In most daily activities, we use many different joints or even both hands to achieve our goals. If something goes wrong – for example we spill a cup of coffee – the motor system needs to adjust and learn from the error. With many muscles and joints involved the question is: “Who’s fault was it?”

The situation facing the brain is comparable to that of a teacher who is supervising two students working on a joint project. If something goes wrong, the teacher could let the more experienced student determine and repair the mistake. Whereas this may be an efficient remedy for the situation, it is more likely that the less experienced student caused the error. Therefore, it may be a good idea to let that student correct and learn from the mistake.

Our study shows that the motor system is “fair”, and assigns the responsibility for motor errors to the less experienced and noisier player – here the left hand. In the study, participants moved a virtual object with two hands. When the object was deflected, right-handed participants corrected the error preferentially with the left hand. In contrast, left-handers corrected more with the right hand.

The study also shows that the “scapegoat” of the motor system can be changed. In further experiments we manipulated the feedback to make the motor system believe that the right hand was noiser than the left. When the two hands moved together again, suddenly the right hand corrected more for the error.

The study shows that the brain assigns responsibility for motor errors to the most likely culprit. This is an important principle to understand compensation after injury. For example after a stroke, some joints may become less reliable. Our results make predictions about how correction and learning changes in such situations.

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