Jonathon Whitlock¹, Ryan Hubbard², Lili Sahakyan³; ¹Mississippi State University, ²University at Albany, SUNY, ³University of Illinois Urbana Champaign

In episodic memory, pupil size increases are greater for correctly recognized old items compared to correctly rejected new items—a phenomenon known as the pupil old-new effect. This effect is often explained by the memory strength account, which posits that stronger memory traces, formed through deeper encoding or memories that involve enriched contextual details, elicit greater pupil dilation. However, challenges to this account arise with the subjective pupil old-new effect, where falsely recognized new items also elicit larger pupil sizes than missed old items. To disentangle these effects, we investigated pupil responses across the entire time course of retrieval during single-item and associative recognition tasks. In a single-item recognition task, participants studied and were tested on faces, providing recognition and confidence judgments. We obtained both the traditional (objective) and the subjective pupil old-new effects and found that confidence judgments dissociated the two effects by moderating the strength of the objective effect but not the subjective effect. Temporal principal component analyses further dissociated these two components across the time course of retrieval, demonstrating the dynamic unfolding of these effects over time. In an associative recognition task, participants studied object-scene pairs and were tested on original pairs, recombined pairs, and novel pairs. Early in the time course, pupil sizes differentiated between original and recombined pairs, despite both involving familiar (studied) information, demonstrating that the early pupil signal reflects the extent to which test stimuli matches the memory representation for studied information. Together, these findings suggest that pupil responses during recognition testing reflects a dynamic rapid match signal between the memory representation formed during learning and test stimuli. By leveraging pupil dilation as a physiological marker, this research advances our understanding of the mechanisms underlying episodic memory retrieval and highlights the complexity of distinguishing between objective and subjective recognition processes.