Left posterior inferior frontal gyrus is causally involved in reordering during sentence processing

- TMS over left pIFG disrupts argument reordering during sentence processing.
- pIFG-TMS increases performance decline for object- vs. subject-first sentences.
- pIFG-TMS selectively impairs sentences with a long argument-verb distance.
- Left pIFG is necessary for reordering during sentence comprehension.

Storage and reordering of incoming information are two core processes required for successful sentence comprehension. Storage is necessary whenever the verb and its arguments (i.e., subject and object) are separated over a long distance, while reordering is necessary whenever the argument order is atypical (e.g., object-first order in German, where subject-first order is typical). Previous neuroimaging work has associated storage with the left planum temporale (PT), and reordering with the left posterior inferior frontal gyrus (pIFG). Here, we tested the causal role of the PT and pIFG in storage and reordering using repetitive transcranial magnetic stimulation (rTMS). We applied either effective rTMS over PT or pIFG, or sham rTMS, while subjects listened to sentences that independently varied storage demands (short vs. long argument-verb distance) and reordering demands (subject- vs. object-first argument order). We found that rTMS over pIFG, but not PT, selectively affected reordering during the processing of sentences with a long argument-verb distance. Specifically, relative to sham rTMS, rTMS over pIFG significantly increased the performance difference between object- and subject-first long-distance sentences. These results demonstrate a causal involvement of left pIFG in reordering during sentence comprehension and thus contribute to a better understanding of the role of the pIFG in language processing.