Professor of Neuroscience
Professor of Neuroscience
Michael Shadlen is a Professor of Neuroscience at Columbia University he studies neurons in the association cortex that process information from the visual cortex to give rise to interpretations, decisions, and plans for behavior. His experiments combine electrophysiology and behavioral and computational methods to advance our knowledge of higher brain function.
As a neurology resident at Stanford, Shadlen often treated violent and confused young men in the emergency room. Even across the room, he could see these men bouncing their eyes between two positions. This bizarre movement indicated intoxication with angel dust (phencyclidine), the recreational drug of choice in the early 1990s. The inability to keep the eyes still indicated that the brain stem was having difficulty integrating eye speed signals to achieve a steady eye position. So Shadlen wondered if mental confusion occurred because some part of the cerebral cortex was having trouble integrating other types of signals.
While Shadlen was a postdoc in the lab of William Newsome (now an HHMI Investigator) at Stanford in the mid-1990s, he recorded the electrical activity of individual neurons while macaque monkeys sat in front of a computer and responded to images on the screen. Such activity enables neurons to talk to one another. "For the kinds of things we want to study, we need animals that are capable of pretty sophisticated tasks," Shadlen says. He stresses that the monkeys are perfectly comfortable performing such tasks while they have electrodes in the brain because the brain has no pain receptors.
These experiments provided the first evidence that a collection of neurons in the parietal cortex (toward the rear of the top of the brain) might integrate information to make decisions. They suggested that those neurons accept information from the part of the brain that processes visual information and then decide how other neurons should shift the eyes. "If this ultimately proves to be the case, several fascinating issues in cognitive neuroscience will be brought under rigorous physiological scrutiny," Shadlen wrote in a 1996 paper in Proceedings of the National Academy of Sciences.
After Shadlen established his own lab at the University of Washington in 1995, he continued to study the parietal cortex. This lobe is part of the association cortex, which encompasses the many areas between the regions of the brain that process sensory information or send commands to muscles. Neurons in the association cortex spring into action when sensory information needs to be interpreted, which is much of the time. People who have a tumor or stroke in the parietal cortex can process information from their surroundings, but have trouble using it in a rational way.
Shadlen and his students discovered that decision-making neurons in the parietal cortex can store and integrate information for at least a few seconds, unlike neurons that control muscles and give commands in split seconds. The group's later work revealed that neurons also obey a termination rule. "They figure out when they have enough information to make a decision, and then pull the trigger," Shadlen says, adding that the neurons reach this point regardless of whether the evidence is good or bad, the task is easy or hard, or the decision is right or wrong. He discovered that decision-making ends when the rate at which neurons fire electrical impulses reaches a critical level. In the long-term, Shadlen would like to discover how to preserve neurons' ability to integrate information and track time.