Following previous research on the self-organising nature of human movement (Kelso, 1984; Zanone & Kelso, 1992; Scholz & Kelso, 1990; Haken, Kelso & Bunz, 1985), the current study investigated the phenomenon of non-equilibrium phase transitions in bimanual finger tapping. Prior research has identified two phase locked movement patterns of differing stability for bimanual oscillation of the index fingers: In-phase (0º relative phase), and anti-phase (180º relative phase). Below a critical value of the frequency control parameter, participants are able to stably perform in either phase locked mode. Beyond the critical frequency however, only the in-phase movement can be performed stably.
The present study examined the role of multiple control parameters acting in combination on pattern change, using a bimanual finger tapping paradigm. Participants were required to initiate an anti-phase tapping pattern under three different frequency conditions (600, 400, & 200 ms), and were required to increase the force on one finger at the onset of a randomly presented stimulus. By holding the frequency parameter constant and manipulating the force parameter, changes in the tapping pattern were measured.
The results suggest that an increase in the force parameter at low frequencies did not result in a change in the phase relation between fingers, however, at high frequencies, the introduction of a force parameter could bring about a change in tapping pattern. Under these conditions, participants experienced fluctuations of the relative phase of their fingers, which were followed by phase transitions from anti-phase to in-phase tapping. The results reflect the context dependent nature of movement, with stable patterns emerging as a function of the combination of differing control parameters acting on the system at any given time.