Functional near-infrared spectroscopy shows that object relative clauses are more difficult to process than subject relative clauses in Turkish.

It was suggested that processing subject relative clauses (SRCs) is universally easier than processing object relative clauses (ORCs) based on the studies carried out in head-initial languages such as English and German. However, studies in head-final languages such as Chinese and Basque contradicted this claim. Turkish is also a head-final language. Existing relative clause processing literature in Turkish is based solely on behavioural metrics. Even though an ORC processing disadvantage was suggested for Turkish, the results were not conclusive. Therefore, we aimed to investigate the neural dynamics of relative clause processing in Turkish. We asked 14 native Turkish speakers to answer yes/no questions about 24 sentences each containing either a SRC or ORC while their prefrontal hemodynamic activity was recorded with functional near-infrared spectroscopy. Our findings revealed hemodynamic activity in the lateral portions of the left prefrontal cortex for both conditions. However, hemodynamic activity was more widespread in prefrontal regions in ORC compared to SRC condition. Even though the behavioural metrics failed to produce a significant difference between the conditions, direct ORC > SRC contrast revealed significant activity in the left inferior frontal cortex, a region heavily involved in language processing, as well as in left and right dorsolateral prefrontal cortices, which are also known to be involved in language processing-related and conflict monitoring-related processes, respectively. Our findings indicate that processing ORCs is more difficult and requires further prefrontal resources than processing SRCs in Turkish, thus refuting the head-directionality-based explanations of relative clause processing asymmetries.