When a Dog is a Cat and How it Changes Your Pupil Size: Pupil Dilation in Response to Information Mismatch

In the present study, we investigate pupil dilation as a measure of lexical retrieval. We captured pupil size changes in reaction to a match or a mismatch between a picture and an auditorily presented word in 120 trials presented to ten native speakers of Swedish. In each trial a picture was displayed for six seconds, and 2.5 seconds into the trial the word was played through loudspeakers. The picture and the word were matching in half of the trials, and all stimuli were common high-frequency monosyllabic Swedish words. The difference in pupil diameter trajectories across the two conditions was analyzed with Functional Data Analysis. In line with the expectations, the results indicate greater dilation in the mismatch condition starting from around 800 ms after the stimulus onset. Given that similar processes were observed in brain imaging studies, pupil dilation measurements seem to provide an appropriate tool to reveal lexical retrieval. The results suggest that pupillometry could be a viable alternative to existing methods in the field of speech and language processing, for instance across different ages and clinical groups.