Defective representation of knowledge in Parkinson's disease: evidence from a script-production task

The deficits seen in frontal-lobe patients and in the elderly show clearly that spontaneous script generation depends on good frontal-lobe function. Shallice, however, has proposed that one aspect of script generation (contention scheduling, CS) which is involved in the activation and maintenance of overlearned or routine scripts may depend more on the basal ganglia. Patients with Parkinson's disease would thus be expected to manifest deficits somewhat different from those observed in frontal-lobe patients when generating scripts. The performances of 16 nondemented and nondepressed patients with idiopathic Parkinson's disease were compared to those of 16 age-matched normal control subjects under two experimental conditions; routine, forward script generation and nonroutine, backward script generation. Parkinsonian patients generated scripts significantly deprived of contextual elements in the forward condition and made significantly more sequencing and perseverative errors in both forward and backward conditions than did normal subjects. They also produced a significantly higher number of irrelevant intrusions, in both conditions, than did controls. These results support, in a general sense, Shallice's notion that the basal ganglia are important in script generation; however, other specific predictions of Shallice's model were not supported by our findings.