This weird, sound-induced illusion makes you feel that your finger has grown longer

Adults are vulnerable to all kinds of body illusions. We can be made to feel that a fake hand, or even a fake body, is our own; that we've left our body; even that we're the size of a doll. These illusions work because our brains use information from various senses to create mental representations of our bodies. Mess with some of these sensory signals, and you can alter those representations, sometimes drastically.

Work to date suggests that young children don't show the same susceptibilities to body illusions, presumably because the systems that underpin them are still developing. Now a new study, published in Scientific Reports, has found that a bizarre auditory-induced illusion that affects adults doesn't work in quite the same way in young kids, either.

Ana Tajadura-Jiménez, now at Charles III University of in Madrid, has led all kinds of fascinating work showing that body illusions can be triggered through the manipulation of sound. Take her "magic shoes", for example. Microphones record the sound of the wearer walking. But this sound is then filtered, so only the the higher-pitched portions are played on to them, through earphones. This has the effect of making the wearer sound lighter, which leads them to report feeling lighter and more sprightly.

For the new study, Elena Nava at the University of Milano-Bicocca and Tajadura-Jiménez turned to what's known as the "auditory Pinocchio illusion". In earlier work, Tajadura-Jiménez and colleagues had found that if adults pull up on their index finger while listening to brief sounds of rising pitch, they have the feeling that their finger is getting longer.

Why? Other research has found that we associate a "high" pitch with a high position in physical space, and so, Tajadura-Jiménez suspects, this metaphor is influencing our perceptions. Like adults, even babies get the "high" vs "low" pitch metaphor — so perhaps young children would experience this illusion, too. But when the pair tried a version of it on a group of 30 four- to six-year-olds, and on a group of 30 adults, they found some differences between the responses of the two groups.

For this study, the participants didn't pull on their own fingers. Rather, they pressed their palm up against a vertical wooden surface, and their hand was screened with a cloth, so they couldn't see it. An experimenter then repeatedly either pulled or pressed on their middle finger while they heard a tone that ascended in pitch, descended in pitch or remained constant.

Each time, the participant used a scale on the side of the board to indicate where they felt the tip of their middle finger to be in space. They were also asked about whether they perceived their finger to be getting longer, shrinking, or staying the same length.

The team found that when ascending notes were played during finger-pulling, adults indicated that their middle finger tip was higher (and so the finger was longer) than it actually was.

Children, on the other hand, did not indicate that their finger tip was higher during these ascending notes — and yet like the adults, they reported feeling that their finger was getting longer.

This suggests that by around the age of four to six, a child's thoughts and feelings about what's happening to its body — its "body image" — is adult-like (at least, in this context). However, its unconscious body "schema" — the brain's map of the body in space — is not yet being calibrated in the same way as an adult's.

Interestingly, the researchers did not find that tones of descending pitch played while the finger was being pressed made adults or children think that their finger was getting smaller. "Why can the brain 'accept' a longer finger but not a shorter one?" the researchers ask. "This asymmetry could reflect the fact that the brain experiences, throughout development, the enlargement of most parts of the body, while it never experiences shrinkage," they suggest.

In a subsequent study, the pair then tried a version of the same experiment, but with the participants' arms and hands positioned horizontally, rather than vertically. In this orientation, the longer finger effect that they had just found in adults was not repeated. As the high/low metaphor for pitch is vertical in nature, not horizontal, this "supports our hypothesis that crossmodal 'metaphoric' correspondences influence the binding of multisensory information," the researchers write.

They would now like to see further work to establish when a child's body map becomes adult-like, and also to explore whether individual differences in the way signals from various senses are integrated might predict our susceptibilities to body illusions.

Further reading

– Auditory-induced body distortions in children and adults

About the author

Emma Young (@EmmaELYoung) is a staff writer at BPS Research Digest