The pupillary light response has been shown not to be a purely reflexive mechanism, but to be sensitive to higher order perceptual processes, such as covert visual attention. In a new study we examined whether the pupillary light response is modulated by stimuli which are not physically present, but maintained in visual working memory. The results reveal that the encoding of task-relevant and physically present information in visual working memory is reflected in the pupil. In contrast, the pupil is not sensitive to the maintenance of task-relevant, but no longer visible stimuli. One interpretation of our results is that the pupil optimizes its size for perception of stimuli during encoding; however, once stimuli are no longer visible (during maintenance), an “optimal” pupil size no longer serves a purpose, and the pupil may therefore cease to reflect the brightness of the memorized stimuli.
Blom, T., Mathot, S., Olivers, C. N. L., & Van der Stigchel, S. (in press). The pupillary light response reflects encoding, but not maintenance, in visual working memory. Journal of Experimental Psychology: Human Perception and Performance.