Dreisbach Laura E, Leek Marjorie R, Lentz Jennifer J
School of Speech, Language, and Hearing Sciences, San Diego State University, San Diego, CA 92182-1518, USA.
J Speech Lang Hear Res. 2005 Aug;48(4):910-21. doi: 10.1044/1092-4388(2005/063).
The ability to discriminate the spectral shapes of complex sounds is critical to accurate speech perception. Part of the difficulty experienced by listeners with hearing loss in understanding speech sounds in noise may be related to a smearing of the internal representation of the spectral peaks and valleys because of the loss of sensitivity and an accompanying reduction in frequency resolution. This study examined the discrimination by hearing-impaired listeners of highly similar harmonic complexes with a single spectral peak located in 1 of 3 frequency regions. The minimum level difference between peak and background harmonics required to discriminate a small change in the spectral center of the peak was measured for peaks located near 2, 3, or 4 kHz. Component phases were selected according to an algorithm thought to produce either highly modulated (positive Schroeder) or very flat (negative Schroeder) internal waveform envelopes in the cochlea. The mean amplitude difference between a spectral peak and the background components required for discrimination of pairs of harmonic complexes (spectral contrast threshold) was from 4 to 19 dB greater for listeners with hearing impairment than for a control group of listeners with normal hearing. In normal-hearing listeners, improvements in threshold were seen with increasing stimulus level, and there was a strong effect of stimulus phase, as the positive Schroeder stimuli always produced lower thresholds than the negative Schroeder stimuli. The listeners with hearing loss showed no consistent spectral contrast effects due to stimulus phase and also showed little improvement with increasing stimulus level, once their sensitivity loss was overcome. The lack of phase and level effects may be a result of the more linear processing occurring in impaired ears, producing poorer-than-normal frequency resolution, a loss of gain for low amplitudes, and an altered cochlear phase characteristic in regions of damage.
辨别复杂声音频谱形状的能力对于准确的语音感知至关重要。听力损失的听众在嘈杂环境中理解语音时遇到的部分困难，可能与频谱峰谷内部表征的模糊有关，这是由于敏感度丧失以及随之而来的频率分辨率降低所致。本研究考察了听力受损听众对高度相似的谐波复合体的辨别能力，这些复合体在三个频率区域之一中具有单个频谱峰。针对位于2kHz、3kHz或4kHz附近的峰值，测量了辨别峰值频谱中心微小变化所需的峰值与背景谐波之间的最小电平差。根据一种算法选择分量相位，该算法被认为会在耳蜗中产生高度调制（正施罗德）或非常平坦（负施罗德）的内部波形包络。与听力正常的对照组听众相比，听力受损的听众辨别谐波复合体对所需的频谱峰与背景分量之间的平均幅度差（频谱对比度阈值）要大4至19dB。在听力正常的听众中，随着刺激电平的增加，阈值有所改善，并且刺激相位有很强的影响，因为正施罗德刺激总是产生比负施罗德刺激更低的阈值。听力损失的听众没有表现出因刺激相位而产生的一致频谱对比效应，并且一旦克服了他们的敏感度损失，随着刺激电平的增加也几乎没有改善。相位和电平效应的缺乏可能是由于受损耳朵中发生的更线性处理导致频率分辨率比正常情况更差、低幅度增益损失以及受损区域耳蜗相位特征改变的结果。
