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Every day, we face thousands of decisions both major and minor - from whether to eat that decadent chocolate cupcake to when to pursue a new romantic relationship or to change careers. How does the brain decide? A new study suggests that it relies on two separate networks to do so: one that determines the overall value - the risk versus reward - of individual choices and another that guides how you ultimately behave.

"Cognitive control and value-based decision-making tasks appear to depend on different brain regions within the prefrontal cortex," says Jan Glascher, lead author of the study and a visiting associate at the California Institute of Technology in Pasadena, referring to the seat of higher-level reasoning in the brain.

Study co-author Ralph Adolphs, a professor of psychology at Caltech, explains the distinction by way of a grocery shopping example: "Your valuation network is always providing you with information about what's rewarding around you - the things you want to buy - but also lots of distracting things like junk food and other items popping into your vision off the shelves."

Cognitive control is what keeps this network in check. "To be able to get to the checkout counter with what you planned, you need to maintain a goal in mind, such as perhaps only buying the salad you needed for dinner," says Adolphs. "That's your cognitive control network maintaining an overall goal despite lots of distractions."

Understanding how the brain parcels out specific decision-making tasks can offer insight into conditions in which such networks go awry, such as in the case of psychiatric disorders. Depressed people, for example, clearly have difficulty with value-based decision making: because nothing feels good or seems appealing, all options appear equally bleak and making choices becomes impossible. Hoarding disorder, in contrast, may involve overvaluation of certain possessions and impairment of the cognitive control needed to shift one's attention away from them. That explains why hoarding becomes more important than other life goals like maintaining relationships.

To tease out the distinct roles of these brain areas, the researchers analyzed data on nearly 350 people with damage, or lesions, in specific regions of the frontal lobes believed to be involved with particular tasks. Such studies of brain lesions are better at helping scientists understand cause and effect than imaging studies alone: if a damaged region is linked with impairment on a particular cognitive test, you know that task requires involvement from that region; with imaging studies, however, researchers can never be sure whether brain activity in certain regions is crucial to the task at hand, or whether it resulted from extraneous factors like a participant being distracted in the scanner.

For the new study, the authors used data that had been collected over more than 30 years by neurologists at University of Iowa, which has the world's largest lesion registry in the the world. The researchers found that damage to a region known as the dorsolateral prefrontal cortex (dlPFC) appears to profoundly affect cognitive control. The study suggests that the dlPFC forms a network with the anterior cingulate cortex, together keeping focus, switching it when appropriate to the task, and looking for erroneous choices in order to correct them. People with damage to the dlPFC had difficulty on cognitive tests that assessed their attention, including the ability to switch from one category of response to another or to rapidly identify the ink color of a printed word when that word spells out a different color (for example, the word red printed in green lettering).

Referring back to the grocery shopping example, Adolphs notes that people with damage here have great difficulty in stores. "They [will] get distracted by all the many choices available, and have a hard time steering a single course of action," he says, adding that being able to go to a mall and accomplish a few specific tasks has actually been used as a test of such brain damage.

The study found that another brain area, the ventromedial prefrontal cortex (vmPFC), evaluates risk and reward as part of a neural network that also includes the orbitofrontal and frontopolar cortex. Damage here is more insidious. In most respects, people with lesions in the vmPFC appear normal - their language skills, perception and overall intelligence are unimpaired - but their ability to balance rewards versus risks is skewed.

"A vmPFC patient would have trouble assessing the riskiness of different decision options, which can lead to very bad financial decisions," says Glascher, noting also that promiscuity, gambling problems and difficulty keeping a job can result. (One of the most famous patients in neurology, Phineas Gage, a railroad construction foreman who survived having an iron rod pass through his skull, but suffered personality changes afterward, was damaged in this region.) "The deficit is more subtle, but it can lead to quite dramatic consequences in everyday life," says Glascher.

Glascher explains that such patients tend to choose immediate gratification rather than delayed reward, and ignore risks when the reward seems large. Researchers test for damage in the vmPFC by using a card game known as the Iowa gambling task. Four virtual decks of cards are involved: some decks reliably produce more wins than losses, while others seem to offer big rewards at first, but result in loss over time.

In cognitively normal people, the brain's valuation network computes which decks are "good" and "bad" before people are consciously aware of why: they find they "like" the good ones better, even though they haven't consciously done the math to determine that they produce more wins. In people with damage to their valuation networks, however, such intuition fails. "This task mimics some of the complexities of real-life decision-making, with the result that you need to develop some kind of gut feeling for what a good choice and what a bad choice is," says Adolphs. "You can't just solve the task intellectually. Going with their gut feeling about a choice is precisely the thing that these patients have the most difficulty with."

The overall control of impulses is split between the two networks, though they do not counteract each other. When the cognitive control regions are working well, distractions are ignored and behaviors occur in the appropriate context; when valuation is appropriate, choices are made that are likely to be beneficial in the long run. However, says Adolphs, "when either one of them goes offline, impulsive behaviors get stronger and may not be inhibited."

"Both networks need to be in balance to function properly together," Glascher says. "If one fails, then other one gets out of step as well and problems are likely to arise."

Understanding the features of these networks could potentially aid treatment of a variety of psychiatric conditions. "Essentially all psychiatric disorders involve abnormal decision-making, and it is one of the core symptoms in addiction and eating disorders," says Adolphs, noting that prior imaging studies have linked changes in the brain's valuation and cognitive-control regions to those conditions as well.

Aside from helping those with brain impairments, a better understanding of how to improve the function of these regions could also potentially help cognitively healthy folks deal with the constant onslaught of distractions that typifies modern life.

The study was recently published in the Proceedings of the National Academy of Sciences.

Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland's Facebook page and on Twitter at @TIMEHealthland.