Who amongst us hasn’t experienced the exasperation and heartbreak of suffering through punishing diets and staring at the unmoving dial on the bathroom scale? Gina Kolata, the eminent New York Times science journalist, has been looking for evidence for throwing in the towel: The body’s composition, especially the proportion of fat, is just out of our control. Don’t blame the overweight and obese for lack of self-control; brain and metabolic regulatory mechanisms are just too powerful to resist over the long term. But wait, before you feel free to stuff your face with this juicy super-size hamburger, consider this.
The neurobiology self-control
Behavioral studies have shown that self-control is exhaustible in the short term (explaining the phenomenon of dieting failures), can be enhanced by cognitive strategies (so no reason to declare defeat), and is correlated with measures of intelligence (Aha! Is that why physicians and scientists are rarely obese?). But hard-headed scientists insist on “hard” evidence; behavioral studies are susceptible to many “confounding factors”, which make unequivocal interpretation very difficult. Now, we have this rock-solid evidence.
The May issue of Science magazine reported on research out of Caltech which identified the anatomical location of self-control in decision-making. They went about it in a clever way. They scanned the brains (using fMRI) of a group of 37 dieting volunteers while they performed two tasks. First, they rated 50 different food items for taste and health separately. On the basis of these rating, the investigators could select a reference item for each subject that was rated neutral in both taste and health. For the second task, the subjects were asked to choose between each of the foods and the reference item. Participants also indicated the strength of their decision by using a five-point scale (strong no, no, neutral, yes, and strong yes), which provided a measure of the relative value, or the amount of expected reward associated with consuming the food, instead of the reference item. Based on their decisions, 19 of the participants were classified as self-controllers (SC) and 18 as non-self-controllers (NSC). There was a stark difference between the two: SC made decisions based on both health and taste, rejecting most-liked-but unhealthy items, whereas the NSC group made decisions on the basis of taste alone.
What did the scans show? It is well known that the prefrontal cortex (PFC) is the seat of the “executive center”—the site where emotional and cognitive signals come in, get processed, and a decision is then made. This study refined the picture quite a bit. A specific area in the PFC, called the ventromedial (or lower middle) prefrontal cortex (vmPFC), lit up in the fMRI scans. Activity in this area correlated with participants’ expectations of reward regardless of whether or not they exercised self-control. Furthermore, the activity in the vmPFC reflected the health ratings of the SC group but not of the NSC group. But, you may rightly observe, this still does not add up to self-control. Indeed, another area in the PFC, the dorsolateral (or upper outer) prefrontal cortex (DLPFC) became significantly more active during successful self-control than failed self-control trials. In other words, the vmPFC made a decision regarding the choice of food, but the DLPC modulated this decision by exerting self-control. It is the stern parent telling us not to eat the chocolate cream cake because it’s “not good for you”.
But if the DLPFC exerts functional control over the vmPFC, there should be some functional connectivity between the two. Indeed, during successful self-control trials (i.e., high DLPFC activity), the activity of the vmPFC was reduced. You might say that in situations of successful self-control, the DLPC becomes predominant and the vmPFC assumes a diminished role. Interestingly, the same observation was made in gamblers who chose not to gamble in losing conditions.
The implications of this elegant study are wide and deep. The DLPFC areas identified in this study are similar to areas that are involved in cognitive control and in emotional regulation. But lest we absolve ourselves of any responsibility, these findings don’t mean that the DLPFC is immutable; thanks to the plasticity of the brain, it is susceptible to behavioral strategies that can enhance or reduce its activity. So, this brings us back to the argument that dieting is futile; that biology will defeat our most valiant efforts. We now know that “the devil made me do it” is simply not acceptable even on biological grounds.
This study is focused on dieters’ self-control. But the implications for drug addicts are obvious. In fact, many behavioral and neurobiological attributes of drug addiction are shared by compulsive eaters. And the many strategies offered to control both can now be evaluated by their effect on the activity of the DLPC.
Finally, one cannot ignore the implications of this study on the current political climate. We have more than our share of nut cases who would act impulsively on their political convictions. In a New York Times column today, Tom Friedman compared the superheated atmosphere surrounding our President with the atmosphere in Israel preceding the assassination of prime minister Rabin by a right wing religious fanatic. How discouraging it is to see Republican leaders (Michael Steele and Bill Bennet, the morals czar) jumping to the defense of these nuts and labeling (I’m not making this up) Tom Friedman as nutty. My guess is that if anybody bothered to run a fMRI on the nuts that are pushing a military coup on Facebook and are running a poll whether or not President Obama should be assassinated, they will find them totally lacking in self-control and with an atrophied DLPFC. Where are the supposedly enlightened leaders who are expected to modulate rather than exploit this outrageous out of control behavior?