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This thesis has explored the neurochemical and psychological basis of cognitive heterogeneity, particularly within the domains of planning and attention, and the relationship that deficits in these domains have between each other. The neurochemical basis of these deficits was examined by investigating the extent to which a genetic polymorphism in the Catechol O-methyltransferase (COMT) enzyme, an enzyme thought to modulate PFC dopamine levels, influenced participants’ cognitive performance. This thesis was able to establish that, whilst there appears to be an inverted-U shape function between putative PFC dopamine levels and attention set-formation (a measure of attentional structure), there is no evidence for supposing that such a relationship exists between PFC dopamine levels and planning ability. The psychological basis of cognitive heterogeneity in PD patients was probed by examining ability of PD patients to establish a demarcation between relevant and irrelevant information, and rapidly update this demarcation. Overall, it was found that PD patients did not have generic difficulty in distinguishing between relevant and irrelevant information, or in updating this distinction. Rather, such deficits were only found in certain experimental contexts, further underlying the specificity of attentional impairments in PD. Overall planning performance in PD patients and healthy older adults was not found to predict attentional dysfunction. Thus, there appears to be a separate, or relatively independent, basis for impairment in these two domains.