Sound and object motion can help change perceptions about body size

Sound and object motion can be used to change perceptions about body size, according to a new study by an international team led by a researcher from the Universidad Carlos III de Madrid (UC3M). The study, published in PLOS ONE, found that when there is a mismatch between the sensory signals (in this experiment, when the sound made by dropping a ball takes more than we expected), a recalibration of the mental representation of our body height occurs.

Researchers started by an evidence: when an object is dropped, the brain uses internal models (both gravitational movement and our body size) to predict when it will hit the floor. They have proof that artificially lengthening the time it takes to hear the impact can change our perceived body height.

"These results reveal the surprising importance that sound and movement have on body representation. We don't just feel and see our bodies, we also hear ourselves whenever we interact with solid objects," explained the main author of the paper, Ana Tajadura-Jiménez, researcher from the UC3M's Computing Department and UCL Interaction Centre (University College London).

The findings could have implications for studies already using sound for rehabilitation for people with poor proprioception - the sense of the position parts of the body in relation to other parts - including those who have Parkinson's Disease or have suffered a stroke. "This is a really promising avenue for applications for clinical conditions where people suffer from chronic pain or other conditions linked to distorted mental body representations such as anorexia nervosa", added Ana Tajadura-Jiménez.

How humans perceive their body size is highly flexible, even beyond the ages when we stop growing. Most previous studies into this used sensory feedback on or about one's body but this study shows that even the movement of objects around us is used to compute our body size. Although in adults the size of the body does not often change much, the mental representations of these sizes can change very quickly. This "recalibration of the mental representations of the body" has been investigated in many studies that have shown that our bodies try to maintain a representation of the body consistent with the sensory signals received. "As these mechanisms are understood, they inform the design of sound-based technology to support novel therapies for such conditions," added co-author Professor Nadia Berthouze (UCL Interaction Centre and UCL Psychology & Language Sciences).

Sensory experiment

For the study, three groups of people participated in laboratory experiments: the participants, while standing and blindfolded, were invited to drop a ball from head height. The researchers artificially introduced different delays in the time interval that the ball takes to reach the floor and produce sound and vibrations in the floor. "Results show that as the perceived time it took the ball to hit the floor increased, so too did the participants' perception of their body height and leg length", explained co-author Prof Ophelia Deroy, professor at Ludwig Maximilians University (LMU) in Munich (Germany). In other words, participants felt higher and act like their legs were longer.

"This is not only valuable for clinical applications but could also inform the development of technologies for motion controlled games where players take on a larger character on screen" said another researcher, Dr Norimichi Kitagawa, from The Nippon Telegraph and Telephone Corporation (NTT) telecommunication company.