Spencer and colleagues have proposed that the control processes used to control periodic movements in which salient events mark each cycle are distinct from those in which the movements lack such events (2003, 2005). One source of evidence in support of this distinction comes studies of patients with cerebellar damage. These individuals exhibit increased temporal variability when producing repetitive movements with an event structure (eg, the taps on the table during finger tapping), yet are unimpaired when producing rhythmic movements that lack an event structure (eg, continuous circle drawing). For the latter, it is hypothesized that timing is emergent, reflecting the operation of alternative control parameters such as angular velocity. Thus, the cerebellum appears to be essential for event, but not emergent, timing.

While maintaining constant velocity would produce consistently timed circles, it is not clear how this form of control could be used to match a movement rate specified by a metronome. Zelaznik et al (2005) proposed a transformation hypothesis in which an event-based representation is used at the start of a trial and gives way to emergent control as the requisite velocity is established. Here we examine this hypothesis by testing patients with cerebellar lesions. Specifically, we predicted that, during continuous circle drawing, temporal variability would be elevated in the initial interval (s) only.