How the brain shops

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. Understanding 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 Jonathan 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.