Single brain cell helps make decisions

New Yale neuroscience research is shedding light on how people make decisions.

 A new Yale study suggests that a single brain cell exercises outsize influence when it comes to choosing between long- and short-term rewards.
A new Yale study suggests that a single brain cell exercises outsize influence when it comes to choosing between long- and short-term rewards.

According to a recent study compiled by Yale neurobiology researchers Daeyeol Lee, Xinying Cai and Soyoun Kim, the activities of a single brain cell — in the moments immediately before a conscious decision is made — can reveal how a person is likely to respond to potential rewards like food, alcohol and sex. Their findings, published Jan. 12 in the scientific journal Neuron, suggest that humans and animals often prefer smaller but more immediate sources of gratification to larger but delayed rewards. Experts interviewed said they found the study’s results relevant and interesting but added that the question over what exactly happens in our brains when we make such choices still remains.

“Our studies show that before an animal makes a decision, the information about various properties of reward — such as its magnitude, probability and timing — is integrated across multiple brain areas,” said Kim, an associate research scientist at the Lee Lab of Cognition and Decision Making. “Neurons begin to change their activity according to the animal’s ultimate choice before the final action is executed, presumably reflecting a process of deliberation.”

To conduct their research, the scientists presented monkeys with choices that differed by payoff, the larger reward coming only after a period of delay. In repeated tests, the monkeys were more likely to opt for instant gratification, indicating that the value of a reward decreases in direct proportion to its delay.

By looking at information processed both in brain regions and individual cells, Kim said, the team of scientists was able to map dense connections among billions of neurons, whose combined activity results in particular behaviors.

Cai, one of the paper’s co-authors, explained how individual neurons fire off with an analogy.

“A single neuron often encodes full information that is necessary to generate desired behavior,” he said. “This is like the organs in our body. It takes many cells for the organ to work, but each cell contains full genetic information.”

In addition, the study investigated how specific sections of the brain help assess the value of the reward, with a particular focus on the basal ganglia, an area of the brain associated with motor control, and the prefrontal cortex, which is important for decision making. One area of the basil ganglia, the dorsal striatum, seems to identify the reward Cai said, while a second, the ventral striatum, evaluates the reward once chosen. The data, Cai said, suggests that the caudate, another section of the ganglia, may be an area where values are compared, even less than one second before the decision is made.

But as for why humans tend towards instant gratification rather than think about the long-term costs of their actions, the researchers do not yet have a definitive answer. As Lee, associate professor of neurobiology and psychology at the School of Medicine, said, scientists still do not know the anatomical basis for many psychiatric disorders, including problem gambling and impulsive behavior.

Yet other experts said the study needs to be evaluated in the context of current literature, which points to the unclear role that the ventral and dorsal stratia play in decision making.

Tobias Kalenscher, a professor of animal physiology and cognitive neuroscience at the University of Amsterdam, cited how different studies on the topic find different, sometimes downright incompatible results. He mentioned a range of studies with human subjects which show that activity in the ventral striatum correlates with impulsivity — the propensity to prefer small, short-term rewards over large, long-term rewards. Animal studies on malformations in the brain, on the other hand, suggest precisely the opposite, he said: that the ventral striatum is necessary to maintain self-control. In short, evidence in human and animal literature agrees that the ventral striatum does play a role in intertemporal choice, but it is unclear what role it really plays.

“The question remains in how far correlative studies based on single-cell activity are informative about the way we humans make intertemporal decisions,” Kalenscher said. “I think this study provides just another piece in the big jigsaw puzzle. It alone will not solve the problem of intertemporal choice, but by shedding light on the underlying computational mechanisms, it contributed to our understanding.”

An answer to this question may have big implications for policy-making and macroeconomic models, he added, since scientists know from numerous behavioral studies that people are not time-consistent: People break diets and do not go to the doctor. On this basis, he said, social institutions can create mechanisms that exploit human psychology, like retirement policies that allow employers to opt in or opt out.

“In the end, Lee’s article may have an impact on policy making in the future,” Kalenscher said, “but for now, we will have to be patient; this is a long-term investment.”

The study was funded by the National Institutes of Health, a part of the U.S. Department of Health and Human Services and the nation’s official medical research agency.

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