The Brain Trust

Columbia’s Mind, Brain, and Behavior Initiative is assembling the best thinkers in the world to study the most complex object in the known universe. How far can this neurological dream team go?

by David J. Craig Published Fall 2012
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Roots of contemplation

Shadlen, a youthful fifty-three-year-old with a broad smile, is disarmingly gentle-natured. Like many neuroscientists today, he is fluent in a variety of disciplines — applied mathematics, computer science, physics, and electrical engineering, to name a few — but his broad interests seem to have brought him to the field, rather than the other way around. He was inspired to study the brain, he says, by reading French philosopher Maurice Merleau-Ponty’s writings on perception. He was twenty-two when he published his first paper, in the New Physician, an account of the troubling conditions he had witnessed that spring in a camp for El Salvadorian refugees in Honduras.

Michael Shadlen has discovered how brain cells speak to each other by varying the rates of their electrical signals.

“Back in school, I thought I’d wind up smuggling medical supplies to rebels in the hills of South America one day,” says Shadlen, now a professor of neuroscience at the College of Physicians and Surgeons. “But along the way I got distracted by more cerebral concerns, like, how the heck does this gray blob between my ears make me who I am?

That curiosity has brought Shadlen to one of the most vexing questions in brain science: how do neurons talk to each other by varying the rates at which they fire electrical signals? To keep this problem as manageable as possible, Shadlen has restricted himself over the past two decades to studying a small group of neurons that serve as a way station for visual data, in hopes of making sense of the mathematical language they speak.

“Neurons talk to each other in groups,” he says. “And every member of a group is saying something based on the signals it has received from thousands of other neurons. It’s quite complicated.”

To isolate one set of messages, Shadlen monitors the brain activity of subjects performing cognitive tasks, like identifying which way clusters of dots are moving on a computer screen. After recording the neurons’ firing rates, he then analyzes the data with software, like a cryptographer trying to decipher a strange new code.

Several of Shadlen’s breakthroughs trace back to an observation that he and his postdoctoral adviser, Stanford’s William Newsome, made together in 1994: that neurons in a part of the brain called the association cortex, which is where sensory information gets integrated into our thoughts, are capable of assessing information for several seconds before reacting to it. “A neuron that simply delivers information into your brain or carries motor instructions back out will pass along its message more or less immediately,” says Shadlen. “But we saw that the neurons involved in forming your thoughts would wait for a second or two first, as if they were evaluating the information they had received.”

Shadlen has since shown that neurons in this part of the brain do evaluate the information they receive, in order to determine the data’s significance to thoughts you are experiencing at the moment. To do so, they use a mathematical tool called a “diffusion-to-bound” form of sequential analysis. Here’s how it works: if you were asked which way dots were moving on a computer screen — say, to the right or to the left — neurons in your association cortex would assess the firing rates of other groups of neurons that are sensitive to rightward and to leftward motion. And they would take their time doing the comparison, gathering at least a few milliseconds’ worth of data before calling the contest, to guard against error. Only when the difference between the firing rates of the two groups of neurons hits a precise statistical threshold would your brain form a conscious thought, such as, those dots are moving to the left.

“It is awe-inspiring to see this happen in the laboratory,” says Shadlen. “I believe this is nothing less than the roots of contemplation, of deliberation, of thought itself. These brain cells are exhibiting what I call a freedom from immediacy, in the sense that, rather than responding reflexively to information, they’re ruminating on it.”

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