The pupil and the mind’s eye: Portable pupillometry captures robust responses to imaginary light

The pupillary response is driven by a range of factors, most notably by retinal illumination and arousal. Strikingly, the pupil’s diameter also adjusts in reaction to endogenous perceptual features, such as the brightness of imagined objects, the brightness or darkness evoked by certain words, and scene interpretation. Pupil size is, moreover, affected by all manner of cognitive processes, including pattern detection, mental effort, attention, and deception. Reliable measurement of pupil changes, however, usually requires tightly controlled conditions – a dark room lacking exogenous variable light sources is the gold standard. Such requirements effectively limit pupillometry to the laboratory, precluding its use with certain populations (e.g., hospitalized patients) as well as in public spaces like schools or parks. Here, using a commercially available head-mounted display (HMD) and an internally mounted eye tracker, we replicate results from two different groups demonstrating the pupil’s sensitivity to imaginary light (Laeng & Sulutvedt, Psychol Sci, 2014; Kay et al., eLife, 2022). In each trial, subjects were shown triangles with four possible luminance levels and four possible orientations against a mid-gray background. This perceptual phase was immediately followed by a long dark-adaptation period. Then, while staring at a gray screen, subjects were asked to vividly imagine the exact shape they had previously seen (with the same brightness and orientation). In line with previous findings, we show that subjects’ pupillary response is proportional not only to the luminance of the perceived shapes, but also to the brightness of their imagined counterparts. Our use of a portable HMD to limit the impact of ambient lighting conditions demonstrates the feasibility of conducting sophisticated pupillometry outside of the laboratory. If the subject cannot come to the darkness, the darkness must go to the subject.