Overlapping Tasks Methodology as a Tool for Investigating Language Perception

A series of studies is reviewed that investigated the effects of additional tasks on the N400 component of the event-related brain potential (ERP) within the framework of an overlapping tasks paradigm. In all studies the N400 component was elicited by words that could be either synonymous or nonsynonymous to a preceding word and by subtracting the ERP to synonymous from those to non-synonymous words. Additional task stimuli were visual and could be presented at high to low temporal overlap with the words. The N400 was delayed in time when there was high temporal overlap with the additional task stimulus. The delay was more pronounced when the additional task was more difficult and more language-like. Second-language speakers showed very similar interference effects albeit at a globally slower level. Somewhat different interference effects were obtained when the meaning of the eliciting words were irrelevant to the task. In contrast, eye movements and visuo-spatial attention shifts had no effects on N400 latency. Together, the results indicate that the N400-eliciting processes — presumably the access to semantic knowledge — belong to a group of central processes that form a bottleneck in the cognitive system because they can handle only one process at a time. On the basis of these findings dual-task methodology is suggested to be a valuable tool not only for investigating the processes supporting language perception and, more generally, the retrieval of semantic knowledge but also for other language perception processes.