Pupil Dilation as a Dependent Variable in Language Research

In this chapter, we review research in which pupillometry has been used to investigate language processing. We group studies based on three major triggers for pupil dilation: cognitive load, surprise or violated expectations, and decision-making. Research on cognitive load has shown that pupil dilation increases with increasing linguistic complexity and listening effort. Studies on violated expectations or surprise have found that unexpected events lead to a transient increase in pupil dilation, reflecting a cognitive reaction to the unexpected input. Investigations of decision-making, meanwhile, have demonstrated that pupil dilation can reflect listeners’ level of uncertainty or confidence in making a decision about linguistic stimuli, such as deciding whether a string of letters is a word. Finally, we review research on language processing in different populations such as bilingual speakers and language learners and discuss a number of related issues deserving of consideration.