Pupillary adjustments to the illusory depth of rotoreliefs

Recent studies in the perception of visual illusions as well as mental imagery indicate that adjustments of the eye pupils reflect the perception instead of the light input stimulating the eyes. The present study put to test the expectation that similar adjustments of the eye pupil and, possibly, of other oculomotor responses like vergence, would be triggered by the stereokinetic effect, that is, illusory depth. In an eye-tracking study, both gaze direction and pupillary changes were measured while observers viewed "rotoreliefs,"' that is, rotating two-dimensional patterns, that are known to generate a particularly strong impression of depth. Rotoreliefs can be multistable, and in some cases their perceptual interpretations alternate between seeing either a bump or a hole, protruding or extending beyond the screen. We found strong evidence that pupil diameters were larger when the depth illusion was that of a hole (funnel) than of a bump (cone). Additionally, eyes diverged a few seconds after observers reported a reversal from a bump to a hole. Thus, the eyes adjust spontaneously to what looks like a three-dimensional form, regardless of whether this is an illusion or a veridical perception. We propose that such oculomotor adjustments to illusions index the disambiguation of visual input that optimizes possible interactions with external objects in a dynamic 3D world.