The pupil’s response to illusory nearness

The size of the pupil decreases when the eye focusses on a nearby object. Enright (1987) investigated if the near-far response similarly occurs when viewing illusory 3D images in a 2D surface. His observations of the pupillary response were “decidedly not typical”. Furthermore, it was shown that covertly shifting attention towards light or dark areas elicits a pupil response (Mathôt et al., 2013). To investigate if the pupils’ near-far reflex occurs when subjects are viewing illusory 3D images on a flat 2D surface, we conducted an experiment with 30 human participants. Subjects viewed illusory 3D objects on a computer screen while we recorded pupil size with and Eyelink camera. On each trial, 2D renderings of 3D objects were shown close by and far away, on either side of the center. Subjects responded to a target line segment briefly appearing in one of the objects by indicating if the line segment tilted left or right. To direct the subject’s attention to either the far-away or close-by object, an arrow appeared right before the target. In the “eyes fixed” condition subjects were instructed to not move their eyes away from the center but covertly shift their attention towards the cued object, in the “eyes free” condition they shifted their gaze. Pupils were relatively small when subjects viewed the near object in comparison to viewing the far object. However, this was only true in the “eyes free” condition. No significant difference was observed when attention was covertly directed towards near or far objects in the “eyes fixed” condition. We conclude that the pupil near-far response occurs when viewing illusory 3D objects on 2D surfaces, at least when directly looking at the objects.