Separability of human motor memories during reaching adaptation with force cues

Judging by the breadth of our motor repertoire during daily activities, it
is clear that learning different tasks is a hallmark of the human motor
system. However, for reaching adaptation to different force fields, the
conditions under which this is possible in laboratory settings have
remained a challenging question. Previous work has shown that independent
movement representations or goals enabled dual adaptation. Considering the
importance of force feedback during limb control, here we hypothesised
that independent cues delivered by means of background loads could support
simultaneous adaptation to various velocity-dependent force fields, for
identical kinematic plan and movement goal. We demonstrate in a series of
experiments that indeed healthy adults can adapt to opposite force fields,
independently of the direction of the background force cue. However, when
the cue and force field were in the same direction but differed by their
magnitude, the formation of different motor representations was still
observed but the associated mechanism was subject to increased
interference. Finally, we highlight that this paradigm allows dissociating
trial-by-trial adaptation from online feedback adaptation, as these two
mechanisms are associated with different time scales that can be
identified reliably and reproduced in a computational model.