Some tastes--like blue cheese or green olives--are "acquired." But can we conquer our aversion to a food before we even taste it for the first time? Neuroscientist Donald Katz at Brandeis University has shown that it can happen with rats.
Rats like sweet tastes best of all. They also like salt. They dislike sour, and they hate bitter. Armed with this knowledge, Katz and colleagues decided to see if they could get the creatures to change their minds about bitter cocoa if they met a pal who seemed to like the stuff.
First the researchers got a rat hungry enough that it would be willing to nibble at some raw cocoa. Then they put another rat in with it so rat number 2 could smell the first rat's breath. Finally, the researchers placed the second rat in a cage with two unfamiliar and not particularly appetizing dishes to choose from. Despite their innate aversion to bitter taste, the rats went for the cocoa.
Electrode recordings of nerve-firing in the taste circuit showed that the test rats actually altered their evaluation of the cocoa. Taste is evaluated by the brain in three stages: in the first milliseconds, the substance is detected; then it's identified; then the amygdala, the seat of the emotions, sends a message to the cortex telling it whether the taste is noxious or palatable.
While neurons reflecting taste aversion will ordinarily fire at a bitter taste, that didn't happen with the rat conditioned to accept cocoa. Instead, it fired neurons indicating it found the stuff palatable--showing "social transmission of taste preference," said Katz. In the past, he said, scientists thought this kind of preference was "hard-wired--but in fact it's under emotional control." Evolutionarily, this might be a mechanism to persuade a rat that even an unknown nasty substance is okay to eat in a pinch--knowing that what the other rat ate didn't kill it.
--Constance Holden
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