Postdoc Rebecca Yang (right) and mentors Yuh Nung and Lily Jan observed fruit flies making choices for their offspings' survival.

When Shadlen was a postdoc in William Newsome's lab, the two researchers identified nerve circuits in the cerebral cortex of monkeys that became more active when the animals made a decision about visual information with which they were presented. These neurons were located in the lateral intraparietal (LIP) area of the cortex.

More recently, Shadlen designed a set of experiments to test the monkeys' ability to make a correct choice when they had to sum up statistical clues presented in a short series. People do this all the time: think of a poker player calculating the changing odds of winning a pot as she draws additional cards into her hand.

The research, published by Shadlen and Tianming Yang in the June 28, 2007, issue of Nature, found that the monkeys became quite adept at the task, showing that they were capable of probabilistic reasoning.

"No one had ever tried to train a monkey to do this kind of thing," says Shadlen. "We had no idea if the monkey could."

He and his colleagues trained two monkeys to stare at a point on a screen for several seconds, then shift their gaze to either a red spot or a green spot; if they picked the better target, they improved the chances of getting a reward. Sometimes the red spot brought the reward and sometimes the green, but even the better choice did not always pay off—it's a matter of playing the odds.

In each trial, the monkeys were shown four different shapes on the screen, one after another, every half-second. Each shape signified a different probability value of success, alone and in combination with the other shapes. If the monkey correctly combined the probabilities, he was likely to move his eyes to the correct target and improve the odds of winning a reward.

While the monkeys were playing this game, the researchers recorded electrical activity from 64 individual nerve cells in the LIP areas. The rate of firing in the nerve cells increased or decreased whenever the animals saw a new shape on the screen that altered the probability of a particular target being the correct choice. As the subject became more convinced of the right choice, the LIP neurons increased their firing rates. Shadlen and Yang showed that the quantity computed by the monkey's neurons could be described by a mathematical term, the "log likelihood ratio"; in other words, the logarithm of probabilities.

"It was enormously satisfying to watch a monkey add and subtract in his head and to keep a running tally of the state of odds in favor of one proposition and against another," Shadlen says. "To see these operations carried out by neurons in the brain was breathtaking."

Neurobiologist Ranulfo Romo has developed techniques to locate decision-making neurons involved with the monkey's sense of touch—the somatosensory system. Through many years of painstaking work, Romo, an HHMI international research scholar at the National Autonomous University of Mexico, has succeeded in recording neuronal activity simultaneously in several areas of the monkey cortex. His goal is to track sensory signals created by stimulating the animal's finger, following them from one cortical area to another as the monkey forms a response.

Photo: Andrew Nagata

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