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INTRODUCTION
Recent research and clinical findings indicate that motor and executive control systems may develop in a parallel manner. Both systems display a similar protracted developmental trajectory, with periods of rapid growth in elementary years and continued maturation into young adulthood (Diamond, 2000). In addition, development of each system is dependent on the functional integrity and maturation of related brain regions, suggesting a shared neural circuitry that includes frontostriatal systems and the cerebellum (Diamond, 2000; Pennington & Ozonoff, 1996; Rubia et al., 2001). Moreover, deficits in either system (executive or motor control) frequently present with coexisting deficits in the other; for example, approximately half of children with Attention-Deficit/Hyperactivity Disorder (ADHD) demonstrate problems with motor coordination (Carte et al., 1996; Denckla & Rudel, 1978; Kadesjo & Gillberg, 1998; Pitcher et al., 2003; Steger et al., 2001), and approximately half of children with Developmental Coordination Disorder manifest problems with attention (Kaplan et al., 1998).
A number of neurodevelopmental disorders, including ADHD and high functioning autism (HFA), are associated with executive dysfunction (Diamond, 2000; Pennington & Ozonoff, 1996; Roth & Saykin, 2004; Steger et al., 2001), although the patterns of expression appear to be different. While "cognitive" elements of executive dysfunction are often deficient in individuals with HFA (Kleinhans et al., 2005), there is more often relative sparing of response inhibition (Ozonoff & Jensen, 1999; Ozonoff & Strayer, 1997). Compared with measures of "cognitive" inhibitory control, inhibition of a motor response is the most direct expression of inhibitory control, as it involves all-or-none decisions about action or nonaction (Rubia et al., 2001). Motor inhibition refers to the ability to suppress a prepotent motor act in favor of a competing movement (Barkley, 1997). Children with ADHD have consistently displayed deficits on tasks of response inhibition, regardless of paradigm [i.e., cognitive (Barkley et al., 1992) or behavioral (Quay, 1997; Schachar et al., 1995)] or motor system assessed [i.e., skeletomuscular (Berlin et al., 2004; Mostofsky et al., 2003; Schachar et al., 1995) or oculomotor (Feifel et al., 2004; Mostofsky et al., 2001a, 2001b)]. In contrast, children with HFA have displayed generally intact inhibitory skills under demands for cognitively controlled response inhibition (Griffith et al., 1999; Ozonoff et al., 1994; Ozonoff & Jensen, 1999; Ozonoff & Strayer, 1997), whereas results have...