Defining the response of the human basilar membrane

声波进入耳道，引起鼓膜振动。这些振动通过中耳，通过三块小骨头（听骨）传递到内耳的耳蜗。耳蜗是一根细长的管子，盘绕成蜗牛壳的形状，在神经系统中将声波转化为电脉冲。这个过程的一个重要部分涉及到基底膜，这是一种覆盖耳蜗长度的薄膜。基底膜上的每一个地方都被调谐成不同频率的声音，因此高频（铙钹明亮的撞击声）引起耳蜗底部的膜振动，低频（低音鼓沉闷的撞击声）引起耳蜗顶端的膜（螺旋的尖端）振动。通过这种方式，基底膜将声音的不同频率成分分离出来，就像水滴将不同频率的光（即颜色）分离出来形成彩虹一样。频率分离帮助我们识别声音，并将同时出现的声音分离出来（例如，在嘈杂的聚会上听谈话时）。基底膜的振动是由一种叫做外毛细胞的特殊细胞调节的。外毛细胞放大低水平的声音，而不是放大高水平的声音，导致基底膜振动的浅增长，声级称为压缩。这使我们对安静的声音更加敏感，而不会影响我们对大声声音的敏感性。此外，放大仅影响基底膜上每个位置的有限频率范围。通过这种方式，扩音使音质变得清晰，使膜更好地分离声音。我的研究是关于测量外毛细胞对人体基底膜振动的影响。虽然生理实验已经在其他哺乳动物身上进行了许多年，但我们直到最近才开发出精确测量人类基底膜反应所需的技术。在这些实验中，声音通过耳机呈现给参与者，他们的反应被记录下来（因此，这是一个“行为”实验）。参与者被要求检测一个声音（“信号”）出现在另一个声音（“掩蔽器”）之后，这个声音的频率相同或不同。通过测量信号的检测如何依赖于信号和掩膜的水平，可以估计基底膜的反应，而不需要外科手术，这种手术对人类来说是不道德的。除了提供有关人类听觉系统的信息外，研究结果还将与哺乳动物的听力普遍相关。例如，这项研究的目的之一是确定耳蜗螺旋顶端附近的基底膜是否受压，该区域对低频有反应。对其他哺乳动物基底膜振动的直接测量表明，事实并非如此，但由于外科手术的困难，这些实验可能受到了损害。有人认为是外毛细胞受损，导致放大和压缩功能丧失。目前的人类行为实验应该能够解决这个问题，因为我们可以测量健康生理状态下的耳蜗，并将这些结果与那些外毛细胞受损的听力受损听众的结果进行比较。我们将利用我们的研究结果开发一个计算机耳蜗模型，一个计算机程序，它将允许我们模拟基底膜对任何声音的反应。我们对基底膜如何将信息传递给更多的中枢结构的更好理解，将有助于我们理解大脑如何使用这些信息来分析和识别声音。

Sound waves enter the ear canal and cause the eardrum to vibrate. These vibrations are transmitted through the middle ear, via three tiny bones (the ossicles), to the cochlea in the inner ear. The cochlea is a long thin tube, coiled up into a shape like a snail shell, that converts sound waves into electrical impulses in the nervous system. An important part of this process involves the basilar membrane, a thin membrane that runs the length of the cochlea. Each place on the basilar membrane is tuned to vibrate to a different frequency of sound, so that high frequencies (the bright crash of a cymbal) cause the membrane at the base of the cochlea to vibrate, and low frequencies (the dull thud of a bass drum) cause the membrane at the apex of the cochlea (the tip of the spiral) to vibrate. In this way the basilar membrane separates out the different frequency components of a sound, just as water droplets separate out the different frequencies of light (i.e. colours) to produce a rainbow. The frequency separation helps us to identify sounds, and to separate out sounds that occur together (for example, when listening to a conversation at a noisy party). The vibration of the basilar membrane is modified by special cells called outer hair cells. The outer hair cells amplify sounds at low levels but not at high levels, leading to a shallow growth of basilar membrane vibration with sound level called compression. This makes us much more sensitive to quiet sounds, without affecting our sensitivity to loud sounds. Furthermore, the amplification only affects a limited range of frequencies at each place on the basilar membrane. In this way, the amplification sharpens the tuning, making the membrane better at separating sounds. My research is concerned with measuring the effects of the outer hair cells on the vibration of the basilar membrane in humans. Although physiological experiments have been conducted on other mammals for a number of years, we have only recently developed the techniques necessary for accurate measurement of the human basilar membrane response. In these experiments, sounds are presented to participants over headphones and their responses are recorded (hence, this is a 'behavioural' experiment). Participants are asked to detect one sound (the 'signal') presented after another sound (the 'masker') with the same or a different frequency. By measuring how the detection of the signal depends on the levels of the signal and of the masker, it is possible to estimate the response of the basilar membrane without requiring a surgical procedure, a procedure that would be unethical in humans. As well as providing information about the human auditory system, the results will be relevant to mammalian hearing in general. For example, one of the aims of the research is to determine whether the basilar membrane is compressive near the apex of the cochlea spiral, the region that responds to low frequencies. Direct measurements of basilar membrane vibration in other mammals suggest that it is not, but these experiments may have been compromised because of the difficulties involved in the surgical procedure. It has been suggested that the outer hair cells were damaged, so that the amplification and compression were lost. The present behavioural experiments on humans should be able to resolve this issue because we can measure the cochlea in a healthy physiological state, and compare these results to those from hearing-impaired listeners who have damaged outer hair cells. We will use our results to develop a computational cochlear model, a computer programme that will allow us to simulate the response of the basilar membrane to any sound. Our improved understanding of how the basilar membrane works conveys information to more central structures will then help us understand how the brain uses that information to analyse and identify sounds.