Progressive changes in vocal behavior over the course of vocal imitation leaning are often attributed exclusively to developing neural circuits, but the effects of postnatal body changes remain unknown. In songbirds, the syrinx transforms song system motor commands into sound and exhibits changes during song learning. Here we test the hypothesis that the transformation from motor commands to force trajectories by syringeal muscles functionally changes over vocal development in zebra finches. Our data collected in both sexes show that, only in males, muscle speed significantly increases and that supralinear summation occurs and increases with muscle contraction speed. Furthermore, we show that previously reported submillisecond spike timing in the avian cortex can be resolved by superfast syringeal muscles and that the sensitivity to spike timing increases with speed. Because motor neuron and muscle properties are tightly linked, we make predictions on the boundaries of the yet unknown motor code that correspond well with cortical activity. Together, we show that syringeal muscles undergo essential transformations during song learning that drastically change how neural commands are translated into force profiles and thereby acoustic features. We propose that the song system motor code must compensate for these changes to achieve its acoustic targets. Our data thus support the hypothesis that the neuromuscular transformation changes over vocal development and emphasizes the need for an embodied view of song motor learning.

SIGNIFICANCE STATEMENT Fine motor skill learning typically occurs in a postnatal period when the brain is learning to control a body that is changing dramatically due to growth and development. How the developing body influences motor code formation and vice versa remains largely unknown. Here we show that vocal muscles in songbirds undergo critical transformations during song learning that drastically change how neural commands are translated into force profiles and thereby acoustic features. We propose that the motor code must compensate for these changes to achieve its acoustic targets. Our data thus support the hypothesis that the neuromuscular transformation changes over vocal development and emphasizes the need for an embodied view of song motor learning.

在模仿声音的过程中，声音行为的逐渐变化通常仅归因于神经回路的发育，但是产后身体变化的影响仍然未知。在鸣禽中，syrinx将歌曲系统的运动命令转换为声音，并在歌曲学习过程中表现出变化。在这里，我们测试了以下假设：斑马雀科的发声过程中，从运动命令到注射器肌肉的强制轨迹的转换在功能上会发生变化。我们在两性中收集的数据表明，仅在男性中，肌肉速度显着增加，并且超线性求和发生并随肌肉收缩速度而增加。此外，我们表明，以前报道的鸟类皮层中亚毫秒级的刺激时间可以通过超快的注射肌肉来解决，并且刺激时间的敏感性随速度增加而增加。由于运动神经元和肌肉属性紧密相关，因此我们可以预测与皮层活动非常吻合的未知运动代码的边界。在一起，我们表明，在歌曲学习过程中，注射器肌肉发生了必不可少的转变，从而极大地改变了将神经命令转化为力的方式，从而改变了听觉特征。我们建议歌曲系统的马达代码必须补偿这些变化，以实现其声学目标。因此，我们的数据支持以下假设：神经肌肉的转换会随着声音的发展而变化，并强调了对歌曲运动学习的具体观点的需求。

重要声明精细运动技能学习通常发生在出生后的时期，那时大脑正在学习控制由于生长和发育而急剧变化的身体。发育中的身体如何影响运动代码的形成，反之亦然，这一点在很大程度上尚不清楚。在这里，我们表明，在歌曲学习过程中，鸣禽中的声乐肌肉经历了重要的转变，从而极大地改变了将神经命令转换为力的方式以及由此产生的声音特征的方式。我们建议电机代码必须补偿这些变化以实现其声学目标。因此，我们的数据支持这样的假说，即神经肌肉的转换随声音的发展而变化，并强调了对歌曲运动学习的具体观点的需求。