Pupil Dilation Reflects the Creation and Retrieval of Memories

It has long been known that pupils-the apertures that allow light into the eyes-dilate and constrict not only in response to changes in ambient light but also in response to emotional changes and arousing stimuli (e. g., Fontana, 1765). Charles Darwin (1872) related changes in pupil diameter to fear and other "emotions" in animals. For decades, pupillometry has been used to study cognitive processing across many domains, including perception, language, visual search, and short-term memory. Historically, such studies have examined the pupillary reflex as a correlate of attentional demands imposed by different tasks or stimuli-pupils typically dilate as cognitive demand increases. Because the neural mechanisms responsible for such task-evoked pupillary reflexes (TEPRs) implicate a role for memory processes, recent studies have examined pupillometry as a tool for investigating the cognitive processes underlying the creation of new episodic memories and their later retrieval. Here, we review the historical antecedents of current pupillometric research and discuss several recent studies linking pupillary dilation to the on-line consumption of cognitive resources in long-term-memory tasks. We conclude by discussing the future role of pupillometry in memory research and several methodological considerations that are important when designing pupillometric studies.