Frontier: Monkeys Capable of Probabilistic Reasoning

by Kelli Whitlock Burton

July, 2007

A new study in monkeys has identified which part of the brain is likely responsible for probabilistic reasoning, work that may shed light on how humans weigh their options when making a decision.  

Probabilistic reasoning is the process of reaching a decision based on incomplete information, where the choice may not result in a desired outcome. Scientists at the University of Washington in Seattle trained two rhesus macaque monkeys to choose which specific combinations of shapes on a computer monitor might result in a reward—in this case, juice.

The monkeys made their choice by looking at either a red or green light. The researchers ran thousands of trials a  
day, making it virtually impossible for the monkeys to memorize patterns, says lead researcher Michael Shadlen.

During the experiment, published online June 3 in Nature, Shadlen and colleagues recorded the activity of 64 neurons in the lateral intraparietal area of the brain, a region involved in vision and attention. The firing rate of the neurons corresponded to the probability that a specific shape combination would likely yield a reward.  

Watching neurons in action allows scientists to better understand the biological underpinnings of cognition,  
Shadlen says. “The next step is to understand how the brain knows that it’s finished deciding,” he adds.