A classroom-based test of the absolute depth theory of stereopsis.

Stereopsis, the visual experience of palpable depth and solidity, is traditionally thought to depend on the congruence or conflict amongst depth cues. But a more speculative theory is that it depends critically on being able to estimate the absolute depths of objects, and their real-world distances from us. We tested this idea in a perception class, using a picture of some plants, a cardboard box with a monocular viewing hole, and a pair of pinhole glasses. Fourteen of 16 students reported stronger stereopsis with pinhole viewing, contrary to the predictions of the absolute depth theory, but consistent with a traditional account. This classroom exercise offers an empirical challenge for the absolute depth theory, and a vivid teaching tool for the paradoxes of pictorial depth perception.