Focusing on an illusion: Accommodating to perceived depth?

Ocular accommodation potentially provides information about depth but there is little evidence that this information is used by the human visual system. We use the hollow-face illusion, an illusion of depth reversal, to investigate whether accommodation is linked to perceived depth. In Experiment 1 accommodation, like vergence, was in front of the physical surface of the mask when the mask was upright and people reported experiencing the illusion. Accommodation to the illusory face did not differ significantly from accommodation to the physically convex back surface of the same mask. Only accommodation to the inverted mask seen as hollow was significantly less and, like the physical surface, beyond the mid-plane of the mask. The effect on accommodation was the same for monocular as binocular viewing, showing that accommodation is not driven by binocular disparities through vergence, although voluntary vergence remains a possibility. In Experiment 2 a projected random dot pattern was used to flip perception between convex and concave in all presentation conditions. Accommodation was again in front of the physical surface when the illusion was experienced. Experiment 3 showed that projected dots are more effective in disambiguating the illusion as concave when they are sharp and provide a good accommodative stimulus than when they are objectively blurred. We interpret Experiments 1 and 2 as showing that accommodation is tied to perceived depth, directly or indirectly, even in a situation where multiple depth cues are available and feedback is not artificially open-looped. Experiment 3 is consistent with accommodation helping to disambiguate depth while not ruling out alternative explanations.