The brain must encode the relative time between events and it must have a way of globally adjusting rates too. In music, we store the duration of each note, but we can also adjust the rate globally. We can recognize a melody played at different tempos and we can play back a melody at different tempos. The same capabilities are needed for almost all motor sequences. For example, signing your name is very much like singing a song, it is a series of motor movements where each element in the sequence has a duration, and you can speed up and slow down the entire motor sequence.
When we developed the HTM sequence memory these timing issues were always on my mind. I felt that if our sequence memory couldn’t solve these timing issues then it wasn’t correct. I have a hypothesis of how the cortex encodes timing and how it is possible to adjust the rate. The HTM sequence memory can accommodate this hypothesized mechanism. We did not implement it because it didn’t seem important enough at the time and we didn’t need it for the applications we were working on. In short, relative timing is an important component of all time-based inference and motor output, however, adding the timing component was secondary to getting the basic sequence memory working.
In the past I have used the term “absolute timing” to refer to the relative timing of elements in sequences. That might not be the best term.
In the past I have described the mechanism I believe implements timing in the cortex. Briefly, there is a projection from L5 (small pyramidals) to the matrix cells in the thalamus, which project back to L1. This pathway is broad, meaning that the matrix cells receiving input from auditory regions will project broadly back to all the auditory regions. There are similar broad circuits for vision, touch and motor. What happens is for each new event (“note”), a cascade-type clock sends timing info to L1. The sequence memory cells use this signal to learn how much delay occurred since the last “note”. If the cortex is playing back a sequence a sequence element is immediately predicted by the preceding element but doesn’t become active until it also recognizes the time delay on its apical dendrites. The central position of the matrix cells allows for the cascade-like clock to be sped up or slowed down. In some recent papers I have seen evidence supporting this hypothesis.