Acoustic-Emergent Phonology in the Amplitude Envelope of Child-Directed Speech

When acquiring language, young children may use acoustic spectro-temporal patterns in speech to derive phonological units in spoken language (e.g., prosodic stress patterns, syllables, phonemes). Children appear to learn acoustic-phonological mappings rapidly, without direct instruction, yet the underlying developmental mechanisms remain unclear. Across different languages, a relationship between amplitude envelope sensitivity and phonological development has been found, suggesting that children may make use of amplitude modulation (AM) patterns within the envelope to develop a phonological system. Here we present the Spectral Amplitude Modulation Phase Hierarchy (S-AMPH) model, a set of algorithms for deriving the dominant AM patterns in child-directed speech (CDS). Using Principal Components Analysis, we show that rhythmic CDS contains an AM hierarchy comprising 3 core modulation timescales. These timescales correspond to key phonological units: prosodic stress (Stress AM, ~2 Hz), syllables (Syllable AM, ~5 Hz) and onset-rime units (Phoneme AM, ~20 Hz). We argue that these AM patterns could in principle be used by naïve listeners to compute acoustic-phonological mappings without lexical knowledge. We then demonstrate that the modulation statistics within this AM hierarchy indeed parse the speech signal into a primitive hierarchically-organised phonological system comprising stress feet (proto-words), syllables and onset-rime units. We apply the S-AMPH model to two other CDS corpora, one spontaneous and one deliberately-timed. The model accurately identified 72–82% (freely-read CDS) and 90–98% (rhythmically-regular CDS) stress patterns, syllables and onset-rime units. This in-principle demonstration that primitive phonology can be extracted from speech AMs is termed Acoustic-Emergent Phonology (AEP) theory. AEP theory provides a set of methods for examining how early phonological development is shaped by the temporal modulation structure of speech across languages. The S-AMPH model reveals a crucial developmental role for stress feet (AMs ~2 Hz). Stress feet underpin different linguistic rhythm typologies, and speech rhythm underpins language acquisition by infants in all languages.

幼儿在语言习得过程中，可利用言语中的声学谱-时间模式（acoustic spectro-temporal patterns）推导口语中的音系单元（phonological units），例如韵律重音模式、音节、音位（phonemes）。幼儿似乎能在无直接指导的情况下快速习得声学-音系映射（acoustic-phonological mappings）关系，但其背后的发展机制仍未明确。跨语言研究均发现，振幅包络敏感性与音系发展存在关联，这提示幼儿或可借助包络内的振幅调制（amplitude modulation, AM）模式构建音系系统。

本文提出谱振幅调制相位层级（Spectral Amplitude Modulation Phase Hierarchy, S-AMPH）模型，这是一套用于提取儿童导向言语（child-directed speech, CDS）中主导振幅调制模式的算法集。通过主成分分析（Principal Components Analysis），我们证实有节律的儿童导向言语包含由3个核心调制时间尺度构成的振幅调制层级。这三个时间尺度分别对应关键音系单元：韵律重音（重音AM，约2 Hz）、音节（音节AM，约5 Hz）以及首音-韵腹单元（onset-rime units），即音位AM（Phoneme AM），调制频率约为20 Hz。

我们认为，理论上朴素听者（naïve listeners）可借助这些振幅调制模式，在无需词汇知识的前提下完成声学-音系映射的计算。后续研究证明，该振幅调制层级内的调制统计特征可将言语信号解析为一套原始的层级化音系系统，该系统包含重音音步（proto-words，原型词）、音节与首音-韵腹单元。

我们将S-AMPH模型应用于另外两则儿童导向言语语料库，一则为自发式语料，另一则为刻意计时式语料。该模型可准确识别72%~82%的自由朗读式儿童导向言语、90%~98%的节律规整式儿童导向言语中的重音模式、音节与首音-韵腹单元。

这项从理论层面证明可从言语振幅调制中提取原始音系的研究，被命名为声学涌现音系（Acoustic-Emergent Phonology, AEP）理论。该理论提供了一套研究方法，用于探究跨语言场景下，言语的时间调制结构如何塑造早期音系发展。S-AMPH模型揭示了重音音步（约2 Hz的振幅调制）在儿童语言发展中的关键作用。重音音步支撑着不同语言的节律类型学特征，而言语节律则为所有语言环境中的婴儿语言习得提供了基础。