Want to fool a wine expert? Dump some red food coloring into a glass of white wine and some of the snootiest sippers will be deceived.
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You could also make a friend see visions. If you show her a flash on a computer screen while playing two beeps, she’ll swear she saw two flashes. You might even try selling skin lotion using another peculiar illusion: Have your customers rub their hands together while listening to an audiotape of rubbing dry skin, and suddenly their skin will feel rough and parched.
If you think you have five separate senses, think again. Neuroscientists are finding evidence that what you see can influence what you taste, what you touch changes what you see, and other sensory crosstalk can guide our perceptions. At a meeting here of the Society for Neuroscience last month, researchers said they had determined that areas of the brain long thought to respond only to one sense also become active when people engage another.
“People thought the senses were independent. The information goes into different places,” such as ears, nose, or fingertips, says Charles Spence, a lecturer in experimental psychology at the University of Oxford. But ideas about perception are changing, he says, as researchers take note that “your brain glues together the senses.”
The human brain has evolved a remarkable acuity for determining that the deafening roar, the smell of animal breath, and the image of a looming golden-maned cat all come from the same beast. But until recently, scientists thought that perceptions from each sense entered the brain separately at areas specialized to deal with a single sense, and that the brain later combined those signals to form one impression of a lion.
Now scientists are discovering that information from one sense actually influences how the brain experiences other types of inputs -- an effect that can create illusions and alter perception. They are chipping away at the traditional view of how the brain integrates the senses, says Krish Sathian. “Perhaps the boundaries between the senses are blurred.”
Dr. Sathian, an associate professor of neurology and rehabilitation medicine at Emory University, wondered whether people use the visual parts of the brain when they touch things with their fingers. After all, he says, blindfolded subjects in tests of touch told him later that they had envisioned the surfaces they were touching in their “mind’s eye.” So he scanned volunteers’ brains -- using Positron Emission Tomography, also known as a PET scan -- while they touched surfaces bearing raised ridges. He asked them to figure out the direction of the ridges.
As he suspected, a part of the brain that processes visual inputs, called the visual cortex, became more active when the blindfolded subjects ran their fingers over the ridges. But that raised another question: Is the visual-cortex activity critical to the tactile task? To find out, he had to prevent a subject from using that part of his or her brain. Though it may sound like science fiction, a method for doing so exists: transcranial magnetic stimulation, the application of magnetic pulses to certain parts of the brain.
Turning Off the Mind’s Eye
“We knew from previous work that if you applied these magnetic pulses to the visual cortex, you could interfere with visual perception. Could we interfere with tactile perception?” Dr. Sathian found that he could. He reported in Nature in 1999 that the subjects no longer could determine the direction of the ridges. More recently, he discovered that other tactile tasks also require activity in the visual cortex.
“We think this is just one example of how the brain is putting the various senses together,” he says.
Stephen M. Kosslyn, a professor of psychology at Harvard University, says that, although he is unfamiliar with the details of Dr. Sathian’s work, many of the multisensory studies leave open the question of whether using a nonvisual sense -- touching the ridges or hearing a sound -- actually activates the visual cortex or whether the process of imagining the surface triggers the activity. This could affect how scientists think about the brain: “If it’s just a ... transfer via imagery, the current conceptualizations stay intact. Whereas if it’s really the case that you have direct effects on the visual area [of the brain] when you have a loud noise, then the way we characterize what [that area] does will have to change.”
Mr. Spence says that imagination does not always play a role. In some studies, activity in one of the brain’s sense areas comes so soon after stimulating another sense that “it is hard to see how mental imagery effects could kick in that quickly.”
In fact, Gemma A. Calvert, a brain physiologist at the University of Oxford, has found that several different areas of the brain previously thought to receive inputs from just one sense are actually multisensory.
Most recently, she has discovered that the olfactory area of the brain also involves vision. A blue strawberry might not only look funny but, she found, it’s likely to smell strange, too.
The Smell of a Blue Strawberry
She told the neuroscientists at the meeting that, with Robert Osterbauer, a graduate student, she scanned volunteers’ brains while they smelled odors or looked at colors or did both. The combinations were either matched, such as pairing red with strawberry, or mismatched, such as blue with strawberry.
The olfactory areas of the brain became especially active when the color and scent matched, and became quieter when the two clashed. Ms. Calvert suspects that the brain enhances the signal when the two harmonize and suppresses it when they don’t. “That’s probably to prevent you from binding things that shouldn’t go together,” she says.
It could also explain wine experts’ difficulty identifying miscolored wine. “The colors are really changing your sensory experience of smell,” she says.
Seeing a Beep
One sense cannot only affect the intensity of another, but it can actually create new perceptions, according to researchers at the California Institute of Technology. At the meeting, Shinsuke Shimojo and Ladan Shams, a professor and a postdoctoral researcher, respectively, in the computation and neural-systems department, discussed how they have used sound to make people see illusions. All it takes is a couple of beeps and flashes.
They discovered that if the timing is right, hearing two beeps while seeing one flash can make people think they’ve seen two flashes. (For a demonstration, see http://neuro.caltech.edu/~lshams/demo.html.) What’s more, they found that the activity of the brain’s visual area is nearly the same whether the person sees two flashes or just one with an illusion of a second due to the beeps.
“Our results suggest that understanding [multisensory] interactions is an integral part of understanding perception,” Mr. Shimojo says. A large number of other researchers agree. “There is a sort of growing momentum of interest in multisensory issues, particularly from people who traditionally worked on one sense or another,” says Ms. Calvert.
And interest comes from outside basic science as well. Unilever, a large manufacturer of foods and personal-care products, has provided financing for both Ms. Calvert and Mr. Spence’s research at Oxford. One of Ms. Calvert’s students is working on how vision influences the sense of touch; she imagines that someday companies could produce softer towels simply by changing the look of the surface.
The research and the applications all point to the same thing -- that our senses interact far more than we might have realized. So the next time you see something odd, you might stop and ask, “Do my ears deceive me?”
http://chronicle.com Section: Research & Publishing Page: A17