During synchronization with sequences of brief sounds, inter response intervals (IRIs) produced by human operators reflect the preceding inter stimulus intervals (ISIs). In a previous study (Madison & Merker, 2004) sequences consisted of 600 ms base intervals that were lengthened or shortened by a constant in the range 3-96 ms, according to an non-periodic and unpredictable pattern based on the Kolakoski sequence. For such sequences, the IRI’s deviation from the base interval is called tracking response. It corresponded to about 75% of the preceding ISI perturbation for perturbation magnitudes up to 24 ms, regardless of whether the it was consciously detected or not, and decreased for larger magnitudes.

Here, I report three experiments that address (a) a possible breakpoint in the tracking response as a function of the perturbation magnitude, (b) the effects of base interval, across a range from about 200 ms to 2 s, and (c) possible interactions between perturbation magnitude and base interval. Furthermore, the pattern of response intervals and asynchronies related to the different elements of the Kolakoski sequence is examined in order to understand the nature of the varying tracking response. Preliminary results show that the tracking response occurs throughout the entire range of base intervals used, and that it disappears for perturbations exceeding 20-25 percent of the base interval.