Last year, Walter Kaye and colleagues published a report that found women who had recovered from anorexia showed an altered reward response, which was demonstrated in difficulties distinguishing between positive and negative outcomes. In other words, someone with anorexia didn't show positive feelings when they "won" at a simple game: they only showed a lack of feelings that accompanied "loss." This blunted reward (as demonstrated by a lack of response in the anterior ventral striatum ) means that anorexics primarily strive to avoid negative outcomes, which is reflected in their obsessive, perfectionistic behavior*. In contrast, the recovered anorexic women showed an over-activation of the caudate nucleus, the area of the brain involved in planning future actions and evaluating long-term consequences.
One of the main neurotransmitters involved in reward response is dopamine, the so-called "feel good" chemical, and abnormalities in dopamine response has been found in people with binge eating behaviors. So Kaye and co. hypothesized that women recovered from bulimia would have an under-response of the caudate nucleus, the opposite of the recovered anorexics, as bulimia tends to be characterized by difficulties in impulse control.
Instead, the researchers found that the bulimic women, like the anorexics, were unable to distinguish a positive outcome from a negative outcome. Whether the women "won" or "lost" at a card guessing came, their brains responded the same. What distinguished this group from the anorexic women was that they did NOT show an over-activation of the caudate nucleus. Kaye et al. conclude the following:
A recent study reported that behavioral/motoric impulsivity is linked to binge-eating type eating disorders in general, but that the nonplanning dimension of impulsivity was only characteristic of BN individuals. Whether the different activity pattern finding in [recovered bulimic women] reflect some difficulty in foreseeing or integrating consequences is conjectural, but may offer important clues for understanding the biology this behavior. It is worth noting that our group previously found that [recovered anorexic] participants had elevated [caudate nucleus] activation in response to negative and positive feedback, perhaps reflecting symptoms of worrying about feedback and excessive need to control/plan consequences of their actions.
Both [recovered bulimic] and [recovered anorexic] individuals had elevated anxiety and harm avoidance scores, and neither group had altered novelty seeking or sensation seeking scores. In this respect, [recovered anorexics] and [recovered bulimics] appear to be similar. Still, novelty seeking scores were positively associated with [anterior ventral striatum] activation for the win condition within the BN group.
Although this study is still rather preliminary, it begins to provide some clues as to the neurobiology of bulimia.
*Um, hi, Dr. Kaye. Thanks for making me feel that you're looking inside my brain right this very instant! While you're at it, could you remind me where I put my car keys again? Oh, right behind that gob of earwax? Thanks. I knew they were around here somewhere...