Motor theory modulated by task load: Effects of tDCS over the LSTG on connectivity patterns for phonological processing

Abstract Our previous work defended that the motor theory is modulated by task load, claiming that motoric participation in phonological processing plays a changing and complementary role with the “auditory part” of the phonological processing system from perception to production: the LIFG being increasingly relevant and the LSTG, decreasingly relevant. We previously targeted the LIFG with tDCS and observed behavioural and functional connectivity patterns which were consistent with our model. Here we investigated whether neurostimulation of the LSTG with the same tasks would reverse the profile of functional connectivity responses previously obtained for the LIFG. With a within-subject design, we administered categorical perception, lexical decision and word naming tasks in three sessions (under cathodal, anodal and sham). This range of tasks was considered to be decreasingly relevant for the LSTG. The network we focused on consisted of the dorsal stream (LIFG and LSTG) and its right homologues (right inferior frontal gyrus and right superior temporal gyrus). Functional connectivity was analysed with partial correlations. C-tDCS LSTG inhibition and A-tDCS LSTG excitation should have a positive relationship with the relevance of the task to the target (task load). C-tDCS was expected to trigger compensation by other nodes in conditions of low task load, evidenced by a higher count of significant connections than in higher task load conditions or under A-tDCS. Overall, results met predictions. For example, C-tDCS in conditions of lower task load, such as with the word naming task, triggered the highest count of significant connections, suggesting compensation by other nodes.