The shrunken finger illusion: Amodal volume completion can make your finger feel shorter

When we look at a complete ball, only the front half of it is directly visible. Hence, a semi-spherical shell producing the same retinal image is easily mistaken for a complete ball. Given that the backside of a ball (or a corresponding semi-sphere) is hidden from direct view, it appears tempting to conceive of this kind of mistake as a rather mundane failure of cognitive guesswork. A more counter-intuitive hypothesis, however, which can be traced back to Michotte's early work on amodal completion is that the semi-spherical shell is completed into a full ball by processes of a genuinely perceptual nature. Here, we present an illusion of bodily self-awareness that lends strong support to the latter hypothesis. When a semi-spherical shell is put on the observer's own finger and viewed from above, the shell looks like a complete ball despite the availability of conflicting proprioceptive information from the finger in the shell. Instead, the finger is felt to be shortened, as if to make space for the illusory volume of the amodally completed ball. We quantified this illusion by asking observers to point to the felt location of their fingertip while balancing a semi-spherical shell on top of it. We found that the illusory shortening of the finger increases linearly with the radius of the shell, as would be expected if the illusory finger shortening depends on the amount of space necessary for amodal volume completion of the semi-sphere. On average, our measurements indicate that the illusory shortening of the finger corresponds to about 23 percent of the diameter of the shell, irrespective of its absolute size. These results provide strong evidence for the idea that our experience of the hidden backsides of objects is shaped by genuinely perceptual processes. Meeting abstract presented at VSS 2015.status: publishe