MEG evidence that the LIFG effect of object extraction requires similarity-based interference

This study addresses a much-debated effect on a much-debated region: the increase of left inferior frontal gyrus (LIFG) activation associated with object-extracted relative clauses. This haemodynamic result is one of the most central and most cited findings in the cognitive neuroscience of syntax and it has robustly contributed to the popular association of Broca's region with syntax. Our study had two goals: (1) to characterise the timing of this classic effect with magnetoencephalography (MEG) and (2) to connect it to psycholinguistic research on the effects of similarity-based interference during sentence processing. Specifically, behavioural studies have shown that object relatives are primarily only costly when the two preverbal noun phrases are parallel in their surface syntax, for example, both consisting of a definite determiner and a noun (e.g. the reporter who the senator attacked), as opposed to employing, for example, a definite noun phrase and a proper name (the reporter who Bill attacked). This finding suggests that the difficulty of object extraction lies not within its syntax but rather in similarity-based interference affecting working memory processes. Although working memory is a prominent hypothesis for the LIFG engagement in object extraction, the haemodynamic literature has routinely employed stimuli involving parallel as opposed to non-parallel syntax. Using written sentences presented word-by-word, we tested whether an LIFG effect of object extraction is obtained with MEG, allowing us to characterise its timing, and whether it reduces or disappears if the two preverbal noun phrases are non-parallel in their surface syntax. Our results show an LIFG increase for object relatives at around 600 ms after verb onset, but only when the preverbal arguments are parallel. These findings are consistent with memory and competition-based explanations of the LIFG effect of object extraction and challenge accounts attributing it to displacement.