Physiological and Computational-Modeling Studies of Timbre Encoding in the Inferior Colliculus

音色，当声音在音高、音阶和音量上相同时，可以区分声音的质量 时长，是言语理解和音乐享受的一个关键方面。我的提议将填补神经学领域的一个空白 通过检验捕获和离-CF抑制机制导致健壮的假设来研究音色 超阈值合成和自然乐器音色在中脑的表现。为了检验我的假设， 我将记录清醒的荷兰带兔的单个单位下丘(IC)反应。我还将开发一种 基于这些生理反应的新的计算IC模型。 谐音的频谱包络与“亮度”的音色感知相关。我 提出了两种有助于频谱包络编码的机制：捕获和非特征频率 (Cf)抑制。第一种机制是俘获，指的是频谱峰值附近的谐波占优势。 听神经纤维在山顶附近调谐。捕获是由于内毛细胞饱和所致。由一台计算机捕获 谐波降低了听神经纤维中低频神经波动的幅度。IC的速率 神经元对以调制传递函数为特征的低频神经波动非常敏感。 最终，捕捉到调谐到谱峰附近的纤维的听神经反应会导致IC率曲线 对光谱峰值进行编码。初步结果与合成音色的谱峰部分一致 捕捉外周反应的刺激，导致显著光谱特征的速率表示 中脑。然而，另一种可以解释音色IC表征的机制是OFF-CF抑制，这是 已经提出用来解释扫频敏感性和心理物理前向掩蔽。一种皮质下 以捕捉为特征的计算模型能够预测初步反应，而不是非循环抑制。 对合成音色和窄带噪声中音，但无法预测对宽带噪声中音或 自然音色，表示需要更新模型。 我已经开发了实验来测试这样的假设，即音色通过 捕获和脱离-CF抑制。Aim 1.1将测试以下假设：对宽带噪声中音的响应 受到OFF-CF抑制的强烈影响，降低噪声带宽会增加捕获的影响。在……里面 目标1.2我将更新一个计算IC模型，添加OFF-CF抑制来测试捕获和 Off-CF抑制是解释噪声中音刺激所必需的。Aim 2.1将测试光谱的假设 成形的和声复合体的峰值，合成音色，在IC中强健地编码在一系列 超过阈值的声音级别。Aim 2.2通过录音弥合了合成音色和自然音色之间的差距 对真实乐器声音的反应。Aim 2的回应将进一步测试新的IC模型。这个项目将 提供关于超阈值音色的IC表示的见解。这项研究将导致新的战略，以 在助听器和人工耳蜗中恢复音色感知，这不是为音色感知而设计的。

Timbre, the quality that allows sounds to be distinguished when they are identical in pitch, level, and duration, is a critical aspect of speech comprehension and music enjoyment. My proposal will fill a gap in neural studies of timbre by testing the hypothesis that capture and off-CF inhibitory mechanisms lead to robust representations of suprathreshold synthetic and natural-instrument timbre in the midbrain. To test my hypothesis, I will record single-unit inferior colliculus (IC) responses from awake Dutch-belted rabbits. I will also develop a new computational IC model based on these physiological responses. Spectral envelopes of harmonic sounds are correlated with the timbral perception of “brightness”. I propose two mechanisms that contribute to spectral-envelope encoding: capture and off-characteristic frequency (CF) inhibition. The first mechanism, capture, refers to the dominance of harmonics near spectral peaks over auditory-nerve fibers tuned near the peaks. Capture is due to saturation of inner hair cells. Capture by a single harmonic reduces the amplitude of low-frequency neural fluctuations in auditory-nerve fibers. Rates of IC neurons are sensitive to low-frequency neural fluctuations as characterized by modulation transfer functions. Ultimately, capture of auditory-nerve responses for fibers tuned near spectral peaks results in IC rate profiles that encode spectral peaks. Preliminary results are partially consistent with spectral peaks of synthetic timbre stimuli capturing peripheral responses, leading to a rate representation of salient spectral features in the midbrain. However, another mechanism that could explain IC representations of timbre is off-CF inhibition, which has been proposed to explain frequency-sweep sensitivity and psychophysical forward masking. A subcortical computational model that features capture, but not off-CF inhibition, was able to predict preliminary responses to synthetic timbre and narrowband tone-in-noise, but could not predict responses to wideband tone-in-noise or natural timbre, indicating the need to update the model. I have developed experiments to test the hypothesis that timbre is robustly encoded in the midbrain via capture and off-CF inhibition. Aim 1.1 will test the hypothesis that responses to wideband tone-in-noise are strongly influenced by off-CF inhibition, and reducing the noise bandwidth increases the influence of capture. In Aim 1.2 I will update a computational IC model by adding off-CF inhibition to test the hypothesis that capture and off-CF inhibition are necessary to explain tone-in-noise stimuli. Aim 2.1 will test the hypothesis that the spectral peak of a shaped harmonic complex, synthetic timbre, is robustly encoded in the IC over a range of suprathreshold sound levels. Aim 2.2 bridges the gap between synthetic and natural timbre by recording responses to real instrument sounds. Responses from Aim 2 will further test the new IC model. This project will provide insight on the IC representation of suprathreshold timbre. This research will lead to novel strategies to restore timbre perception in hearing aids and cochlear implants, which are not designed for timbre perception.