A research team has reported insights based on behavioral tests and mathematical modeling that may help explain some of the cognitive mechanisms that go awry in bulimia nervosa (BN).

BN is a disabling disorder characterized by recurrent episodes of eating that feel out of control. In BN, these eating episodes are followed by “compensatory” behaviors such as purging, intense exercise, or misuse of laxatives. BN is distinguished from binge eating disorder (BED), which involves recurrent out-of-control episodes of eating that are not followed by compensatory behaviors.

Both eating disorders can be difficult to treat.  It is estimated that 60% of those who receive first-line cognitive behavioral therapies for BN remain symptomatic. Hoping to find a path to more effective therapies, a team led by 2020 BBRF Young Investigator Laura A. Berner, Ph.D., of the Icahn School of Medicine at Mount Sinai, sought to learn more about neurocognitive mechanisms that may maintain BN symptoms.

The team was particularly interested in understanding how it might be that individuals with BN “get stuck” in their symptoms. Because our external environment and internal body states are constantly changing, our actions must be continually adjusted to match new circumstances. However, it has been proposed that over time, BN symptoms become insensitive to such changes and are automatic in response to particular cues, like strong emotions or the sight of certain foods. Therefore, the researchers aimed to measure how the process of adjusting behavior after a change may be altered in people with BN.

Since this process of adjustment involves links between cues, the behaviors they prompt, and resulting outcomes, the researchers tested two possible ways that entrenched symptoms of BN may be maintained: difficulty stopping a behavior after getting new information about the behavior’s outcome, and difficulty stopping a behavior after getting new information about a cue that precedes the behavior.

The researchers recruited 30 women with confirmed BN and 31 demographically matched healthy controls. The women, who ranged between 18 and 35, were, on average, 22 years old. Those with BN reported at least one bulimic episode and compensatory behavior per week for the prior 3 months at the time of recruitment for the study; self-induced vomiting was a compensatory behavior for all women in the BN group. The women were between 85% and 120% of the expected weight for their height based on standard actuarial data.

The behavioral task that participants completed was designed to help unravel a mystery that arises in many different forms of maladaptive behavior including drug addiction and gambling: Why do people continue to engage in behaviors that they have already learned is damaging their health, their social relationships, and/or their ability to function in society?  

In the task, participants first learn associations between cues, behavioral responses, and outcomes. In the final two subtests of the task, participants must repeatedly adjust their responses when different cues or outcomes are “devalued.” In one subtest, participants are told which outcomes are no longer valuable, so they should no longer respond to the cues that led to those outcomes. In the other subtest, participants are told which cues no longer lead to valuable outcomes, so they should no longer respond to those cues. Importantly, the cues or outcomes that are devalued switch many times within each subtest, so participants have to keep adjusting their behavior in light of new information so that they can correctly override their previously learned associations.

Previous studies using this task in other psychiatric conditions have only examined how many times participants incorrectly responded. Dr. Berner and her colleagues similarly examined the frequency of incorrect responses, but they also, for the first time, used mathematical modeling of participants’ behavior to quantify a potential underlying brain computation. This computational modeling approach provides a metric for how quickly participants update their beliefs about what the correct response is.

The researchers found that women with BN were just as able as the healthy control women to use feedback to learn associations among cues, responses, and outcomes. In addition, to the researchers' surprise, women with BN performed as well as healthy control women when they needed to adjust their behavior in response to new information about outcome.

A difference was noted, however, when the participants needed to adjust their behavior in response to new information about cues. Here, women with BN were not as able to make behavioral adjustments as the healthy women. The computational modeling approach revealed that women with BN also showed slower belief updating when cues were devalued. Put another way, women with BN were less able to flexibly update their beliefs when they were given new information about the value of the cues that they had learned about in the first phase of the task. These results suggested to the team an interpretation of participants’ behavior: their impaired ability to adaptively control their responses might be driven by slower belief updates.

The authors found that these deficits in flexibly updating beliefs to withhold responses to devalued cues were correlated with more frequent binge eating and purging. This led the team to propose, in their paper published in Translational Psychiatry, that “perseverative [i.e., repetitive] responses after direct instruction to override” previously learned stimulus-response associations might perpetuate BN symptoms. For example, these cognitive dysfunctions “could result in automatic bulimic behaviors in response to particular cues (e.g., binge foods, strong negative emotion) that persist despite efforts to respond in new ways to those cues,” (e.g., using skills learned in adaptive emotion-regulation therapy).

The results could have important implications for treatment of BN, the team said. “Because our findings indicate that slower updates of stimulus-response associations may underpin bulimic symptoms, more prolonged exposure to stimuli that powerfully evoke binge eating and purging, paired with response prevention, may be helpful for those with more severe BN.”  

In addition, treatments could capitalize on the fact that women with BN did not have difficulty flexibly inhibiting behaviors when they explicitly learned new information about outcome values, the team said.  Explicitly devaluing eating disorder symptom outcomes such as weight loss or thinness that BN patients tend to value, combined with learning to assign higher value to other outcomes such as better relationships, “may be particularly effective for BN,” the researchers suggested.