Changes in locomotion, brain states and pupil size are correlated in time ( Reimer et al., 2014 ), making it difficult to determine whether pupil dilation is driven by a single or multiple neuromodulatory circuits and, if multiple, whether the circuits that relate pupil size and locomotion, and pupil size and brain states act independently. Changes in pupil diameter are primarily driven by luminance and the visual environment, necessitating careful separation of the effectors when seeking to use pupil diameter as a proxy for an internal brain state. Changes in pupil diameter are thought to coincide with the activity of neuromodulators, including noradrenaline and acetylcholine, producing alterations in the brain state and corresponding changes in behavior.

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