Junk-Food-Valuing Brain Cells Pinpointed

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Individual human brain cells can be savvy shoppers, tuning their behavior to

precisely reflect the worth of a candy bar, finds a study published January 5 in

The Journal of Neuroscience.

sciencenewsUnderstanding how these bean-counting neurons operate may help

scientists get a better idea of how the brain assigns value to objects.

Evaluating objects is “something we all do on a moment-to-moment basis,” says

study coauthor Rick Jenison of the University of Wisconsin–Madison, but just how

the human brain tallies up value isn’t clear.

To eavesdrop on the discerning human brain, Jenison and his team took advantage

of a rare opportunity: human volunteers who are undergoing a procedure that uses

electrodes to pinpoint the origin of severe seizures. As a by-product, these

electrodes can also listen to the activity rates of single neurons in the

amygdalae — a pair of almond-shaped structures located on each side of the

brain– as the volunteers assessed the value of junk food.

Experiments with monkeys and rats have shown that amygdala neurons play a role

in evaluating objects, but getting precise value estimates from an animal is

nearly impossible. In contrast, human volunteers can easily assign exact values

to objects and communicate that information to researchers.

“In this study, you can get humans to tell you how much they value something,”

says neurophysiologist Wallis of the University of California,

Berkeley. “You’re not just getting, ‘It’s good,’ or ‘It’s bad.’ You’re getting a

precise estimate of how good it is, or how bad it is.”

After the electrodes were in place, three participants viewed pictures of 50

different kinds of junk food, ranging from chocolate-chip cookies to M & M’s to

salty chips. The participants viewed each image for one second, and then came up

with a subjective value rating of the snack by bidding amounts between zero and

three dollars for the item.

The system was designed to reflect personalized tastes. “With different people,

it’s quite idiosyncratic in terms of what they like and dislike,” Jenison says.

“The real goal here is to get them to give us an honest bid of what they value

the food item to be.”

Throughout the experiment, electrodes caught the activity of single neurons in

the volunteers’ amygdalae. Of the 51 neurons that the researchers tracked in the

three volunteers, 16 performed in lockstep with the value of the food item,

changing their activity in a predictable way as the value increased. As the

value (and corresponding bid) went up, some of these neurons’ activity went up

too. Others showed an inverse relationship, with their activity declining as the

value increased.

Neuroscientist Daeyeol Lee of Yale University says that along with other

studies, the new work “expands the role of the amygdala,” a region that is

traditionally associated with fear. Lee cautions that due to the necessity of

working with human volunteers who have electrodes implanted in their heads, the

sample size is inherently small, precluding many repetitions of the experiment.

“The opportunity is limited, but it’s a really, really exciting opportunity.”

Next, Jenison and his team are testing how these neurons respond to foods that

some people find disgusting, such as oyster juice and liver pâté. They are also

examining how these neurons behave when a person decides between two objects.