Pupil size at encoding predicts the temporal structure of memory

Episodic memories are temporally organized: items experienced close together in time during encoding are later remembered together. We examined how spontaneous attentional fluctuations during encoding, measured with pupil diameter, affect this temporal organization. We hypothesized that smaller pupil sizes would index worse attentional states that disrupt the ability to link items over time. Healthy young adults (N = 71) encoded object images while eye tracking was performed. Memory was tested with free recall. We then examined recall organization as a function of pupil size at encoding. As hypothesized, the temporal structure of recall was disrupted when pupil sizes were smaller vs. larger at encoding. Secondary analyses showed that these results were specific to pupil diameter: fluctuations in response times predicted task errors but not the temporal organization of memory. Together, these results show that pupil size provides a window into the cognitive operations that shape the temporal structure of memory.