Data from: The decay of motor adaptation to novel movement dynamics reveals an asymmetry in the stability of motion state-dependent learning

Motor adaptation paradigms provide a quantitative method to study short-term modification of motor commands. Despite the growing understanding of the role motion states (e.g., velocity) play in this form of motor learning, there is little information on the relative stability of memories based on these movement characteristics, especially in comparison to the initial adaptation. Here, we trained subjects to make reaching movements perturbed by force patterns dependent upon either limb position or velocity. Following training, subjects were exposed to a series of error-clamp trials to measure the temporal characteristics of the feedforward motor output during the decay of learning. The compensatory force patterns were largely based on the perturbation kinematic (e.g., velocity), but also showed a small contribution from the other motion kinematic (e.g., position). However, the velocity contribution in response to the position-based perturbation decayed at a slower rate than the position contribution to velocity-based training, suggesting a difference in stability. Next, we modified a previous model of motor adaptation to reflect this difference and simulated the behavior for different learning goals. We were interested in the stability of learning when the perturbations were based on different combinations of limb position or velocity that subsequently resulted in biased amounts of motion-based learning. We trained additional subjects on these combined motion-state perturbations and confirmed the predictions of the model. Specifically, we show that (1) there is a significant separation between the observed gain-space trajectories for the learning and decay of adaptation and (2) for combined motion-state perturbations, the gain associated to changes in limb position decayed at a faster rate than the velocity-dependent gain, even when the position-dependent gain at the end of training was significantly greater. Collectively, these results suggest that the state-dependent adaptation associated with movement velocity is relatively more stable than that based on position.

运动适应范式（motor adaptation paradigms）为研究运动指令的短期重塑提供了量化方法。尽管学界对运动状态（例如速度）在这类运动学习中的作用已有愈发深入的认知，但针对基于此类运动特征形成的记忆的相对稳定性的研究仍较为匮乏，尤其是与初始适应过程的对比分析。
本研究中，我们训练受试者完成受力模式扰动的伸手运动，该扰动的力模式取决于肢体位置或速度。训练结束后，受试者需完成一系列误差钳制试验（error-clamp trials），以测量学习消退阶段前馈运动输出（feedforward motor output）的时间特征。
补偿性力模式主要基于扰动的运动学参数（kinematic），但同时也受到另一运动学参数的小幅影响。然而，相较于基于速度训练的条件下位置贡献的衰减速率，针对位置型扰动的速度贡献衰减速率更慢，这提示两类记忆的稳定性存在差异。
随后，我们对既往运动适应模型进行修改以体现这一差异，并针对不同学习目标模拟了行为表现。我们的研究兴趣在于：当扰动基于肢体位置与速度的不同组合，进而导致基于运动状态的学习量出现偏差时，学习的稳定性会呈现何种特征。
我们对额外受试者开展了联合运动状态扰动的训练，验证了模型的预测结果。具体而言，我们发现：（1）用于表征适应学习与消退过程的观测增益空间轨迹存在显著分离；（2）对于联合运动状态扰动，即便训练结束时与位置相关的增益显著更高，肢体位置变化相关的增益衰减速率仍快于速度依赖型增益。
综上，上述结果表明：与基于肢体位置的适应相比，与运动速度相关的状态依赖型适应具有相对更高的稳定性。