The frog hand illusion: Distortion of hand shape in inverted presentation

When a photograph of the back of a hand with the fingers extended to the depth is observed upside-down, the hand appears vertically squashed, with extremely short fingers. The first aim of this study was to quantitatively measure the "frog hand illusion (FHI)", named after its bizarre appearance, and the second aim was to examine whether the dominant hand affects the strength of FHI. We measured the apparent shortening of the fingers using the method of constant stimuli. The results showed that the fingers of the inverted hand appeared to be shorter than those of the upright hand by about 5% on average. No effect of the dominant hand was observed. We propose the hypothesis that FHI occurs because of the attenuation of perceptual constancy, which might stem from observing the hand image from an atypical viewpoint.