Harkrider Ashley W, Plyler Patrick N, Hedrick Mark S
Department of Audiology and Speech Pathology, University of Tennessee, Knoxville, TN 37920, USA.
Ear Hear. 2009 Feb;30(1):31-42. doi: 10.1097/AUD.0b013e31818f359f.
The primary purpose of this study was to more clearly define the effects of hearing loss, separate from age, on perception, and neural response patterns of dynamic spectral cues. To do this, the study was designed to determine whether (1) hearing loss affects the neural representation and/or categorical perception of stop-consonant stimuli among young adults and (2) spectrally shaped amplification aimed at increasing the audibility of the F2 formant transition cue reduces any effects of hearing loss. It was predicted that (1) young adults with hearing loss would differ from young adults with normal hearing in their behavioral and neural responses to stop-consonant stimuli and (2) enhancing the audibility of the F2 formant transition cue relative to the rest of the stimulus would not overcome the effects of hearing loss on behavioral performance or neural response patterns.
Behavioral identification and neural response patterns of stop-consonant stimuli varying along the /b-d-g/ place-of-articulation continuum were measured from seven young adults with mild-to-moderate hearing impairment (mean age = 21.4 yr) and compared with responses from 11 young adults with normal hearing (mean age = 27 yr). Psychometric functions and N1-P2 cortical-evoked responses were evoked by consonant-vowel (CV) stimuli without (unshaped) and with (shaped) frequency-dependent amplification that enhanced F2 relative to the rest of the stimulus.
Behavioral identification and neural response patterns of stop-consonant CVs differed between the two groups. Specifically, to the unshaped stimuli, listeners with hearing loss tended to make low-frequency judgments more often (more /b/, fewer /g/) than listeners with normal hearing when categorizing along the /b-d-g/ continuum. Additionally, N1 amplitudes were larger and P2 latencies were longer to all phonemes in young adults with hearing impairment versus normal hearing. Enhancing the audibility of the F2 transition cue with spectrally shaped amplification did not alter the neural representation of the stop-consonant CVs in the young listeners with hearing loss. It did modify categorical perception such that listeners with hearing loss tended to make high-frequency judgments more often (more /g/, fewer /b/). However, shaping the stimuli did not make their psychometric functions more like those of the normal controls. Instead, young adults with hearing loss went from one extreme (low-frequency judgments with unshaped stimuli) to the other (high-frequency judgments with shaped stimuli), whereas judgments from the normal controls were more balanced.
Hearing loss, separate from aging, seems to negatively impact identification and neural representation of time-varying spectral cues like the F2 formant transition. Enhancing the audibility of the F2 formant transition cue relative to the rest of the stimulus does not overcome the effects of hearing loss on behavioral performance or neural response patterns in young adults. Thus, the deleterious effects of hearing loss on stop-consonant perception along the place-of-articulation continuum may not only be due solely to decreased audibility but also due to improper coding by residual neurons, resulting in distortion of the time-varying spectral cue. This may explain, in part, why amplification cannot completely compensate for the effects of sensorineural hearing loss.
本研究的主要目的是更清晰地界定听力损失（与年龄因素分开）对动态频谱线索的感知及神经反应模式的影响。为此，本研究旨在确定：（1）听力损失是否会影响年轻成年人中塞音刺激的神经表征和/或范畴知觉；（2）旨在提高F2共振峰过渡线索可听度的频谱塑形放大是否能减少听力损失的任何影响。研究预测：（1）有听力损失的年轻成年人在对塞音刺激的行为和神经反应方面会与听力正常的年轻成年人不同；（2）相对于刺激的其他部分，增强F2共振峰过渡线索的可听度不会克服听力损失对行为表现或神经反应模式的影响。
从7名轻度至中度听力障碍的年轻成年人（平均年龄 = 21.4岁）中测量了沿/b-d-g/发音部位连续体变化的塞音刺激的行为识别和神经反应模式，并与11名听力正常的年轻成年人（平均年龄 = 27岁）的反应进行比较。通过辅音-元音（CV）刺激诱发心理测量函数和N1-P2皮层诱发反应，刺激分为无（未塑形）和有（塑形）频率依赖性放大两种情况，后者相对于刺激的其他部分增强了F2。
两组之间塞音CV的行为识别和神经反应模式存在差异。具体而言，对于未塑形刺激，在沿/b-d-g/连续体进行分类时，有听力损失的听众比听力正常的听众更倾向于做出低频判断（更多/b/，更少/g/）。此外，与听力正常的年轻成年人相比，有听力损失的年轻成年人对所有音素的N1波幅更大且P2潜伏期更长。用频谱塑形放大增强F2过渡线索的可听度并未改变有听力损失的年轻听众中塞音CV的神经表征。它确实改变了范畴知觉，使得有听力损失的听众更倾向于做出高频判断（更多/g/，更少/b/）。然而，对刺激进行塑形并没有使他们的心理测量函数更接近正常对照组。相反，有听力损失的年轻成年人从一个极端（对未塑形刺激做出低频判断）转变为另一个极端（对塑形刺激做出高频判断），而正常对照组的判断则更为平衡。
与衰老无关，听力损失似乎会对诸如F2共振峰过渡等随时间变化的频谱线索的识别和神经表征产生负面影响。相对于刺激的其他部分，增强F2共振峰过渡线索的可听度并不能克服听力损失对年轻成年人行为表现或神经反应模式的影响。因此，听力损失对沿发音部位连续体的塞音感知的有害影响可能不仅完全归因于可听度降低，还归因于残余神经元的编码不当，导致随时间变化的频谱线索失真。这可能部分解释了为什么放大不能完全补偿感音神经性听力损失的影响。
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