The Role of the Efferent System in Auditory Gain Reduction

人耳检测声级的微小差异的能力依赖于内耳耳蜗内的基底膜（BM）的响应特性。当声波进入耳蜗时，它们引起BM振动，膜上的每个地方对特定频率（其特征频率）的声音反应最大。对于频率接近特征频率的声音，BM响应被放大。这种放大或增益随着声音的级别而变化，对低级别声音的BM响应比对高级别声音的BM响应应用更多的增益。由于响应于声音而应用的增益的量随其电平而变化，因此所应用的增益被描述为电平相关的。虽然我们现在在一定程度上了解了增益是如何随声级变化的，但我们仍然没有完全了解增益是如何随声音的持续时间变化的。对人类的行为研究表明，与声音结束时相比，声音开始时的增益最大。这种增益的减少被认为是由于来自大脑中更高听觉中心的信息的神经反馈，并且通常被称为传出效应。以前的行为研究对人类涉及的声音呈现耳机，试图测量增益变化与声音持续时间使用一种技术称为同步掩蔽。在该方法中，在同时存在掩蔽声音（掩蔽物）的情况下测量声音（信号）的可检测性。如果信号和掩蔽声具有相同的频率，并且针对一定范围的声级测量信号可检测性，则可以针对不同的掩蔽声持续时间获得BM的特征频率处的增益变化的估计。然而，这些研究的解释可能会被BM上其他复杂相互作用的存在所混淆，因此可能无法反映增益和传出效应的准确估计。本研究使用了一种称为掩蔽的加和性的技术，其中包括前掩蔽。在前向掩蔽中，测量掩蔽之后不久出现的信号的信号可检测性。由于信号和掩蔽物不同时存在，因此该方法减少了沿BM的其它复杂相互作用的参与，所述其它复杂相互作用沿着BM可能影响增益的真实估计。本提案将使用掩蔽技术的可加性来发现i）增益如何随着声音的持续时间增加和减少; ii）声音的频率是否影响增益如何随着持续时间变化; ii）声音水平的增加如何影响增益随着声音持续时间的变化。这些发现将增加我们对正常听力个体传出效应的理解。一个关键领域，将受益于人类传出效应的知识增加是听力障碍的治疗。传出效应对于检测和理解语音至关重要。然而，许多有听力障碍的人缺乏传出效应。通过获得一系列听觉刺激的传出效应的准确估计，我们将能够改进助听器和人工耳蜗植入的信号处理策略。

The ability of the human ear to detect small differences in sound level relies on the response properties of the basilar membrane (BM) within the cochlea of the inner ear. When sound waves enter the cochlea, they cause the BM to vibrate, with each place on the membrane responding most to a sound of a particular frequency (its characteristic frequency ). The BM response is amplified for sounds with a frequency close to that of the characteristic frequency. This amplification or gain varies with the level of a sound, with more gain applied to the BM response to low-level sounds than to higher-level sounds. Since the amount of gain applied in response to a sound varies with its level, the applied gain is described as level-dependent . Although we now understand to some extent how gain varies with sound level, we still do not fully understand how gain varies with the duration of a sound. Behavioural studies on humans suggest that the gain is greatest at the start of a sound compared to the end of a sound. This reduction of gain is thought to be due to neural feedback of information from higher auditory centres in the brain, and is often referred to as the efferent effect. Previous behavioural studies on humans involving sound presentation by headphones, have attempted to measure the gain variation with sound duration using a technique called simultaneous masking . In this method the detectability of a sound (signal) is measured in the presence of a masking sound (masker) presented at the same time. If the signal and masker are of the same frequency and signal detectability is measured for a range of sound levels, an estimate of the change in gain at the characteristic frequency of the BM can be obtained for different masker durations. However the interpretation of such studies may be confounded by the presence of other complex interactions on the BM and so may not reflect an accurate estimate of the gain and efferent effect. The present study uses a technique called the additivity of masking which involves forward masking . In forward masking signal detectability is measured for a signal presented soon after a masker. Since the signal and masker are not present at the same time, this method reduces the involvement of other complex interactions along the BM which could affect the true estimate of the gain. The present proposal will use the additivity of masking technique to find i) how the gain increases and decreases with the duration of a sound; ii) If the frequency of the sound affects how the gain varies with duration; ii) How increases in sound level affect the variation of gain with sound duration. The findings will increase our understanding of the efferent effect in normal-hearing individuals. One key area that would benefit from an increased knowledge of the human efferent effect is that of the treatment of hearing impairment. The efferent effect is vital for detecting and understanding speech. However many individuals with hearing impairment lack an efferent effect. By obtaining accurate estimates of the efferent effect for a range of auditory stimuli we will be able to improve signal processing strategies for hearing aids and cochlear implants.