The cerebral areas activated during the perception of tastes have been identified by means of nuclear magnetic resonance imaging.
JEAN-ANTHELME BRILLAT-SAVARIN OBSERVED in Meditation 2 of The Physiology of Taste, “Up to the present time there is not a single circumstance in which a given taste has been analyzed with stern exactitude, so that we have been forced to depend on a small number of generalizations, such as sweet, sugary, sour, bitter, and other like ones which express, in the end, no more than the words agreeable or disagreeable, and are enough to make themselves understood and to indicate, more or less, the taste properties of the sapid body which they describe. Men who will come after us will know much more than we of this subject; and it cannot be disputed that it is chemistry which will reveal the causes or the basic elements of taste.”
Brillat-Savarin was prescient. Today biochemists and neurobiologists know a good deal about the function of receptor molecules for tastes, which are located on the surface of papillary cells on the tongue. Even so, nuclear magnetic resonance imaging (MRI) techniques are helping them better understand how information perceived by the taste receptors travels up into the brain for processing.
With the development of these techniques, which record cerebral activity by detecting changes in blood flow in the brain, neurobiologists have paid special attention to cognitive activities such as language, calculation, and memorization. Olfaction has been less studied and the perception of tastes completely neglected. Barbara Cerf and Annick Faurion of the Laboratoire de Neurobiologie Sensorielle in Massy and Denis Le Bihan of the Centre Hospitalier d’Orsay in Paris have identified the cerebral areas activated by taste molecules.
Basic knowledge was rudimentary. The only thing that was known, from observation of people whose brains had been partly destroyed by wartime injuries, was that the parietal operculum, located near the central sulcus (Rolando’s fissure), undoubtedly played a role in the perception of tastes. Nonetheless, electrophysiological studies yielded contradictory results, which pointed instead to another area located in the insula.
Because an MRI requires subjects to lie down inside a tunnel-like machine, they were fed with solutions transmitted through flexible tubes. These constraints determined the sapid substances that were tested: The subjects received solutions of aspartame (a sweetener), sodium chloride, quinine (bitter taste), glycyrrhizic acid (licorice taste), guanosine monophosphate (the umami taste, similar to that of monosodium glutamate, used in Asian cooking), and D-threonine (indescribable—you have to taste it for yourself). The experimenters first gave the subjects water, then sapid solutions, then water again, and so on, in order to forestall habituation while sustaining stimulation for several dozen seconds, the time needed for the MRI device to record a signal.
Lateralization of Taste
The subjects who received these solutions were instructed to concentrate on their taste in order to minimize interfering activations of other parts of the brain. The subjects described the intensity of their sensations by moving a cursor along a graduated scale. By calculating correlations between the various perceptual profiles and activations of the different areas of the brain, the neurobiologists were able to determine which activations were linked to the perception of tastes. Individual differences were pronounced and the images noisy, so that many experiments had to be analyzed in order to pinpoint the areas that were specifically associated with the perception of taste.
The first studies showed that four cerebral areas are activated by sapid solutions: the insula and the frontal, parietal, and temporal opercula. There is no single taste center in the brain nor any cerebral areas that are specifically linked to particular tastes. On the other hand, certain areas that were not systematically activated are known to play a role in language comprehension, hence the hypothesis that the detection of taste may be associated with the act of naming it.
A second study comparing five left-handed and five right-handed people found that the four cerebral taste areas were not systematically inverted between the two groups. By contrast, activation of the insula differed according to handedness. This area is composed of two regions. The upper part is activated in both hemispheres, for right-handers as well as left-handers; the lower part is activated unilaterally in the subject’s dominant hemisphere. The perception of taste therefore is lateralized in a way that is analogous to language use and motor activity.
A third series of studies compared subjects’ reaction to molecules that have only taste and to molecules that have both taste and astringency (or pungency). This time the activated areas were analogous, which explains why flavor—the overall sensation that is registered in the course of eating—is so all-encompassing and so difficult to describe: The brain constructs a global sensation through the synthesis of signals coming from various types of receptors.