Reduction of excitability in the left inferior frontal gyrus by cathodal transcranial direct current stimulation facilitates emotion recognition

Abstract

Facial emotion recognition is a prerequisite of successful social cognition and interaction (Haxby et al., 2002). The left inferior frontal gyrus (IFG) is critically involved in the neuronal network subserving emotion recognition (Dal Monte et al., 2014). Transcranial direct current stimulation (tDCS) can be used to modulate cortical excitability and associated behavioral functions in a polarity-specific and activity-dependent manner (Dayan et al., 2013; Wolkenstein and Plewnia, 2013; Wolkenstein et al., 2014; Zwissler et al., 2014). In a sham-controlled crossover design, excitability enhancing anodal and excitability decreasing cathodal tDCS of 2mA were applied to the left IFG in 32 healthy subjects performing the ‘Reading the Mind in the Eyes’ (RME) test. The RME is widely used to measure emotion recognition in healthy subjects and psychiatric disorders (Baron-Cohen et al., 2001). Prior to the two (verum/sham) stimulation sessions, near-infrared spectroscopy (NIRS) was applied to measure brain activity in the IFG during RME performance (Ehlis et al., 2014). NIRS indicated a deactivation in the IFG during emotion recognition that was associated with faster responses. Consistently, cathodal, inhibitory tDCS of 2mA clearly accelerated emotion recognition in the RME test. Moreover, the change in response correctness induced by cathodal tDCS was significantly correlated with the emotion-specific IFG activity measured with NIRS during RME performance. This is first evidence for a beneficial, state-dependent effect of excitability-decreasing cathodal tDCS on emotion recognition. The correlation with individual imaging data supports the concept of a focusing effect of cathodal tDCS that reduces neural noise facilitating signal detection (Antal et al., 2004; Dockery et al., 2009; Miniussi et al., 2013).