Guo Zhe-Chen, McHaney Jacie R, Parthasarathy Aravindakshan, McFarlane Kailyn A, Chandrasekaran Bharath
Roxelyn and Richard Pepper Department of Communication Sciences and Disorders, Northwestern University, Evanston, IL, USA.
Department of Communication Science and Disorders, University of Pittsburgh, Pittsburgh, PA, USA.
Neurobiol Lang (Camb). 2025 Jun 18;6. doi: 10.1162/nol_a_00169. eCollection 2025.
Speech perception can decline in middle age even when hearing thresholds remain normal, and the underlying neurobiological mechanisms are not well understood. In line with the age-related neural dedifferentiation hypothesis, we predicted that middle-aged adults show less distinct cortical representations of phonemes and acoustic-phonetic features relative to younger adults. In addition to an extensive audiological, auditory electrophysiological, and speech perceptual test battery, we measured electroencephalographic responses time-locked to phoneme instances (phoneme-related potential) in naturalistic, continuous speech and trained neural network classifiers to predict phonemes from these responses. Consistent with age-related neural dedifferentiation, phoneme predictions were less accurate, more uncertain, and involved a broader network for middle-aged adults compared with younger adults. Representational similarity analysis revealed that the featural relationship between phonemes was less robust in middle age. Electrophysiological and behavioral measures revealed signatures of putative cochlear neural degeneration (CND) and speech perceptual deficits in middle-aged adults relative to younger adults. In line with prior work in animal models, proxies of CND were associated with greater cortical dedifferentiation, explaining nearly a third of the variance in PRP prediction accuracy together with measures of acoustic neural processing. Notably, even after controlling for CND proxies and acoustic processing abilities, age-group differences in cortical PRP prediction accuracy remained. Overall, the results reveal "fuzzier" cortical phonemic representations in middle age, suggesting that age-related neural dedifferentiation may underlie speech perceptual challenges despite a normal audiogram.
即使听力阈值保持正常，言语感知能力在中年时仍可能下降，其潜在的神经生物学机制尚未完全明确。根据与年龄相关的神经去分化假说，我们预测中年成年人相对于年轻成年人而言，对音素和声学语音特征的皮层表征较不清晰。除了一系列广泛的听力学、听觉电生理学和言语感知测试外，我们还测量了在自然连续语音中与音素实例锁时的脑电图反应（音素相关电位），并训练神经网络分类器根据这些反应预测音素。与年龄相关的神经去分化一致，与年轻成年人相比，中年成年人的音素预测准确性较低、不确定性更高，且涉及更广泛的神经网络。表征相似性分析表明，音素之间的特征关系在中年时不太稳固。电生理学和行为测量结果显示，相对于年轻成年人，中年成年人存在假定的耳蜗神经变性（CND）和言语感知缺陷的迹象。与先前在动物模型中的研究结果一致，CND的指标与更大程度的皮层去分化相关，与声学神经处理指标共同解释了PRP预测准确性中近三分之一的方差。值得注意的是，即使在控制了CND指标和声学处理能力之后，皮层PRP预测准确性的年龄组差异仍然存在。总体而言，研究结果揭示了中年时期皮层音素表征“更模糊”，这表明尽管听力图正常，但与年龄相关的神经去分化可能是言语感知挑战的潜在原因。
