AS A rule, I don’t remove my shoes in public. But today I’m making an exception. Surrounded by engineers and psychologists, I pull off my Converse and step into a pair of rather ordinary-looking brown leather sandals.

I begin to walk slowly around the room, and that’s when I experience the most peculiar sensations. The sound of my footsteps changes, and suddenly my lower legs feel lighter and longer. My knees feel looser, and I begin to raise them higher and higher as I walk. My walking speed increases until it’s all I can do not to break into a trot. I feel slimmer, stronger, and full of energy. These are unlike any shoes I have ever worn. (Watch a video of the shoes in action).

Such footwear sounds fantastical, but these shoes are just one of a number of new experiments revealing how the noises we make have an immediate and profound effect on the way we experience our bodies, on our emotions and our behaviour. The trick here is not in the shoes themselves, but in the way they change the sound of my footsteps.

Perception-skewing shoes can make you feel slimmer, happier and full of energy by retuning your body’s soundtrack Andrew Lyons Advertisement

This phenomenon raises the tantalising prospect that simply donning a particular pair of shoes or a headset could help us feel energised, more inclined to exercise – or just happier.

The discovery also sheds light on the way the brain builds your sense of self. This could have big implications for people who struggle with how it feels to live inside their bodies, not least those with chronic pain or psychological problems with their body image.

The sensation of inhabiting our own body is so familiar we barely give it a second thought. But over the last decade it has become clear what a complex construction this feeling is.

Much of what we know about it comes from studying the brain circuits of people subjected to the rubber hand illusion – one of the most well-established experiments into our sense of embodiment. In the classic version, a rubber hand is placed on a table in front of a volunteer, in a natural-looking position, and the person’s own hand is hidden from their view. Then they see the rubber hand being stroked, while their real hand is simultaneously stroked in the same way. The experience results in the person perceiving that the fake hand is their actual hand.

These experiments suggest that we carry a mental model of our body – how many limbs we have, for example, and where they are in relation to one another. This model is then constantly updated with sensory information from the outside world, and from special receptors inside the body that sense the movement and position of joints and muscles – called proprioceptors. In the case of the rubber hand, it is the combined visual and tactile input that convinces the volunteer’s brain that the hand is in fact part of their body.

We now know that much of this integration happens in an area of the brain called the right temporoparietal junction. This area collects and processes information about body movement, and sensory information such as sight, touch and sound.

When it is disrupted using magnetic stimulation, people suddenly find it hard to figure out where their body ends and the outside world begins.

Variations of the rubber hand illusion have since shown that as long as enough information from different senses reaches the brain, it is surprisingly easy to change what people feel to be the limits of their physical form. With a bit of technology, it is even possible to fool someone into perceiving an entire other body as their own.

Personal soundtrack

These perception-skewing studies have so far relied on vision, touch and proprioception to understand the way in which the brain constructs this sense of an embodied self. Until now, the role of one sense had been widely overlooked. “Sound is a fundamental yet under-investigated dimension of body representations,” says psychologist Manos Tsakiris at Royal Holloway, University of London, who has conducted much of the embodiment research. “For hearing people, there is a continuous, ever-present soundtrack to our bodily actions.”

We use this soundtrack to infer information about other people: just hearing someone’s footsteps hints at their size, for example. Ana Tajadura-Jiménez, who studies perception at University College London, is now focusing on the sounds we make ourselves. “What do they tell us about our own bodies?” she asks.

In trying to answer this question, she is finding that hacking our hearing can have profound and surprising effects. In an early experiment, Tajadura-Jiménez’s team at UCL’s Hearing Body Project asked a group of volunteers to do a trial that involved tapping a desk while speakers played the sound of the taps at increasing distances. After each trial, participants were subjected to a touch test on each arm (see illustration).

When the sound originated from double the distance away from the tapping, a curious phenomenon occurred – the volunteers significantly overestimated the distance between two touch points on the tapping arm. Their brain perceived that arm to be longer, even though they weren’t consciously aware of it having stretched (Current Biology, DOI: 10.1016/j.cub.2012.04.028).

In a similar experiment, blindfolded volunteers were asked to drop a ball, which was surreptitiously caught in a net. Instead of the true sound of the ball hitting the floor, a recording was played, with a varying time lag. Similarly to the arm experiment, the longer the lag, the longer they felt their legs to be.

Tajadura-Jiménez suspected that manipulating the audio feedback in this way disrupts the brain’s internal model of the body by causing a mismatch between the input it gets from the different senses. Strange bodily sensations result, as the brain tries to account for the discrepancies.

These kinds of manipulations not only alter how we perceive the body, but can also affect behaviour. Touching surfaces generates sounds, however subtle. In another test, blindfolded volunteers stroked a smooth plastic board, but through a headset heard the sound of touching either sandpaper or velvet.

The sound they heard affected their actions – participants became hesitant to stroke the board when they heard sandpaper, but not when they heard velvet. (To experience this for yourself see the online version of this story here: bit.ly/NSsoundFX)

Slimming shoes

Tajadura-Jiménez now wants to know if it is possible to harness these peculiar experiences in more profound ways. This is why I find myself standing in my socks. A part of her team’s most elaborate experiment yet, I am handed a pair of sandals fitted with microphones linked to a set of earphones. I don the shoes, put the earphones in, and begin to walk on the vinyl floor.

It is an unnerving experience. At first, I simply hear the unmodified sound of my footsteps. But then Tajadura-Jiménez filters the noise using an equaliser. Emphasising the higher frequencies mimics a lighter person’s footsteps, causing them to sound higher-pitched, even slightly hollow. In a matter of seconds, I experience a sensation of lightness in my lower legs. And that’s when I start to feel the other changes in my body too. The opposite effect also occurs – lower frequencies cause people to drag their feet like a heavier person.

What’s going on? Tajadura-Jiménez thinks that what I experience is a conflict between two senses: hearing and proprioception. My brain, quite rightly, expects my body to be 156 centimetres tall and weigh about 55 kilograms. But thanks to the get-up, my ears pick up the sound of a considerably lighter person – and my brain updates my body representation accordingly, putting an added spring in my step.

This idea fits with a fairly recent theory of how the brain works, called predictive coding. This depicts the brain not as a passive recipient of sensory information but a complex prediction machine, which makes sense of the world by forecasting the probability of various events occurring.

Those predictions are based on prior experience, but they are continually updated. This way of calculating how likely something is to happen is known as Bayesian statistics. The Bayesian model of the brain explains how we can instinctively work out, for instance, whether there is time to cross the road in front of an approaching car – we make a prediction based on past experiences, but the brain will update that calculation if new information comes in, such as the car suddenly accelerating.

Predictive coding is a compelling explanation of how these experiments work their magic. “If the sensory feedback you get is not what you were expecting, you update your predictions,” says Tajadura-Jiménez. My brain predicted that my footsteps would sound the way that they usually do. These shoes, though, provided unexpected sensory information, and so my brain quickly “corrected” its error, creating an illusion of lightness.

Tapping into these expectations could affect much more than the way we walk. Tajadura-Jiménez says people who try the shoes often report feeling happier. What’s more, when participants are asked to adjust the size of a virtual avatar to reflect their body shape, after walking in the shoes, they consistently depict their virtual selves as slimmer than before.

The effects are temporary – it only takes a few seconds without the shoes to return the wearer to their original state. Still, such footwear could be just the ticket for anyone in need of an instant confidence boost.

Could they also help people with serious issues with their body image? Tajadura-Jiménez is now teaming up with medics interested in how sound manipulations could help people with body dysmorphia to adjust their distorted body image. “We are finding that sound is affecting not just the perception of our body shape but our physical capabilities,” she says. “Understand these sounds, and we could change our feelings in a positive way.”

There’s reason to think this group could be particularly responsive to interventions with sound and touch. People with eating disorders have already been shown to be more susceptible to tricks like the rubber hand illusion. And in as-yet-unpublished research, Tajadura-Jiménez’s team has found that experimental sound manipulations can change the size volunteers feel their waist to be. People who were more concerned about their body shape felt the illusion more strongly.

“Manipulating sounds changes the size people feel their waist to be”

These experiments suggest that you could consider body dysmorphia to be a disrupted sense of embodiment, says Anil Seth, who studies consciousness and identity at the University of Sussex in Brighton, UK – and that it might well be amenable to treatment with some of these new approaches.

The findings clearly run deeper than the contours of the body – perhaps even to our fundamental sense of identity, he says. “I believe that our sense of self is at least partly determined by the brain’s inferences about the shape and physiological conditions of what it considers to be its body.”

So, change the shape, change the person? Work by Amy Cuddy of Harvard Business School and her colleagues shows that simply getting people to change their posture – even if they don’t realise they have done so – can make them feel more powerful and, in turn, more likely to take risks. For instance, her team found that people who drive cars whose seats are designed to encourage a more expansive driving posture were more likely to park illegally.

In other experiments, when adults wearing a virtual reality headset were made to inhabit the body of a child, they started to identify more with childlike qualities in themselves, rather than adult ones.

This is an idea known as embodied cognition, whereby the way our body looks, feels and moves influences the way we think and behave. Tajadura-Jiménez plans to investigate whether her sound manipulations could be used to trigger similar responses, changing the way people think about themselves more fundamentally.

In the meantime she is focusing on people with chronic pain. Feeling pain in one part of the body often leads to distortions in a person’s sense of movement and positioning, leading to reduced awareness of their actual movement. For example, they tend to think their body is stretching more than it really is, and this makes it hard to carry out physiotherapy exercises. Some of their proprioceptors also switch to transmitting pain.

Tajadura-Jiménez is collaborating with teams at UCL’s Emotion and Pain Project and the University of Genoa in Italy who have developed an app that works as a kind of sensory prosthesis, giving people with chronic pain real-time information about their body movement and location. The hope is that it will help them to stretch further, and will reset their proprioceptors to accurately represent their body position. This might give them a better idea of their true physical capabilities and stop the proprioceptors from transmitting pain.

Another app, which would allow people to modify the sounds their footsteps make is also in development, Tajadura-Jiménez says. The idea is that anyone could get the benefits of her sandals, “when walking or running, perhaps in the gym, to feel lighter and more motivated”.

This is the kind of thing that should appeal to makers of sports equipment, says sports engineer Steve Haake at Sheffield Hallam University, UK. “Something that enhances runners’ perception would be very popular.”

From my short time in Tajadura-Jiménez’s shoes, I would certainly agree. The feeling of lightness I experienced vanished as soon as I took them off, so I would be keen to have such a trick regularly at my disposal. The experience has also left me with a newfound appreciation of the role sounds play in forming my sense of identity. Walk a mile in these shoes, and you’ll never feel the same again.

Tongue trick As with sound, taste manipulations can have surprising effects on the way we experience the world. Earlier this year, a team at the University of Oxford’s Crossmodal Research Laboratory asked volunteers to stick their tongue through a hole in the side of a box, which was equipped with a strategically placed mirror. This made them see a fake, rubber tongue in place of their own. Participants then watched as a cotton bud was stroked along the fake tongue while the same thing happened to their tongue. Seventy per cent of them perceived the fake tongue to be theirs. Next, the volunteers saw a cotton bud get dipped in lemon juice and applied to the rubber tongue, while a water-soaked bud was simultaneously applied to their own. To the researchers surprise, a few participants reported a sour taste. “It’s the first time that vision has been shown to influence perception of an internal organ,” says study co-author Charles Michel. We already know that how food looks affects how much we like it. If we can make people perceive taste on a virtual representation of their tongue, Michel says, it raises an interesting question: “How much of our everyday experience of food is an illusion?”

This article appeared in print under the headline “Hear yourself happy”