Povel and Essens use the model of an internal clock in their 1985 study. An internal clock is a mechanism that, if induced by a rhythm, organizes and predicts time. Two main components of the clock are the periodic pulse and the counter.
Povel and Essens mention three possible models of clocks:
Absolute Clock: measures and stores in the memory the interval between pulses in absolute time, i.e. msec. This model, however, fails to account for different tempos of songs to be perceived as the same.
A clock with a time unit derived form the sequence: relates all inter-onset intervals to the shortest interval duration, which is stored in the memory as the time unit for the pulse. While it accounts for changes in tempo, it fails to explain why some rhythms, i.e. 200 msec-400 msec-400 msec, are more difficult to produce than others, i.e. 200 msec-200 msec-400 msec.
A hierarchical clock: similar to the previous clock, but uses an interval of medium duration, rather than of the shortest duration, as the unit of time. This allows the unit to be both multiplied and subdivided, creating more flexibility.
Clocks are induced by accents, which Povel and Essens call “perceptually marked” (415).
In modern Western musical notation, meter and beat are manifestations of this concept.
(Povel and Essens, 413-415).