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 heir 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-bytrial 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. Author summary The conditions under which humans can adapt reaching movements to different force fields in parallel remain the subject of active debates. Mounting evidence highlights that contextual factors linked to movement planning are necessary to form different motor memories. Here we show that background forces indicative of the direction of a force field could play the role of contextual factors, and enable dual adaptation. However, we uncovered that when the cue and the force field were in the same direction but of different magnitudes, the motor memories were still distinct but subject to increased interference. We further show that different timescales of adaptation observed can be explained by a combination of offline and online adaptation in a model of reaching control.

Automated summary

The ability of humans to adapt reaching movements to different force fields in parallel remains a subject of active debates. Independent movement representations or goals enable dual adaptation. Independent cues delivered by means of background loads can support simultaneous adaptation to various velocity-dependent force fields. When the cue and force field are in the same direction but differ in magnitude, there are different motor representations observed but subject to increased interference.