Executive dysfunction or slowed information-processing speed in Parkinson’s disease?
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Recent evidence has shown that PD patients can exhibit impaired performance in tests such as verbal fluency that are sensitive to frontal lobe dysfunction. Two accounts have been proposed to explain the observed PD impairments; slowed information-processing speed (bradyphrenia), or specific executive dysfunction. The current study aimed to explore verbal fluency performance in PD patients in an effort to determine which account is most pertinent. Eight PD patients were compared to eight controls on experimental measures of verbal fluency and numerical information-processing (NIP), as well as background neuropsychological tests. In addition to word output measures, patterns of clustering (generating words within categories) and switching (shifting between categories) were assessed in verbal fluency tasks. A novel paradigm was employed that manipulated the executive load of fluency tasks by constraining the number of syllables. In addition, a novel measure of information-processing speed was employed that had three executive load levels. PD patients produced fewer words than controls in almost all the fluency tasks. The clustering and switching analyses revealed that patients had disproportionately slower switching times than controls in one of the letter conditions. The PD group was impaired relative to controls in the both measures of syllabic fluency. The NIP task revealed that PD patients were significantly slower than controls in the high executive demand condition only. It is argued that the observed pattern of results support the presence of a specific executive impairment in PD rather than general cognitive slowing. Furthermore, the techniques used in the current study show potential for development in this line of investigation.