Popescu F C, Rymer W Z
Northwestern University, Evanston 60208, USA.
J Neurophysiol. 2000 Nov;84(5):2670-9. doi: 10.1152/jn.2000.84.5.2670.
A single force pulse was applied unexpectedly to the arms of five normal human subjects during nonvisually guided planar reaching movements of 10-cm amplitude. The pulse was applied by a powered manipulandum in a direction perpendicular to the motion of the hand, which gripped the manipulandum via a handle at the beginning, at the middle, or toward the end the movement. It was small and brief (10 N, 10 ms), so that it was barely perceptible. We found that the end points of the perturbed motions were systematically different from those of the unperturbed movements. This difference, dubbed "terminal error," averaged 14.4 +/- 9.8% (mean +/- SD) of the movement distance. The terminal error was not necessarily in the direction of the perturbation, although it was affected by it, and it did not decrease significantly with practice. For example, while perturbations involving elbow extension resulted in a statistically significant shift in mean end-point and target-acquisition frequency, the flexion perturbations were not clearly affected. We argue that this error distribution is inconsistent with the "equilibrium point hypothesis" (EPH), which predicts minimal terminal error is determined primarily by the variance in the command signal itself, a property referred to as "equifinality." This property reputedly derives from the "spring-like" properties of muscle and is enhanced by reflexes. To ensure that terminal errors were not due to mid-course voluntary corrections, we only accepted trials in which the final position was already established before such a voluntary response to the perturbation could have begun, that is, in a time interval shorter than the minimum reaction time (RT) for that subject. This RT was estimated for each subject in supplementary experiments in which the subject was instructed to move to a new target if perturbed and to the old target if no perturbation was detected. These RT movements were found to either stop or slow greatly at the original target, then re-accelerate to the new one. The average latency of this second motion was used to estimate the voluntary RT for each subject (316 ms mean). Additionally, we found that the hand neither exerted target-oriented force against the handle nor drifted toward the desired end point just before coming to rest, making it unlikely that the mechanical properties of the manipulandum prevented the hand from reaching its intended target.
在5名正常受试者进行幅度为10厘米的非视觉引导平面伸手动作过程中，意外地向其手臂施加了单个力脉冲。该脉冲由动力操作器在垂直于手部运动的方向上施加，手部在运动开始时、中间或接近结束时通过手柄握住操作器。该脉冲较小且短暂（10牛，10毫秒），几乎难以察觉。我们发现，受扰运动的终点与未受扰运动的终点存在系统性差异。这种差异被称为“终端误差”，平均为运动距离的14.4±9.8%（平均值±标准差）。终端误差不一定与扰动方向一致，尽管它会受到扰动的影响，并且不会随着练习而显著减小。例如，虽然涉及肘部伸展的扰动导致平均终点和目标获取频率在统计学上有显著偏移，但屈曲扰动并未受到明显影响。我们认为，这种误差分布与“平衡点假说”（EPH）不一致，该假说预测最小终端误差主要由命令信号本身的方差决定，这一特性被称为“等效最终性”。据称，这一特性源于肌肉的“弹簧样”特性，并通过反射得到增强。为确保终端误差不是由于中途的自愿修正造成的，我们只接受那些在对扰动做出这种自愿反应开始之前就已经确定了最终位置的试验，也就是说，在一个比该受试者的最小反应时间（RT）更短的时间间隔内。这个RT是在补充实验中为每个受试者估计的，在实验中，如果受到扰动，受试者被指示移动到新目标，如果未检测到扰动，则移动到旧目标。发现这些RT动作在原始目标处要么停止，要么大幅减速，然后重新加速到新目标。第二次运动的平均延迟用于估计每个受试者的自愿RT（平均316毫秒）。此外，我们发现，手部在即将静止之前既没有对手柄施加面向目标的力，也没有向期望的终点漂移，这使得操作器的机械特性不太可能阻止手部到达其预期目标。
