It behooves us for our brains to have a solid working knowledge of our bodies: to know where our limbs are, what they feel like, and how they’ll interact with objects in the world. Even as you perform the simple act of lifting a glass of water to your mouth, you rely on the assumptions your brain makes about your body to know how hard your muscles should pull to move your arm, where your fingers end, and how hard you should grip the glass. But can these assumptions about your body ever change? A new study from PLOS ONE by researchers in Italy and Germany uses a powerful illusion to show just how fast our brains can update our perceptions of our bodies, given the right sensory cues.

Participants in the experiment first completed a questionnaire about how their right hand felt: its stiffness, heaviness, hardness, temperature, naturalness, and sensitivity. Next, they donned headphones and placed one hand behind a screen. Researchers then repeatedly tapped each participant’s hand with a small hammer for five minutes, and every time they did, they played the sound of a hammer striking stone. To maximize the illusion, the stone-hammer sound started at a very low volume and became louder over time. After the hammer session, participants filled out the same survey about their hand. Survey results showed that test subjects felt their hand was harder, heavier, stiffer, and less natural than before. In other words, as the brain started perceiving a strong auditory indication that the arm was hard and stone-like, it seems it also started updating its assumptions about the arm’s properties very quickly.

To validate these findings, researchers also took physical measurements of skin sensitivity on a subset of the group, both before and after the hammer hits. Our skin conducts electricity, and as it responds to stimuli—a painful prick, a change in temperature, or even an emotion—its conductivity varies. Measuring the resistance between electrodes connected to two points on the skin gives us a physiological measure of skin sensitivity and arousal, which is sometimes called the Galvanic skin response. The authors found that after the hand-hammering illusion, participants’ physiological response to a threatening stimulus (in this case, watching a needle approach their hand) increased significantly.

The authors conducted several other control experiments to better understand the mechanism behind the illusion. They repeatedly struck the hands of participants in a control group with a hammer and played the same hammer-on-stone sound, but did not time the hammer hits and the sound to sync up perfectly. This group did not report the same change in hand feeling or perception that the experimental group did, nor did it display a change in Galvanic skin response. The team also tested the effect of playing a pure tone with each hammer tap, rather than a hammer-and-stone sound, and found that this also had no significant effect on hand perception or Galvanic skin response. Finally, participants who heard the natural, unaltered sound of the hammer hitting their skin did not report any changes in their hand perception after their hand was hammered.

The authors state that these controls help demonstrate how the illusion works. When incoming signals do not appear related—for instance, when the hammer hits and sounds don’t come at the same time—our brains can easily keep them separate. It is only when signals come at the same time and seem to be related that the illusion occurs. Rather than using static information about your body, your brain can take the extraordinary step of updating its understanding of the body to match the incoming signals, even when the new body perception is at odds with what we know to be true. Seeing is believing, and so too, it seems, is hearing and feeling.

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Citation: Senna I, Maravita A, Bolognini N, Parise CV (2014) The Marble-Hand Illusion. PLoS ONE 9(3): e91688. doi:10.1371/journal.pone.0091688

Images: Figures are panels A and B of Figure 1 of the full manuscript