A circuit for pupil orienting responses: implications for cognitive modulation of pupil size

• Transient pupil dilation is evoked by the presentation of visual or auditory stimuli.
• The evoked pupil responses are systematically modulated by stimulus saliency.
• Similar transient pupil dilation is evoked by weak microstimulation of the SC.
• SC projections to the pupil circuitry can mediate pupil modulation by cognitive processes.

Pupil size, as a component of orienting, changes rapidly in response to local salient events in the environment, in addition to its well-known illumination-dependent modulation. Recent research has shown that visual, auditory, or audiovisual stimuli can elicit transient pupil dilation, and the timing and size of the evoked responses are systematically modulated by stimulus salience. Moreover, weak microstimulation of the superior colliculus (SC), a midbrain structure involved in eye movements and attention, evokes similar transient pupil dilation, suggesting that the SC coordinates the orienting response which includes transient pupil dilation. Projections from the SC to the pupil control circuitry provide a novel neural substrate underlying pupil modulation by various cognitive processes.