Pupil size as a window on neural substrates of cognition

Pupil modulations provide a non-invasive window onto how the brain performs many cognitive tasks. Here we review recent work that has begun to identify specific neural underpinnings of these cognitively driven pupil signals, including key roles for: 1) the superior colliculus (SC), which mediates responses to salient stimuli that evoke orienting responses; and 2) the locus coeruleus (LC), the source of a major neuromodulator (norepinephrine) that mediates arousal and cognition. We discuss how these findings can inform the interpretation of pupil measurements in terms of specific neural operations involving the SC and LC. We also highlight caveats, open questions, and key future experiments for improving the interpretation of pupil measurements in terms of the underlying neural dynamics throughout the brain.