Synchronization

Studying the synchronization of repetitive finger taps with a stream of regular external events has a long history in experimental psychology (Aschersleben and Prinz, 1995; Dunlap, 1910). Synchronization requires the ability to control motor output based on the prediction of external events (Hary and Moore, 1985, 1987). It is thought to entail both open- and closed-loop processes: the former in that the responses are generated in advance of the metronome signals, and the latter in that an error signal associated with the asynchrony between the responses and metronome signals is used to modify future responses. We first outline a general model of the hypothetical processes required for synchronized tapping and then turn to a review of previous attempts to link these processes to neural structures.

19.3.1 Components Involved in Synchronization

A schematic of the component operations involved in synchronized tapping is presented in Figure 19.1. A clock-like system that represents the predicted interval emits a timing signal (tk) for the next motor command. The timing of this command is set such that the resulting tap occurs approximately at the same time as the stimulus tone (sk). The internal timing signal triggers a motor implementation process that adds a random motor delay component Mk conceptualized as an independent noise source (Wing and Kristofferson, 1973).

To ensure that the taps occur in simultaneity with the metronome, the system must contain a closed-loop component. Two types of mechanisms have been proposed (Mates, 1994a, 1994b). In period correction, the internal representation of the interval t is adjusted when a significant and consistent mismatch is detected between the metronome and the produced responses. This mechanism is thought to be relatively slow and dependent on a conscious perception of the mismatch of expected and perceived tempo (Repp, 2001).

The other process, phase correction, ensures that the error of the central clock does not accumulate over a number of taps. Such accumulation would lead to a loss of phase stability between the metronome signals and the responses. Phase correction

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FIGURE 19.1 Process model of synchronization (see Vorberg and Schulze, 2002; Vorberg and Wing, 1996). Lowercase variables indicate time points of events, uppercase variables indicate the length of the intervals between events, and subscripted variables are conceptualized as random variables.* An external pacing signal occurs with period P at the time points sk. An internal clock is emitting signals to the motor system at times tk. In absence of error correction, the clock produces timing signals separated by the clock intervals Tk. The motor implementation process produces the kth taps at time rk by adding a random motor delay Mk to the time of the internal timing signal. The real synchronization error Ak between the tap and the pacing signal is perceived (A'k) by a comparator system. The perceived asynchrony is influenced by the perceptual delays, with which the comparator perceives the occurrences of the tap (Fk) and the pacing signal (Sk). The perceived asynchrony is then used to correct the next timing signal with a gain of a, the error correction parameter.

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