The perceived size of the implicit representation of the dorsum and palm of the hand

Abstract

The perception of the body and its parts has traditionally been studied using the conscious body image. Here, we determine the implicit representation of the hand. Participants were sequentially shown two life-size images of either the dorsal or palmar surface of their hand. 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 hand. The staircase honed in on the distorted image that was equally likely to be judged as matching their own hand as the accurate image. The implicit representation was taken as midway between these two images. The experiment was repeated with different hand orientations. Perceived width depended on the orientation, with differences found between the upright and right orientations. Interestingly, the perceived length of the dorsum and palm were different from each other-length of the dorsum was overestimated whereas palm length was perceived accurately. This study reveals distortions of the implicit representation of the hands in healthy individuals.

Fig 1. Sample images for the dorsum and palm in each orientation used.

The four orientations are shown, from left to right, in the order of (1) upright (0°), (2) to the right (90°), (3) upside down (180°), and (4) to the left (270°) for the dorsum (top row) and palm (bottom row).

Fig 2. Perceived width of the dorsum and palm.

Mean differences from accurate when dorsum of the hand (left panel) or palm (right panel) was distorted in the width dimension for each viewing orientation. Positive and negative scores represent overestimation and underestimation respectively. Error bars represent ±1 SEM.

Fig 3. Perceived dorsum and palm length.

Mean differences from accurate when dorsum (left panel) or palm (right panel) was distorted in the length dimension for each viewing orientation. Positive and negative scores represent overestimation and underestimation respectively. Error bars represent ±1 SEM.

Fig 4. Dorsum vs palm.

Relationship between dorsum and palm distortions for width (pink triangles) and length (blue circles). The lines plotted through the data are the goodness-of-fit lines for the width and length.

Fig 5. Actual vs perceived length.

Relationship between actual and perceived lengths for (A) dorsum length and (B) palm length. The line plotted through the data represents accurate perception.