Close observation of tiny movements in the eyes may help researchers better understand and perhaps eventually improve assessment of attention-deficit/ hyperactivity disorder (ADHD), according to a new study published in the journal Psychological Science.
Growing evidence suggests that small involuntary eye movements, called saccades and microsaccades, are promising new tools for revealing the underlying mechanisms of mental processes like attention and anticipation — cognitive processes that are often impaired in individuals with ADHD.
The researchers believe that carefully tracking these eye movements may offer a new method for empirically monitoring temporal expectation (an expectation with respect to the timing of a certain event) in people with ADHD.
“The eye is restless and eye movements occur constantly, even when observers try to avoid them. Our study shows that this continuous stream of eye movements is temporarily paused before an anticipated visual event,” says psychologist and neuroscientist Dr. Shlomit Yuval-Greenberg, Assistant Professor at Tel Aviv University, senior author of the study.
“This attenuation in eye movements can be used as an estimate for whether and when the occurrence of regular events was indeed predicted.”
The researchers discovered that neurotypical individuals (those without a diagnosis of ADHD) tended to have different patterns of eye movements compared with individuals with ADHD.
“We found that individuals with ADHD tended to not attenuate their eye movements before a predictable event, which suggests that they were not able to predict the event and/or to act upon predictions,” said Yuval-Greenberg.
The findings suggest that careful analysis of eye movements may offer a complimentary and objective measure for ADHD diagnosis and treatment.
For the study, the research team collected data from a group of 20 individuals who had an ADHD diagnosis and a group of 20 controls without ADHD. The ADHD participants were asked to refrain from taking any ADHD-related medication for 24 hours prior to the testing sessions.
On two different days, participants were shown a series of colored shapes on a screen while the researchers monitored their eye movements. They were instructed to press a key whenever they saw a red square (which appeared around 25 percent of the time).
On one day, the shapes were shown at predictable intervals (every two seconds the next shape would appear). On the other day, the time between shapes varied from one to 2.5 seconds. Participants did not know that the timing would be different between the two sessions.
When the shapes appeared at regular, predictable intervals participants without ADHD responded more quickly than when they appeared at varied intervals. However, the reaction times of ADHD participants did not improve under predictable conditions.
The research team also found that those in the control group tended to have fewer eye movements immediately before a predicted event. In contrast, ADHD participants did not show the same eye movement slowdown in preparation for an upcoming stimulus.
One surprising finding was that an ADHD diagnosis was not the best predictor of a person’s ability to stay focused on the task.
“It is well documented that ADHD is a heterogeneous disorder. It is also documented that only some of the individuals with ADHD experience difficulties in maintaining focused attention throughout a monotonous task,” Yuval-Greenberg says.
“Yet we were most surprised to reveal that the individual ability to stay focused throughout the task was a better predictor for the attenuation of eye movements than whether or not that individual was diagnosed with ADHD.”