Previous research in primates had suggested that expectation had little effect on how taste registers in the brain. Neuroscientist Jack Nitschke and his colleagues at the University of Wisconsin lined up 30 college-age volunteers to see whether the same holds true for humans.
The neuroscience team prepared five drinks containing water mixed with varying amounts of quinine or sugar and paired them with five symbols: water with a strong concentration of quinine was linked to a minus sign; water with a milder concentration got a crossed-out minus sign; simple distilled water received a zero; and water with either a mild or strong concentration of sugar got the plus sign equivalents of their negative counterparts. After three trial runs, the students had learned the associations.
The researchers then loaded the subjects into an fMRI machine for the next round of tastings. This time, however, they mixed the signs and drinks. For example, the crossed-out minus sign--initially paired with the milder quinine mixture--sometimes preceded the bitterest drink during the eight tastings in the machine.
When the participants were shown a sign indicating that the bitter drink was going to be less bitter, the same regions of the brain that fired when the subjects believed they were going to taste the bitterest drink still fired, but they did so less strongly. The subjects also reported that the drink tasted less bitter to them. The same influence of expectation on taste perception was evident when the subjects received the sugared water.
"These data show that neural responses to taste in the primary taste cortex are modulated by expectations and not solely by the objective quality of taste," the researchers write in a paper published online today by Nature Neuroscience. In other words, taste is partly in your mind.