Laurie Santos/NCBI/NIH

Decisions, decisions ...
Macaques can make them using probabilistic reasoning.

Monkey See, Monkey Choose

Beware of making bets with a monkey. A study has shown that macaques, just like humans, know how to play the odds. Neurons in one part of the brain appear to have an important role in this process; they integrate what a macaque has seen to help its brain make a decision, researchers report.

When they make decisions, few animals have a firm idea what the consequences will be. Still, there are all kinds of predictors of what might happen after a choice is made--deliberately step on a banana peel, bust your bum. Somehow, the brain integrates these clues, but how it reaches a conclusion is far from clear. Based on recent studies, some scientists had speculated that neurons in the brain region called the lateral intraparietal area can add and subtract information to estimate the chances of various outcomes.

Hoping for their own positive outcome, neuroscientists Michael Shadlen and Tianming Yang of the University of Washington, Seattle, spent several months training two macaques to perform a complex task. In each session, they showed the monkeys a sequence of four shapes (out of a possible 10) on a screen. The monkeys then had to choose either a red target or a green target; if they picked the "right" one, they increased the odds they would receive a small drink reward. It wasn't entirely a game of skill, however. Whether red or green was the winning color was determined by the shapes that preceded them in a probabilistic manner; for instance, a diamond shape meant a higher chance that green would be rewarded, whereas a semicircle meant that red was a better bet. Both monkeys picked the color most likely to be rewarded, based on the "evidence" they had seen, 75% of the time, the researchers report online 3 June in Nature.

The team also recorded the activity of 64 neurons in the lateral intraparietal area and found that neurons reacted to the first shape and then adjusted their firing rates after each of the next three shapes were shown; in other words, the animals were combining new clues with existing evidence to estimate the probability of a sugary treat.

The study is the first to convincingly demonstrate that the lateral intraparietal area is involved with calculating evidence, says neuroscientist Anna Ipata of Columbia University. And it is a clear demonstration that monkeys, like humans, can make rational decisions based on accumulation of evidence, she says.

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