A small set of brain cells that transmit the chemical messenger dopamine to various neural destinations works as an uncertainty meter, at least in monkeys, a new study finds. The electrical activity of these cells rises sharply when monkeys find themselves unable to predict whether a familiar visual signal heralds a food reward, say Christopher D. Fiorillo of the University of Fribourg in Switzerland, and his coworkers.
This brain response may stimulate risky, exploratory acts in natural settings where such behaviors can reap big rewards, Fiorillo’s group theorizes. In people, they add, it may also contribute to the allure of gambling.
Electrodes implanted in the brains of two adult monkeys tracked electrical responses of 188 dopamine-making neurons in an area called the midbrain. Over a series of trials, the monkeys learned to associate each of five distinctive visual patterns shown for 2 seconds on a computer screen and the probability of receiving a taste of syrup from a dropper. Specific patterns were accompanied by a dose of syrup either in all, three-quarters, half, one-quarter, or none of the presentations.
After training, dopamine neurons displayed peak electrical activity as monkeys viewed the most unpredictable pattern, which denoted a 50-50 chance of reward, the scientists report in the March 21 Science. Smaller surges in neuron activity occurred in response to patterns that predicted a reward three-quarters and one-quarter of the time. Dopamine neurons showed no response to patterns that signified certain reward or denial.
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