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Summary.What is the nature of the human timing mechanism for perceptual judgements about short temporal intervals? One possibility is that initial periodic events, such as tones, establish internal beats which continue after the external events and serve as reference points for the perception of subsequent events. A second possibility is that the timer records the intervals produced by events. Later, the stored intervals can be reproduced or compared to other intervals. A study by Schulze (1978) provided evidence favoring beat-based timing. In contrast, our two experiments support an interval theory. The judgements of intervals between tones is not improved when the events are synchronized with internal beats established by the initial intervals. The conflict between the two sets of results may be resolved by the fact that an interval timer can recycle from one interval to the next, thus operating in a beatlike mode. However, a timer of this sort is just as accurate when comparing intervals that are off the beat.A great deal of research has been concerned with the perception of time. Although "time" is a term in common, many of the problems addressed in different studies are quite different. Much of the work has been concerned with subjective time -that is, judgements of how much time has transpired. Typically those judgements concern seconds, minutes, and even hours. In contrast, little work has been concerned with relatively short intervals measured in terms of a few hundred ms. Such short intervals are characteristic of the timing of a rapid series of movements, as in playing a musical instrument or in speaking, and are also characteristic of the perceptual experiences emanating from rapid production of this sort, such as the music or the speech sounds.The present study is concerned with possible mechanisms that underlie the perception and production of such short intervals as may be relevant to skill. In particular, the study compares two general mechanisms whereby humans distinguish slight irregularities in the timing of otherwise periodic events. The events occur at short intervals, in this case one about every 300 ms. The two mechanisms were described by Schulze (1978). One mechanism, a beat-based timer, essentially involves the synchrony of events. The second type of mechanism, an interval timer, involves Offprint requests to: S. W. Keele comparing the intervals between events. The general idea of a beat-based timer is that an initial series of periodic events establishes an internal beat that persists after the initial events. Whether or not subsequent events are perceived as occurring at the proper time will depend on the degree to which they are synchronized with the internal beat. In contrast, an interval timer registers the duration of an interval between events, and that temporal memory is then compared to other intervals in order to judge whether or not they are the same.Schulze (1978) devised a test between the theories using a paradigm which we repeat here. On each trial he presented a series of...

Previous work (Keele & Hawkins, 1982; Keele, Pokorny, Corcos, & Ivry, 1985) has suggested two general factors of coordination that differentiate people across a variety of motor movements, factors of timing and maximum rate of successive movements. This study provides comparable evidence for a third general factor of coordination, that of force control. Subjects who exhibit low variability in reproducing a target force with one effector, the finger, tend to show low variability with two other effectors, the foot and forearm. In addition, ability in force control cuts across different force ranges and across situations where force control is either the primary goal or the secondary goal. Force records obtained during a periodic tapping task show that, although force control is largely independent of timing, there are some interactions between the two factors. Force variation appears to distort timing a small amount in part because larger forces speed up implementation of movement, thereby shortening preceding intervals and lengthening following ones, and in part because force variation alters central-timing mechanisms.

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