Where in the brain decisions are made

October 13, 2004

While neurobiologists have long suspected that certain regions of the brain are specifically involved in making decisions, the challenge has been to develop rigorous laboratory behavioral experiments that could pinpoint those areas.

Now, Paul Glimcher and colleague Michael Dorris have used a game-playing approach to demonstrate that a region of the cortex called the lateral intraparietal (LIP) area is active when monkeys are making subjective internal decisions about the desirability of an action--in this case, moving their eyes to a target. According to the researchers, their findings represents a step toward understanding the machinery by which the brain processes decisions.

In their studies, the researchers first tested the behavior of humans competing in a game in which a player was asked to click on a computer mouse to choose one of two buttons to receive either a certain monetary reward or a risky choice that could yield a larger reward. An opponent, meanwhile, was asked to select an option that would prevent the reward, and the researchers could vary the cost to the opponent of making that selection.

The basic aim of the researchers was to create a situation in which there was no single correct choice, so that subjects adopted a mixed strategy, reflecting that the subjective desirability of the choices was equal.

Once the researchers determined that the humans playing the game adopted just such a strategy, they then developed a computer "opponent"--a program designed to elicit the same mixed strategy. They tested the computer program against human players, determining that it elicited the same mixed strategy.

They then trained monkeys to play a version of the game against the computer, in which the animals were required to choose to glance at specific colored targets to receive sips of water. The computer program would analyze their responses and vary the parameters to elicit the same strategy. The researchers determined that the monkeys adopted the same strategy as humans--presumably reflecting the same neural processes.

The researchers had the monkeys play the game while they recorded electrical activity from the lateral intraparietal area.

A complex array of experiments in which the monkeys played the game under different parameters revealed that the monkeys were, indeed, making subjective decisions about which target to look at. And, the researchers found, the electrical activity of neurons in the lateral intraparietal region did indicate that it was involved in making those decisions. Analysis of the experiments also indicated that the neurons were encoding the relative subjective desirability of a choice, rather than absolute subjective desirability.

"While we cannot ask nonhuman primates to report their subjective impressions of the decision-making process during these experiments, the data described here may still shed light on the mechanisms of human voluntary decision making," wrote the researchers. "The neural data presented here suggest that the neurons of area LIP encode the relative subjective desirability of saccadic eye movements.

"If human choices are guided by circuits involving neurons like those in monkey area LIP, then it is tempting to speculate that the average firing rates of these neurons may also encode the subjective desirability of actions in humans."
Michael C. Dorris and Paul W. Glimcher: "Activity in Posterior Parietal Cortex Is Correlated with the Relative Subjective Desirability of Action"

Published in Neuron, Volume 44, Number 2, October 14, 2004, pages 365-378.

Cell Press

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