Watching the brain pick junk food over healthy food

Last year, we covered research that examined how well people's dietary intentions lined up with what they actually ate. It turns out that a significant portion of those with good intentions—those planning to eat healthy—end up eating unhealthy junk food instead. Of course, over half the study participants had no intention of ever trying to eat a healthy snack in the first place.

New research from a team at CalTech, published in the current edition of Science, has now examined the brains of self-proclaimed dieters as they are forced to make a decision about what foods to eat. The researchers used functional magnetic resonance imaging (fMRI) to look at a few specific areas of the participant's brain. The areas under study are hypothesized to play a key role in making decisions that balance tastiness and healthiness of various food choices.

The experiment was carried out in three parts. In the first two parts, each subject was shown fifty different foods and asked to rate them first on how good they thought each would taste and how healthy they felt each was. From these two measurements, the researchers picked an "index" food, one that rated right in the middle of both taste and healthiness for that participant. The subject was then shown each of the fifty foods again, but asked to rate how much they would like to eat it relative to their "index" food item, using a five point scale.

The team hypothesized that value decisions—the amount one would like to eat a given food item—would arise in an area of the brain known as the ventromedial prefrontal cortex (vmPFC). They also hypothesized that this raw value signal would be modulated by the dorsolateral prefrontal cortex (DLPFC), with the DLPFC providing a degree of self-control.

Since all three tests were carried out inside an fMRI machine, the researchers were able to monitor the blood-oxygen levels in specific areas of the brain; this signal is used as a proxy for brain activity. They were able to identify two distinct groups of individuals: those who exhibited a high level of self-control, and those who exhibited a low level of self-control.

The authors expected that, for the 19 participants who were identified as self controllers, decisions on which foods to eat were made on the basis of both taste and health. Those in the low self-control group (18 in total) made their decisions on the basis of taste alone.

Analysis of the fMRI data suggested the authors' hypothesis was correct. Those who made good decisions and employed self-control exhibited more activity in the DLPFC region of the brain, where as the amount of activity in the vmPFC was similar in both groups. According to the lead author of the paper Todd Hare, "the vmPFC works during every decision. The DLPFC, on the other hand, is more active when you're employing self-control."

Based on their findings, the authors speculate on the evolution of our brains and the nature of self-control. They argue that the vmPFC originally evolved to predict or forecast the "short-term value of stimuli," and that humans gained the ability to examine the long-term considerations as structures such as the DLPFC evolved to modulate the short-term desire signal. While the experiments were limited to diet choices, the authors state that understanding the origins of self-control can have implications for areas as diverse as addiction science, economic policy, and even into determining whether someone is in full command of their decision-making facilities under the eyes of the law.