Asymmetry in kinematic generalization between visual and passive lead-in movements are consistent with a forward model in the sensorimotor system

In our daily life we often make complex actions comprised of linked movements, such as reaching for a cup of coffee and bringing it to our mouth to drink. Recent work has highlighted the role of such linked movements in the formation of independent motor memories, affecting the learning rate and ability to learn opposing force fields. In these studies, distinct prior movements (lead-in movements) allow adaptation of opposing dynamics on the following movement. Purely visual or purely passive lead-in movements exhibit different angular generalization functions of this motor memory as the lead-in movements are modified, suggesting different neural representations. However, we currently have no understanding of how different movement kinematics (distance, speed or duration) affect this recall process and the formation of independent motor memories. Here we investigate such kinematic generalization for both passive and visual lead-in movements to probe their individual characteristics. After participants adapted to opposing force fields using training lead-in movements, the lead-in kinematics were modified on random trials to test generalization. For both visual and passive modalities, recalled compensation was sensitive to lead-in duration and peak speed, falling off away from the training condition. However, little reduction in force was found with increasing lead-in distance. Interestingly, asymmetric transfer between lead-in movement modalities was also observed, with partial transfer from passive to visual, but very little vice versa. Overall these tuning effects were stronger for passive compared to visual lead-ins demonstrating the difference in these sensory inputs in regulating motor memories. Our results suggest these effects are a consequence of state estimation, with differences across modalities reflecting their different levels of sensory uncertainty arising as a consequence of dissimilar feedback delays.

日常生活中，我们常会完成由联动动作构成的复杂行为，例如伸手取一杯咖啡并送至嘴边饮用。近期研究表明，这类联动动作在独立运动记忆（motor memory）的形成中扮演关键角色，会影响学习速率与对抗力场（opposing force fields）的学习能力。在上述研究中，独特的前置引导动作（lead-in movements）可使后续动作适应对抗性动力学特性。当对前置引导动作进行调整时，纯视觉或纯被动的前置引导动作会展现出该运动记忆的不同角度泛化（angular generalization）特性，提示其背后存在不同的神经表征（neural representations）。

然而，目前我们尚不清楚不同的运动学（kinematics）参数——包括距离、速度或时长——会如何影响该记忆提取过程与独立运动记忆的形成。本研究针对被动与视觉型前置引导动作，探究此类运动学泛化（kinematic generalization）特性，以揭示二者各自的特征。在受试者通过训练用前置引导动作适应对抗力场后，我们在随机试次中调整前置引导动作的运动学参数，以测试泛化能力（generalization）。

无论是视觉还是被动模态（modality），提取的运动补偿均对前置引导动作的时长与峰值速度敏感，且偏离训练条件时补偿效应会逐渐衰减。但随着前置引导动作的距离增加，力补偿几乎未出现衰减。有趣的是，我们还观察到前置引导动作模态间存在不对称迁移（asymmetric transfer）：被动模态向视觉模态存在部分迁移，而反向迁移则极为有限。总体而言，相较于视觉型前置引导动作，被动型前置引导动作的调节效应更为显著，这表明两类感觉输入在调控运动记忆方面存在差异。

本研究结果表明，上述效应源于状态估计（state estimation）过程，而不同模态间的差异则反映了由反馈延迟（feedback delays）差异所导致的感觉不确定性（sensory uncertainty）水平差异。