Pupillometric evidence favors an active attentional zoom lens account over passive filtering.

Most contemporary accounts of selective attention assume limited perceptual resources that need to be allocated through attention. These accounts, however, differ in how exactly irrelevant stimuli are filtered out. Suggestions include passive exhaustion of resources (e.g., load theory) or actively shaping the attended region (attentional window) such that irrelevant items remain excluded (e.g., attentional zoom account). We use pupillometry to test predictions of these accounts: Observers are asked to simultaneously monitor and compare two streams of rapidly presented items, either in a low-load (compare color) or high-load (compare the conjunction of color and orientation) condition. Flickering areas are placed between or beside the streams, falling either inside or outside the postulated attentional window. Observers respond faster and more accurately in the low-load condition, with no behavioral effects of the flicker position. In contrast, pupil size is more strongly modulated by the flicker in the inside than in the outside condition, but the modulation strength is independent of load. Exploiting the fact that the strength of the pupil modulation at the flicker frequency is an index of covert attention to the flickering region, we interpret this result as evidence for a continuous attentional window that encompasses the space between the streams. Together, our data provide evidence for an active filter mechanism, supporting theories like the attentional zoom account, rather than a passive, load-dependent spreading of residual attentional resources.