One of the most important functions of the brain is to make decisions. Even the simplest animals need to make choices based on sensory information: is that thing over there food or a predator, and should I eat it, or run from it? Making the right decision is literally a matter of life or death. In humans, our decision-making can grow far more complex, encompassing matters from choosing where to eat dinner to whether a defendant is guilty or innocent.
How those decisions are made has fascinated philosophers as far back as Plato, and been the subject of science books as recently as Jonah Lehrer’s How We Decide. Plato’s metaphorical tug of war between the rational mind and the emotional mind has been updated with brain imaging and psychological experiments to a balance between emotional centers of the brain (the amygdala, the insula) and the “rational” calculations of the prefrontal cortex. But as Lehrer nicely illustrates via airplane pilots, poker players, and Tom Brady, the prefrontal cortex doesn’t always make the best decisions, and human choices are often the product of an argument between a Congress of different brain regions.
One brain area that doesn’t appear in Lehrer’s book is the parietal lobe, a region located roughly in the upper middle of the human brain. An old-fashioned name for the parietal lobe is the “association cortex,” named for the region’s role in integrating sensory and motor information primarily mediated by other parts of the brain. As a hub between incoming sensory inputs (such as the sight of an attacking tiger) to behavioral motor outputs (such as RUN!!!), the parietal lobe would seem an obvious place for decision-making. But it was a different phenomenon that first led David Freedman, assistant professor of neuroscience at the University of Chicago, to the decision-making role of the parietal lobe.
As a graduate student and postdoctoral researcher, Freedman was interested in visual categorization, how the brain classifies the many objects around us into useful groups. Freedman trained subjects to form categories about shapes they were shown, such as the difference between dogs and cats. By recording from different areas of the brain, including the prefrontal cortex that is often implicated in decision-making, Freedman found that learned categories could be encoded in the activity of individual neurons – some began responding preferentially to cats, others responded to dogs.
“We’re not born knowing about things like furniture and vehicles, you have to learn about that through experience,” Freedman said. “So the experiments we did suggested that learned information about these kinds of visual categories can be encoded in the activity of individual neurons at the highest stages of the visual system and the frontal lobe.”
While at Harvard working with John Assad, Freedman found another area of the brain that seemed to play a role in categorization: a region of the parietal lobe called lateral intraparietal area, or LIP. The region had already attracted the attention of scientists studying decision-making at the University of Washington, Stanford, and NYU, who found activity there when monkeys performed a simple visual choice task.
In those experiments, subjects were shown an array of dots moving in one direction, and were rewarded when they darted their eyes (a motion called a saccade) in the same direction as the dots. When the researchers looked in the LIP – previously known to play a role in saccades – they saw an increase in activity as the subjects processed the dot stimulus, peaking with the decision to move their eyes in the correct direction. The dynamics suggested that such decisions might be encoded in an intentional framework, in which making a decision is inseparable from the motion to execute it. In other words, both deciding on a Diet Coke and moving your arm to press the right button on the pop machine originate from the same brain region and activity.
But Freedman wondered if the real job of LIP was obscured by the simplicity of the eye movement task. Because of its quickfire look-and-respond nature, the experiment could give a false impression about the tightness of the link between decision-making and action. So Freedman designed a more complex experiment, where subjects first learned to group the movement of dots into two directional categories, and then were shown two different arrays of moving dots one second apart. If the movement of the two arrays belonged to the same category, the subjects released a lever; if they were from different categories, they maintained their hold.
“It’s a much more complicated, more cognitive task where the decision is not just based on the direction of stimuli but what they learned about them in the past,” Freedman said. “The decision is about what category the subject just saw, independent of what action that instructs for later. That’s how we dissociated the motor part from the sensory part.”
The more difficult experiment yielded an interesting result, discussed recently in Nature Neuroscience by Freedman and Assad. LIP activity increased after the first dot array stimulus was presented, and encoded the category to which it belonged. Furthermore, LIP activity was observed whether or not a motor response was required, since half of the “correct” responses required the subject to maintain a hold on the lever rather than a movement. So while the intentional framework may exist when snap judgments, such as evading a predator or oncoming car, are needed, the LIP contains the capacity to make and hold abstract decisions independent of movement.
The results also suggest decision-making may be tightly related to categorization – Freedman’s original area of interest – as the LIP forms categories and chooses between them. To make a choice, whether about soda, dinner, or guilt, our brains must first form categories to distinguish between options – and the parietal lobe may be where those categories are sorted out and decisions are made.
“It make us think what we’re seeing is more related to the rules the monkeys have learned, more related to monkeys making decisions between alternatives, which is something we do all the time,” Freedman said. “If I wanted to basically read the mind and only had one chance, I would chose to record from the parietal cortex.”
Freedman DJ, & Assad JA (2011). A proposed common neural mechanism for categorization and perceptual decisions. Nature neuroscience, 14 (2), 143-6 PMID: 21270782