Eyes wide open: Object-scene congruency and the pupillary response

The pupil response has long been considered a robust marker of cognitive load. In the context of semantic processing, research has demonstrated that the pupil dilates in response to stimuli which violate contextual expectations (e.g. events presented out of chronological order). However, the scope of this relationship has yet to be fully elucidated. For example, incongruent object-scene relationships, while comprehensively explored by eye-tracking and electrophysiology research, have yet to be investigated via pupillometry. In this study, we measured pupil size in response to an object-scene congruency task. Participants were presented with a photorealistic background scene and instructed to fixate their gaze on a cued point within the scene. Upon recovery of pupil size to baseline, a congruent object (i.e. an object which fit into the overall meaning of the scene) or an incongruent object appeared at the cued fixation point for the remainder of each trial. We hypothesized that incongruent objects would result in greater mean pupil dilation from baseline than congruent objects, due to the increase in cognitive effort required for semantic processing of incongruent objects within a scene. Yet, in opposition to our hypothesis, the results of a time-course analysis revealed that pupil size was significantly greater for the congruent condition than the incongruent condition. The resulting implications for understanding pupil dilation as a physiological marker, both independently and in comparison to other markers, for high-level cognitive processes such as semantic integration are discussed.