Category learning difficulties in ADHD across modalities and multiple learning systems.

Attention-deficit/hyperactivity disorder (ADHD) has been associated with suboptimal functioning of both the prefrontal cortex and the striatum. These abnormalities may impede the acquisition of perceptual categories, important for fundamental abilities such as object recognition and speech perception. While prior research has shown that children with ADHD perform comparably to neurotypical peers in visual category learning despite using suboptimal strategies, much remains unknown about how adults with ADHD acquire perceptual categories, where more mature functions may shape learning processes differently than in childhood. To address this gap, we investigated auditory and visual category learning in adults with ADHD compared with neurotypical controls. Specifically, we focused on two types of category structures: rule-based categories, which are believed to rely on hypothesis-testing mechanisms mediated by the prefrontal cortex, and information-integration categories, thought to depend on reinforcement learning processes governed by the striatum. Our findings revealed consistent impairments in both rule-based and information-integration category learning among adults with ADHD across sensory modalities. Furthermore, category learning performance was negatively associated with ADHD symptom severity. Computational modeling analyses showed that individuals with ADHD were slower to adopt optimal learning strategies than their neurotypical counterparts, regardless of the category type or sensory modality. These findings point to disruptions in multiple learning systems in young adults with ADHD that extend across sensory modalities and arise from impairments in domain-general mechanisms.