Beyond Arousal: Pupil Fluctuations, Neural Activity, and Behavior

Spontaneous fluctuations in pupil size are often interpreted as a direct readout of arousal-related neural activity. When pupil size correlates with behavior, this is therefore assumed to reflect arousal. However, pupil fluctuations also have optical consequences: larger pupils allow more light in the eye, whereas smaller pupils improve focus. These effects are rarely considered. Generally, the relationships between pupil fluctuations, neural activity, and behavior are complex and context-dependent. We propose four mechanisms to better understand these relationships. The first is mediated by arousal. The second relates to the optical effects of pupil fluctuations. The third relates to visual stimulation, which both constricts the pupil and activates visual brain areas. The fourth relates to brightness constancy with respect to pupil size.