Representation of the incorrect use of an object is supported by the DLPFC: The tDCS effect on ERP N400 for visual and linguistic stimuli

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 (transcranial direct current stimulation). First, we sought to analyse the direct effect of tDCS on the ERP profile in response to the semantic task. Two different tasks were examined: a first object-related action representation induced by an action sequence (Experiment 1) and a second object-related action representation induced by a sentence (Experiment 2). The effect of tDCS when subjects processed congruent/incongruent object-related actions or sentences 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, 28 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). A significant reduction of the N400 was observed for incongruent stimuli in the case of cathodal (inhibitory) stimulation of the DLPFC compared with pre-stimulation conditions for Experiment 1 and Experiment 2. Moreover, ERs were increased, and RTs were reduced in response to incongruent conditions after tDCS, but not after sham stimulation in Experiments 1 and 2. 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 (Balconi et al., 2012).