Case Studies in Neuroscience: The central and somatosensory contributions to finger inter-dependence and coordination: A study of a "deafferented person".

We tested finger force interdependence and multi-finger force-stabilizing synergies in a patient with large-fiber peripheral neuropathy ("deafferented person"). The subject performed a range of tasks involving accurate force production with one finger and with four fingers. In one-finger tasks, non-task fingers showed unintentional force production (enslaving) with an atypical pattern: Very large indices for the lateral (index and little) fingers and relatively small indices for the central (middle and ring) fingers. Indices of multi-finger synergies stabilizing total force and of anticipatory synergy adjustments in preparation to quick force pulses were similar to those in age-matched control females. During constant force production, removing visual feedback led to a slow force drift to lower values (by about 25% over 15 s). The results support the idea of a neural origin of enslaving and suggest that the patterns observed in the deafferented person were re-organized based on everyday manipulation tasks. The lack of significant changes in the synergy index shows that synergic control can be organized in the absence of somatosensory feedback. We discuss the control of the hand in deafferented persons within the alpha-model of the equilibrium-point hypothesis and suggest that force drift results from an unintentional drift of the control variables to muscles toward zero values.