Overestimation of force during matching of externally generated forces

Non-technical summary If a weight is applied to a finger and the subject asked to produce the same force, the subject generates a force larger than the weight. That is, subjects overestimate the force applied by an external target when matching it. Details of this force overestimation are not well understood. We show that subjects overestimate small target weights, but not larger ones. Furthermore we show for the first time that the force overestimation consists of two components. The first component is a constant. The second component depends on the precise magnitude of the weight and is only present when subjects hold the target weight against gravity. We suggest that the two components are generated in different phases of the force-matching task, are due to different processes, and must have an influence on all proprioceptive judgements of force.To make accurate movements the brain must differentiate between forces it commands and forces imposed by the environment. This requires afferent information and signals related to central commands. If subjects match an externally generated target force with a self-generated force, they produce a force that is larger than the target. It has been proposed that this is due to simple attenuation of afferent force signals produced by the body's own actions, but the mechanisms are unclear. Four studies of forces applied to the index finger in 14 subjects investigated this force overestimation. We determined which sensory signals are involved, if handedness is important, if overestimation is present at high forces, and which muscle actions can generate it. Subjects overestimate an externally generated target force by 2-3 N when matching it with a voluntary force using a simple contraction or complex muscle synergy. This 'offset' occurs at low but not high forces. The effect occurs when only cutaneous inputs, or when only combined inputs from muscle and central command sources can signal force. We report a novel central factor that increases the gain, or gradient of the relationship, between the matching and target forces to similar to 1.20. This increased gain is present only if the target force is received on an active finger and persists after the 'offset' is abolished. It may reflect processing of reactive forces during the target phase of the task. Overall, the previously described simple model of force attenuation cannot explain fully the overestimation of external forces.