A Feast for the Eyes
As we approach the holiday season, more opportunities emerge for social engagement. Circuits of typical animal and human brains are wired to experience social engagement as rewarding. However for some of us, these engagements are not so rewarding and may even be aversive. Early disruptions in the circuitry that mediates social attention and reward can lead to the atypical social communications observed in autism spectrum disorders (ASD) and a surprisingly-related disorder anorexia nervosa.
Social reward seems to be different than other types of reward in ASD. It may be that social rewards uniquely tickle some reward circuits, but not others. However, clear differences between circuits subserving social versus other rewards have yet to emerge. Autism Speaks-funded postdoctoral fellow, Karli Watson, conducted a series of recordings in frontal brain regions of monkeys trained to tell how much they wanted to look at an image by trading drops of their favorite juice. In this way, Karli and her postdoctoral advisor, Michael Platt, both of Duke University, could compare different types of rewards—both social and nonsocial—in a common reference frame and relate the value of rewards to neural activity. A detailed understanding of the neural hardware that drives our response to social stimuli was sorely needed and only accessible in an animal model with sufficiently complex and human-like behaviors.
But Karli also wanted to get closer to understanding social attention in people. At that time, available clinical research opportunities in autism at Duke were few. A unique opportunity emerged from Nancy Zucker, a Duke colleague who studies anorexia nervosa. Zucker approached Watson and Platt with an idea.
Although most people would not consider anorexia nervosa (AN) and ASD as related, there are eerie parallels that have led some to suggest that AN might be a female analog of autism. Contrary to the strong male bias found in ASD, AN has a very strong female bias. Symptoms can appear suddenly, although later in development than ASD. Obsessions and rigidity in behaviors and preferences are common to both disorders. Atypical eating preferences are, obviously, defining for AN, but also quite common in ASD. Even more suggestive is a co-morbidity of ASD symptoms in individuals with AN and a clustering of both disorders in families. Recently at an IACC meeting during public comments, a woman raised the issue of ASD co-morbidity with eating disorders (videocast, beginning around minute 180).
Intrigued by these connections, Watson, Zucker, and Platt designed a study to analyze patterns of gaze when women with weight-restored AN viewed images of other women. Although the women had previously received a diagnosis of AN, their weights were restored to a healthy value for at least the past 9 months. The researchers observed the same result so often reported in ASD—individuals with AN spent less time looking at the eyes in the images. “Women with anorexia nervosa avoid the eye region when looking at faces and avoid the face region when looking at full body images. These results are similar to those you would expect in studies of participants with autism, and highlights the possibility that there may be an overlapping set of neural circuitry that is dysfunctional in the two disorders,” says Watson.
The research team also observed differences in reward value of different types of images using a similar how much would you pay to look at this image task previously used in their monkey research. Although mean attractiveness of the images were rated similarly by control subjects and women with AN, the women with AN found images judged to be unattractive much less valuable to view. They also greatly preferred to look at images from women who were underweight compared with typical or overweight women, again compared with control women. “This task allows us to measure, in an actual dollar amount, how much subjects will pay to look at different kinds of images,” explains Watson. “Whereas women with no history of an eating disorder will actually pay to view images of faces (think paying for a copy of People magazine at a newsstand), women who have had anorexia nervosa will not. On the other hand, women who have had anorexia nervosa will pay to look at full body images of emaciated women, but require a bribe to look at full body images of overweight women.”
The similarities in eye tracking findings between ASD and AN go beyond gaze on static images. Women with AN have previously been characterized as having difficulty in maintaining eye contact during interviews and also having difficulty reading emotion in faces, both characteristics commonly observed in ASD .
The next steps for the research are two-fold. The team plans to conduct a similar study while imaging the brain in individuals with AN. These data would permit comparisons of functional brain circuitry during social engagement between AN and ASD. The human research has also inspired the next experiments in non-human primates. Although creating an anorexic monkey is out of the question, the research team can compare responses to aversive taste stimuli with scenarios where animals choose to withdraw from social images. But perhaps most important, Karli wants increase public awareness of the link between AN and ASD, noting “an appreciation of symptom co-morbidity and family clustering will help with understanding both disorders”.