Non-image forming vision as measured through ipRGC-mediated pupil constriction is not modulated by covert visual attention

When exposed to light, the pupil constricts, whereas in darkness, the pupil dilates: this is the pupillary light response (PLR), which, for a long time, had been considered to be a reflex. The PLR is driven by all photoreceptors—rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs)—where rods and cones cause the pupil to immediately constrict in response to light, whereas ipRGCs cause the pupil to remain constricted for as long as light is on. Recent studies have shown that the initial PLR is modulated by covert attention; however, it remains unclear whether the same holds for the sustained PLR that is driven by ipRGCs. In our study, we investigated the effect of covert attention on the sustained PLR. To do so, we leveraged the fact that ipRGCs are predominantly responsive to blue light, causing the most prominent sustained constriction in response to blue light. We found that the pupil constricted more when covertly attending to bright as compared to dim stimuli (with the same color), an effect that emerged rapidly after stimulus onset, thus replicating the effect of covert attention on the initial PLR. However, we did not find any difference in pupil size when covertly attending to blue as compared to red stimuli (with the same luminosity), whereas we did observe this difference when participants directly looked at the same blue or red stimuli. This suggests that the sustained PLR is not modulated by covert attention. This finding implies that non-image forming vision, as measured through ipRGC-mediated pupil constriction, is not modulated by covert visual attention.