Order effects in response times of parkinsonian patients and normal controls.

Response latencies were measured in 6 parkinsonian patients and 6 normal subjects in a choice reaction task requiring the discrimination of two different tones with different probabilities of occurrence (frequent and rare). Response latency was measured from stimulus onset to onset of electromyographic activity in the responding muscle. Rare-tone responses were separated on the basis of the number of frequent tones intervening between the rare tone of interest and the immediately preceding rare tone (defined as rare-tone position). Frequent-tone responses were separated by the number of consecutive frequent tones occurring either before or after a rare tone (defined as frequent-tone position). Rare- and frequent-tone position had a significant impact on response latency. Both patients and controls had the shortest response latencies to rare tones when four frequent tones (the median interval for these experiments) intervened. Similarly, the response latency to frequent tones increased at approximately this same median interval after a rare event for both patients and controls. These findings suggest that normal controls utilize probability information about both global probabilities and their immediate past experience in order to modify upcoming responses. Our findings also indicate that patients with Parkinson's disease do not differ from normal subjects in this regard, and thus that even subtle attributes of preprogramming are not affected in Parkinson's disease, despite suggestions by others to the contrary.