Transcranial Direct Current Stimulation and Simultaneous Functional Magnetic Resonance Imaging

Summary

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique. It has successfully been used in basic research and clinical settings to modulate brain function in humans. This article describes the implementation of tDCS and simultaneous functional magnetic resonance imaging (fMRI), to investigate the neural basis of tDCS effects.

Abstract

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that uses weak electrical currents administered to the scalp to manipulate cortical excitability and, consequently, behavior and brain function. In the last decade, numerous studies have addressed short-term and long-term effects of tDCS on different measures of behavioral performance during motor and cognitive tasks, both in healthy individuals and in a number of different patient populations. So far, however, little is known about the neural underpinnings of tDCS-action in humans with regard to large-scale brain networks. This issue can be addressed by combining tDCS with functional brain imaging techniques like functional magnetic resonance imaging (fMRI) or electroencephalography (EEG).

In particular, fMRI is the most widely used brain imaging technique to investigate the neural mechanisms underlying cognition and motor functions. Application of tDCS during fMRI allows analysis of the neural mechanisms underlying behavioral tDCS effects with high spatial resolution across the entire brain. Recent studies using this technique identified stimulation induced changes in task-related functional brain activity at the stimulation site and also in more distant brain regions, which were associated with behavioral improvement. In addition, tDCS administered during resting-state fMRI allowed identification of widespread changes in whole brain functional connectivity.

Future studies using this combined protocol should yield new insights into the mechanisms of tDCS action in health and disease and new options for more targeted application of tDCS in research and clinical settings. The present manuscript describes this novel technique in a step-by-step fashion, with a focus on technical aspects of tDCS administered during fMRI.

Introduction

Transcranial direct current stimulation (tDCS) is a noninvasive method of brain stimulation in which cortical functioning is modulated by means of a weak electrical current (typically 1-2 mA) projected between two scalp-affixed electrodes. Physiologically, tDCS induces a polarity-dependent shift in neuronal resting membrane potential (RMP) within the targeted cortical region through the manipulation of sodium and calcium channels, thereby promoting changes in cortical excitability¹. Specifically, anodal stimulation (atDCS) has been shown to increase cortical activity via depolarization of neuronal RMP while cathodal stimulation (ctDCS) reduces cortical excitability². Compared to other types of brain stimulation (e.g. transcranial magnetic stimulation) safety has been well established and thus far no serious side effects have been reported even in vulnerable populations^3,4. Also, at least for lower stimulation intensities (up to 1 mA), an effective placebo (“sham”) stimulation condition exists⁵, allowing effective blinding of participants and investigators to the stimulation conditions, rendering tDCS an attractive tool in experimental and clinical research settings.

Numerous studies so far have shown that these changes in cortical excitability may result in behavioral modulations. In the motor system, consistent polarity dependent effects have been reported^1,6 for both atDCS and ctDCS. In cognitive studies, the majority of studies that employed atDCS to enhance cognitive functions reported beneficial effects on performance⁷, while ctDCS frequently did not result in impaired cognitive processing. The latter may be explained by the greater redundancy of neural processing resources underlying cognition⁶. The majority of tDCS studies have employed cross-over designs to study the immediate effects of the stimulation, which outlast the termination of the current only for short periods of time¹. However, it has been suggested that repeated stimulation impacts on protein synthesis, i.e. the neural mechanism underlying skill acquisition⁸. Indeed, motor or cognitive training success may be enhanced when combined with repeated tDCS sessions and long-term stability of these improvements have been reported to last up to several months in healthy adults^8-10. Such findings have also sparked an interest in the use of tDCS in clinical contexts and preliminary data suggests that it may also be useful as a primary or adjunct treatment approach in various clinical populations³. However, while a relatively large number of studies addressed neurophysiological effects of tDCS in the motor system, little is known about the underlying neural mechanisms of tDCS effects on cognitive brain functions in health and disease. A better understanding of the mode of action of tDCS is a necessary prerequisite for more targeted applications of tDCS in research and clinical settings.

This issue can be addressed by combining tDCS with functional brain imaging techniques like electroencephalography (EEG) or functional magnetic resonance imaging (fMRI). The majority of studies investigating the neural mechanisms underlying cognition and motor functions have chosen to employ fMRI¹¹. In particular, fMRI is the most widely used brain imaging technique to investigate the neural mechanisms underlying cognition and motor functions¹¹. Moreover, when combined with concurrent application of tDCS, fMRI allows examination of the neural mechanisms underlying behavioral tDCS effects with higher spatial resolution across the entire brain compared to EEG (for recent descriptions of combined tDCS-EEG see Schestatsky et al.¹²). The present manuscript describes the combined use of tDCS during simultaneous fMRI. This novel technique has successfully been used to study the neural mechanisms underlying tDCS-induced modulations of motor and cognitive functions^13-19. In the future, this combined protocol will yield new insights into the mechanisms of tDCS action in health and disease. Understanding the impact of tDCS on large-scale neural networks as assessed with this technique may lay the groundwork for more targeted application of tDCS in research and clinical settings.

The manuscript will focus on differences between behavioral tDCS experiments and the combined use of tDCS during simultaneous fMRI, with a specific emphasis on hardware requirements, implementation of the technique, and safety considerations. As an example, a single session of tDCS administered to the left inferior frontal gyrus (IFG) during task-absent resting-state (RS) fMRI and during a language task^14,15 will be described, though many other applications are possible^16,19. Details of the experimental design, participant characteristics and fMRI data analysis procedures have been described in detail in the original publications^14,15 and are beyond the scope of the present manuscript. Moreover, in these studies, an additional fMRI scan that involved sham tDCS was acquired and compared to the results of the atDCS session (see "Representative results" for details). This session was identical to the one described in the present manuscript, except that the stimulation was discontinued prior to the start of the scanning session (see Figure 1 for details). The present procedure has been successfully implemented at a 3-Tesla Siemens Trio MRI scanner at the Berlin Centre for Advanced Imaging (Charité University Medicine, Berlin, Germany), and should in principle be applicable to other scanners as well¹³.

Protocol

1. Contraindications and Special Considerations

1. Thoroughly screen participants for MRI contraindications (e.g. pacemakers, claustrophobia, etc.)  and exclude if necessary. Acquire standard questionnaires at clinical or research institutions that operate MRI scanners. Always obey standard safety procedures when entering the scanner room.
2. Thoroughly screen participants for contraindications for tDCS. These may overlap with contraindications for MRI. See Villamar et al.²⁰ for an example.
3. Consult with the operating facility regarding local safety and ethics regulations and obtain necessary permissions. Test for potential imaging artifacts induced by the stimulation current or tDCS equipment prior to commencement of the actual experiment (e.g. by testing the impact of tDCS on signal-to-noise ratio^17,18).

2. fMRI Setup, Experimental Design, and Materials

Note: The use of tDCS inside an MRI scanner requires special equipment. In particular, specific MRI-compatible cables, filter boxes, electrodes and straps to attach electrodes to subjects’ head are required. Figure 2 illustrates (A) standard tDCS equipment and (B) components for use with MRI. The latter components are necessary to prevent the possibility of heating under the electrodes due to radio-frequency pulses emitted during MRI. In addition, high-frequency imaging artifacts may be induced by the tDCS device. Both can be prevented by using filter boxes positioned outside and inside of the scanner room, cables equipped with resistors and dedicated MRI-compatible conductive rubber electrodes.

1. Perform general experimental set-up and sequences for the fMRI experiment. Both depend on the aims of the study. Note: the protocol below is specific to this experiment, but can be revised to apply to a number of different experimental situations.
2. Use a desktop computer with stimulus presentation software installed for a language task that involves visual presentation of semantic categories inside the scanner. Present these stimuli on a screen inside of the scanner via a projector connected to the computer and a system of mirrors.
3. Use an MRI-compatible microphone for transmission of overt verbal responses. Acquire two functional sequences during tDCS: a five-minute task-absent RS-sequence and an overt semantic word-generation task. Note: additional details of the experimental set-up, fMRI sequences and stimuli have previously been described in detail^14,15 and Figure 1 illustrates the experiment.
4. To set up the tDCS device, program the device to deliver a constant direct current of 1 mA for 20 min to cover the entire duration of the two functional scans, including short breaks and time for instructions in between scans^14,15. Ensure sure that the stimulator is sufficiently charged; otherwise it may shut down during the experiment.
5. Make sure that all necessary materials are available (Figure 2).

3. tDCS Setup Outside and Inside of the Scanner (See Figure 3 for a Schematic Overview)

1. Place the outer filter box close to the radio-frequency filter (RF) tube (i.e. the penetration point in the radio frequency shield of the MRI scanner that can be used to insert cables from outside of the scanner). Connect stimulator with the outer box using stimulator cable. Inner and outer filter box must not be mixed up. Note: Figure 4A illustrates the tDCS set-up outside of the scanner. The outer box is clearly marked in Figure 4B.
2. Measure cable length required to connect inner with outer box using box cable (see next point regarding positioning of cable in scanner room). Insert box cable into RF tube from the outside of the scanner and connect with outer filter box (Figure 4A).
3. Place inner filter box inside the rear end of the scanner bore (Figure 5); use adhesive tape to keep it in place. Connect box cable with inner filter box and avoid loops in any cables as these may induce RF heating. Note: The cable should be aligned with the walls of the scanner room and attached with adhesive tape (Figure 3).

4. Participant Preparation and Positioning of Participant in Scanner

1. As with conventional tDCS set-ups, inspect the skin of the participant for any pre-existing lesions, move hair away, clean skin with alcohol to remove hairspray, body lotion, etc. to improve skin conductivity underneath the electrodes^12,21.
2. Soak sponge pockets with saline solution and insert MRI-compatible electrodes into pockets (see DaSilva²¹ for general considerations of participant preparation and electrode positioning).
3. Mark electrode positions on subjects’ heads using a pen that leaves no ferromagnetic traces (e.g. don’t use eyeliner). Determine target position for anode using 10-20 EEG system (here: left IFG, 5 x 7 cm²)^14,15. To do so, locate (a) the intersection of T3-F3 and F7-C3 and (b) the midpoint between F7-F3. The target position is at the center of a line connecting points (a) and (b). Place cathode (10 x 10 cm²) over right supraorbital position (for details of electrode placement see Meinzer et al.^14,15). Attach electrodes to head using rubber band.
4. Guide the participant behind the scanner and connect electrode cable with the inner filter box. Turn on stimulator and test impedance by pressing the upper right and lower left button of the stimulator simultaneously. If the impedance limits are reached, then the stimulator will stop automatically. If this occurs, check whether electrodes have contact with the scalp, clean skin again or apply more saline solution if sponges have become too dry, and then check if any cable is broken. Note: Impedance is typically higher compared to conventional set-ups because of additional cables and filter boxes between stimulator and electrodes.
5. Guide participant into the scanner room (after a final safety check). Position the participant on the scanner gantry and make sure that electrodes are still in the correct position. Close the head coil. The electrode cable should be fed through the lower left part of the head coil (see Figure 6) or according to recommendations of the manufacturer.
6. Move participant into scanner bore. Make sure that the cable does not catch on the gantry and break (see Figure 6 for a possible secure position of the cable during this stage). When the participant has reached the final position inside the scanner, reach for the electrode cable from the rear end of the scanner and connect it to the inner filter box. Hand over emergency button to participant and leave the scanner room.

5. Starting the Stimulation

1. Use scanner intercom to inform the participant about the start of scanning session. Start the structural localizer scan (to determine head position of participant in scanner and allow for planning of subsequent functional and structural scans) using scanning console. Inspect localizer scan for high-frequency artifacts: Double-click on localizer scan after the end of the acquisition period and adjust contrast (for Siemens Trio by holding right mouse button and moving mouse to left and right; for examples see Figures 7A and 7B).
2. Use scanner intercom to communicate to the subject that the stimulation will commence and that he/she might feel a tingling sensation on the scalp for a short time. Repeat instructions for first functional scan. In this example, instruct the participant to keep the eyes closed for the duration of the scan (5 min), move as little as possible and think of nothing in particular. Make sure that projector is turned off (screen inside the scanner bore is black) to avoid visual stimulation during RS-scan.
3. Start stimulation manually approximately 1-2 min prior to the start of the first functional scan (RS-scan). Use scanner console to load RS-sequence. Double click on RS-sequence to open field-of-view (FOV), adjust position to cover the entire brain and align approximately with the anterior-posterior commissure. Start the first scan (using the START scan button).
4. Monitor impedance throughout the experiment. Note: If the experiment is conducted in a double-blind mode (participant and researcher are blinded to the stimulation), a second researcher may be necessary to monitor the impedance.
5. While the RS-sequence is running, load second functional imaging sequence (for subsequent language task) and adjust FOV, using scanner console as above, to reduce time required in between scans. After the end of the RS-sequence, turn on projector to allow for visual display of experimental stimuli during language task. Double click on presentation software icon and load language paradigm. Use scanner intercom to repeat instructions for task-related fMRI paradigm and commence with task^14,15.
6. After the end of the stimulation/fMRI experiment, continue with planned structural scans. Do not disconnect electrode cables until the end of the scanning session.
7. At the end of the experiment, disconnect electrode cable from inner filter box before moving participant out of scanner bore. Remove participant from the scanner, detach head coil and ask the participant to sit up and remove electrodes carefully.

Results

Functional MRI is the most widely used functional imaging technique to address the underlying neural mechanisms of motor or cognitive functions. More recently, fMRI has also been used to evaluate tDCS effects on cortical activity and connectivity. However, most of these studies administered tDCS outside of the scanner and evaluated offline effects of the stimulation (i.e. administered tDCS prior to scanning^22,23). Only a few studies so far have administered tDCS during simul...

Discussion

The combined application of tDCS with simultaneous fMRI has shown potential for elucidating the neural underpinnings of the immediate effects of the stimulation across the entire brain with high spatial resolution^13-19. In the future, such studies may be complemented by combined EEG-tDCS studies, to exploit the superior temporal resolution of the latter technique. In addition, intrascanner stimulation allows verification of correct positioning of the electrodes on the scalp (e.g. using T-weighted...

Materials

Name	Company	Catalog Number	Comments
DC-Stimulator Plus	NeuroConn, Illmenau, Germany	21
Hardware extension DC-Stimulator MR (2 MRI compatible rubber electrodes, electrode and box cable and inner filter box; outer filter box and stimulator cable)	NeuroConn, Illmenau, Germany
2 sponge pads for rubber electrodes (7x5 and 10x10 ccm)	NeuroConn, Illmenau, Germany
Rubber head band
NaCL solution
Measurement tape			To determine electrode position using the EEG 10-20 system
Pen			Used during electrode positioning