before it's in the papers"
August 03, 2010
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Brain cells that track value of objects
and World Science staff
Researchers say they have identified brain cells that track the track the value of different items, as the brain sees it.
“We have long known that different neurons [brain cells] in various parts of the brain respond to separate attributes, such as quantity, color, and taste. But when we make a choice… we assign a value to each available item,” said Camillo Padoa-Schioppa of Harvard Medical School in Cambridge, Mass., lead author of a paper on the findings.
“The neurons we have identified encode the value individuals assign to the available items when they make choices based on subjective preferences, a behavior called ‘economic choice.’”
Everyday examples of such choice include choosing between working and earning more or enjoying more leisure time, or choosing to invest in bonds or in stocks.
Economists and have long studied how such decisions affect economies. But recent research has found that the choices often aren’t perfectly rational, as economists, for simplicity, tend to assume they are.
This has motivated a growing interest among scientists in how the brain makes such choices, an emerging field called “neuroeconomics.” Researchers believe economic choice involves assigning values to available options, but the underlying brain mechanisms are not well understood.
In the new study, Harvard’s Padoa-Schioppa and John Assad found brain cells in monkeys that assign values based on a “common value scale”—a scale that allows for comparison between goods of different types, like apples and oranges. The cells were in the orbitofrontal cortex, a brain area just behind the forehead.
Previous research had found that damage to this area can result in “choice deficits” such as eating disorders, compulsive gambling, and abnormal social behavior, the researchers said. This area is also part of brain circuitry implicated in drug abuse, which can also be thought of as a choice deficit, they added.
The new findings, they said, establish a more direct link between this area and valuation processes underlying choices. “Various choice deficits may result from an impaired or dysfunctional activity of this population [of cells], though this hypothesis remains to be tested,” Padoa-Schioppa said.
Padoa-Schioppa had macaque monkeys choose between two types of juice offered in different amounts. In some trials, a monkey chose between a drop of grape juice, which monkeys prefer, and a drop of apple juice. In other trials, the monkey chose between one drop of grape juice and two drops of apple juice, and so on.
Padoa-Schioppa found a tradeoff between juice type and quantity. For instance, a monkey would assign roughly the same value to one drop of grape juice and three drops of apple juice. Padoa-Schioppa correlated the electrical activity of the neurons with the value assigned to the two juices.
For example, he explained, a particular neuron would have little activity when the monkey chose one drop of grape juice or three drops of apple juice. Its activity would increase when the animal chose two drops of grape juice or six of apple juice. Activity would rise still further when the monkey chose three drops of grape juice or 10 of apple juice.
Padoa-Schioppa also found that other neurons encode the value of only one of the two juices. “The monkey’s choice may be based on the activity of these neurons,” says Padoa-Schioppa. The findings appear in the April 23 issue of the research journal Nature.
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