Emerging and disappearing synergies in a hierarchically controlled system

The purpose of the study was to explore the ability of the central nervous system (CNS) to organize synergies at two levels of a hypothetical control hierarchy involved in two-hand, multi-finger tasks. We investigated indices (ΔV) of finger force co-variation across trials as reflections of synergies stabilizing the total force (F _TOT). Subjects produced constant force with one or two finger-pairs (from one hand or two hands). In trials starting with one finger-pair, subjects added another finger-pair without changing F _TOT. In trials starting with two finger-pairs, subjects removed one of the finger-pairs without changing F _TOT. Adding or removing a finger-pair resulted in a transient drop in ΔV computed for the finger-pair that remained active throughout the trial. This drop in ΔV was seen simultaneously with force changes. Compared to the original steady-state, addition of a finger-pair led to a significant drop in ΔV at the newly established steady-state. This drop eliminated negative co-variation among finger forces that had stabilized F _TOT. In contrast, in trials starting with two finger-pairs, no negative co-variation between finger forces within-a-pair was seen. Removing a finger-pair led to the emergence of negative co-variation between finger forces at the new steady-state. The ΔV index computed across two finger-pairs confirmed the existence of negative force co-variation. The emergence and disappearance of force stabilizing synergies within a finger-pair may signal limitations in the ability of the CNS in forming synergies at two different hierarchical levels.