FIG. 8.

Sensitivity analysis of force covariance mapping to noise source and strength. The ability of the different hypotheses to generate target-directedness as high, as observed, was analyzed by varying unknown parameters. A: signal-independent noise (Sig.-Indep. Noise) at the muscle level forces both hypotheses to generate low values of target directedness. B: under the assumption of signal-dependent noise at the muscle level (Muscle SDN), parameter values exist for the flexible activation hypothesis that are consistent with the data, but no parameter values were found for the synergistic activation hypothesis that can reproduce the target directedness of the data. C: a similar pattern emerges if there are equal amounts of SDN at the synergy (Synergy SDN) and muscle levels. D: if the synergy-level SDN is generating 10 times more variance in muscle force than the muscle-level SDN, some parameter values can be found so that the target directedness of the synergistic activation hypothesis is consistent with the data. However, the synergistic activation simulations consistent with the data also have much stronger correlations (average 0.75, maximum 0.95, minimum 0.41, averaged across all muscle pairs).