Monday, 3 November 2008

As Above, So Below

Isn't the ear fascinating? It's an incredible bit of kit. Sound waves go into the ear canal and strike against the eardrum (tympanic membrane) and they start it thrilling or vibrating, just as a guitar string vibrates when you thrum it. These little vibrations are carred across the hollow behind the drum by a chain of bones, known as the ear-bones (called from their shapes , the hammer, the anvil, and the stirrup) and passed onto the keyboard of the cochlea.

The cochlea is made up of a long row of tiny little hair cells, laid side by side like the keys of a piano, only there are about 3,000 of them. They are organised tonotopically (by sound frequency) in a coiled , spiral shape, and is how it derives its name cochlea (Greek for 'snail-shell'). It is also called, because it is the deepest or innermost part of the hearing apparatus, the inner-ear. The base of the cochlea responds to high frequency sounds, and the apex responds to low frequency sounds. These inner hair cells are innervated by a rich array of afferent nerve fibers (10-20 fibers per hair cell) that synapse with the auditory division of the vestibulocochlear nerve at the spiral ganglion. The vestibulocochlear nerve (also known as the auditory or acoustic nerve) is the eighth of twelve cranial nerves, and is responsible for transmitting sound and equilibrium (balance) information from the inner ear to the brain. Cranial nerves are nerves that emerge directly from the brain stem in contrast to spinal nerves which emerge from segments of the spinal cord. In short then, by banging on the keys of the cochlea we are blowing noises through the pipes of our minds.

People who have normal hearing actually hear far more than they percieve. Where hearing is a function of the ear, auditory processing (listening) is a function of the brain. Something inside the brain is censoring, and organising what sounds we are conscious of within our enviroment, based on a level of priority for whatever task we are completing. Startle responses are important to survival, and they illustrate how our emotions are directly connected to the auditory system. We might be busy watching the telly but someone leaping into the room screaming will demand our full attention. We can also be selective about what we are conscious of hearing. Whilst in a conversation with someone we might listen to them, or a car in the road, or the voice in our own head.... depending on which one we find the most interesting. We are not tuning in to different sounds taking place outside ourselves, but rather we are turning a dial inside our minds to what is preferential at that particular moment.

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