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INTRODUCTION
Attention Deficit Hyperactivity Disorder (ADHD), Combined Type, is an externalizing disorder characterized by inattention, disinhibition, impulsivity, and excessive motor activity (Goodyear & Hynd, 1992). These motor symptoms are evident through developmentally inappropriate levels of fidgetiness, difficulties staying seated, inappropriate/excessive movement, difficulties waiting one's turn, and excessive manipulation of objects (Barkley, 1997a). Anecdotal and observational evidence indicates that children with ADHD have great difficulty restricting their behavior to conform to instructions/rules and with deferring gratification/resisting temptation (Barkley, 1997a). In addition to demonstrating higher activity levels, these children also exhibit qualitatively more inappropriate and intrusive motor behaviors (Porrino et al., 1983). Hyperactive children appear less able to inhibit their motor activity when asked to do so (Ullman et al., 1978), and demonstrate more off-task behavior, out of seat behaviors, and forbidden "touching of objects" than non-hyperactive children (Barkley, 1991, 1997a). Given the above-noted symptoms, some theorists have proposed that ADHD is uniquely characterized by overactivity and difficulties with motor regulation (Halperin et al., 1992), with these symptoms distinguishing ADHD from other psychiatric conditions.
It has been proposed that some of the challenging behaviors seen in children with ADHD closely resemble those seen in individuals with documented frontal lobe pathology (Barkley, 1997a; Chelune et al., 1986; Goodyear & Hynd, 1992). Neuropsychological investigations have supported a "frontal lobe hypothesis" of ADHD (Barkley, 1997a; Chelune et al., 1986; Gorenstein & Mammato, 1989; Grodzinsky & Diamond, 1992; LaPierre et al., 1995; Pennington & Ozonoff, 1996; Shue & Douglas, 1992). In addition, neuroimaging investigations have identified abnormal metabolism within frontal-lobe systems in children with ADHD (Sieg et al., 1995; Zametkin et al., 1990, 1993), and both structural and functional neuroimaging studies have suggested dysfunction within frontal-striatal systems in these children (Alexander et al., 1990; Casey et al., 1997; Casey, 2001; Cummings, 1993; Groenewegen et al., 1997; Lou et al., 1989; Roeltgen & Schneider, 1991; Tannock et al., 1989; Zametkin et al., 1993) .
Frontal-striatal circuits, including basal ganglial-thalamocortical pathways running from the prefrontal cortex through the basal ganglia and thalamic nuclei, have been associated with the voluntary control of motor activity and the suppression of inappropriate action/behavior (Kroptov & Etlinger, 1999). Statistically significant correlations between behavioral inhibition and frontal-striatal volumes have been obtained in children with ADHD (Casey, 2001), suggesting that...