A LOOK UNDER THE HEAD|
What Can Brain Mechanics Tell Us About Decision-Making?
Researchers are closer than ever to understanding how you make up your mind. What’s more, they may be able to quantify your thought process and perhaps even why you make the decisions you do. If that prospect freaks you out, don’t worry. The research tells us about how the brain achieves these feats, not what any individual’s brain is doing as you muse about this or that.
For much of the past two decades, Dr. Michael Shadlen, professor and HHMI investigator in the department of neuroscience at Columbia University, and his colleagues have been studying decision- making from the inside out—examining mechanisms of the brain known to be involved in choice. Now his group is pushing the boundaries of that research.
“A decision is a commitment to a proposition among alternatives that arises through a process of deliberation,” Shadlen said at a recent NIH Neuroscience Seminar, “and I think the neuromechanisms of decision- making offer a window on higher brain function and cognition.”
Calling himself a “kind of a grandchild of the [NEI] Lab of Sensorimotor Research,” which sponsored his lecture, Shadlen described two studies, one on reasoning from symbols and the other that connects memory to decision-making.
NIH’ers, he pointed out, provided crucial building blocks for the studies and for a large share of research in the field. Former LSR scientists Dr. Mickey Goldberg and Dr. Carol Colby conducted work to characterize the parietal cortex—the region of the brain that Shadlen studies.
Shadlen himself worked as a postdoc with Stanford’s Dr. Bill Newsome (of BRAIN Initiative renown), who spent 4 years as a postdoc with NEI’s Dr. Bob Wurtz, cofounder in 1973 of the LSR and its chief until 1997.
At Columbia, Shadlen’s lab uses rhesus monkeys to study the neural mechanisms of perceptual decisions. They train the monkey to view information on a computer monitor and use that information to make a decision. Scientists track the monkey’s eye movements because that’s the way the monkey communicates its decision—an eye movement to one of two targets.
“As neurologists,” he said, “we study decision-making in the lab because we think the basic principles that arise by studying these highly contrived, non-naturalistic tasks will help us understand what goes awry in disorders that affect higher brain functions.”
“The region is suited to take information from vision and construe it as evidence leading to what the monkey is going to do with his eyes,” Shadlen explained.
The scientists measure the electrical activity of neurons during deliberation. The researchers can trace, for example, the choices the monkey makes as well as how long he takes to make them and why. They can also predict the choices using a mathematical formula based on reaction time.
The amount of time used to decide, Shadlen explained, is controlled by the same mechanism that controls which choice is made. He demonstrated this by predicting the monkey’s accuracy simply by measuring the reaction times.
“What we learned is that the neurons don’t just tell us which eye movement the monkey is going to make, but also why he made it,” Shadlen said. “Noisy information from the environment comes in through the eyes—the visual cortex—where it’s converted into ‘momentary evidence’ in favor of right versus left.” The information keeps piling up until the brain has enough to make a decision.
“Many of us believe that it’s this persistent activity—which is a notable property for the association cortex—that holds the key to cognition because it confers a kind of freedom from immediacy,” Shadlen continued. “It’s not like sensory neurons that keep up with the changing world or motor neurons that have to control the body in real time. It’s the stuff in between that is the stuff of thought and the site at which many diseases—whether caused by genes or toxins—infiltrate or negatively affect our cognitive capacity.”
Shadlen shared several movies from his experiments—decisions, accompanied by neural recordings.
“You’re witnessing the brain doing additions and subtractions—actually the running sums of positive and negative numbers approximating an integral,” he explained.
He said such videos refute “the idea that information in the brain will need billions of neurons recording in order [for scientists] to see things. We have to reconcile that intuition with the fact that it’s so easy to see computations arising at the level of a single neuron in a single trial.”
Shadlen concluded his lecture with a slide showing a size progression of three brains from mouse to monkey to human. Findings from decision-making research have already broadened the field’s notions about the mechanics of more complex thought.
“As we evolved cognition and got bigger and bigger cortical mantle, what was growing essentially was the association cortex,” he said. “I think the basic principles of decision-making that can be studied in these very simple direct paths could be telescoped out to decisions about decisions about decisions—that’s sort of what abstraction is—and that the principles understood at the level of the monkey brain will have some relevance to the kind of complicated things we do with the human brain.
“And I think ultimately to treat diseases one day,” he concluded, “and restore these kinds of operations, integrations and bounds-setting—and lots of things we don’t yet understand obviously—will require manipulating brains at the levels of molecules and circuits. I know in my lab, and for many of you, there’s already at least a dialogue with [scientists] working in all of these levels, interaction with people who do human [brain] imaging and circuit dissection.”