Gambling monkeys may provide clues to why humans take risks

Like most rhesus monkeys, Aragorn is small and smart. Unlike most of his peers, however, Aragorn is also a practiced gambler.

He’s one of several rhesus monkeys – all named after “Lord of the Rings” characters – that Johns Hopkins University neuroscientist Veit Stuphorn has trained to perform what is formally known as “decision-making under risk.” The goal has been to look at how those choices unfold in the brain. And in a new study, Stuphorn says his lab’s gambling monkeys have led to the discovery that an area of the brain is associated with high-risk tendencies – a finding that could eventually help scientists better understand such behavior in humans.

Stuphorn began working with monkeys about a decade ago, and Aragorn was the first. While the research doesn’t involve poker or blackjack, it has nonetheless been illustrative.

Veit starts by teaching the monkey to understand both color schemes and probability. Then the monkey is put in front of a computer screen and presented with two square boxes: one on the left and another of equal size on the right. Inside each box are colors – red, blue, green or a green-blue called cyan – that correspond to increasing rewards. In this case, the monkey plays for drinks of water.

The proportion of the colors in the box indicates the probability of a particular payoff. For instance, an all-blue box guarantees a medium amount of water. A box that’s 80 percent cyan and 20 percent green indicates a high chance of getting small amounts of water but a low chance of getting a lot of water. With each round, the monkey must decide between two options.

“The monkey used his eyes to make the choice,” said Stuphorn, explaining that researchers use sensors to track the animal’s eye movements. Whichever box its gaze fixes on indicates the animal’s choice, and the monkey then receives the associated reward – more water if the gamble pays off, less if it doesn’t. The trials last until the monkey has gotten its fill of water or gotten bored, which it sometimes makes very clear, Stuphorn said.

“They close their eyes and snore,” he said.

Over many sessions and thousands of repetitions, Stuphorn and his colleagues found that the monkeys were risk-takers: They routinely opted to gamble for more water rather than settle for less. Those initial findings, Stuphorn said, aligned with what other researchers in the field had seen. So five years ago, Stuphorn and then-Hopkins graduate student Xiaomo Chen decided to take the work a step further.

For their recent study, scientists focused on a part of the brain’s frontal cortex known as the supplementary eye field (SEF), an area that helps control movement and that their earlier research indicated might also be involved in risky behavior. But the details were murky, so they decided to see what would happen if they deactivated the SEF.

Chen and Stuphorn worked with Aragorn and another monkey named Isildur. By positioning tiny metal plates over the SEF region of the monkey’s brain, they cooled the tissue until it became inactive. (Stuphorn said the process is harmless and reversible, and that other scientists have used it in previous studies.)

“If it really has an impact, the behavior of the monkey should change,” Stuphorn said.

Then the monkeys got back to gambling, and their behavior did indeed change. After a few minutes of cooling, the SEF became inactive, and Aragorn and Isildur suddenly became much less likely to take the riskier bets.

“The preference for the uncertainty was reduced,” and significantly so, Stuphorn said. “This study definitely helps in understanding what this brain area is doing.”

Darby Proctor, a comparative psychologist at the Florida Institute of Technology who was not involved in the study, said it’s “not super surprising” to find that the frontal cortex is involved in the decisions.

“That’s where we know that humans make their complex decisions,” said Proctor, who has done similar work with primates. “But I think it’s very interesting that they are able to change the responses by knocking out some of those regions. That helps us really to refine where the biological basis for those decisions are.”

Proctor noted that there were only two monkeys in the study, and that there was very little at stake in the gambling trials, which could influence the animals’ decisions.

Stuphorn points out that this research provides new clues, but not definitive answers. The SEF, for example, is related to eye function, not other motor movements. So the findings have no direct implications for a casino game such as slots, which relies on arm motions that are controlled by a different part of the brain.

But Daeyeol Lee, a neuroscientist at Yale University, said the study’s importance shouldn’t be underestimated, because other areas of the brain near the SEF are known to control limb function.

“These areas all have similar properties,” he said, meaning that there’s a high probability that more research could show a wider applicability of Chen and Stuphorn’s work.

Until then, Stuphorn seems satisfied that the gambling monkeys have provided research that adds to a broader shift in how scientists view primate, and consequently human, perceptions of risk.

“People thought of risk-taking as a character trait of the person — that some people are risk-taking and others are not,” Stuphorn said. “Now, people think of risk-taking as more flexible.”