Control of the corridor illusion in baboons (Papio papio) by gradient and linear-perspective depth cues.

The corridor illusion was recently demonstrated in baboons with background pictures containing rich depth information (Barbet and Fagot 2002, Behavioural Brain Research 132 111-115). In the current research we determined the contribution of gradient texture and perspective lines to that illusion. In experiment 1, the corridor illusion was tested in two baboons, with pictures of a hallway as backgrounds, or the same image of the hallway represented by perspective lines. Findings confirmed that the baboons experience the corridor illusion with the picture of the hallway, and showed that this illusion remained with the perspective-line backgrounds. The same procedure was adopted in experiment 2, but with a hallway drawn from gradient textures. The two baboons experienced the illusion in this experiment too. Thus both gradient and perspective line cues convey sufficient information to control the corridor illusion in baboons. In baboons, the processing of these two kinds of depth cues interacts with the perception of object size, suggesting homologous processes of pictorial depth perception in humans and non-human primates.