Neuromodulation of prefrontal cortex promotes deep processing during language comprehension: a tDCS/EEG study

In this study, we used transcranial direct current stimulation (tDCS), a noninvasive neuromodulation technique, to test a set of hypotheses about the extent to which the prefrontal cortex (PFC) contributes to revision and updating processes during language comprehension. Following 20 min of PFC-targeted, Active Control, or Sham tDCS, EEG was recorded while participants performed a widely used paradigm in which they read sentences containing plausible and implausible thematic roles (e.g. The actress/film-maker was directed by the film-maker/actress on set). This linguistic manipulation allowed us to examine comprehension under conditions when shallow processing and deeper processing yield conflicting meaning representations, which previous work has shown often engages revision and updating processes upon detection of the conflict. A different pattern of event-related potential responses was elicited when the same participants encountered implausible thematic roles during reading after receiving Sham compared with PFC-targeted tDCS. Specifically, N400 effects were found after Sham tDCS, whereas robust P600 effects were found after PFC-targeted tDCS (and to a significantly lesser extent, after Active Control tDCS). This suggests that while readers tended to treat implausible thematic roles as semantic anomalies after Sham tDCS, those same readers were more likely to detect conflict and engage in revision and updating in response to implausible thematic roles when in a state of heightened PFC stimulation. These results provide a novel demonstration of within-individual variability in language processing depending on current neurocognitive state and have implications for psycholinguistic theory about PFC contributions to revision and updating processes during language comprehension.