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A sudden change in dynamics produces large errors leading to increases inmuscle co-contraction and feedback gains during early adaptation. We previouslyproposed that internal model uncertainty drives these changes, whereby thesensorimotor system reacts to the change in dynamics by up regulating stiffnessand feedback gains to reduce the effect of model errors. However, thesefeedback gain increases have also been suggested to represent part of theadaptation mechanism. Here, we investigate this by examining changes invisuomotor feedback gains during gradual or abrupt force field adaptation.Participants grasped a robotic manipulandum and reached while a curl forcefield was introduced gradually or abruptly. Abrupt introduction of dynamicselicited large initial increases in kinematic error, muscle co-contraction andvisuomotor feedback gains, while gradual introduction showed little initialchange in these measures despite evidence of adaptation. After adaptation hadplateaued,there was a change in the co-contraction and visuomotor feedbackgains relative to null field movements, but no differences (apart from thefinal muscle activation pattern) between the abrupt and gradual introduction ofdynamics. This suggests that the initial increase in feedback gains is not partof the adaptation process, but instead an automatic reactive response tointernal model uncertainty. In contrast, the final level of feedback gains is apredictive tuning of the feedback gains to the external dynamics as part of theinternal model adaptation. Together, the reactive and predictive feedback gainsexplain the wide variety of previous experimental results of feedback changesduring adaptation.

Feedback Gains modulate with Motor Memory Uncertainty