Targeting anterior cingulate cortex with tDCS: an overview and analysis of electric field magnitude
The anterior cingulate cortex (ACC) is a brain region with a key role in various cognitive, emotional, and sensory processes. Given its extensive functional repertoire, modulating ACC activity to enhance cognitive functioning and alleviate symptoms of certain clinical conditions holds great potential. Over the past decades, transcranial direct current stimulation (tDCS) has gained popularity due to its ability to non-invasively affect the cortical excitability. Nevertheless, using tDCS to target regions located beneath superficial cortical areas, such as the ACC, could pose a challenge due to the unpredictable distribution of the electric field (E-field). To systematize the current state of evidence regarding the use of tDCS to modulate ACC activity, we conducted a systematic review focusing on analyzing the E-field distribution across the brain and its magnitude within the ACC as the region of interest. Our goal was to review the stimulation parameters used thus far and examine whether the E-field characteristics are linked to the observed effects. After the literature search and study selection, 14 studies were included in the review. Most of the studies were sham-controlled single-session experiments aiming to modulate specific cognitive processes in healthy adults. The most prominent process of interest was cognitive control, and most of the included studies observed behavioural effects. Our results show that cathodal stimulation led to significant results more frequently, regardless of E-field magnitude in ACC or specificity of E-field distribution throughout the brain. When it comes to anodal tDCS, more focal stimulation of the ACC was associated with a higher frequency of significant effects. We discuss these findings considering diverse methodological designs, the (un)specificity of ACC targeting, and other factors that could contribute to the variability of tDCS effects across studies.