ADHD Pupils May Reveal Shifting Attention Rhythms

The eyes are often described as windows to the soul. Researchers now think they could also provide a view of something rather different: the tempo of our attention.

The signal may not lie in the direction of our gaze. Instead, it could be concealed in minute pupil changes while the brain attempts to recall information, block out a distraction or reach a decision.

ADHD and the changing rhythm of attention

A new study indicates that this rhythm could be subtly altered in children with attention deficit hyperactivity disorder (ADHD).

ADHD is a neurodevelopmental disorder marked by ongoing difficulties with attention, hyperactivity or impulsivity. Attention, however, is not an on-off switch: it may remain stable at one point and drift at another.

No individual test can diagnose ADHD. Diagnosis draws together medical evaluations, personal history and observations of behaviour across a range of settings.

Researchers at the Indraprastha Institute of Information Technology Delhi asked whether pupils might contribute another detail to that overall picture.

The researchers reanalysed existing eye-tracking records from 50 Chilean children aged between 10 and 12. Of these, 28 had combined-type ADHD and 22 had no neurodevelopmental diagnosis.

The children completed what was effectively a memory game. They saw dots appear on a grid and were asked to remember where they were located. Once a distracting image briefly appeared on screen, a further dot was shown. The children then decided whether it was in a position they had previously seen.

They carried out the task 160 times, as an eye tracker recorded pupil diameter 1,000 times per second.

Pupils do not simply adapt to changing light. Their size can also shift with effort, arousal, tiredness and engagement.

Earlier studies have frequently examined average pupil size. This research team instead investigated whether each child's pupils reacted consistently when faced with the same challenge.

"Looking at how pupil responses vary from trial to trial may provide information that is missed when we only look at average pupil size or behavioral performance," first author and computer scientist Saumya Yadav told ScienceAlert.

Pupil variability in children with ADHD

Having adjusted for age, IQ and task difficulty, the researchers' statistical model calculated roughly 23 percent more trial-to-trial variation in pupil responses among unmedicated children with ADHD than among the controls.

The result depended on the way variation was assessed, however. Pupil responses showed greater differences from one separate trial to another, but less fluctuation during each individual trial, in the unmedicated ADHD group. This implies that the two measures reflect distinct elements of pupil dynamics.

Within the unmedicated ADHD group, children with more variable pupil responses also generally gave less accurate answers. Their pupils seemed to reflect changes in attention or engagement.

This does not mean that an unsettled pupil necessarily signals an unsettled mind. Pupil size is influenced by numerous factors, so it is only an indirect measure of attention.

Another element of the experiment nevertheless made the finding more interesting.

Seventeen children with ADHD performed the task on two occasions: once after stopping methylphenidate for 24 hours, and once after taking their normal medication.

When they were medicated, their pupils reacted more strongly during the task and varied less between rounds. The general form and timing of these responses also more closely resembled those in the control group.

It might be appealing to conclude that the medication had 'normalized' their attention. The researchers caution that this conclusion cannot be drawn.

Methylphenidate can itself act directly on the physiological systems that affect pupil size. The experiment was unable to determine whether the difference resulted from better attention, the drug's direct effects on the body, or a combination of both.

"We would not interpret this as evidence that medication 'normalized' attention," Yadav said.

"We could not completely separate these pharmacological effects from cognitive effects."

The researchers lacked information on each child's dose and drug formulation. The two testing sessions were also, on average, around six months apart, meaning maturation, practice or other effects linked to the time period could have affected the findings.

Why pupil measurements cannot diagnose ADHD yet

There was a further reason to be careful. Although a model using thousands of trials found increased variability, the direct comparison of individual participants fell just short of statistical significance after adjustment for multiple tests.

Higher pupil variability was associated with poorer accuracy in the unmedicated ADHD group, but this association was not significantly stronger than the comparable pattern observed in the controls.

"It is still too early to consider pupil measurements as a clinical diagnostic tool," Yadav emphasized.

The study was limited in size, relied on existing data and used a tightly controlled laboratory task. An everyday classroom would be considerably more complicated.

Bigger independent studies conducted over longer periods would be required to establish whether these pupil patterns remain dependable among different people and in everyday settings.

If they do, eye tracking would not take the place of doctors, interviews or behavioural assessments. Rather, it could support them by illustrating how an individual's attention changes with time or reacts to treatment.

At present, the eyes cannot diagnose ADHD.

They may, though, expose something that is ordinarily unseen: attention moving, diminishing and returning from one moment to the next.

The study was published in Scientific Reports.

This article was fact-checked by Peter Dockrill and edited by Peter Dockrill. While we take pride in our process, we are only human. If you notice an error, please let us know.