The role of the l-IPS in the comprehension of reversible and irreversible sentences: an rTMS study

Thematic roles can be seen as semantic labels assigned to who/what is taking part in the event denoted by a verb. Encoding thematic relations is crucial for sentence interpretation since it relies on both syntactic and semantic aspects. In previous studies, repetitive transcranial magnetic stimulation (rTMS) over the left inferior intraparietal sulcus (l-IPS) selectively influenced performance accuracy on reversible passive (but not active) sentences. The effect was attributed to the fact that in these sentences the assignment of the agent and theme roles requires re-analysis of the first-pass sentence parsing. To evaluate the role of reversibility and non-canonical word order (passive voice) on the effect, rTMS was applied over l-IPS during a sentence comprehension task that included reversible and irreversible, active and passive sentences. Participants were asked to identify who/what was performing the action or who/what the action was being performed on. Stimulation of the l-IPS increased response time on reversible passive sentences but not on reversible active sentences. Importantly, no effect was found on irreversible sentences, irrespective of sentence diathesis. Results suggest that neither reversibility nor sentence diathesis alone are responsible for the effect and that the effect is likely to be triggered/constrained by a combination of semantic reversibility and non-canonical word order. Combined with the results of previous studies, and irrespective of the specific role of each feature, these findings support the view that the l-IPS is critically involved in the assignment of thematic roles in reversible sentences.