High-Definition Transcranial Direct Current Stimulation During Sleep

We use neuromodulation during sleep to investigate the relationships between targeted brain regions and sleep physiology. High-definition transcranial direct current stimulation with simultaneous electroencephalography advances research on regulation of and restoration from sleep. To begin, gather all necessary equipment and materials required for the procedure.

Ensure the conventional tDCS device has an adequate power supply, and if required, change the batteries. Turn on HD-tDCS and EEG devices and verify the power status according to the manufacturer's specifications. Select an appropriately sized EEG cap based on the participant's head circumference.

Use bilateral frontal lateral positions F3 and F4 to target the prefrontal cortex. Next, input the electrode positions and stimulation parameters. Insert the HD-tDCS electrodes or the electrode holders into the designated positions on the EEG cap and reverse the EEG cap inside out and place it on a foam head for better access.

Then, apply the high viscosity, chloride-free adhesive conductive paste to the cap. Using a tongue depressor, cover each EEG electrode with paste, avoiding application on HD-tDCS stimulation electrodes. Shape the conductive paste into dome-like structures.

Apply a smaller amount of paste in the front of the scalp, where there is less hair, and a greater amount at the crown and back of the head, where hair density is higher. Now, clean each electrode site using an alcohol swab. Exfoliate the site with mildly abrasive preparation gel using a cotton swab, then remove any residual fluid with a dry gauze square.

Target sites include the forehead and mastoids behind the ears for reference electrodes. Outer canthi near the eyes to record eye movements, chin for electromyography electrodes, and clavicle for the ground electrode. Then, fill each electrode with adhesive conductive paste and apply it to the prepared site.

Secure each electrode in place using medical grade tape. Similarly, apply all the facial and ground reference electrodes using gold cup electrodes filled with adhesive conductive paste. For the HD-tDCS ground electrode, position the stimulation electrode on the collarbone and secure it with tape.

Now, measure the distance from the nasion to the inion and mark the halfway point along this line. Then, measure the distance between the left and right preauricular points, and mark the halfway point on this line with a visible line. Identify the CZ location at the intersection of the nasion, inion and preauricular lines.

After that, lightly spray the participant's hair with water using a spray bottle to assist in EEG cap placement. Invert the prepared EEG cap and place it snugly on the participant's head, aligning the CZ electrode with the marked CZ site. Secure the cap in place using the chin straps for a comfortable and stable fit.

To fill each electrode site with conductive gel, pull up each electrode slightly and insert the large gauge blunt tip needle into its center. Gently wiggle the needle to displace hair and dispense approximately 0.5 milliliters of conductive gel. Push the electrode back down to establish firm contact between the gel, paste and scalp.

Position the silver or silver chloride centered ring electrodes into each HD-tDCS electrode holder with the rough surface facing downward and the smooth rounded surface facing upward. Add more high viscosity conductive gel over each inserted electrode to fully cover its surface. Lock each electrode in place using the caps provided.

Turn on the amplifier after connecting the required cords and launch the EEG recording software on the connected computer. Check the electrode impedances using the software. Set the impedance limit to 25 kiloohms and aim to achieve values below 10 kiloohms for all electrodes.

Ask the participant to open and close their eyes for approximately 10 seconds. Observe the presence of alpha wave activity, typically more prominent in posterior channels than in anterior ones. Monitor the scalp for amplitude variations.

Ask the participant to look to the left, right, up and down, then instruct them to clench their teeth while observing the recordings from the eye and chin electrodes for movement signals. Ask the participant to blink several times. Connect the cables of the silver or silver chloride centered ring electrodes to the corresponding receivers on the 4x1 adapter output cable Switch on the stimulator using its power button.

Once the stimulator is on, plug the electrode cable securely into the back of the device. Set the stimulation parameters according to the study design. Configure the intensity.

Select the waveform as direct current for transcranial direct current stimulation protocols, and set the duration and ramp times. Select either active stimulation or sham mode as directed by the study protocol. If the HD-tDCS electrodes show high impedance, deliver a brief test stimulation under the sham setting to verify connectivity.

Start the recording and mark the time in the EEG recording when the lights are turned off. Monitor the EEG for signs of sleep onset and specific sleep stages to trigger stimulation according to the study protocol. Mark the start of stimulation in the EEG recording with a clear notation.

Once the final stimulation period ends, unplug the stimulation cord from the device. Mark the time in the EEG recording when the lights are turned on. After waking the participant from sleep, stop the EEG recording using the software interface.

Gently remove the silver or silver chloride ring electrodes from the participant's scalp. Clean the used electrodes with a damp paper towel and allow them to air dry before storing them. Once the EEG cap is removed from the participant's head, clean all electrode sites thoroughly to remove residual conductive gels.

Bilateral frontal lateral HD-tDCS produced a focal electric field distribution, highlighting the spatial precision achievable with a 4x1 ring electrode montage. Clean EEG signals with minimal artifacts were observed during the ramp-up phase, confirming correct protocol implementation with optimal electrode preparation. HD-tDCS provides enhanced spatial precision and focal targeting versus conventional tDCS, minimizing off-target effects during sleep.

This enables mechanistic studies of region-specific sleep physiology and development of targeted interventions for neuropsychiatric disorders. Future research will develop adaptive closed-loop systems and refine computational models for improved targeting.