fMRI and Transcranial Electrical Stimulation (tES): A systematic review of parameter space and outcomes

Abstract

The combination of non-invasive brain stimulation interventions with human brain mapping methods have supported research beyond correlational associations between brain activity and behavior. Functional MRI (fMRI) partnered with transcranial electrical stimulation (tES) methods, i.e., transcranial direct current (tDCS), transcranial alternating current (tACS), and transcranial random noise (tRNS) stimulation, explore the neuromodulatory effects of tES in the targeted brain regions and their interconnected networks and provide opportunities for individualized interventions. Advances in the field of tES-fMRI can be hampered by the methodological variability between studies that confounds comparability/replicability. In order to explore variability in the tES-fMRI methodological parameter space (MPS), we conducted a systematic review of 222 tES-fMRI experiments (181 tDCS, 39 tACS and 2 tRNS) published before February 1, 2019, and suggested a framework to systematically report main elements of MPS across studies. We have organized main findings in terms of fMRI modulation by tES. tES modulates activation and connectivity beyond the stimulated areas particularly with prefrontal stimulation. There were no two studies with the same MPS to replicate findings. We discuss how to harmonize the MPS to promote replication in future studies.

Transcranial direct current stimulation with functional magnetic resonance imaging: a detailed validation and operational guide

“…The MRI compatible electrodes are made from an electrically conductive rubber. It is possible that circumferential RF currents could be set up directly within these relatively large pads but for the low SAR sequences used heating is negligible, as confirmed by previous experiments (Holland et al, 2011). As a further risk mitigation strategy, in our Lab only low power imaging sequences are used.…”

“…For example, one study demonstrated evidence of BOLD signal within brains of two cadavers during a concurrent tDCS and fMRI protocol (Antal et al, 2014). Whilst a previous study from our lab demonstrated visual evidence of change in echo-planar imaging (EPI) field maps that was limited to the scalp/surface near to the electrode site (Holland et al, 2011). These contrasting cases demonstrate the need for careful consideration of concurrent tDCS-fMRI data, and acquisition of appropriate field map data to allay concerns over false positive functional results from perturbation of the magnetic field.…”

“…It was originally developed for a series of experiments using tDCS-fMRI investigating language recovery after stroke, but can be adapted for any study which uses fMRI to investigate the mechanisms underlying tDCS effects. At the time of writing, we have collected data from over 36 stroke patients with aphasia with no reported adverse events or tolerance issues (Ondobaka et al, 2020), and the same fMRI-tDCS procedure has been found to be well tolerated by healthy older adults (Holland et al, 2011). In both these studies, participants were not able to reliably detect differences between the stimulation conditions (anodal 2mA and sham tDCS).…”

Effects of tDCS Dose and Electrode Montage on regional cerebral blood flow and motor behavior

“…They highlighted that the tDCS dose effects studies had focused their attention on the current intensity range from 0mA to 2mA but saw a need to examine effects at higher doses to improve our understanding of the dose-response relationship (Batsikadze et al, 2013; Clark et al, 2012; Ho et al, 2016; Jamil et al, 2017; Zheng et al, 2011). Some studies over the last several years have expanded tDCS current intensity range up to 3mA (Agboada et al, 2020, 2019; Jamil et al, 2020) and even 4mA (Chhatbar et al, 2017), although more studies, in particular concurrent tDCS-fMRI studies, are necessary to examine relationships between stimulation dose and physiological signals (Esmaeilpour et al, 2020; Ghobadi-Azbari et al, 2020).…”

A Checklist for Assessing the Methodological Quality of Concurrent tES-fMRI Studies (ContES Checklist): A Consensus Study and Statement

“…Over the last 20 years, low intensity tES has been used extensively to study and modulate the neural mechanisms underlying basic physiological and cognitive processes (Bachtiar et al, 2015; Bikson and Rahman, 2013; Jamil et al, 2020; Keeser et al, 2011; Kuo et al, 2016; Minhas et al, 2010; Nitsche and Paulus, 2000; Stagg and Nitsche, 2011; Zoefel et al, 2018). Initial studies combining tES with fMRI were limited to sequential tES-fMRI recording, which primarily provides an avenue to investigate the neural mechanisms underlying tES offline (after) effects (Almeida et al, 2017; Antal et al, 2014b; Esmaeilpour et al, 2019; Ghobadi-Azbari et al, 2020; Lee et al, 2019; Meeker et al, 2019; Ruttorf et al, 2019).…”