The Active Cochlea

The cochlea is a hydromechanical frequency analyzer located in the inner ear (Fig. 1 a). Its principal role is to perform a realtime spectral decomposition of the acoustic signal in producing a spatial frequency map. The frequency analysis may be understood with the aid of Figure 1 b, which shows a straightened cochlea with a snapshot of its basilar membrane displaced in response to a single-frequency sound (pure tone). Upon delivery of an acoustic signal into the fluid-filled cochlea, the basilar membrane undergoes an oscillatory motion at the frequency of the sound, resulting in a wave traveling toward its distal end. The drawing shows an instantaneous view ofthis traveling wave. The wave is spatially confined along the length of the basilar membrane, and the location of its maximum amplitude is related to the frequency of the sound. The higher the frequency, the more restricted the disturbance to the proximal end. The vibrating membrane supports the sense organ of hearing, the spiral organ of Corti (Fig. 2) which is deformed maximally in the region of the peak of the traveling wave. In this location, the sensory receptor cells of the organ of Corti receive maximal mechanical stimulation, transduce it into maximal electrical signals, and thus produce maximal afferent sensory outflow from the cochlea. Thus, mechanical frequency analysis is performed by matching particular frequencies with particular groups of auditory receptor cells and their nerve fibers. Understanding of frequency analysis in the inner ear progressed through three main epochs. The first was dominated by Helmholtz’s suggestions that lightly damped, spatially ordered, mechanically resonant elements in the cochlea perform the spectral analysis (this period was reviewed by Wever, 1949). The second epoch, lasting from the late 1940s to the early 1970s was dominated by von Bekesy’s description of the traveling wave (von BtkCsy, 1960). We are now in the third epoch (an overview of its evolution is given in Dallos, 1988) during which a fundamentally different paradigm has emerged. According to this paradigm, von Btktsy’s traveling wave is boosted by a local electromechanical amplification process in which one of the mammalian ear’s sensory cell groups, outer hair cells, function as both sensors and mechanical feedback elements. The ideas that the operation of a sense organ is dependent upon local mechanical feedback modification by what resembles a sensory receptor cell, that such mechanical feedback may operate at audio frequencies utilizing some novel cellular motor, and that