Motor theory modulated by task load: strategies of phonological processing in frontal aphasia revealed by tDCS over the LIFG

In our previous studies we supported the claim that the motor theory is modulated by task load. Motoric participation in phonological processing increases from speech perception to speech production, with the endpoints of the dorsal stream having changing and complementary weightings for processing: the left inferior frontal gyrus (LIFG) being increasingly relevant and the left superior temporal gyrus (LSTG) being decreasingly relevant. Our previous results for neurostimulation of the LIFG support this model. In this study we investigated whether our claim that the motor theory is modulated by task load holds in (frontal) aphasia. Person(s) with aphasia (PWA) after stroke typically have damage on brain areas responsible for phonological processing. They may present variable patterns of recovery and, consequently, variable strategies of phonological processing. Here these strategies were investigated in two PWA with simultaneous fMRI and tDCS of the LIFG during speech perception and speech production tasks. Anodal tDCS excitation and cathodal tDCS inhibition should increase with the relevance of the target for the task. Cathodal tDCS over a target of low relevance could also induce compensation by the remaining nodes. Responses of PWA to tDCS would further depend on their pattern of recovery. Responses would depend on the responsiveness of the perilesional area, and could be weaker than in controls due to an overall hypoactivation of the cortex. Results suggest that the analysis of motor codes for articulation during phonological processing remains in frontal aphasia and that tDCS is a promising diagnostic tool to investigate the individual processing strategies.