July 16, 1999 Findings By Glimcher and Platt Challenge Paradigm First Proposed 300 Years Ago by Descartes
How do the brains of humans and animals make decisions - what direction to move, what food to eat, where to sleep? Decision theory, a branch of the social sciences developed by economists and psychologists to explain behavior, has long proposed that humans and animals decide what to do in a given situation by first assessing the relative value of each possible option and then selecting the option which is of greatest value. In contrast, biological studies of the mind have relied upon Descartes' conception of the 'reflex', which, like the knee-jerk response to a physician's hammer, simply connects a single sensation and a single action. While this discrepancy between complex social scientific theories of behavior and simple biological reflexes has troubled neurobiologists for much of this century, until now no researchers have been able to identify specific neurobiological mechanisms for decision-making.
In the July 15th issue of Nature, NYU neuroscientists Michael L. Platt and Paul W. Glimcher have provided evidence of a true decision-making mechanism, of the type advocated by social scientists, within the brains of macaque monkeys. These findings raise the possibility that biological mechanisms which can account for the complex and often upredictable behavior of living animals have been identified. Platt and Glimcher have found neurons in the parietal cortex of monkeys that, although previously thought to transform visual signals into eye movements in a reflexive way, actually carry information about the amount of reward a monkey expects to receive for making the movement. They found that the activity of these neurons was, like the behavior of the monkey, not predictable simply from the appearance of the visual world, but reflected the value the monkey placed on the movement.
Glimcher said, "For over three hundred years, the Cartesian reflex, which proposed a direct connection between sensation and movement, has served as the fundamental paradigm for understanding the nervous system. Contemporary neurobiologists, for example, still parcel the brain into 'sensory' and 'motor' areas but overlook the possibility that much of the brain must be devoted to subjective evaluation and decision-making. Over the last 5 years our laboratory has identified signals that seem to participate in this decision-making, and in a way not predicted by the reflexological paradigm."
"Our research has now demonstrated that the theories of decision-making developed by social scientists present a viable biological alternative to the Cartesian reflex. It has revealed that neurons in parietal cortex carry signals correlated with both the probability that a particular eye movement response will yield a fruit juice reward and the amount of reward that can be expected. More importantly, when we permit animals to choose freely amongst two alternative responses, both the choices they make, at a behavioral level, and the brain activity we record at the neuronal level, are correlated with the probability and size of an upcoming fruit juice reward. Thus, the activation of parietal cortex really does appear to reflect the decision processes that behavioral scientists suggest humans and animals use to guide their behavior.
"It is also important to point out that our research has implications for the treatment of neurological disorders like stroke and brain cancer. Our data suggests that the inability of a patient suffering from a parietal stroke to make a particular movement may be more closely related to an inability to decide to make a movement than to an inability to contract the muscles that produce a movement. If that turns out to be true, then we will have made an important advance in understanding just what goes wrong in these patients."
Glimcher and Platt's Nature article is based on two experiments, both of which measured the activity of dozens of parietal neurons. In the first experiment, during a series of tests macaque monkeys were shown two lights illuminated against a dark backround. The animals were free to look at either light, but on each trial the experimenters indicated to the animals that if they looked at one of the lights they would receive a fruit juice reward and if they looked at the other light they would receive nothing. Over groups of these tests, Platt and Glimcher varied the amount of juice reward the animals would receive for making the correct movement. The researchers found that parietal neurons kept careful track of the amount of juice each movement was worth, encoding each change in juice reward imposed by the researchers.
In the second experiment, the monkeys were given the opportunity to freely choose to look at either light for their fruit-juice reward. Although the monkeys were rewarded for looking at either light, the juice reward obtained for choosing one of the points was always greater than the other. Under these conditions the researchers found that the monkeys behaved more unpredictably, most often choosing the movement that yielded more reward, but not always. Most importantly, the researchers found that there was a high correlation between the frequency with which monkeys looked at a particular target and the neuronal activity associated with that movement, exactly the correlation that would be expected if the monkey's unpredictable choice behavior was being produced by these neurons.
Paul Glimcher is an assistant professor of neural science and psychology at NYU's Center for Neural Science. He is the principal investigator for the Laboratory for Sensory-Oculomotor Research, which is funded by the National Eye Institute. Glimcher received his Ph.D. from the University of Pennsylvania and his B.A. from Princeton University.
Michael Platt is a post-doctoral fellow in the Laboratory for Sensory-Oculomotor Research. He also received his Ph.D. from the University of Pennsylvania and received his B.A. from Yale University.
The Center for Neural Science (CNS) is the focus for inquiry in the brain sciences at the Washington Square Campus of New York University. Formed in 1987, CNS is regarded as an international center for research and teaching. The research interests of the faculty span a broad range of topics in neural science, and utilize techniques ranging from molecular and cellular analyses to fully integrated systems, computational, and cognitive studies.
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