during the 3D digitizer measurement. This procedure can be used for of thought localizations or for any other localizations needed to study the function of the brain. This technique can also be used in personalized brain stimulation for precise positioning.