Force cues flexibly separate motor memories in human reaching adaptation

Current evidence suggests that population dynamics combined with initial neural states associated with movement planning underlines the generation of motor commands1. From this perspective, the ability to learn multiple and potentially opposite force field disturbances applied during reaching may require that preparatory activity be distinct2. Here we tested whether applying a background force prior to a reaching movement could separate motor memories by exposing healthy volunteers to opposite velocity dependent force fields (clockwise and counter-clockwise). Our results confirmed that a background force cue about the direction of the upcoming force field could form independent motor memories of the randomly interleaved opposite force fields. Interestingly, this association was flexible, as upon reversing the relationship between the background and force field directions midway through the experiment, participants were able to relearn the association and form novel, independent motor memories of the force field cued by the background force applied in the opposite direction. To check if this associated was sensitive to the magnitude of the cue, we went further and performed a second experiment where we manipulated the magnitude of the force field (only one direction), cued by the magnitude of the background force. We found a partial separation of motor memories: first interference occurred such that participants zeroed the effect of the heavier perturbation, at the cost of overcompensating for the lighter one during the first few trials. Nevertheless, a statistical analysis indicated that over the course of several tens of trials, the anticipation of force field changed independently across the different magnitudes, suggesting that the memories were indeed separated in spite of the interference. To conclude, the formation of motor memories facilitated by force cues of different directions suggests that muscle afferent feedback prior to movement can put preparatory cortical activity into different states, thereby allowing simultaneous formation of independent motor memories. 1Shenoy et al., 2013, Ann Rev Neuro, 36:337–59; 2 Sheahan et al., Neuron, 92:1–7