I had expected the taps to be approximately correct, but jittered either 
side of the correct points by a roughly normal distribution. However, 
this is not what I observe.

Take a look at the first attachment. There's an obvious U-shapred trend 
in the error graph, and I have no idea why. It appears very, very 
frequently though. But aside from that, the errors look much more like a 
random walk than a normal distrubition. The timing error in each tap 
appears to be highly correlated with its neighbors.

The second attachment shows a similar pattern (although the U-shapred 
trend is now inverted). Note the difference in tempo between the two 
examples.

I hypothesize that what actually happens is that my tapping is actually 
running off an internal mental clock, which is being adjusted by a 
feedback loop that keeps it reasonably in-phase with the actual 
drumbeat. Hence the random-walk variations.

Of course, it's possible that the music I'm tapping to actually contains 
subtle tempo variations. Or that some of these patterns are prediction 
errors. I think what I need to do is perform a tapping experiment with a 
timing source of known tempo, so I can get genuine error measurements. 
(Currently it's tricky to reperate human tapping errors from computer 
prediction errors.)

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Attachments:
Download 'bpm1.png' (14 KB) Download 'bpm2.png' (16 KB)

Preview of image 'bpm1.png'

Preview of image 'bpm2.png'