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Paola Binda, Maria Pereverzeva, Scott Murray; Attention enhances the Pupillary Light Reflex. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3400. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The reflexive pupillary constriction in response to light (Pupillary Light Reflex, PLR) depends on a well characterized subcortical circuit. However, cortical signals have been suggested to modulate this simple behavior. We investigated whether the behavioral relevance of the luminance stimuli affects pupillary responses. Specifically, we asked whether a pupillary constriction can be evoked without any luminance increase, by deploying attention to the bright elements of a constant visual scene.
We measured pupil diameter in 6 normal adult humans while they fixated at the center of a display composed of two fields of moving dots, bright and dark, generating the percept of two overlapping surfaces rotating in opposite directions. Subjects were cued to attend to either the bright or the dark surface and detect threshold-level speed increments, while ignoring speed increments in the other surface.
Even though retinal illumination was constant across trials and across space, pupil diameter was smaller when attention was directed to the bright surface versus the dark surface. The difference in pupil size between the two conditions was about 30% of the difference observed in a control experiment, where a single surface was presented (bright or dark), causing a physical increment or decrement of retinal illumination.
These findings extend our recent observation that attending to a luminance increment enhances the evoked pupillary constriction. Pupil diameter affects visual acuity and sensitivity; the PLR ensures that pupil diameter remains optimal across a wide range of illumination conditions. The attentional modulation of the PLR that we report may serve to optimize pupil diameter for the light level of behaviorally relevant visual features. Our findings suggest that cortical signals related to the distribution of attention modulate the output of the subcortical PLR circuit. They reveal a new mechanism through which attention optimizes visual processing, not only by modulating thalamic and cortical visual responses but also by changing the optics of the eye.
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