Pupil size reflects the content of covertly attended afterimages

Does attention operate within afterimages? Here we show that it does, using a novel pupillometry-based paradigm. Participants fixated centrally while bright and dark peripheral stimuli were presented, and a central cue directed attention to one of them. Over time, the stimuli perceptually faded due to adaptation and were then removed, leaving strong, negative afterimages. We found that pupil size tracked the brightness of the attended stimulus both during perceptual fading, when stimuli were present but perceptually weakened, and during perception of afterimages, when no physical stimuli were present. In the latter case, pupil size reflected the brightness of the negative afterimage rather than the preceding physical stimulus. This finding shows that covert attention can be directed within afterimages. More broadly, the results suggest that attention to afterimages bridges the gap between external and internal attention, challenging the notion of a strict dichotomy and supporting the view that this distinction is better understood as a continuum.