Observing a Movement Correction during Walking Affects Evoked Responses but Not Unperturbed Walking

Seeing an action activates neurons in the premotor, motor, and somatosensory cortex. Since a significant fraction of these pyramidal neurons project to the spinal motor circuits, a central question is why we do not automatically perform the actions that we see. Indeed, seeing an action increases both cortical and spinal excitability of consistent motor patterns that correspond to the observed ones. Thus, it is believed that such imitative motor patterns are either suppressed or remain at a sub-threshold level. This would predict, however, that seeing someone make a corrective movement while one is actively involved in the same action should either suppress evoked responses or suppress or modulate the action itself. Here we tested this prediction, and found that seeing someone occasionally stepping over an obstacle while walking on a treadmill did not affect the normal walking pattern at all. However, cutaneously evoked reflexes in the anterior tibial and soleus muscles were modulated as if the subject was stepping over an obstacle. This result thus indicates that spinal activation was not suppressed and was neither at sub-threshold motor resonance. Rather, the spinal modulation from observed stepping reflects an adaptive mechanism for regulating predictive control mechanisms. We conclude that spinal excitability during action observation is not an adverse side-effect of action understanding but reflects adaptive and predictive motor control.观察某一动作时，运动前皮层（premotor cortex）、运动皮层（motor cortex）与躯体感觉皮层（somatosensory cortex）内的神经元会被激活。由于此类锥体细胞（pyramidal neurons）中有相当比例会向脊髓运动环路（spinal motor circuits）投射，学界的核心问题在于：为何我们不会自动执行所观察到的动作？诚然，观察动作会增强与所观察动作匹配的一致性运动模式的皮层与脊髓兴奋性（cortical and spinal excitability）。因此，学界普遍认为，此类模仿性运动模式要么受到抑制，要么维持在阈下水平。不过，这一结论可作出如下预测：当个体正主动执行某一动作时，若观察到他人做出矫正动作，要么会抑制自身的诱发反应，要么会抑制或调节正在进行的动作本身。本研究对该预测进行了验证，结果发现：当受试者在跑步机（treadmill）上行走时，偶尔观察到他人跨越障碍物的动作，并未对其正常行走模式产生任何影响。但胫骨前肌（anterior tibial）与比目鱼肌（soleus muscles）的皮肤诱发反射（cutaneously evoked reflexes）却出现了调节，其表现与受试者自身尝试跨越障碍物时一致。该结果表明，脊髓激活并未受到抑制，也未处于阈下运动共振状态。相反，由观察到的迈步动作引发的脊髓调节，实则为调节预测控制机制（predictive control mechanisms）的适应性机制。我们由此得出结论：动作观察（action observation）过程中的脊髓兴奋性并非动作理解的负面副作用，而是反映了适应性与预测性运动控制过程。