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

## TL;DR

Looking at an upside-down hand image makes fingers appear shorter, and this 'frog hand illusion' is not affected by hand dominance.

## Contribution

Quantitatively measures the frog hand illusion and explores its relation to hand dominance for the first time.

## Key findings

- Inverted hand images cause a 5% average shortening illusion in finger length.
- No significant difference in illusion strength between dominant and non-dominant hands.
- The illusion may result from disrupted perceptual constancy due to atypical viewpoints.

## Abstract

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.