Review Of Existing Studies

While temporal regularities are manifest at many timescales, our review will be restricted to tasks involving intervals in the hundreds of milliseconds range. We opt for this limited range for two reasons. First, given the role of the cerebellum and basal ganglia in motor control, this range reflects the temporal extent of the component movements that form human actions such as walking, reaching, or speaking. Second, similar methodologies have been applied in studies looking at timing in this range, providing an empirical basis for comparison. By restricting our review to short intervals, we do not imply that the timing of intervals in the range of multiple seconds does not entail similar processes and neural structures as timing in the millisecond range. At present, we see this issue as one in need of further study (for discussion of this issue, see Gilden et al., 1995; Ivry, 1996; Malapani et al., 1998; Mangels and Ivry, 2001).

19.2.1 The Production of Timed Sequences

Time production studies in this time span that have involved patients with either cerebellar lesions or Parkinson's disease (PD) are summarized in Table 19.1. The PD patients have been generally viewed as a model for studying basal ganglia dysfunction. Most of these studies have used a continuation task introduced by Wing and Kristofferson (1973). Trials begin with the presentation of a periodic signal, usually an auditory metronome. After an initial synchronization phase, the metronome is terminated and the participants are asked to continue tapping, attempting to separate each response by the interval specified by the metronome.

This task is appealing for its simplicity. The instructions are intuitive and comprehensible by people with a range of neurological or psychiatric disorders. The motor requirements are minimal. More important, this task has provided a process model for evaluating component sources of temporal variability in performance (Wing and Kristofferson, 1973). The model postulates two component sources of noise: a central clock that provides the timing signals for the series of successive

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