Interactions of force and time control investigated through state transitions

In producing repetitive rhythmic movements such as finger tapping or pulse production, there is ambiguity regarding how independently force and timing are controlled at the central level. Tapping studies involving Parkinson and cerebellar patients report increased variability in one parameter but the preservation of the other, indicating independent control. However, when producing a force accent within a sequence of responses, simultaneous changes in timing suggest coupled control. We hypothesized that force and timing control behave independently but will exhibit parallel transient changes during execution of an anticipated state transition, and subsequently revert to independence. A transition is a change in state, where stress is introduced in the system, possibly loading cognitive resources and subsequently affecting control behaviour.
Twelve participants were trained on a pulse production task (tapping without lifting finger off the surface) at ‘Fast’ (600ms) and ‘Slow’ (1000ms) speeds as well as ‘Hard’ (2.5N) and ‘Soft’ (1.5N) forces. They then performed either an up-switch or a down-switch on the manipulated parameter (Time manipulation: fast to slow and slow to fast; Force manipulation: hard to soft and soft to hard). Observations on the non-manipulated parameter show transient changes occurring about the point of transition before returning to a stable state, suggesting coupling. However, differences in variability patterns of time and force during stable state indicate distinct control pathways. We conclude that force and timing control is generally independent but can be dependent during preparation and execution of an anticipated state transition, perhaps when cognitive resources are consciously engaged.