Chen Yunru, Chen Chih-Ting, Gui Yuhan, Kanold Patrick O
bioRxiv. 2025 Aug 26:2025.08.26.672221. doi: 10.1101/2025.08.26.672221.
Sound harmonicity is foundational in complex auditory stimuli like music and vocalizations but it remains unclear how such spectrally complex stimuli are processed in the auditory cortex (ACtx). Subregions of the auditory cortex process are thought to process harmonic stimuli differently, and secondary ACtx (A2) layer (L) 2/3 is believed to be the most selective. Selective responses to sound features in ACtx are thought to emerge hierarchically starting from A1 L4. Since the spectral complexity of harmonic stacks can range from two to more than ten components, harmonic selectivity and sensitivity might also arise hierarchically across layers and areas. We studied responses to simple and complex harmonic stacks across A1 L4, A1 L2/3, and A2 L2/3 in adult mice using in vivo two-photon microscopy. We found harmonic-sensitive neurons (HN) responding only to harmonic stacks but not to their pure-tone components in all areas at similar proportions. HNs showed non-linear processing of component tones with onset-responsive HNs showing greater nonlinearity, which decreased with harmonic complexity. Co-tuned HNs in A1 L4 exhibited the highest signal correlation, regardless of harmonic complexity. A1 L4 HNs also showed the lowest noise correlation with other neurons. Moreover, A1 L4 HNs achieve robust spectral integration and harmonic sensitivity by receiving diverse inputs and maintaining high signal correlation, ensuring independent, strong responses to harmonic stimuli. Therefore, harmonic sensitivity is present in A1 L4 and is not a unique feature of A2. Thus, tuning to complex spectral sounds is a fundamental property of ACtx and is already established in A1 L4.
Harmonics are essential in auditory perception, influencing how we process complex sounds like music and speech. This study reveals that neurons in the primary auditory cortex (A1) and secondary auditory cortex (A2) integrate simple and complex harmonic structures with distinct mechanisms of neuronal recruitment. A1 L4 harmonic-sensitive neurons (HNs) demonstrated strong, independent responses through high signal correlation and minimal noise correlation, suggesting a robust mechanism for spectral integration. Our results show that harmonic relationships are already extracted at the input layers of A1, and that HNs show non-linear facilitative integration. Thus, tuning to sounds of complex spectral contents might be a fundamental processing function of the auditory cortex and is already established in A1 L4, which receives major thalamic inputs.
声音谐波在音乐和发声等复杂听觉刺激中是基础，但目前尚不清楚这种频谱复杂的刺激在听觉皮层（ACtx）中是如何被处理的。听觉皮层的各个子区域被认为对谐波刺激的处理方式不同，并且二级听觉皮层（A2）第2/3层被认为是最具选择性的。听觉皮层中对声音特征的选择性反应被认为是从A1第4层开始分层出现的。由于谐波叠加的频谱复杂性可以从两个成分到十多个成分不等，谐波选择性和敏感性也可能在各层和各区域中分层出现。我们使用体内双光子显微镜研究了成年小鼠A1第4层、A1第2/3层和A2第2/3层对简单和复杂谐波叠加的反应。我们发现谐波敏感神经元（HN）在所有区域中以相似的比例仅对谐波叠加做出反应，而不对其纯音成分做出反应。HN对成分音表现出非线性处理，起始反应性HN表现出更大的非线性，且随着谐波复杂性的增加而降低。无论谐波复杂性如何，A1第4层中的共调谐HN表现出最高的信号相关性。A1第4层的HN与其他神经元的噪声相关性也最低。此外，A1第4层的HN通过接收多样化的输入并保持高信号相关性来实现强大的频谱整合和谐波敏感性，确保对谐波刺激有独立、强烈的反应。因此，谐波敏感性存在于A1第4层，而不是A2的独特特征。因此，对复杂频谱声音的调谐是听觉皮层的基本属性，并且已经在A1第4层中确立。
谐波在听觉感知中至关重要，影响着我们处理音乐和语音等复杂声音的方式。这项研究表明，初级听觉皮层（A1）和二级听觉皮层（A2）中的神经元通过不同的神经元募集机制整合简单和复杂的谐波结构。A1第4层的谐波敏感神经元（HN）通过高信号相关性和最小噪声相关性表现出强烈、独立的反应，表明存在一种强大的频谱整合机制。我们的结果表明，谐波关系在A1的输入层就已经被提取，并且HN表现出非线性促进整合。因此，对复杂频谱内容声音的调谐可能是听觉皮层的基本处理功能，并且已经在接收主要丘脑输入的A1第4层中确立。
