Massachusetts Eye & Ear Infirmary, Eaton-Peabody Laboratory of Auditory Physiology, Boston, MA 02114, USA.
Ear Hear. 2013 Jan-Feb;34(1):42-51. doi: 10.1097/AUD.0b013e31825f9bd3.
Presently available nonbehavioral methods to estimate auditory thresholds perform less well at frequencies below 1 kHz than at 1 kHz and above. For many uses, such as providing accurate infant hearing aid amplification for low-frequency vowels, an accurate nonbehavioral method to estimate low-frequency thresholds is needed. A novel technique was developed to estimate low-frequency cochlear thresholds based on the use of a previously reported waveform. It was determined how well the method worked by comparing the resulting thresholds to thresholds from onset-response compound action potentials (CAPs) and single-auditory-nerve (AN)- fibers in cats. A long-term goal is to translate this technique for use in humans.
An electrode near the cochlea records a combination of cochlear microphonic (CM) and neural responses. In response to low-frequency, near threshold-level tones, the CM is almost sinusoidal whereas the neural responses occur preferentially at one phase of the tone. If the tone is presented again but with its polarity reversed, the neural response keeps the same shape, but shifts ½ cycle in time. Averaging responses to tones presented separately at opposite polarities overlaps and interleaves the neural responses and yields a waveform in which the CM is canceled and the neural response appears twice each tone cycle, that is, the resulting neural response is mostly at twice the tone frequency. The resultant waveform is referred to as "the auditory nerve overlapped waveform" (ANOW). In this study, ANOW level functions were measured in anesthetized cats from 10 to 80 dB SPL in 10 dB steps using tones between 0.3 and 1 kHz. As a response metric, the magnitude of the ANOW component was calculated at twice the tone frequency (ANOW2f). The ANOW threshold was the sound level where the interpolated ANOW2f crossed a statistical criterion that was higher than 95% of the noise floor distribution. ANOW thresholds were compared with onset-CAP thresholds from the same recordings and single-AN-fiber thresholds from the same animals.
ANOW and onset-CAP level functions were obtained for 0.3 to 1 kHz tones, and single-AN-fiber responses from cats. Except at 1 kHz, typical ANOW thresholds were mostly 10 to 20 dB more sensitive than onset-CAP thresholds and 10 to 20 dB less sensitive than the most sensitive single-AN-fiber thresholds.
ANOW provides frequency-specific estimates of cochlear neural thresholds over a frequency range that is important for hearing but is not well accessed by nonbehavioral, objective methods. Results suggest that with further targeted development, the ANOW low-frequency threshold estimation technique can be useful both clinically in humans and in basic-science animal experiments.
目前可用于估计听觉阈值的非行为方法在 1 kHz 以下的频率表现不如 1 kHz 以上的频率。对于许多用途，例如为低频元音提供准确的婴儿助听器放大，需要一种准确的非行为方法来估计低频阈值。开发了一种新的技术，基于以前报道的波形来估计低频耳蜗阈值。通过将得到的阈值与起始反应复合动作电位 (CAP) 和猫的单个听觉神经 (AN) 纤维的阈值进行比较，确定了该方法的工作效果如何。长期目标是将该技术转化为人类使用。
耳蜗附近的电极记录耳蜗微音 (CM) 和神经反应的组合。对于低频、接近阈值水平的音调，CM 几乎是正弦波，而神经反应则优先在音调的一个相位发生。如果再次呈现相同的音调，但极性反转，神经反应保持相同的形状，但时间上偏移了 ½ 周期。对分别以相反极性呈现的音调的反应进行平均，会重叠并交织神经反应，并产生一个 CM 被抵消且每个音调周期出现两次神经反应的波形，即得到的神经反应主要在音调频率的两倍处。该波形称为“听觉神经重叠波形” (ANOW)。在这项研究中，使用 0.3 至 1 kHz 之间的音调，以 10 dB 的步长，在麻醉猫中测量了从 10 至 80 dB SPL 的 ANOW 水平函数。作为响应指标，计算两倍音调频率处的 ANOW 分量的幅度 (ANOW2f)。ANOW 阈值是插值 ANOW2f 穿过高于噪声基底分布 95%的统计标准的声级。将 ANOW 阈值与来自同一记录的起始-CAP 阈值和来自同一动物的单个 AN-纤维阈值进行比较。
获得了 0.3 至 1 kHz 音调以及猫的单个 AN-纤维的 ANOW 和起始-CAP 水平函数。除了 1 kHz 之外，典型的 ANOW 阈值通常比起始-CAP 阈值敏感 10 至 20 dB，比最敏感的单个 AN-纤维阈值敏感 10 至 20 dB。
ANOW 提供了对听觉非常重要但非行为性、客观方法无法很好地访问的频率范围内的耳蜗神经阈值的频率特异性估计。结果表明，通过进一步有针对性的开发，ANOW 低频阈值估计技术在临床和基础科学动物实验中都可能有用。
