ADHD and perceptual learning: insights from the intermixed-blocked effect

Perceptual learning, which is typically considered an implicit and automatic process, has received little attention in the study of attention deficit hyperactivity disorder (ADHD). In this study, we investigated whether children and adolescents with ADHD show intact perceptual learning using the intermixed-blocks effect, whereby alternating exposure to similar stimuli improves subsequent discrimination compared to block exposure. Eighty-eight participants aged 8 to 16 years (44 with ADHD and 44 controls) completed a flower counting task while incidentally exposed to two types of visually similar flowers that differed only in the number of petals, without being informed of this distinction. In the interleaved condition, the two types of flowers were alternated within each screen; in the blocked condition, only flowers of one type were presented during the first half of the task and all flowers of the other type during the second half. Discrimination was assessed using a no-prior-warning same-different test. The results showed a robust intermixed-blocked effect in both groups, indicating that implicit learning is preserved in ADHD. However, while control participants in the intermixed group often identified the specific distinguishing feature, participants in the same condition but with ADHD more frequently failed to do so, despite correctly judging the stimuli as different. This dissociation suggests that implicit perceptual learning is preserved in ADHD, but that explicit access to learned information may be impaired. These findings highlight the value of perceptual learning paradigms for unravelling automatic and controlled processes in neurodevelopmental conditions.