The iris sphincter muscle is stronger than the dilatory pupillary muscle, making the iris sphincter muscle the primary controller of pupil diameter. The diameter of the pupil is controlled by the iris sphincter muscle that constricts the pupil and the dilatory pupillary muscle that promotes pupil dilation (Borgdorff, 1975 Levin and Kaufman, 2011). Pupil dilation is altered by luminance in the visual environment as well as during arousal, defined here as periods of heightened sensory responsiveness and perception that involve autonomic and endocrine activation. Mechanisms controlling pupil diameter and the effects of luminance

Here we review evidence that changes in pupil dilation reflect and, in some instances, may be caused by the activity of neuromodulatory pathways.

It has been suggested that these changes are linked by the activities of neuromodulatory systems (Jones, 2004 Lee and Dan, 2012) and changes in the activity of noradrenergic and cholinergic circuitry correlate with changes in pupil dilation. As a result of these correlations, pupillometry has been used as an indirect measure of brain state.Ĭhanges in the behavior of an animal, the pupil dilation (mydriasis), and the brain state often coincide on the scale of seconds. Changes in network activity are often accompanied by changes in neuronal responsiveness and pupil diameter (McGinley et al., 2015). Similarly, sharp wave ripples are observed in hippocampus during periods of reduced activity such slow wave sleep, quiet wakefulness, and grooming (Buzsáki, 1986). For example, high frequency fluctuations in pyramidal cell membrane potential, local field potential (LFP), and electroencephalogram (EEG) are observed in sensory cortices during attentive or activated states (Steriade, 1997 Crochet and Petersen, 2006). Brain states can be defined as periods of neuronal network activity that correlate with behaviors such as periods of arousal, locomotion, exploration, and attention (Lee and Dan, 2012). An animal's behavior and neuronal responses are modulated by changes in brain state.

Eye pupil constriction causes