Eye tracking refers to the use of an external device to follow an individual's gaze and eye movement. Researchers use eye tracking for various studies, including studies of the visual system, cognition and psychology.
Eye tracking has long been used in human perception experiments. In 1977, psychologists at the University of Durham in England tracked the eye movements of three healthy adults as they looked at black-and-white photographs of human faces. They found that people tend to gaze mostly on 'core features' of the face ― the nose, the mouth and especially the eyes1.
Relevance to autism:
Eye tracking is particularly useful for autism for two main reasons. First, what an individual chooses to pay attention to is believed to be fundamentally different for individuals who have autism compared with typical controls — especially for socially relevant information and face processing.
In 2002, researchers at the University of North Carolina published the first eye-tracking study of individuals with autism2. They tracked the eye movements of five adults with autism looking at photographs of facial expressions, and found that they spend a smaller fraction of time looking at the nose and eyes than do controls.
In another famous study, researchers tracked the gaze of adults as they watched scenes from the movie Who's afraid of Virginia Woolf3. They found that individuals with autism watch distinctly different parts of a scene than do typical controls, preferring to look at the actors' mouths, or even at movements far in the periphery of the scene, as opposed to watching the actors' eyes.
Researchers have also used eye tracking to inform studies of theory of mind — the ability to understand others’ beliefs and intentions. Failing theory of mind tests is often associated with language deficits. However, individuals who have intact social cognition will still look at the correct location while failing to give the accurate answer4.
Eye tracking as a biomarker:
Eye tracking is especially useful for autism research because it can be used on young infants and children. This might allow researchers to develop biomarkers to diagnose autism at a young age, when early interventions are the most beneficial.
In a 2008 study, researchers tested Helen, a 15-month-old child, on two eye-tracking paradigms. In the first, she watched a video of a woman making friendly cooing noises. Unlike typical babies, Helen focused much more on the woman's mouth than on her eyes5.
The second paradigm tested Helen's interest in biological motion by using point-light animations of body movement. Typically developing babies prefer to look at a waving human figure rather than at the same animation presented upside-down. But Helen didn't make this social connection. She showed a preference for the right-side-up version of the patty-cake animation only when it was accompanied by clapping noises.
Researchers can also use eye tracking to monitor other measures, such as blink rate. Healthy toddlers refrain from blinking as they watch scenes with high emotional content, such as when the toddler-actors fight over a toy, according to a study presented at the 2010 International Meeting for Autism Research. Toddlers with autism, by contrast, are just as likely to blink during emotional scenes as during dull ones.
A 2011 study also showed that, unlike healthy controls, individuals with autism do not synchronize their eye blinks with those of other people6.