Department of Hearing and Speech Sciences, Vanderbilt University Medical Center, Nashville, TN 37232, USA.
Cereb Cortex. 2020 Mar 21;30(2):618-627. doi: 10.1093/cercor/bhz112.
Speech perception involves mapping from a continuous and variable acoustic speech signal to discrete, linguistically meaningful units. However, it is unclear where in the auditory processing stream speech sound representations cease to be veridical (faithfully encoding precise acoustic properties) and become categorical (encoding sounds as linguistic categories). In this study, we used functional magnetic resonance imaging and multivariate pattern analysis to determine whether tonotopic primary auditory cortex (PAC), defined as tonotopic voxels falling within Heschl's gyrus, represents one class of speech sounds-vowels-veridically or categorically. For each of 15 participants, 4 individualized synthetic vowel stimuli were generated such that the vowels were equidistant in acoustic space, yet straddled a categorical boundary (with the first 2 vowels perceived as [i] and the last 2 perceived as [i]). Each participant's 4 vowels were then presented in a block design with an irrelevant but attention-demanding level change detection task. We found that in PAC bilaterally, neural discrimination between pairs of vowels that crossed the categorical boundary was more accurate than neural discrimination between equivalently spaced vowel pairs that fell within a category. These findings suggest that PAC does not represent vowel sounds veridically, but that encoding of vowels is shaped by linguistically relevant phonemic categories.
言语感知涉及将连续且可变的言语信号映射到离散的、具有语言意义的单元。然而，目前尚不清楚在听觉处理过程中，言语声音的表示从真实（忠实地编码精确的声学属性）转变为类别（将声音编码为语言类别）的位置。在这项研究中，我们使用功能磁共振成像和多元模式分析来确定音调拓扑主听觉皮层（PAC）是否真实或类别地表示一类言语声音——元音。对于 15 名参与者中的每一名，生成 4 个个体化的合成元音刺激，使得元音在声学空间中等距，但跨越类别边界（前 2 个元音被感知为[i]，后 2 个元音被感知为[i]）。然后，每个参与者的 4 个元音在一个块设计中呈现，同时进行一项无关但需要注意力的水平变化检测任务。我们发现，在双侧的 PAC 中，跨越类别边界的元音对之间的神经辨别比落在类别内的等距元音对之间的神经辨别更准确。这些发现表明，PAC 没有真实地表示元音，而是元音的编码受到语言相关的音位类别的影响。
