Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania.
Institute of Electronics and Computer Science, Riga, Latvia.
J Neurophysiol. 2020 Dec 1;124(6):1625-1636. doi: 10.1152/jn.00322.2020. Epub 2020 Sep 30.
When a person tries to press with a finger, other fingers of the hand produce force unintentionally. We explored this phenomenon of enslaving during unintentional force drifts in the course of continuous force production by pairs of fingers of a hand. Healthy subjects performed accurate force production tasks by finger pairs Index-Middle, Middle-Ring, and Ring-Little with continuous visual feedback on the combined force of the instructed (master) fingers or of the noninstructed (enslaved) fingers. The feedback scale was adjusted to ensure that the subjects did not know the difference between these two, randomly presented, conditions. Across all finger pairs, enslaved force showed a drift upward under feedback on the master finger force, and master force showed a drift downward under feedback on the enslaved finger force. The subjects were unaware of the force drifts, which could reach over 50% of the initial force magnitude over 15 s. Across all conditions, the index of enslaving increased by ∼50% over the trial duration. The initial moment of force magnitude in pronation-supination was not a consistent predictor of the force drift magnitude. These results falsify the hypothesis that the counter-directional force drifts reflected drifts in the moment of force. They suggest that during continuous force production, enslaving increases with time, possibly due to the spread of excitation over cortical finger representations or other mechanisms, such as increased synchronization of firing of α-motoneurons innervating different compartments of extrinsic flexors. These changes in enslaving, interpreted at the level of control with referent coordinates for the fingers, can contribute to a variety of phenomena, including unintentional force drifts. We report a consistent slow increase in finger enslaving (force production by noninstructed fingers) when visual feedback was presented on the force produced by either two instructed fingers or two noninstructed fingers of the hand. In contrast, force drifts could be in opposite directions depending on the visual feedback. We interpret enslaving and its drifts at the level of control with referent coordinates for the involved muscles, possibly reflecting spread of cortical excitation.
当一个人试图用手指按压时，手的其他手指会无意识地产生力。我们在手的成对手指在连续用力过程中，探索了这种在无意用力漂移过程中被奴役的现象。健康受试者通过手指对 Index-Middle、Middle-Ring 和 Ring-Little 进行准确的力产生任务，通过连续视觉反馈对指示（主）手指或非指示（奴役）手指的合力进行反馈。反馈刻度进行了调整，以确保受试者无法区分这两种随机呈现的条件。在所有手指对中，在主手指力反馈下，奴役力向上漂移，而在奴役手指力反馈下，主手指力向下漂移。受试者没有意识到力漂移，这些漂移在 15 秒内可以达到初始力大小的 50%以上。在所有条件下，奴役指数在试验持续时间内增加了约 50%。旋前-旋后力大小的初始时刻不是力漂移大小的一致预测因子。这些结果否定了反方向力漂移反映力矩漂移的假设。它们表明，在连续用力过程中，随着时间的推移，奴役会增加，这可能是由于兴奋在皮质手指代表区域的扩散，或者是由于支配外在屈肌不同节段的α运动神经元的放电同步增加等机制所致。这些奴役变化，在控制水平上用手指的参考坐标进行解释，可以导致各种现象，包括无意的力漂移。我们报告了当视觉反馈呈现在手的两个指示手指或两个非指示手指产生的力上时，手指奴役（非指示手指产生的力）持续缓慢增加。相比之下，力漂移的方向可能取决于视觉反馈。我们用涉及肌肉的参考坐标来解释奴役及其漂移，这可能反映了皮质兴奋的扩散。
