Event-related potentials and brain oscillations reflect unbalanced allocation of retrieval and integration efforts in sentence comprehension

Empirical studies have found a processing asymmetry between Chinese subject-extracted relative clauses (SRCs) and object-extracted relative clauses (ORCs). Still, there is no consensus on how this SRC-ORC asymmetry occurs. Thus, aiming to elucidate how the neural activity, in the forms of both event-related potentials (ERPs) and brain oscillations (i.e. event-related synchronisation/desynchronisation, ERS/ERD), attuned to sentences with different levels of processing difficulty, we conducted an electroencephalography (EEG) study to examine the comprehension of Chinese SRCs and ORCs. The results showed an N400 and a P600 effect when comparing SRCs and ORCs. Simultaneously, delta ERS was associated with N400 during the processing of both types of relative clauses and theta ERS with P600 during the processing of SRCs. By incorporating the ERP and ERS indexes, we propose that the dissociation between the integration and retrieval effort involved in sentence comprehension may account for the processing asymmetry between sentences.