DLPFC and left supramarginal gyrus activation increases the recognition of instrumental and functional violations in action representation

The correct outcome of many actions depends on the objects use, considering both the instrumental and functional features. Evidence that people have a dedicated system that links objects to possible uses or functions comes from lesion studies and experimental research on normal subjects. Indeed, object-related actions are blind to both the correct instrumental use of the object and to the functional use of the object. We directly compared two different object-related semantic violations, that is the incorrect object-action relationship (i.e., instrumental incongruence), in which the object was incorrectly used to perform the action in violation of the instrumental object-related features (grasping a bat upside-down), and an unusual object-action relationship (i.e., functional incongruence), in which the object was incompatible with the goal-related and intention-related requirements to perform that action (using a bat to mince meat). The contribution of frontal (Experiment 1) and posterior (Experiment 2) areas to process semantic violations (instrumental vs functional) in action representation was explored. The DLPFC and supramarginal gyrus activity was appositely modulated by tDCS (transcranial Direct Current Stimulation). The effect of tDCS (anodal) stimulation when subjects processed congruous/incongruous object-related actions was verified by measuring ERs (Error Rates) and RTs (Response Times) modulation. Forty-three subjects performed the detection task within a dynamic context (video tapes representing a sequence of four action frames ending with a congruous vs incongruous action). A constant current of 2 mA was applied for 13 min. The anode was placed above the DLPFC (with the centre above FCz, Experiment 1), and above the left supramarginal gyrus (with the center above P3, Experiment 2), and the cathode above the right supraorbital region. Significant decreased RTs were observed for incongruous stimuli in the case of DLPFC stimulation in comparison with sham condition. This effect was mainly related to instrumental semantic violations. It was suggested that activation of the DLPFC may increase the ability to analyse the semantic anomaly, limiting the cognitive costs. Moreover, parietal (supramarginal gyrus) stimulation reduced RTs only in response to functional semantic violations: this “reduction effect” was related to a specific mechanism induced by the action observation in case of contextual-constraint violation. Thus, different cortical generators were supposed respectively for the instrumental (more frontally distributed) and the functional (more frontally and parietally distributed) semantic incongruence.