Cognitive slowing in Parkinson's disease fails to respond to levodopa treatment: the 15-objects test.

To evaluate clinically the slowing of cognitive processing in Parkinson's disease, we used a visual discrimination task consisting of 15 superimposed images of objects. The time needed to identify 12 objects increased by 58% in 70 patients withdrawn from levodopa treatment compared with 20 controls matched for age and education. Perceptual, motor, and psycholinguistic factors, as well as mood, only partially accounted for the slowness of performance. The 15-objects test scores of the parkinsonian patients correlated significantly with both their intellectual impairment and the severity of their parkinsonian disability, but not with the duration of the disease. The scores did not correlate with depression. Levodopa had no effect on the score, although the parkinsonian motor disability score was improved by 54%. The results indicate a cognitive slowing in Parkinson's disease which is probably related to abnormalities of nondopaminergic neuronal systems in the brain.