The logic of misperceived distance (or location) theories of the Poggendorff illusion

What Are Visual Illusions?

Visual illusions have been a popular topic of study for a long time, but in recent years, a number of authors have questioned the coherence of this notion. This article deals in depth with ways in which illusions have been, could be, and should be defined and with various criticisms and different conceptions of the notion of illusions. After a review of definitions of illusions in the relevant literature, a more comprehensive but also more restrictive framework is presented, involving both veridicality and illusoriness, and is illustrated using two variants of a 2 × 2 scheme for the presentation of illusions. Many different aspects of illusions are discussed. A set of criteria for illusionhood is listed. Criticisms of the notion of illusions are presented, commented upon, and responded to. Conceptions of illusions differing from the one advocated here are discussed. Throughout the paper, novel variations of illusions are shown, and problems with accounts of some well-known illusions are noted. Examples of strong context effects which are not considered to be illusions are presented. It is concluded that the notion of visual illusions, suitably reformulated, is still viable.

Applying Emmert's Law to the Poggendorff illusion

The Poggendorff illusion was approached with a novel perspective, that of applying Emmert's Law to the situation. The extensities between the verticals and the transversals happen to be absolutely equal in retinal image size, whereas the registered distance for the verticals must be smaller than that of the transversals due to the fact that the former is assumed to occlude the latter. This combination of facts calls for the operation of Emmert's Law, which results in the shrinkage of the occluding space between the verticals. Since the retinal image shows the transversals to be in contact with the verticals, the shrinkage must drag the transversals inwards in the cortical representation in order to eliminate the gaps. Such dragging of the transversals produces the illusory misalignment, which is a dictation of geometry. Some of the consequences of this new explanation were tested in four different experiments. In Experiment 1, a new illusion, the tilting of an occluded continuation of an oblique line, was predicted and achieved. In Experiments 2 and 3, perceived nearness of the occluding entity was manipulated via texture density variations and the predicted misalignment variations were confirmed by using a between-subjects and within-subjects designs, respectively. In Experiment 4, tilting of the occluded segment of the transversal was found to vary in the predicted direction as a result of being accompanied by the same texture cues used in Experiments 2 and 3.