tDCS effect on N400 ERP in case of action or sentence representation

In the present study, we explored the representation of an incongruent action (instrumentally incorrect use of an object) in comparison with sentences ending with an incongruent action word, taking into account the role of left dorsolateral prefrontal cortex (DLPFC) activation. This activity was appositely modulated by tDCS. The effect of tDCS when subjects processed congruent/incongruent object- related actions (Experiment 1) or sentences (Experiment 2) was verified by measuring changes in the ERP (event-related potential) N400, ERs (Error Rates) and RTs (Response Times). In Experiment 1, thirty subjects performed the detection task within a dynamic context (video tapes representing a sequence of four action frames). In Experiment 2, twenty-eight subjects read sentences that represented object related actions. The stimulation effect (a cathode applied to the DLPFC and an anode to the right supraorbital region) was analysed by comparing the ER, RT and ERP profiles before and after stimulation (or sham treatment). As shown by ERP analysis and LORETA a significant reduction of the N400 was observed for incongruent stimuli in the case of cathodal stimulation of the DLPFC compared with pre-stimulation conditions for Experiment 1, but not Experiment 2. Moreover, ERs were increased, and RTs were reduced in response to incongruent conditions after tDCS, but not after sham stimulation in Experiment 1. It was suggested that perturbation of the DLPFC may limit the ability to analyse a semantically anomalous action sequence, with a reduced N400 ERP effect and increased random responses being observed. Finally, the contribution of the frontal area to the semantic processing of action was discussed, comparing action with sentence representation