Park Se-Woong, Marino Hamal, Charles Steven K, Sternad Dagmar, Hogan Neville
Department of Biology, Northeastern University, Boston, Massachusetts;
Research Center "E. Piaggio," University of Pisa, Pisa, Italy.
J Neurophysiol. 2017 Jul 1;118(1):69-83. doi: 10.1152/jn.00643.2016. Epub 2017 Mar 29.
Mounting evidence suggests that human motor control uses dynamic primitives, attractors of dynamic neuromechanical systems that require minimal central supervision. However, advantages for control may be offset by compromised versatility. Extending recent results showing that humans could not sustain discrete movements as duration decreased, this study tested whether smoothly rhythmic movements could be maintained as duration increased. Participants performed horizontal movements between two targets, paced by sounds with intervals that increased from 1 to 6 s by 200 ms per cycle and then decreased again. The instruction emphasized smooth rhythmic movements without interspersed dwell times. We hypothesized that ) when oscillatory motions slow down, smoothness decreases; ) slower oscillatory motions are executed as submovements or even discrete movements; and ) the transition between smooth oscillations and submovements shows hysteresis. An alternative hypothesis was that ) removing visual feedback restores smoothness, indicative of visually evoked corrections causing the irregularity. Results showed that humans could not perform slow and smooth oscillatory movements. Harmonicity decreased with longer intervals, and dwell times between cycles appeared and became prominent at slower speeds. Velocity profiles showed an increase with cycle duration of the number of overlapping submovements. There was weak evidence of hysteresis in the transition between these two types of movement. Eliminating vision had no effect, suggesting that intermittent visually evoked corrections did not underlie this phenomenon. These results show that it is hard for humans to execute smooth rhythmic motions very slowly. Instead, they "default" to another dynamic primitive and compose motion as a sequence of overlapping submovements. Complementing a large body of prior work showing advantages of composing primitives to manage the complexity of motor control, this paper uncovers a limitation due to composition of behavior from dynamic primitives: while slower execution frequently makes a task easier, there is a limit and it is hard for humans to move very slowly. We suggest that this remarkable limitation is not due to inadequacies of muscle, nor to slow neural communication, but is a consequence of how the control of movement is organized.
越来越多的证据表明，人类运动控制使用动态基元，即动态神经机械系统的吸引子，其需要最少的中枢监督。然而，控制的优势可能会被通用性的降低所抵消。扩展最近的研究结果表明，随着持续时间的缩短，人类无法维持离散运动，本研究测试了随着持续时间的增加，是否能够维持平滑的节律性运动。参与者在两个目标之间进行水平运动，由声音节奏控制，间隔从1秒增加到6秒，每个周期增加200毫秒，然后再次减少。指令强调平滑的节律性运动，没有穿插停留时间。我们假设：（1）当振荡运动减慢时，平滑度会降低；（2）较慢的振荡运动以子运动甚至离散运动的形式执行；（3）平滑振荡和子运动之间的转换表现出滞后现象。另一种假设是：（1）去除视觉反馈可恢复平滑度，这表明视觉诱发的校正导致了不规则性。结果表明，人类无法进行缓慢而平滑的振荡运动。谐波性随着间隔时间的延长而降低，周期之间的停留时间出现并在较慢速度下变得突出。速度分布图显示，随着周期持续时间的增加，重叠子运动的数量也增加。在这两种运动类型之间的转换中，有微弱的滞后证据。消除视觉没有效果，这表明间歇性的视觉诱发校正不是这种现象的基础。这些结果表明，人类很难非常缓慢地执行平滑的节律性运动。相反，他们“默认”为另一种动态基元，并将运动组成一系列重叠的子运动。补充了大量先前的研究工作，这些工作显示了组合基元在管理运动控制复杂性方面的优势，本文揭示了由于从动态基元组成行为而产生的一个局限性：虽然较慢的执行通常会使任务更容易，但存在一个限度，人类很难非常缓慢地移动。我们认为，这种显著的局限性不是由于肌肉不足，也不是由于神经通信缓慢，而是运动控制组织方式的结果。
