Perceptual Illusions in Imagery

A mental image is in many ways analogous to a percept but it is not completely identical to it. In some respects, visual perception and visual imagery work in different ways. One area which is worth examining with regard to similarities and asymmetries between perception and imagery is the initial phases of visual information processing. The literature includes some references to the equivalence of imagery and perception in optical illusions, but data are contradictory. In our view, a mental image should not be particularly sensitive to variables which are critical in producing an optical illusion, i.e., variables affecting the early phases of information processing and field global effects. Our hypothesis is that an optical illusion will be present in a perception condition but not in an equivalent imagery condition. To test this, we carried out two experiments using the Ebbinghaus illusion and the Ponzo illusion. The results confirmed our hypothesis, demonstrating that there are indeed asymmetries between perception and imagery and that such differences mainly concern specific perceptual processes that differ from those involved in the generation of a mental image.