Thematic Reanalysis in the Left Posterior Parietal Sulcus: A TMS Study

Understanding who does what to whom is at the core of sentence comprehension. The actors that contribute to the verb meaning are labeled thematic roles. We used transcranial magnetic stimulation (TMS) to verify the possible impact of verb semantics on the thematic role encoding process that has been shown to involve the posterior portion of the left posterior parietal sulcus (PPS; Finocchiaro et al., 2015). Sixteen participants underwent TMS and sham stimulation sessions while performing an agent-decision task, in which they had to decide by key press which of the two arguments was the agent of visually presented sentences or pseudo-sentences. The (pseudo)sentences were all reversible and were presented in the active or passive diathesis. Double pulse TMS was delivered to the posterior part of the intraparietal sulcus in an event-related fashion, at two different time windows: 200-400 ms (T1) or 600-800 ms (T2) time-locked to the presentation of the (pseudo)sentence. Results showed that TMS increased accuracy on passive sentences and pseudo-sentences as compared to active sentences and to the baseline, sham condition. Indeed, the presence of a verb with a full semantic representation was not a necessary precondition for the TMS-induced facilitation of passive (pseudo)sentences. Stimulation timing had no effect on accuracy for sentences vs. pseudo-sentences. These observations support the idea that the posterior parietal site is recruited when the correct interpretation of a sentence requires reanalysis of temporarily encoded thematic roles (as in reversible passive sentences) even when the verb is not an entry in the lexicon and hence does not have a semantic representation. Results are consistent with previous evidence and deserve further investigation in larger experimental samples. Increasing the number and variety of stimulus sentences, and administering TMS to additional control sites will be key to further articulate the conclusions allowed by these initial findings.