Selective delayed response deficits in Parkinson's and Alzheimer's disease.

Experimental paradigms adopted from animal models were used to contrast the functional anatomy of frontal systems involved in Alzheimer's disease and the dementia of Parkinson's disease. Patients with Parkinson's disease with dementia were compared with patients with Alzheimer's disease, and nondemented patients with Parkinson's disease, and normal controls. The tasks administered, delayed alternation and delayed response (DR), are sensitive to frontal system damage. Different aspects of each task are mediated by separate frontosubcortical neuronal networks. Patients with Parkinson's disease with dementia were significantly impaired only on DR, whereas patients with Alzheimer's disease were impaired on DR and delayed alternation, even though both groups were equated for severity of dementia. Although dysfunction in the frontal lobes, and/or their subcortical connections, is implicated in both Parkinson's and Alzheimer's disease, dorsolateral frontal systems appear primarily impaired in Parkinson's disease, whereas dorsolateral frontal impairment combined with other regions, possibly orbitofrontal, are prominent in Alzheimer's disease.