Abstract
The pupillary light response is often assumed to be a reflex that is not susceptible to cognitive influences. In line with recent converging evidence, we show that this reflexive view is incomplete, and that the pupillary light response is modulated by covert visual attention: Covertly attending to a bright area causes a pupillary constriction, relative to attending to a dark area under identical visual input. This attention-related modulation of the pupillary light response predicts cuing effects in behavior, and can be used as an index of how strongly participants attend to a particular location. Therefore, we suggest that pupil size may offer a new way to continuously track the focus of covert visual attention, without requiring a manual response from the participant. The theoretical implication of this finding is that the pupillary light response is neither fully reflexive, nor under complete voluntary control, but is instead best characterized as a stereotyped response to a voluntarily selected target. In this sense, the pupillary light response is similar to saccadic and smooth pursuit eye movements. Together, eye movements and the pupillary light response maximize visual acuity, stabilize visual input, and selectively filter visual information as it enters the eye.
Highlights
The pupil adjusts its size in order to optimize visual acuity under varying levels of luminance [1,2]
Modulation of the pupillary light response (PLR) by Covert Visual Attention The main results are shown in Fig. 2, in which mean pupil size is plotted over time
We have shown that attending to a bright area induces a pupillary constriction relative to attending to a dark area under identical visual input (Fig. 2)
Summary
The pupil adjusts its size in order to optimize visual acuity under varying levels of luminance [1,2]. Acuity is limited by the number of photons that fall on the retina, and the pupil becomes enlarged to increase the influx of light. Acuity is limited by optical artifacts, and the pupil constricts to reduce spherical and chromatic aberrations. The PLR is a constriction of the pupil in response to luminance increases. When going from brightness to darkness, the pupil gradually ‘unconstricts’ back to a resting state [4]. When a stressor is removed or when an organism habituates, the pupil gradually shrinks back to a resting state [6]. This type of ‘undilation’ is fundamentally different from the constriction of the PLR
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