Summary

While parietal lobe function in ADHD has not been extensively studied, some evidence does exist for possible right parietal dysfunction in children with ADHD (Aman et al., 1998). There is stronger evidence for deficits in frontal lobe function in children with ADHD, who are fairly consistently found to have difficulties with aspects of task performance thought to be mediated by the frontal lobes, including motor control, problem solving, formulating and testing hypotheses, using feedback to modify responding, organizing responses, and adhering to task constraints (Barkley, 1997; Barkley & Grodzinsky, 1994; Boucugnani & Jones, 1989; Gorenstein et al., 1989; Pennington & Ozonoff, 1996; Shue & Douglas, 1992). This combination of deficits is similar to that found for patients with frontal lobe lesions and suggests that ADHD may be associated with comprehensive frontal lobe deficits in planning, hypothesis testing and inhibitory control (Shue & Douglas, 1992).

One of the most consistent findings from neuropsychological studies is that of deficits in the executive function of response inhibition in children with ADHD, providing compelling evidence that ADHD involves impaired behavioural inhibition (Barkley, 1997; Barkley & Grodzinsky, 1994). However, inhibitory deficits may not be sufficient to explain the range of executive dysfunction in children with ADHD, who are also found to have difficulties in problem solving, effective use of feedback, and generation and use of strategies (Shue & Douglas, 1992). The integrative function of the frontal lobes may be impaired in ADHD as it is in patients with frontal lobe lesions, and impairment of higher order cognitive processing may result from difficulties in integrating information (Shue & Douglas, 1992). Alternatively, these other deficits may arise from the influence of deficient inhibition on other executive functions as suggested by Barkley (1997).

While the majority of studies do find executive function deficits in children with ADHD, there are conflicting findings for most of the frontal lobe tasks that have been used (Pennington & Ozonoff, 1996). Some of the inconsistencies in the results of neuropsychological studies of ADHD may be due to methodological differences such as selection criteria and type of tests used, or to comorbidity and heterogeneity of ADHD subject groups (Barkley & Grodzinsky, 1994; Seidman et al., 1995a).

These inconsistencies in methodology and results also plague the literature on studies of attention in ADHD. While many studies find that children with ADHD perform poorly on attentional tasks, it is generally concluded that this poor performance cannot be explained by attention deficits (Barkley, 1997; Schachar, 1991; Swanson et al., 1990; Van der Meere, 1996). Studies that have isolated various aspects of information processing have failed to find deficits in orienting of attention, encoding of information, selective attention or divided attention, but have found deficits in motor processes (Sergeant & Scholten, 1983,1985a,b; Van der Meere & Sergeant, 1987, 1988c). These findings suggest that the inattention that is characteristic of ADHD may also be related to deficits in inhibition and self-regulation (Barkley, 1997; Van der Meere, 1996). Children with ADHD fairly consistently demonstrate performance deficits on continuous performance or vigilance tasks in terms of slower and more variable reaction times and increased errors of omission and of commission. These findings have been interpreted as reflecting concentration problems (Corkum & Siegel, 1993; Oades, 1998), compromised regulation of effort or activation (Dinklage & Barkley, 1992; Van der Meere, 1996; Van der Meere & Sergeant, 1988), and poor inhibition (Barkley, 1997). So, executive function deficits may also be related to the poor performance of children with ADHD on the CPT. This is further supported by evidence that the frontal lobes are involved in vigilance tasks (Pardo et al., 1991).

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