The stability of bimanual performance of the frequency ratios 3:8 and
5:8 was examined from the perspective of the sine circle map and the a
ssociated Farey mode-locking hierarchy. By gradually increasing moveme
nt frequency, abrupt transitions from the initial frequency ratios to
other frequency ratios were induced. In general, transitions occurred
to frequency ratios that were near the initial frequency ratio but low
er in the Farey ordering and, hence, of higher stability in the sine c
ircle map. A fair percentage of these transitions were to unimodularly
related ratios. The transition routes from 3:8 and 5:8 remained large
ly unaffected by extensive practice of the lower-order ratios 2:5 and
3:5. Collectively, these results suggest that(i) bimanual tapping occu
rs in a domain in which frequency-locked states either overlap or are
located sufficiently close to each other to make stochastic switching
possible (coupling parameter K > 1 or close to I); (ii) the overall st
ability of these frequency-locked states decreases as movement frequen
cy increases (due to a decrease in K); and, consequently, (iii) the pr
obability of transitions to nearby frequency ratios increases as movem
ent frequency increases, due to the differential stability of the freq
uency locks.