Spatial and feature-based attention enhance the Pupillary Light Reflex

Attention optimizes the processing of behaviorally relevant visual information by acting through multiple mechanisms, including neural response enhancement and sharpened tuning. One longstanding question is how early in the visual system attention exerts its influence. Here we investigate the effects of attention at the earliest possible level, the pupil of the eye. The diameter of the pupil modulates the amount of retinal illumination and the optical quality of the visual image, affecting visual acuity and sensitivity. The pupil constricts in response to light increments, mainly under the control of a subcortical reflex pathway; this ensures that the pupil diameter remains optimal across a wide range of illumination conditions. Our experiments show that attending to a bright surface results in a pupil constriction. On each trial, we presented a bright and a dark surface, either at separate peripheral locations (a bright and a dark disk to the left and right of fixation) or at the same central location (overlapping fields of bright and dark dots, rotating in opposite directions). Subjects were cued to attend to one surface and detect threshold-level events, while ignoring the other surface. Though retinal stimulation was the same across trials, a relative pupil constriction was observed when attention was directed to the bright surface. The effect was the same, irrespective of whether the attended surface was selected based on its spatial location or its features. Control experiments showed that attention selectively modulates the pupillary constriction evoked by light increments, leaving unaffected the pupillary dilation in response to light decrements. Our results reveal a new mechanism through which attention optimizes visual processing, by acting at the point where information enters the visual system to modulate the pupillary light response.Meeting abstract presented at VSS 2013