Pupil size correlates with stress levels, researchers say—and may one day be used to prevent mistakes on the job.

One of the most frustrating things about mental health issues is that they are often invisible: Unlike a physical injury or ailment, it’s usually not possible to tell—just by looking at someone—if he or she is depressed, or mentally ill, or dealing with an unhealthy level of stress.

Researchers at the University of Missouri want to change that, and they say they’ve come up with a physical way to measure one of the most ubiquitous mental health problems in today’s society: stress in the workplace. According to a new study in the International Journal of Human-Computer Interaction, pupil size can be a good indicator of how overwhelmed people are when they have to multitask on the job.

“Basically, there are many ways you can measure physical workload,” said study author Jung Hyup Kim, PhD, assistant professor of industrial and manufacturing systems engineering, in a video interview on the university’s website. “Nowadays, there are many people suffering from mental stress, but there’s no objective way to measure the mental workload.”

Kim and his colleagues knew that a human’s pupils respond to changes in the environment by changing size. They grow and shrink based on the amount of light available, but they also react to unexpected sounds and other external stimuli. Pupils also change in response to the cognitive processes going on inside a person’s head, as well.

The researchers wondered if pupil response correlated to mental workload—a variable similar to attention—in a multitasking setting, so they designed a simulation that mimicked an oil and gas refinery plant control room. Participants were asked to complete work-related tasks while responding to unexpected changes (such as alarms going off around them), and eye-tracking technology was used to monitor them as they worked.

The researchers noticed that as the work the participants were asked to do got more complex, their eye behaviors became more erratic. They also discovered a negative relationship between the dimension of pupil dilation and the person’s mental workload. In other words, the more stressed they became, the smaller their pupils were.

It’s not a huge surprise that multitasking would raise stress levels: There have been plenty of studies that suggest that when people try to focus their attention on several things at once, it usually backfires. But the fact that researchers found a way to actually measure this effect—and in a fairly unobtrusive way—could be a big deal for science, and for businesses as well.

It will likely be several years before a tool like this is available for the general public, Kim says, but he’s hopeful that he and his colleagues will be able to develop a low-cost, portable eye-tracking device that can be used in many different workplaces—including industrial settings, office buildings, 911 dispatch centers, and air-traffic control towers.

Their goal is for employers to one day be able to use this technology to create safer and more productive work environments. “This is a breakthrough,” Kim tells Health. “If we can measure the workload objectively, we can design better working schedules and optimize their tasks so that workers aren’t getting tired or overwhelmed. And if we can monitor their well-being, we can hopefully prevent mistakes from happening.”

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