Haptic illusions and inappropriate constancy scaling.

Abstract

THE widespread use of the term “optical” illusion has implied both that spatial illusions are restricted to vision and that the explanation of illusions is to be sought in terms of the operating characteristics of the visual system. Equivalent visual and haptic illusions are, however, obtained with many of the well known illusion figures1. During haptic inspection a blindfolded subject is required to trace his finger over the raised contours of a figure. Some variables known to affect the magnitude of a visual illusion similarly influence the haptic illusion found with the same figure. Varying the angle of the arrowheads of the Muller–Lyer figure modifies visual and haptic judgments in the same way2. The magnitude of the illusion diminishes over practice trials at the same rate for the two modalities, and crossmodal transfer of the practice decrement is found3. These data suggest that it is unlikely that quite different processes coincidentally produce equivalent spatial illusions in the two modalities. Haptic illusions are found with congenitally blind subjects1,4; thus it cannot be argued that haptic judgments are controlled by a framework developed through visual learning.