IISc Mathematics Initiative, Indian Institute of Science, Bengaluru, India.
Centre for Neuroscience, Indian Institute of Science, Bengaluru, India.
J Neurophysiol. 2023 May 1;129(5):1094-1113. doi: 10.1152/jn.00522.2022. Epub 2023 Mar 29.
Understanding how motor plans are transformed into appropriate patterns of muscle activity is a central question in motor control. Although muscle activity during the delay period has not been reported using conventional electromyographic (EMG) approaches, we isolated motor unit activity using a high-density surface EMG signal from the anterior deltoid muscle to test whether heterogeneity in motor units could reveal early preparatory activity. Consistent with our previous work (Rungta SP, Basu D, Sendhilnathan N, Murthy A. 126: 451-463, 2021), we observed early selective recruitment of small amplitude size motor units during the delay period for hand movements similar to the observed early recruitment of small-amplitude motor units in neck muscles of nonhuman primates performing delayed saccade tasks. This early activity was spatially specific and increased with time and resembled an accumulation to threshold model that correlated with movement onset time. Such early recruitment of ramping motor units was observed at the single trial level as well. In contrast, no such recruitment of large amplitude size motor units, called nonrampers, was observed during the delay period. Instead, nonrampers became spatially specific and predicted movement onset time after the delay period. Interestingly, spatially specific delay period activity was only observed for hand movements but was absent for isometric force-driven cursor movements. Nonetheless, muscle activity was correlated with the time it took to initiate movements in both task conditions for nonrampers. Overall, our results reveal a novel heterogeneity in the EMG activity that allows the expression of early motor preparation via small amplitude size motor units that are differentially activated during movement initiation. We studied the spatial and temporal aspects of response preparation in the anterior deltoid muscle using high-density surface EMG. Our results show that early spatially specific ramping activity that predicted reaction times could be accessed from muscle activity but was absent during isometric force-driven cursor movements. Such ramping activity could be quantified using an accumulator framework across trials, as well as within single trials, but was not observed in isometric reach tasks involving cursor movements.
理解运动计划如何转化为适当的肌肉活动模式是运动控制的一个核心问题。尽管使用传统的肌电图（EMG）方法尚未报告延迟期间的肌肉活动，但我们使用前三角肌的高密度表面 EMG 信号分离运动单位活动，以测试运动单位的异质性是否可以揭示早期预备活动。与我们之前的工作一致（Rungta SP、Basu D、Sendhilnathan N、Murthy A. 2021 年，126：451-463），我们观察到手部运动的延迟期间小幅度大小运动单位的早期选择性募集，类似于在执行延迟扫视任务的非人类灵长类动物的颈部肌肉中观察到的早期小幅度运动单位募集。这种早期活动具有空间特异性，随着时间的推移而增加，并且类似于与运动起始时间相关的累积到阈值模型。这种早期募集的斜坡运动单位也在单个试验水平上观察到。相比之下，在延迟期间没有观察到大振幅大小的运动单位（称为非斜坡器）的这种募集。相反，非斜坡器在延迟期后变得具有空间特异性，并预测运动起始时间。有趣的是，仅在手部运动中观察到具有空间特异性的延迟期活动，而在等距力驱动光标运动中则不存在。尽管如此，对于非斜坡器，肌肉活动与启动运动所需的时间相关，两种任务条件下均如此。总的来说，我们的结果揭示了 EMG 活动中的一种新的异质性，这种异质性允许通过在运动起始时具有不同激活的小幅度大小运动单位来表达早期运动准备。我们使用高密度表面 EMG 研究了前三角肌中反应准备的空间和时间方面。我们的结果表明，可以从肌肉活动中获取预测反应时间的早期空间特异性斜坡活动，但在等距力驱动光标运动中则不存在。这种斜坡活动可以使用累加器框架在试验之间以及单个试验内进行量化，但在涉及光标运动的等距到达任务中则观察不到。
