Defining the response of the human basilar membrane

声波进入耳道，导致耳膜振动。这些振动通过中耳，通过三块细小的骨头(听小骨)传递到内耳的耳蜗处。耳蜗管是一根细长的管子，盘绕成蜗牛壳的形状，在神经系统中将声波转化为电脉冲。这一过程的一个重要部分涉及到基底膜，这是一种贯穿耳蜗长的薄膜。基底膜上的每个地方都被调谐为振动到不同的声音频率，因此高频(铿锵的响声)导致耳蜗基底部的膜振动，而低频率(低音鼓的迟钝的轰鸣声)导致耳蜗尖(螺旋尖端)的膜振动。通过这种方式，基底膜分离出声音的不同频率分量，就像水滴分离出不同频率的光(即颜色)来产生彩虹一样。频率分离帮助我们识别声音，并区分同时出现的声音(例如，在嘈杂的派对上听对话时)。基底膜的振动被称为外毛细胞的特殊细胞所改变。外毛细胞在低水平放大声音，但不在高水平放大声音，导致基底膜振动浅增长，声音水平称为压缩。这使我们对安静的声音更加敏感，而不会影响我们对响亮的声音的敏感性。此外，放大只影响基底膜上每个位置的有限频率范围。通过这种方式，扩音器使调谐变得尖锐，使薄膜更好地分离声音。我的研究涉及测量外毛细胞对人类基底膜振动的影响。虽然在其他哺乳动物身上进行生理学实验已经有很多年了，但我们直到最近才开发出准确测量人类基底膜反应所需的技术。在这些实验中，通过耳机将声音呈现给参与者，并记录他们的反应(因此，这是一个“行为”实验)。参与者被要求以相同或不同的频率检测一个声音(信号)之后出现的另一个声音(掩蔽者)。通过测量信号的检测如何依赖于信号和掩蔽物的水平，就有可能在不需要外科手术的情况下估计基底膜的反应，而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总体听力相关。例如，这项研究的目的之一是确定基底膜是否在耳蜗螺旋顶端附近受压，耳蜗螺旋是对低频做出反应的区域。对其他哺乳动物的基底膜振动的直接测量表明并非如此，但由于手术过程中涉及的困难，这些实验可能已经受到影响。有人认为，外毛细胞受损，因此失去了放大和压缩。目前在人类身上进行的行为实验应该能够解决这个问题，因为我们可以在健康的生理状态下测量耳蜗，并将这些结果与听力受损的听者的外毛细胞受损的结果进行比较。我们将利用我们的结果来开发一个计算耳蜗模型，这是一个计算机程序，可以让我们模拟基底膜对任何声音的反应。我们对基底膜如何工作的更好的理解，将信息传递到更多的中央结构，然后将帮助我们理解大脑如何利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.