Tracking Eye Movements Can Measure Chronic Desire to Overeat

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by Steve Bryson PhD |

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Tracking the eye movements of children with Prader-Willi syndrome (PWS) may be a non-invasive, low-cost, and reliable method to assess their chronic desire to overeat, known as hyperphagia, a study suggests.

The study, “Eye tracking as an objective measure of hyperphagia in children with Prader‐Willi syndrome,” was published in the American Journal of Medical Genetics.

One of the hallmarks of PWS is an insatiable appetite that leads to hyperphagia and obesity.

The primary source of information about hyperphagia comes from questionnaires which, while specifically focusing on symptoms, are an indirect measure and can be subject to bias. As such, more objective ways of measuring hyperphagia are needed to help support research and clinical trials. 

Eye tracking is a non-invasive way of measuring specific physiological processes. Research has shown that stimuli in the visual field that attract one’s attention are linked to a higher number and longer duration of eye gaze fixations, or looking at a single point. 

A recent food-based eye tracking study investigating children and adults with PWS found an increased number of gaze fixations and repeated gazes at the same item (perseverations) was associated with higher (more severe) hyperphagia scores, as reported by caregivers. Notably, this was only observed when food items were presented along with animals, given that people with PWS have a well-known strong desire to care for babies and animals.

In the new study, researchers at Vanderbilt University Medical Center, along with investigators at Case Western Reserve University and Istanbul Medipol University, in Turkey, designed an eye-tracking study to test its ability to measure hyperphagia in young children with PWS, ages 3–11. 

As hyperphagia develops gradually over a wide age range, examining gaze patterns in response to food may be a sensitive way to determine hyperphagia-related changes, the scientists said.

A total of 57 children were recruited along with 47 typically developing children as a comparison group. While there were no differences in age or body mass index (BMI) between the two groups, children with Prader-Willi had lower IQ, as assessed with the Kaufman Brief Intelligence Test-2.

Hyperphagia was assessed using the 9-item Hyperphagia Questionnaire-Clinical Trials filled out by parents, which measures hyperphagic behaviors, drive, and severity — including persistence in food-seeking and reactions to food restriction. Responses were based on behaviors across different environments, such as home, school, and the community, during the two weeks before the assessment. 

In addition, parents in the PWS group completed a survey to determine their child’s nutritional phase — ranging from failure to thrive to full hyperphagia. 

Color images of common foods, animals (in non-aggressive poses), and household objects served as the visual stimuli.

Eye movements were tracked using software that followed the reflections from the front part of the eye, known as the cornea. No head-mounted equipment or physical restraints were used. The procedure took, on average, around 30 seconds and was repeated as needed. 

Children with PWS explored a significantly lower number of items and had shorter fixation times than controls.

Among the PWS group, boys had longer fixation times than girls on food items compared to household objects. Only girls with PWS spent more time fixated on high-interest items such as animals than on low-interest items such as household objects.

Compared to controls, the PWS group showed less gaze returns (revisiting items) to both food and non-food items, and revisited household objects less often than food.

A comparison of PWS genetic subtypes found that children with deletion on paternal chromosome 15 had shorter fixations than children with maternal uniparental disomy (mUPD), in which both copies of chromosome 15 are inherited from the mother.

Finally, the team examined the associations among eye-tracking measures and IQ, hyperphagia scores, BMI, and nutritional phase. 

PWS children in the later nutritional phase and those with more severe hyperphagia explored a higher number of food items, and had longer fixation times and a greater number of gaze returns.

For participants with PWS caused by chromosomal deletion, eye-tracking measures correlated with high-interest items (animals), while in children with mUPD such measures correlated with exploring food versus household objects.

“The results indicate that gaze characteristics derived from eye tracking may be a promising objective marker of hyperphagia in PWS for use in research and clinical trials,” the scientists wrote. 

“Increasing accessibility of eye tracking equipment in combination with the current evidence of successful paradigm implementation across research groups and testing settings make gaze data a cost-effective measure for use in future treatment studies,” they added.