The tDCS effect on ERP profile for the semantic representation of action. The role of DLPFC

Representation of action within an object-related dynamic context was considered as a semantic task similar to word comprehension within a linguistic context. Indeed it has been hypothesized that the mechanisms involved in the perception of action sequences may be similar to those associated with the processing of language. That is, action comprehension was seen as a specific semantic processing where context-action representation resulted integrated as in the word-sentence representation. The aim of the present research was firstly to investigate the effect of tDCS on a specific ERP deflection (N400 effect) when subjects processed a congruous/incongruous object-related action. Secondly, the contribution of tDCS to modulate the cortical response of the dorsolateral prefrontal cortex (DLPFC) was explored. We supposed that the inhibition of anterior DLPFC area may induce a decreased ability to produce a concomitant N400 effect. Thirty subjects performed a detection task when congruous or incongruous sequences of action (video tapes). The procedure was subdivided into three phases. Prior to tDCS stimulation, a baseline task was conducted on a separate day, and EEG was registered (phase 1). The participants were required to press a left or right pulse of the mouse depending on whether the final action target frame represented a congruous or an incongruous ending scene. Successively tDCS/sham stimulation was induced (phase 2) on the subjects. Finally, immediately after tDCS/sham stimulation (phase 3) subjects were submitted to the same experimental task of phase 1. The EEG was registered during the task execution using the same procedure of EEG acquisition adopted in phase 1. The stimulation effect (cathode applied on the DLPFC and anode on the control site) was tested comparing the behavioural (RTs) and ERP profile before and after the stimulation (or sham effect) applied on the frontal areas. A significant N400 reduction was observed for incongruous stimuli in case of cathodic stimulation of DLPFC compared with the pre-stimulation condition. Also RTs were modified in case of tDCS application when subjects processed incongruous actions. It was suggested that the inhibition of DLPFC may limit the ability to analyze the semantic anomaly induced by action representation. Secondly, the contribution of the frontal areas for the semantic processing of action was supported. Finally, the N400-like effect was largely modulated by tDCS, as shown by comparing pre- and post-stimulation ERP profiles. The effect of tDCS applied over prefrontal cortex to explore action semantic processing was demonstrated and currently we tested for changes in the RT and ERP cortical responses induced by direct current stimulation