P115 A tale of two networks: a simultaneous Fmri-tDCS study of anodal versus cathodal stimulation

Abstract

Introduction The right inferior frontal gyrus (rIFG) is important in co-ordinating changes in network activity, which underlie normal cognitive control ( Hampshire, 2015 ). TDCS can modulate cognition, though little is known about its neurophysiological effects. Objectives To investigate the neurophysiological effects of rIFG (F8) anodal (a-tDCS) and cathodal (c-tDCS) tDCS. Materials & methods 26 controls participated in a novel simultaneous fMRI/tDCS study. Subjects performed a blocked Choice Reaction Time task (CRT) (fixation cross between task blocks) and had 3 types of tDCS (1.8mA: a-tDCS, c-tDCS or sham), giving 6 possible combinations (Fig. 1) Download : Download high-res image (563KB) Download : Download full-size image . Subjects then performed 3 runs of a Stop Signal Task (SST), an inhibitory control task requiring subjects to withhold an automatic response when an infrequent ‘Stop’ signal appears. Each run had sham, a-tDCS or c-tDCS (2mA), in a counterbalanced order across participants. FMRI was analysed in the FMRIB FSL package. Results CRT performance activated the Dorsal Attention Network, sensorimotor and primary visual areas and deactivated the Default Mode Network (DMN), as seen in ( Sharp, 2011 ) (Fig. 2A). The effect of tDCS depends on: 1. Brain state: tDCS/fixation and tDCS/task had opposing effects on DMN and task network activity (Fig. 2B Download : Download high-res image (1MB) Download : Download full-size image ) 2. Electrode polarity: c-tDCS/CRT produced greater activation within task regions and deactivation of DMN regions than a-tDCS/CRT (Fig. 2C). Inhibiting a response in the SST led to expected DMN deactivation ( Aron, 2006 ). Compared to sham tDCS, when inhibiting a response, c-tDCS resulted in: • A failure to deactivate the medial frontal cortex, a key DMN node (p = 0.03). • Increased activation of a right frontal-subthalamic network important in inhibitory control ( Bonnelle, 2012 )(p = 0.02). Conclusions Short periods of tDCS can modulate brain network activity. The neurophysiological effect of tDCS is strongly dependent on underlying brain state. C-tDCS and a-tDCS have distinct effects on brain network activity.