Seeking cognitive markers of specific language impairment and attention-deficit/hyperactivity disorder

Specific Language Impairment (SLI) and Attention-Deficit/Hyperactivity Disorder (ADHD) frequently co-occur. Surface similarities (e.g., not “listening”) can complicate discriminating between them. However, underlying basic processing dysfunctions may differentiate these disorders. Promising candidates from the literature included impaired processing of brief acoustic information (rapid temporal processing, RTP) or generalized slowing in SLI, and impaired inhibition or extremely variable response speed in ADHD. However, the uniqueness of these dysfunctions to each disorder was unclear.; Using key design elements not well-considered in past research, primary objectives were to (1) clarify the uniqueness of dysfunctional RTP and generalized slowing to SLI by excluding ADHD from a sample of children with SLI (Study One), confirming these dysfunctions in SLI (Study One), and examining these processes in children with ADHD (Study Two), and (2) clarify the uniqueness of impaired inhibition and extreme response variability to ADHD by excluding SLI from a sample of children with ADHD (Study Two), confirming these dysfunctions in ADHD (Study Two), and examining these processes in SLI (Study One).; In Study One, fourteen 7–11 year olds who had receptive-expressive SLI without concurrent ADHD and 14 children with typical development (TD) were administered measures of language and behaviour, and performed experimental tasks targeting the four candidate processes. In Study Two, fourteen 6–11 year olds who had ADHD without concurrent SLI and 14 children with TD participated in an identical protocol.; As predicted, only children with SLI differed significantly from children with TD in RTP. However, some individual children with ADHD and TD had substantial RTP impairments. Unexpectedly, both children with SLI and children with ADHD were slow on some speeded tasks. As predicted, only children with ADHD showed extreme response variability. Unexpectedly, only children with SLI differed significantly from children with TD in inhibition, although many individual children with ADHD showed impaired inhibition.; Findings suggested that further pursuit is warranted of extreme response variability as a cognitive marker of ADHD; the role of RTP impairments in SLI, ADHD, and TD requires further examination; process-specific slowing and poor inhibition may be present in both SLI and ADHD; and these disorders may not be fully dissociable.