The tongue in the title of Oxford University’s Crossmodal Research Laboratory’s new paper, “The Butcher’s Tongue Illusion,” does not come from a butcher shop. “I actually just ordered the most normal-looking rubber tongue from a magic store,” Charles Michel, the report’s lead author and a professionally trained chef, said. “Magicians put them in their mouths and tie them in knots and things like that.”
Michel and his co-authors put their magic tongue to use in a simple but provocative experiment, carried out late last year and described in the current issue of the scientific journal Perception. Although the involvement of a stretchy pink latex tongue makes it easy to mistake the experiment for a cheap gag, it’s actually an important addition to a distinguished tradition of psychological research that studies illusions for what they can reveal about how the brain constructs reality.
The team’s goal was to replicate a famous experiment called the “rubber hand illusion.” In 1998, the psychologists Matthew Botvinick and Jonathan Cohen demonstrated that people could be tricked into believing that a rubber left hand sitting on a table in front of them was actually their own, simply by stroking the rubber hand with a paintbrush at exactly the same time as the subject’s real hand, which was hidden out of sight, behind a screen. (This makes a fun party trick, if you happen to own a rubber hand.)
The rubber-hand illusion helps scientists explore a fundamental issue in psychology: how the brain discriminates between self and other. By deconstructing the ways in which simultaneous inputs from across the senses are combined by the brain, scientists can begin to understand what goes into creating the elusive yet fundamental feeling of being a body in the world. For example, in the past five years, by adding a finger-lifting routine and fMRI brain-scanning to the rubber-hand illusion, researchers have found that the sense of bodily ownership (“this body is mine”) and that of bodily agency (“I am in control of this body”), which are usually experienced together as an over-all awareness of one’s own body, are actually generated more or less independently, in different areas of the brain. Participants whose actual and illusory rubber forefingers were simultaneously lifted by a thread still felt that the rubber hand was their own, but didn’t feel as though they had any control over it. Neuroimaging showed activity confined to the insula, the frontal operculum, and the cortical midline. Meanwhile, participants who lifted their own forefinger while watching the fake forefinger be lifted by a thread reported feeling both ownership and agency over the rubber hand—and, inside their brains, the motor preparatory areas and the inferior parietal lobe joined the party.
Since Botvinick and Cohen’s landmark paper, other psychologists have successfully recreated the experiment with other parts of the body, creating rubber-leg, rubber-arm, and even rubber-face illusions. One evening a couple of years ago, Charles Spence, the experimental psychologist who directs the Crossmodal Research Laboratory, was conducting an experiment to map brain activity during the basic rubber-hand illusion, and found himself wondering whether the investigation could also be extended to the tongue. He was curious as to whether the illusion would work on a part of the body that we usually experience through touch and taste rather than through vision. There was also an undercurrent of frustration.
“A lot of my colleagues don’t want to think about the mouth and flavor,” he said. “They all want to study hearing and vision.” Spence’s work, by contrast, is concerned with how the brain combines input from multiple senses to create perception, and he regularly finds himself arguing for the importance of touch, taste, and smell in the construction of our day-to-day experiences. And, while the hand is limited to two senses, the tongue offers the possibility of testing four sensory modes: touch, sight, taste, and smell (through a process called retronasal olfaction).
Spence discussed his idea with Michel, who promptly requisitioned the cardboard box that held the lab’s printer paper, spent an afternoon reconfiguring it with an X-Acto knife and glue, and started recruiting test subjects. One by one, participants were instructed to put their faces against the end of the box and stick their tongues into the box’s interior through a hole that Michel had cut. Michel then inserted a mirror at the other end of the box, in which participants could see a reflection of a tongue—the participant’s own, it would seem—poking into a white-walled cuboid.
Unbeknownst to the participants, the mirror was positioned to reflect a fake tongue; their real tongues were hidden behind a false foam-board wall. Each participant was instructed to watch the mirror and hold his tongue completely still, steadying it between his teeth if necessary. Michel then took a wet Q-tip in each hand, and stroked them along the two tongues—real and fake—simultaneously. A statistically conclusive seventy per cent of the participants experienced the illusion: they felt that the rubber tongue they saw being stroked was their own. (Typically, rubber-hand illusions have a success rate of eighty-five per cent, Spence said.)
Indeed, even when only the fake tongue was stroked, forty per cent reported experiencing the touch on their own tongue. In a final flourish that didn’t make it into the paper because it hadn’t been run by the ethics board in advance, Michel then opened a pair of scissors around the fake tongue, as if to chop it off. “We did that just for fun,” he said. “They flinch and shout, ‘Whoa!’ ” Barry Smith, a co-director of the Center for the Study of the Senses at the University of London, confirmed that it was the first time that vision had been shown to influence perception of an internal organ as strongly as it does external body parts.
Spence and Michel’s experiment raises an obvious question: If you can feel things with a fake tongue, can you also taste things? “Yes, yes, of course,” Michel said. “In fact, I tried it on Heston Blumenthal the other day,” he told me, referring to the English celebrity chef known for his multi-sensory culinary experiments. “When he was in the box, I showed him a lemon. Then I dipped the Q-tip in the lemon and I touched the fake tongue, and at the same time I touched his tongue with a Q-tip that was just wet with water. And he looked at me and said, ‘I don’t believe this is not sour.’ ” Spence and Michel plan to continue this line of investigation, with the goal of understanding how the brain integrates taste, smell, sight, and tactile sensations to create flavor experience.
It’s easy to dismiss questions of flavor and “mouthfeel” as foodie hedonism, but recent research by anthropologists and neuroscientists alike has suggested that the way we perceive flavor has played an outsized role in the evolutionary development of the human brain, by regulating the kinds and amounts of food that we eat. We must eat to survive, and the tongue and the eyes are essential in guiding that consumption. One theory holds that, by understanding how the brain integrates input from those senses, we glimpse the forces that helped to shape our more celebrated abilities, such as language, reasoning, and planning ahead.
This kind of research is not without practical application. The results of the butcher’s-tongue illusion suggest that, with further testing, it might be possible to one day enjoy out-of-body flavor experiences. To a person wearing Michel’s box, a Slim-Fast shake could be made to taste like a chocolate-fudge sundae, delivering a fraction of the calories and a great deal more pleasure. It’s the holy grail of dieters: a virtual-reality headset that makes those undressed salad leaves you’re dutifully eating taste like French fries or dark chocolate. At last, we could have nutritional restraint combined with sensory decadence, with no sacrifice required.
But, despite the ease with which Michel and Spence deceived Blumenthal into “virtually” tasting a lemon, they agree that the box needs a technological upgrade. The lab has joined forces with a team of Japanese augmented-reality researchers who recently developed a virtual-reality headset that delivers visual and tactile sensations. By combining the headset with the butcher’s-tongue illusion, they hope to create what Spence refers to as “augmented sushi”—a feat of technological trickery that will fool a diner’s brain into believing that farmed tilapia is bluefin tuna.
In the meantime, the box has many uses, given the sheer number of experiments the illusion makes possible. Some of Spence’s earlier rubber-hand research showed that if arthritis patients view the fake hand through back-to-front binoculars, making it appear smaller, the pain they feel through it is diminished. Following that logic, it’s possible that simply incorporating a magnifying glass into Michel’s cardboard box might boost flavor perception, making a food seem to taste more like itself—a sort of neuro-M.S.G., without the headaches. “I’ll have to make another box to try it, though,” Michel said, with a little pride. “The London Science Museum asked for my original prototype—it’s going to be on display next year.”
To read more about the mind and the brain, visit our collection of archive selections and new stories on the subject.