Distortions in reproduction of two-interval rhythms : when the "attractor ratio" is not exactly 1:2

When rhythms consisting of two unequal intervals are reproduced cyclically, their interval ratio tends to be distorted in the direction of 1:2 (= 0.5), which thus seems to function as an "attractor ratio" (AR). However, recent results for musicians in a synchronization task (Repp, London, & Keller, 2011) have suggested an upward-shifted AR (USAR) somewhat greater than 0.5. Three new experiments suggest that this shift is not due to synchronization versus continuation tapping, the range of interval ratios employed, unimanual versus bimanual tapping, intensity differences between taps, or mental subdivision of the long interval, although some of these factors may affect its size. The new results also show that the USAR is found more consistently in musicians than in nonmusicians and seems to arise in rhythm production, not in perception. While the exact causes of the USAR remain unclear, the results suggest that the AR is not necessarily the mathematically simplest interval ratio.