Sarah D'Amour, Laurence Harris; Perceived size of the face and arm depends on visual orientation. Journal of Vision 2016;16(12):738. doi: 10.1167/16.12.738.
The perception of body size has traditionally been studied using subjective, qualitative measures that assess only one type of body representation - the conscious body image. Previous research has typically focused on measuring the perceived size of the entire body rather than individual body parts, such as the face and arms. Here, we present a novel psychophysical method for determining perceived body size that taps into the implicit body representation. Using a two-alternative forced choice (2AFC) design, participants were sequentially shown two life-size images of either their own arm or their own face. In one interval either the horizontal or vertical dimension of the image was varied using an adaptive staircase, while the other interval contained the full-size, undistorted image. Participants reported which image most closely matched their own perceived size. The staircase honed in on the distorted image that was equally likely to be judged as matching their perception as the accurate image, from which the perceived size could be calculated. The visual orientation of the image was varied to compare performance for familiar and unfamiliar views. When the face was viewed upright or upside down, the width was overestimated and length underestimated whereas perception was accurate for the on-side view. Arm length was significantly overestimated when shown in either horizontal or vertical orientations. These results indicate that participants' representation of their face is wider and shorter than actual size and that they represent their arms as longer than actual size, although of accurate width. The method reveals distortions of the implicit body representation independent of the conscious body image.