Eaters and cooks know that flavor, in the jargon of neuroscientists, is multi-modal.
Taste is all important, to be sure. But so is the look of food and its feel in the mouth — not to mention its odor and the noisy crunch, or juicy squelch, that it may or may not make as we bite into it. The perception of flavor demands that we exercise a suite of not only gustatory, but also visual, olfactory, tactile and auditory sensitivities.
Neuroscientists are now beginning to grasp some of the ways the brain enables our impressive perceptual power when it comes to food.
Traditionally, scientists represent the brain's sensory function in a map where distinct cortical areas are thought of as serving the different senses. But it is increasingly appreciated that brain activity can't quite be segregated in this way.
Cells in visual cortex may be activated by tactile stimuli. This is the case, for example, when Braille readers use their fingers to read. These blind readers aren't seeing with their fingers, rather, they are deploying their visual brains to perceive with their hands. And, in a famous series of studies that had a great influence on my thinking on these matters, Miriganka Sur at MIT showed that animals whose retinas were re-wired surgically to feed directly into auditory cortex do not hear lights and other visible objects presented to the eyes, rather, they see with their auditory brains. The brain is plastic, and different sensory modalities compete continuously for control over populations of cells.
An exciting new paper on the gustatory cortex from the laboratory of Alfredo Fontanini at Stony Brook University shows that there are visual-, auditory-, olfactory- and touch-sensitive cells in the gustatory cortex of rats. There are even some cells that respond to stimuli in more than one modality. But what is more remarkable is that when rats learn to associate non-taste qualities — tones, flashes of lights, etc. — with food (sucrose in their study), there is a marked transformation in the gustatory cortex. Fontanini and his colleague Roberto Vincis found that the number of cells responding to cross-modal stimuli — that is, to stimuli other than taste — increases and, more remarkably, the number of cells that are responsive to more than one kind of stimulus increases.
And here's the kicker: These new multi-modal cells don't only respond to sensory stimuli associated with sucrose, but are sensitive to range of different psychological qualities.
It is as if learning to integrate different kinds of information teaches the brain to tune itself more broadly to the sorts of compounds and variety that in fact shape an animal's experience.
In an email in response to questions, Fontanini elaborates on this:
"Our findings also showed that not all the modalities are the same, some are more attuned to taste than others. For instance, in rats odors and tactile stimuli of the face are more represented in the gustatory cortex and more easily associated with taste."
Which in a way is just what you'd expect. Rats explore the worlds with their noses and whiskers; it makes good ecological senses that their taste experience would be tied to the nose and whisker in this way.
Neuroscience, by probing the activity of individual cells with an electrode, is pretty good at telling a linear story. A visual stimulus turns on this cell. An auditory stimulus produces a different effect over here. But we don't encounter the world one sense at a time. Experience isn't really linear in this way.
The multi-modality of gustation is just a single example of a general phenomenon: Sights are tied to smells and sounds and also to other strictly non-sensory qualities such as tastiness, friendliness, desirability and usefulness.
Alva Noë is a philosopher at the University of California, Berkeley where he writes and teaches about perception, consciousness and art. He is the author of several books, including his latest, Strange Tools: Art and Human Nature (Farrar, Straus and Giroux, 2015). You can keep up with more of what Alva is thinking on Facebook and on Twitter: @alvanoe