Time estimation in Parkinson's disease: normal long duration estimation despite impaired short duration discrimination.

It has been claimed that patients with Parkinson's disease (PD) are deficient in estimating and reproducing time intervals in the range of seconds. This deficit is more severe when subjects are requested to count internally during the demanded intervals, and when the rate of internal counting is fast. The observed deficit might therefore reflect slow internal counting, i. e. motor slowness, rather than a specific deficit of temporal processing. However, PD patients also have a higher temporal discrimination threshold for sensory stimuli, a finding purportedly indicating a slow 'internal clock'. In this study we examined PD patients' processing of short durations (approx. 1s,'psychological present') and long durations (up to 48 s,'extended present'). In the first experiment the ability to discriminate between temporally overlapping presentations of two visual stimuli (darkened rectangles on the computer screen) in the range of one second was tested. In the second experiment PD patients' ability to estimate time intervals between 12 and 48 s was investigated. During these intervals, subjects were required to tap and read a random number on the computer screen at a rate of 1 Hz. We found that the patients were impaired at discriminating between short intervals. This deficit was independent of task difficulty and appeared to be based on an impairment of divided attention. Despite this deficit, the PD patients estimated time intervals up to 48 s as accurately as the controls. We suggest that time estimation, i. e. the feeling for the flow of time, is normal in PD patients despite impaired discrimination of brief intervals in the range of seconds.