Beyond arousal: pupil fluctuations, neural activity, and behavior.

Spontaneous fluctuations in pupil size are correlated with neural activity and behavior in many ways. This is generally attributed to a single underlying cause: arousal. This unitary account disregards other potential underlying causes. To address this gap, we propose a comprehensive framework of four mechanisms that jointly underlie the complex and context-dependent relationships between spontaneous pupil-size fluctuations, neural activity, and behavior. In addition to arousal, we consider optical effects, brightness constancy, and self-generated visual stimulation. Based on this framework, we argue that pupil fluctuations should be considered beyond arousal, as a functionally important, yet still poorly understood, component of visual processing.