Brightness perception changes related to pupil size

Dilating the pupils allow more quanta of light to impact the retina. Consequently, by “forcing” dilation of one pupil with a pharmacological agent (Tropicamide), the brightness of a surface under observation should increase proportionally to the dilation of the eye pupil. In a psychophysical procedure of brightness adjustment and matching, we presented to one eye geometrical patterns with a central square (the reference pattern) that differed in physical brightness within backgrounds of constant luminance. Subsequently, with the other eye, participants adjusted to the same luminance a similar pattern (target) whose central square luminance was randomly set higher or lower in brightness than the reference. As only one eye was treated with Tropicamide, we assessed whether the subjective brightness of the target square shifted in a consistent direction when viewed with the dilated pupil compared to the untreated eye. We found that, as the eye pupil increased post drug administration, so did the sense of brightness of the pattern (i.e., higher brightness adjustments followed viewing the reference with the treated eye). A reversed effect was observed when the untreated (control) eye viewed the reference first. The results confirm that even slight dilations of the pupil can result in an enhanced perceptual experience of brightness of the attended object, corresponding to an average increase of 2.09 cd/m2 for each 1 mm increase in pupil diameter