The typical finding is that pupil size follows the dominant percept, with a relative constriction when the brighter stimulus dominates conscious perception ( 14– 16), and an attenuation of pupillary responses to light flashes when these were presented to the suppressed eye ( 14, 17– 19). Another line of experiments showing pupil changes in conditions of constant retinal stimulation examined the phenomenon of binocular rivalry between stimuli of different luminance.
By the same token, the pupil constricts in response to visual illusions of brightness ( 10) and stimuli that evoke the idea of bright objects, like pictures of the sun ( 11, 12) or words conveying brightness ( 13).
For example, it has been shown that covert shifts of attention to brighter surfaces cause pupillary constrictions ( 6– 8) and that similar changes in pupil diameter can be induced even in the absence of visual stimulation by asking participants to mentally visualize a bright scene ( 9). More recently, it has been argued that high-level visual processing, including attention, mental imagery, and contextual modulation, can also influence pupillary responses under conditions of constant retinal illumination, demonstrating that the pupil diameter is not solely determined by physiological factors. However, since pupillometry-i.e., the measurement of the diameter and rate of reactivity of the pupil-was introduced more than 50 years ago, it soon became evident that pupillary responses can be used to index cognitive operations, such as thinking and emotional processing. This is known as the pupillary light reflex, and has been traditionally considered as a low-level mechanism that simply regulates the amount of light that enters the eye to optimize vision. Increments or decrements of light are associated with pupillary constrictions or dilations, respectively. This was not the case when the pictures were successfully suppressed from awareness, demonstrating that pupil size is highly sensitive to the contents of consciousness. Results revealed that when participants were aware of the sun pictures their pupils constricted relative to the control stimuli. A stream of Mondrian patterns was displayed to the right eye to produce continuous flash suppression, which rendered the left-eye stimuli invisible on some trials. Is conscious appraisal of these stimuli necessary for the pupillary constriction to occur? Participants' pupil diameter was recorded while sun pictures and their phase-scrambled versions were shown to the left eye. For example, stimuli that evoke the idea of brightness (e.g., pictures of the sun) induce pupillary constriction compared to control stimuli of matched luminance.
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