Abnormal perceptual functions in attention deficit hyperactivity disorder (ADHD).

Background: Visual crowding, collinear facilitation, and contour integration are visual integration functions (VIF) that are critical for perceptual and cognitive processing. Our recent results support a common early neural basis for the development of VIF in young healthy children. The results show that there is a developmental cascade in which the existence of high levels of crowding in young children limits the developmental onset of collinear facilitation until about the age of 6 years when the crowding decreases to nearly an adult's level. Similarly, the onset of the contour integration skill developed after the maturation of collinear facilitation, between 6 and 9 years. Since development of some of the cognitive functions overlaps with this age period, we hypothesized that the development of VIF in ADHD subjects might be affected. The aim of this study was to explore whether VIF in ADHD subjects during the age of 6-18 years is normal. Results: We found a high degree of crowding, higher thresholds for contour integration, and reduced collinear facilitation in the ADHD group compared with the age-matched control group. We also found that their thresholds do not improve with age and remain immature, matching the threshold levels of younger children. Conclusions: Contrary to the visual processing in young normal children, which improves with age, the neural mechanisms underlying global perception are immature in ADHD individuals. These results may suggest that abnormal development contributes to the perceptual processing in ADHD, which in turn may limit the development of some higher cognitive processing.