Reaction Time Variability in Children With ADHD Symptoms and/or Dyslexia

Reaction time (RT) variability on a Stop Signal task was examined among children with attention deficit hyperactivity disorder (ADHD) symptoms and/or dyslexia in comparison to typically devel-oping (TD) controls. Children’s go-trial RTs were analyzed using a novel ex-Gaussian method. Children with ADHD symptoms had increased variability in the fast but not the slow portions of their RT distributions compared to those without ADHD symptoms. The RT distributions of children with dyslexia were similar to those of TD-controls. It is argued that variability in respond-ing may be underpinned by impairments in response preparation or timing during Stop Signal tasks. Attention deficit hyperactivity disorder (ADHD) is a prevalent and debilitating neurodevelopmen-tal disorder characterized by difficulties with attention, hyperactivity, and impulsivity (American Psychiatric Association, 2000). A deficit in response inhibition has been identified as one of several possible endophenotypes of ADHD (Castellanos & Tannock, 2002; Doyle et al., 2005) and current models of ADHD emphasize impaired response inhibition as either the core deficit in ADHD (Barkley, 1997, 1999; Quay, 1997) or as one of three independent pathways to the disorder (Sonuga-Barke, Bitsakou, & Thompson, 2010)