Performance on more complex attentional tasks, those that require the self-allocation of attentional resources, divided attention, and selective attention, is impaired in PDD (73,74). As the disease progresses, patients with PDD may show difficulty even on those attention tasks in which external cues are provided (75).
Executive functions are tied to frontal-striatal-thalamic circuit integrity, especially to the dorsolateral circuit (76). Frontal lobe dysfunction in PDD most likely stems from nigrostriatal dopaminergic deficits, resulting in a striatocortical deaf-ferentation effect (77). However, cholinergic dysfunction secondary to neuronal loss in the septal and basal nuclei likely also plays a role in executive dysfunction (78). Executive deficits are particularly evident on tasks that require patients to develop, deploy, and maintain efficient information processing strategies. It has been hypothesized that the basal ganglia and frontal-subcortical circuits function as a subcogni-tive, internal navigational system that limit PDD patients' available options for efficient problem solving (77,79).
Poor performance on tasks that require coordination of complex mental and motor functions (e.g., operation of an automobile) may be conditioned by visuospa-tial deficits, leading to the defective planning and execution of strategies to accomplish a task (e.g., turning a corner while walking or driving) (80).
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