Linear perspective and the Ponzo illusion: a comparison between rhesus monkeys and humans1

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Abstract
Rhesus monkeys trained to classify bars of different lengths as “long” or “short” were tested for their bias of this classification when photographic linear perspective was superimposed on the bar. The photograph was either an upright or an upside down picture of the landscape that provided good linear perspective to human eyes. In Experiment 1, these photographs appeared together with the upward‐converging lines. The rhesus monkeys reported “long” more often when the target was located above the center of the stimulus, that is, near the apex of the converging line, than when it was located below the center, irrespective of the orientation of the photograph. In Experiment 2, the photographs appeared without the converging lines. The subjects showed a bias similar to, but less than, Experiment 1 only for the upright photograph. No bias was found for the upside down photograph. It was concluded that in nonhuman primates photographic linear perspective induces the Ponzo illusion, but the effect is so small that it is overshadowed by the converging lines typically used to produce the Ponzo illusion. In Experiment 3, humans tested in a comparable procedure perceived the illusion as long as the upright picture was there, irrespective of the presence or absence of the converging lines. This suggested that linear perspective provided by natural photographs strongly induces the illusion in humans.