How convincing is the evidence for cognitive slowing in Parkinson's disease?

The issue of whether or not Parkinson's disease (PD) patients process information more slowly than normal is unresolved. Across a range of paradigms the evidence for slowing is scant, and many show no slowing. Even the results of a single experimental paradigm can be conflicting; some studies have obtained evidence for slowed memory scanning in subgroups of PD patients whereas others have not. The present study sought the cause of these discrepant results. Old-old (n = 11) and young-old (n = 7) PD subjects and controls (n = 16, n = 9) completed both fixed and varied set versions of the memory scanning task. No evidence for slowing in PD was found. These results indicated that PD did not interact with age or task version in its effects on information processing rates in the memory scanning task. It was concluded that PD patients do not have any cognitive slowing that is detectable with the memory scanning task.