Detection of Visual Stimuli located within Angles

Abstract

THE upper member of two horizontal lines, identical in length, drawn within an inverted V-shaped bracket appears to be somewhat longer than the lower. This figure is one example of many two-dimensional spatial patterns known as “the geometrical illusions”. These patterns undergo distortion in such a way as to assume an appearance markedly at variance with their physical form. This particular figure is usually described as the Ponzo illusion, the distortion characterizing it being seen clearly in Fig. 1. The geometrical illusions have provided the topics for a very large number of experimental enquiries. In these, attempts have been made to understand the spatial distortions seen in them, according to features of the structural organization and functional repertoire of the visual projection system. The purpose of the present communication is to initiate consideration of the problem of perceiving visual stimuli seen in the context of illusions, the Ponzo figure being used as a convenient example.