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 exci....

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 per....

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 simultaneo.......

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

List of materials used in this article

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 (7 cm x 5 cm and 10 cm x 10 cm)	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