Studies in Perception and Action V MA. Grealy & J. A. Thomson (Eds.) © 1999 Lawrence Erlbaum Associates, Inc.
Thomas R. Alley
Department of Psychology, Clemson University, USA
A concern for estimates of linear extent arises in many contexts, including basic research in perception (e.g., judgments of distance), clinical studies (e.g., judgments of "body image") and industry. From an ecological perspective, size and distance are most meaningful in terms of body-scaled potential activities. Natural selection and everyday activities guarantee that humans are able to judge linear extents under many conditions with sufficient accuracy to support a great variety of actions. For perception to serve action, distance and size must be perceived in body-scaled terms, and to do this we must be able to accurately judge limb lengths, maximum body width, and other aspects of body size. In apparent contradiction of this expectation stands a sizeable literature suggesting that many people grossly mis-perceive their body size. This literature on "body-image" is fraught with problems, including questionable measurement techniques, and rarely includes comparison of accuracy in body image with that of other objects ( Schlundt & Johnson, 1990; Thompson, 1996).
This paper reports results obtained with two new techniques that allow precise assessment of judgments of perceived or remembered linear dimensions. The primary technique utilizes observer-controlled projection of a laser beam to vary the distance between the beam and a reference line. The second technique involves adjustment of a web belt to indicate circumference. To test the usability and reliability of these methods, 11 judgments of size were made on two occasions one week apart for targets that varied in familiarity and shape.