Abstract
Pupil diameters were recorded with an eye-tracker while participants observed cruciform patterns of gray-scale gradients that evoked illusions of enhanced brightness (glare) or of enhanced darkness. The illusions were either presented as static images or as dynamic animations which initially appeared as a pattern of filled squares that—in a few seconds—gradually changed into gradients until the patterns were identical to the static ones. Gradients could either converge toward the center, resulting in a central region of enhanced, illusory, brightness or darkness, or oriented toward each side of the screen, resulting in the perception of a peripheral ring of illusory brightness or darkness. It was found that pupil responses to these illusions matched both the direction and intensity of perceived changes in light: Glare stimuli resulted in pupil constrictions, and darkness stimuli evoked dilations of the pupils. A second experiment found that gradients of brightness were most effective in constricting the pupils than isoluminant step-luminance, local, variations in luminance. This set of findings suggest that the eye strategically adjusts to reflect in a predictive manner, given that these brightness illusions only suggest a change in luminance when none has occurred, the content within brightness maps of the visual scene.
Highlights
In a 1969 Peanuts comic strip by Schulz, Charlie Brown’s little sister Sally is drawing a sun before the eyes of Linus van Pelt
Laeng and Endestad (2012) have recently shown that the pupil’s aperture is modulated not solely by the physical light intensity of a target region and by the perceived intensity of the region or () by its brightness. These pupillometry experiments made use of visual illusions caused by luminance gradients, such as the Asahi configuration (Kitaoka, 2005), or cognitive contours, in which the brightness of a target region appears enhanced with respect to an identically luminous background or to control stimuli made of the same elements
Such findings may strike one as being altogether astonishing for two main reasons: (a) The reflexive adjustments of pupil’s aperture, mediated by the parasympathetic system and functional to the protection of retinal receptors, are traditionally thought to be independent from cortical mechanism dedicated to processing visual information and (b) visual illusions, because of their arousing power and aesthetic value (Noguchi, 2003; Stevanov, Markovic, & Kitaoka, 2012) are considered to be ‘‘eye-catching’’ stimuli, and when something engages the attention of the observer (Beatty & Lucero Wagoner, 2000; Kahneman, 1973), this typically results in an increase in pupil size
Summary
In a 1969 Peanuts comic strip by Schulz, Charlie Brown’s little sister Sally is drawing a sun before the eyes of Linus van Pelt. Laeng and Endestad (2012) have recently shown that the pupil’s aperture is modulated not solely by the physical light intensity of a target region and by the perceived intensity of the region or () by its brightness. These pupillometry experiments made use of visual illusions caused by luminance gradients, such as the Asahi configuration (Kitaoka, 2005), or cognitive contours (as for the well-known ‘‘illusory triangle’’ by Kanizsa, 1979), in which the brightness of a target region appears enhanced with respect to an identically luminous background or to control stimuli made of the same elements. Such findings may strike one as being altogether astonishing for two main reasons: (a) The reflexive adjustments of pupil’s aperture, mediated by the parasympathetic system and functional to the protection of retinal receptors, are traditionally thought to be independent from cortical mechanism dedicated to processing visual information and (b) visual illusions, because of their arousing power and aesthetic value (Noguchi, 2003; Stevanov, Markovic, & Kitaoka, 2012) are considered to be ‘‘eye-catching’’ stimuli, and when something engages the attention of the observer (Beatty & Lucero Wagoner, 2000; Kahneman, 1973), this typically results in an increase in pupil size
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