ADHD subjects fail to suppress microsaccades and eye blinks during anticipated stimulus presentation.

Background: ADHD is the most prevalent psychiatric disorder, estimated to affect about 10% of the population. Currently, the diagnosis is based mainly on subjective parameters, though there are several computer-based tools, such as TOVA, to support the diagnosis. However, no objective tool based on physiological measurements is available for diagnosing ADHD. It was previously shown that several ocular parameters correlate with attention. We thus aimed to search for objective markers of ADHD based on ocular parameters. Methods: Twenty subjects diagnosed with ADHD and 20 control subjects performed 2 sessions of TOVA, while their eye movements were tracked. ADHD subjects performed the first session un-medicated and the second session 1.5 hours after taking medication (Ritalin). Control subjects performed the second session 1.5 hours after the first, both without medication. We measured the average rates of eye blinks and microsaccades during the time interval -75 to 150 milliseconds relative to stimulus onset (peri-stimulus). Results: We found that un-medicated ADHD subjects have a significantly higher average blink rate than control group during the peri-stimulus interval; this average rate is significantly reduced with medication, but still remains significantly above the control rate. We also found that the average rate of microsaccades in ADHD group in the un-medicated session during the peri-stimulus interval is significantly higher than in the control group and that is reduced to the level of the control group after medication. Conclusions: These results suggest that un-medicated ADHD subjects fail to suppress both microsaccades and eye blinks during anticipating the stimulus presentation. We also show that medication increases the suppression capacity of ADHD subjects towards the normal range.