Although the field of decision neurology is relatively young, a recent paper by two Yale researchers may provide a major step forward.

The duo worked with monkeys to find the exact mechanisms by which the primates calculated expected returns in the form of drops of juice from a simple game of chance. They found that after close examination, they could parse through data collected from the activity of individual neurons to decode both the stimuli the primates were facing and previous stimuli and rewards. The study was published in the journal Nature Neuroscience on Jan. 12.

“In this field, we are at the very beginning. We really have no idea how decisions are made,” said Ueli Rutishauser, professor of neurosurgery at Cedars-Sinai Medical Center, Los Angeles, who is not affiliated with the study.

The researchers had the monkeys play a game in which they were shown red and green tokens. The monkeys were then trained to understand that one of the colors corresponded to an 80 percent chance of a reward, and the other corresponded to a 20 percent chance of reward. The researchers also trained the monkeys to understand how big their reward would be if they chose successfully. The researchers then swapped the percentages associated with each color and retrained the monkeys.

Throughout the process, researchers monitored the neurons in the primates’ dorsolateral prefrontal cortices. The researchers found that from only looking at the data collected from neural activity, they could tell which colors the monkeys had previously selected and whether or not they had been rewarded.

“[This study] gives us background on how our brains are able to make optimal decisions in the face of uncertainty and of changing requirements is really one of the big questions in neuroscience … and it’s essentially completely unknown how we make such decisions,” Rutishauser said.

The researchers found that although the monkeys were not formally computing expected values, they were using an additive heuristic, treating as more important trials that had occurred more recently.

“[The monkeys] did a pretty good job, but they did not calculate the probability mathematically,” said Daeyeol Lee, professor at the Yale School of Medicine and senior author of the paper. “If you think about it, that sounds pretty dumb. But on the other hand, they are not even doing a symbolic math problem. They are doing this empirically and it was actually a pretty efficient strategy in that they were able to secure the amount of juice they were getting by using heuristics that are close enough.”

Chris Donahue YSM ’13, a postdoctoral researcher at Gladstone Institutes and lead author of the paper, said one of the key elements of the paper is that they randomized which side of the screen each color token would appear on. This way, the researchers were able to watch how the monkeys’ brains responded both to task relevant and irrelevant data.

The researchers chose to use macaque monkeys because their brain structure is complex enough for the findings to be applicable to humans but simple enough to be observed with high resolution.

Correction, Feb. 2: A previous version of this article incorrectly stated that the study senior authored by Daeyeol Lee was published in Nature. It was published in Nature Neuroscience.