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HY = 2.4

Note: Thresholds are 1 SD of a logistic distribution fit to the observer's response. K = Weber fraction of 1 SD/PSE; Sig. = significance (* indicates a statistically reliable difference between the experimental and control groups); HY = Hohn and Yahr scale; CCA = cortico cerebellar atrophy; OPCA = olivio pontine cerebellar atrophy.

Note: Thresholds are 1 SD of a logistic distribution fit to the observer's response. K = Weber fraction of 1 SD/PSE; Sig. = significance (* indicates a statistically reliable difference between the experimental and control groups); HY = Hohn and Yahr scale; CCA = cortico cerebellar atrophy; OPCA = olivio pontine cerebellar atrophy.

a This task required the participants to classify single intervals as short or long; a standard interval was not presented on each trial. The PSEs were 274 and 282 for controls and cerebellar patients, respectively.

Jueptner et al. (1995, 1996) have conducted two positron emission tomography (PET) studies to investigate the neural correlates of duration discrimination, one with auditory stimuli (Jueptner et al., 1995) and a second with tactile stimuli (Jueptner et al., 1996). In both studies activation of the left inferior cerebellum (hemisphere of lobule VII) and vermis (VI and VII) was found during the duration discrimination task, compared to a control task in which the same stimuli were presented but the responses simply required alternating key presses. Increased blood flow was also seen bilaterally in the basal ganglia, but only in the experiment with auditory stimuli.

In summary, the patient and imaging studies of time production and perception yield a somewhat unsatisfying picture. The existing data indicate that cerebellar lesions are consistently associated with deficits on both production and perception tasks. While the initial studies involving PD patients reported no impairments, more recent reports have shown that PD patients also exhibit increased variability on time production and perception tasks. Thus, while Ivry and colleagues had argued that the dissociation between cerebellar and PD patients provided evidence of a specialized role for the cerebellum in the representation of temporal information, the current literature does not offer strong support for this dissociation.

Nonetheless, the fact that both groups can be impaired on similar tasks need not lead to the conclusion that temporal processing involves a distributed network that includes both the cerebellum and basal ganglia, as well as other structures such as the prefrontal cortex (Harrington et al., 1998b; Wittmann, 1999). It may well be that the analytic power of the tasks used in these studies is insufficient. As noted above, the estimate of clock variability in the Wing-Kristofferson model is really a composite of all nonmotor implementation sources of variability. Similarly, the perception tasks used in the patient studies have generally failed to provide a means for evaluating different sources of variability (Ivry and Hazeltine, 1995). Two exceptions are Mangels et al. (1998) and Casini and Ivry (1999), in which an attempt was made to separate the effects of timing from those associated with attention and working memory. In both studies, the results from the cerebellar group were consistent with an impairment in timing, whereas patients with prefrontal lesions were primarily influenced by the attentional demands of the tasks, temporal or nontemporal. It would be useful to apply a similar strategy in a direct comparison of cerebellar and PD patients.

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