Imagine the sight, sound, smell, taste, and feeling of peeling and eating an orange. Scientists had long thought the brain handled such multisensory experiences via brain regions known as primary sensory cortices that are exclusively unimodal—that is, each devoted solely to a single sensory system. But researchers increasingly have found that these areas are actually cross-modal, in that each can respond to other senses as well. Now scientists find that in rats, the primary sensory cortex for taste can actually be trained to react to sensations other than taste, according to findings detailed online August 30 in the journal eLife.
Previous research in humans and monkeys found that the visual cortex could respond to sounds, and that the auditory cortex and the primary sensory cortex underlying touch could both respond to light and other visual cues. This holds true even when subjects are not, say, deaf or blind—meaning they’re not using one primary sensory cortex to compensate for the loss of another sense.
“We do not believe that the senses are used in isolation from each other, but rather integrated and modulated by the context,” says study co-author Alfredo Fontanini, a neuroscientist at the State University of New York at Stony Brook. “Taste,” he says citing one example, “cannot be considered as a sense solely devoted to analyzing chemicals dissolved into saliva, but rather a sense representing the whole experience of eating.”
To learn more about cross-modal activity, the researchers analyzed the rat’s gustatory cortex, which responds to taste. They first found that in ordinary rats, most neurons in the gustatory cortex only responded to taste.
The scientists next trained some rats to associate the taste of sugar with four other kinds of sensory stimuli—odors, puffs of air on their whiskers, lights, and tones. They found that afterward, gustatory cortex neurons did change the rate at which they fired when exposed to only those odors, air puffs, lights, and sounds. Such training also increased the number of neurons in the gustatory cortex that responded to multiple sensory cues.
The researchers discovered that more gustatory neurons responded to odors and air puffs than to lights and sounds. Moreover, of those four kinds of stimuli, they most easily learned to associate odors and air puffs with the taste of sugar. “We think this has to do with the fact that rats preferentially use their nose and their whiskers to locate and explore food,” Fontanini says.
The researchers suggest this cross-modal activity is the foundation for forming expectations. “Our ability to anticipate the taste of an orange on the basis of its smell depends on the activation of the gustatory cortex,” says Fontanini, adding that his lab has shown that expectation can lead to more rapid detection of tastes. Fontanini also says his group has preliminary evidence indicating that expectation can bias the perception of taste toward the expected stimuli.
Neuroscientist Sidney Simon at Duke University in Durham North Carolina, who did not take part in this research, said the result was surprising and that future work could explore whether the changes the researchers saw are permanent. Simon would like to see the experiment repeated under more natural conditions, such as when the animals are freely licking objects and not restrained.