7Pupil size changes under different light conditions. Whereas this pupillary light response (PLR) has 8 long been regarded to be influenced by luminance only, recent studies indicated the PLR is also 9 modulated by cognitive factors such as the allocation of spatial attention. This attentional modulation 10 of the PLR has previously been hypothesized to facilitate detection and discrimination of visual 11 information. Here, we replicated the finding that the pupil dilates when a cue is presented at the dark 12 side of a screen and constricts when the cue is presented at the bright side, even when the eyes are 13 fixated at the center. Furthermore, we investigated whether this modulation of the PLR, evoked by 14 exogenous shifts of covert attention, facilitates perception operationalized as detection performance for 15 threshold stimuli. Results showed that a larger pupil was indeed related to increased detection 16 performance, although this effect was restricted to conditions in which both cue and target appeared on 17 a dark surface. Our findings are in line with the notion that pupil dilations improve detectability, 18 whereas pupil constrictions enhance discriminability of small stimuli. 19