What is wrong with force magnitude ? Study of interferences in dual adaptation of human reaching movements with force cues

When people try to learn to adapt in parallel to two force fields of same direction and different magnitudes, interferences are observed. This happens even if they are helped by cues in the form of background loads of different magnitudes and same direction. This work aims to understand why those interferences are observed. Thirty volunteers took part in two experiments to find out if the interferences are due to the fact that the used clues are not efficient, or if it is just harder for human to adapt to force field of different magnitudes than of different directions. After analysis of different learning indicators, we concluded that background loads of different magnitudes are rather inefficient clues to enable the formation of distinct motor memories. The magnitude information does not seem to enable dual adaptation. Moreover, we discovered different learning behaviours in the population, some people seem to have more difficulties than other to adapt to force fields of different magnitudes in parallel.