3.2.2. Audition

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Hearingauditory perception, or audition is the ability to perceive sound by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ such as the ear. Sound may be heard through solid, liquid, or gaseous matter. It is one of the traditional five senses. The inability to hear is called deafness.

In humans and other vertebrates, hearing is performed primarily by the auditory system: vibrations are detected by the ear and transduced into nerve impulses that are perceived by the brain(primarily in the temporal lobe). Like touch, audition requires sensitivity to the movement of molecules in the world outside the organism. Both hearing and touch are types of mechanosensation.

The outer ear includes the visible part of the ear (or the pinna), the auditory canal and the eardrum. The eardrum is made of an airtight flap of skin. Sounds travel in waves, and when these waves arrive at the eardrum, they cause it to vibrate, following the waveform of the sound. The eardrum simplifies incoming air pressure waves to a single change with a certain amplitude. This allows for the differentiation of sound.

The middle ear consists of a small air filled chamber that is located behind the eardrum. Within this chamber are three smallest bones in the body, known collectively as the ossicles. They aid in the transmission and amplification of the vibrations from the ear drum to the inner ear.

The inner ear contains the cochlea, which is a spiral shaped, fluid filled tube that is considered the organ of auditory transduction. It is divided lengthwise by the basilar membrane, a structure that vibrates when waves from the middle ear propagate through the cochlear fluid–membrane system. The basilar membrane is tonotopic, so that each frequency has a characteristic place of resonance along it. Characteristic frequencies are high at the basal entrance to the cochlea, and low at the apex. Basilar membrane motion causes the movement of the hair cells, specialized auditory receptors located within the basilar membrane. [4] The space–time pattern of vibrations in the basilar membrane is converted to a spatial–temporal pattern of firings on the auditory nerve, which transmits information about the sound to the brainstem

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