Academic journal article Psychonomic Bulletin & Review

The Role of Expectancies in the Size-Weight Illusion: A Review of Theoretical and Empirical Arguments and a New Explanation

Academic journal article Psychonomic Bulletin & Review

The Role of Expectancies in the Size-Weight Illusion: A Review of Theoretical and Empirical Arguments and a New Explanation

Article excerpt

Published online: 16 April 2014

© Psychonomic Society, Inc. 2014

Abstract The size-weight illusion (SWI) refers to the phenomenon that objects that are objectively equal in weight but different in size or volume are perceived to differ in weight, such that smaller objects feel heavier than larger ones. This article reviews studies trying to support three different viewpoints with respect to the role of expectancies in causing the SWI. The first viewpoint argues for a crucial role; the second admits a role, yet without seeing consequences for sensorimotor processes; and the third denies any causal role for expectancies at all. A new explanation of the SWI is proposed that can integrate the different arguments. A distinctive feature of the new explanation is that it recognizes the causal influence of expectancies, yet combines this with certain reactive and direct behavioral consequences of perceiving size differences that are independent of experience-based expectancies, and that normally result in the adaptive application of forces to liftor handle differently sized objects. The new account explains why the illusion is associated with the repeated generation of inappropriate lifting forces (which can, however, be modified through extensive training), as well as why it depends on continuous visual exposure to size cues, appears at an early age, and is cognitively impenetrable.

Keywords Perceptual illusions . Size-weight illusion . Internal representation . Evolution

The size-weight illusion (SWI) refers to the phenomenon that objects that are objectively equal in weight but different in size or volume are perceived to differ in weight, such that smaller objects feel heavier than larger ones. The most compelling way to demonstrate the SWI is to first let research participants liftequal-weight but differently sized objects blindly and in the same manner via an attached handle or string (so that size cannot be haptically determined) and to establish that no differences in weight are perceived. When, during visual exposure to size differences, objects are again lifted and size appears to negatively influence perceived weight (see, e.g., Ellis & Lederman, 1993), we must assume that perceived size rather than some other difference between the stimulus objects (e.g., differences in mass distribution) was causally responsible for the illusion. That the illusion strongly depends on visual exposure to size cues is suggested by the tendency of perceivers to avoid looking at hand-held objects when trying to accurately determine weight differences (Ross, 1969). Note, however, that the illusion also occurs when information about size is obtained entirely haptically (Ellis & Lederman, 1993).

Although the illusion has attracted research attention since the 19th century (for reviews, see Amazeen & Turvey, 1996; Murray, Ellis, Bandomir, & Ross, 1999; Nicolas, Ross, & Murray, 2012), researchers continue to disagree about its explanation. A dominant explanation for the SWI is that it is based on a general expectancy, derived from previous experience, that larger objects are heavier than smaller ones of similar material, and that this expectancy is violated when differently sized objects of equal physical weights are lifted or supported. This unexpectedness, or "contrast," with the initial expectancy is what determines perceivers' incorrect weight judgments.

At phenomenological and verbal levels, the role of expectancies and expectancy violation in illusory weight perception is strongly supported. For example, Dijker (2008) measured perceived size, perceived weight, and expected weight in separate groups of participants and found that, in terms of variation between objects, the effect of perceived size on the perceived weight of equal-weight objects was completely mediated by expected weight. Furthermore, in many early studies of the SWI (e.g., Davis, 1973; Koseleff, 1957; Usnadze, 1931), it was mentioned that participants reported being surprised that the small object felt relatively heavy and the large one relatively light. …

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