Phase transitions in rhythmic tracking movements: A case of unilateral coupling

Previous research has demonstrated that both within and between persons two limbs coordinated in an anti-phase mode (relative phase of 180 degrees) spontaneously switch to an in-phase mode (relative phase of 0 degrees) when movement frequency is increased. These phase transitions satisfy the criteria of physical bifurcations. The present experiments generalize this finding to movements of a single limb (lower arm) coordinated with the motion of an externally driven visual signal. The results support the idea that phase transitions in coordinated rhythmic movements are due to the presence of informational resolution limitations of the system, and are therefore to be understood as 'threshold non-linearities'. Once in a particular behavioural mode a threshold is reached due to changes in a system parameter, a phase transition occurs if, and only if, there is another behavioural mode available under which the global task goals can be satisfied at lower informational costs. If the spatial conditions of the task are manipulated in such a way that the relation between the information specifying the behaviour and the information generated by the behaviour is identical for both behavioural modes (in-phase, anti-phase), no phase transitions occur.