Vocal development through morphological computation

The vocal behavior of infants changes dramatically during early life. Whether or not such a change results from the growth of the body during development—as opposed to solely neural changes—has rarely been investigated. In this study of vocal development in marmoset monkeys, we tested the putative causal relationship between bodily growth and vocal development. During the first two months of life, the spontaneous vocalizations of marmosets undergo (1) a gradual disappearance of context-inappropriate call types and (2) an elongation in the duration of context-appropriate contact calls. We hypothesized that both changes are the natural consequences of lung growth and do not require any changes at the neural level. To test this idea, we first present a central pattern generator model of marmoset vocal production to demonstrate that lung growth can affect the temporal and oscillatory dynamics of neural circuits via sensory feedback from the lungs. Lung growth qualitatively shifted vocal behavior in the direction observed in real marmoset monkey vocal development. We then empirically tested this hypothesis by placing the marmoset infants in a helium–oxygen (heliox) environment in which air is much lighter. This simulated a reversal in development by decreasing the effort required to respire, thus increasing the respiration rate (as though the lungs were smaller). The heliox manipulation increased the proportions of inappropriate call types and decreased the duration of contact calls, consistent with a brief reversal of vocal development. These results suggest that bodily growth alone can play a major role in shaping the development of vocal behavior.幼崽的发声行为在生命早期会发生显著变化。这类变化究竟是源于发育过程中的身体生长（而非仅由神经变化导致），相关研究迄今仍较为匮乏。本研究以狨猴为对象，探究身体生长与发声发育之间的推定因果关系。在出生后的前两个月内，狨猴的自发发声呈现出两项特征变化：(1) 与情境不匹配的鸣叫类型逐渐消失；(2) 与情境匹配的联系性鸣叫的时长逐渐延长。我们提出假设：这两项变化均为肺部生长的自然结果，无需神经层面发生任何改变。为验证该假设，我们首先构建了狨猴发声的中枢模式发生器（central pattern generator）模型，证明肺部生长可通过肺部的感觉反馈影响神经回路的时序与振荡动力学特性。肺部生长可使发声行为在质向上朝着真实狨猴发声发育过程中观察到的方向发生改变。随后我们通过实验验证该假设：将狨猴幼崽置于氦氧混合气（heliox）环境中——该环境下空气密度更低。此举可降低呼吸所需的做功，进而提升呼吸频率（模拟肺部更小的发育状态），以此实现发育状态的逆转模拟。氦氧混合气干预使得不匹配情境的鸣叫类型占比上升，同时缩短了联系性鸣叫的时长，这与发声发育短暂逆转的预期结果相符。上述研究结果表明，仅靠身体生长即可在塑造发声行为发育的过程中发挥核心作用。