Mental body representations retain homuncular shape distortions: Evidence from Weber’s illusion

• We compared the shape of body part representations underlying Weber’s illusion.
• We found that the represented shape of both the arm and hand was highly distorted.
• The shape of the arm representation was more distorted than the hand.
• Mental body representations retain shape distortions characterizing the somatosensory homunculus.

Mental body representations underlying tactile perception do not accurately reflect the body’s true morphology. For example, perceived tactile distance is dependent on both the body part being touched and the stimulus orientation, a phenomenon called Weber’s illusion. These findings suggest the presence of size and shape distortions, respectively. However, whereas each morphological feature is typically measured in isolation, a complete morphological characterization requires the concurrent measurement of both size and shape. We did so in three experiments, manipulating both the stimulated body parts (hand; forearm) and stimulus orientation while requiring participants to make tactile distance judgments. We found that the forearm was significantly more distorted than the hand lengthwise but not widthwise. Effects of stimulus orientation are thought to reflect receptive field anisotropies in primary somatosensory cortex. The results of the present study therefore suggest that mental body representations retain homuncular shape distortions that characterize early stages of somatosensory processing.