Background- Normal Hearing and the Cochlear Implant
Normal Hearing
Sound is produced when an object vibrates in matter. Most of the time, for us humans, those vibrations are in the air. Our body senses sound through the interaction of a number of components that form the ear (See Figure 1). The outer ear collects the waves in the air, focusing the energy via the auditory canal to the eardrum, which then mechanically vibrates the bones of the middle ear. These bones drive against the cochlea, a fluid filled organ called the organ of hearing. Fluid pressure waves in the cochlea cause movement in the basilar membrane (see cross-section, Figure 2). When the basilar membrane deflects, the attached hair cells become stimulated to produce an electro-chemical signal. The resultant signal stimulates specific fibers on the auditory nerve.
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Figure 2 Cross-section of the cochlear. The vibrating basilar membrane moves the hair cells, which electrochemically create the signal that excites the auditory nerve.
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The shape and structure of the cochlea play a critical role. The fluid wave inside the cochlea does not cause the same reaction at all points on the basilar membrane. If the sound is high in pitch, the
base of the cochlea (the wide part) tends to vibrate more, while low pitch sounds will cause the apex (the tip of the spiral) to respond (Loizou, 1998). The structure of the cochlea is such that the frequency of the sound affects which location on the membrane will vibrate most vigorously, providing the ability to differentiate frequency/pitch in the incoming signal. The pattern of electrical signals is transmitted via the auditory nerve to the brain, where they are processed and interpreted as sound.
