How We Hear

Eardrum (Tympanic Membrane) Auditory cortex of the temporal lobe How We Hear How We Hear Outer Ear Auditory Canal Eardrum (Tympanic Membrane) Hammer, Anvil, Stirrup three tiny bones Oval Window outer membrane of the cochlea Basilar Membrane whose axons form the auditory nerve Hair Cells lining the surface of the basilar membrace Thalamus Auditory cortex of the temporal lobe

How We Hear The sense of audition, or hearing, results from sound waves created by vibrating objects being converted into neural messages. Sound waves, like light waves, have three physical dimensions and equivalent psychological properties. Frequency determines pitch (highness or lowness of a sound) Measured in Hertz (Hz); Longer wavelengths or low frequencies create lower pitched sounds. Shorter wavelengths or high frequencies create higher pitched sounds.

How We Hear Amplitude (intensity) determines loudness Measured in Decibels (dB) Higher amplitude or tall waves create louder sounds. Lower amplitudes or short waves create softer sounds. Purity determines timbre Pure tones are possible in lab experiments but difficult in the natural world

How We Hear Sounds waves first enter the outer visible portion of the ear, called the pinna, and then move down the auditory canal (ear canal) to the tympanic membrane (eardrum) that vibrates in response to the funneled sound waves.

How We Hear The middle ear (between the tympanic membrane and the cochlea) contains the ossicles (the three smallest bones in the body) which help amplify sound waves traveling to the inner ear.

How We Hear The inner ear contains two sense organs: the cochlea concerned with hearing and the semicircular canals concerned with balance. The cochlea is a fluid-filled and coiled tube where sound waves are converted into neural impulses. Covering the opening to the inner ear is a membrane called the oval window. Pressure here causes waves in the fluid to move to the basilar membrane. The sound waves cause the cilia (hair cells) to bend, initiating the process of transduction.

How We Hear The signals travel along the auditory nerve through the thalamus to the temporal lobe’s auditory cortex.

Hearing Theories Frequency Theory According to the frequency theory, the basilar membrane vibrates at the same frequency as the sound wave. The frequency theory explains how low-frequency sounds are transmitted to the brain. However, since individual neurons cannot fire faster than about 1,000 times per second, the frequency theory does not explain how the much faster high-frequency sounds are transmitted. The volley theory suggests that sounds above 1,000 hertz require the activity of multiple neurons working together.

Hearing Theories Place Theory According to the place theory, different frequencies excite different hair cells at different locations along the basilar membrane. High-frequency sounds cause maximum vibrations near the stirrup end of the basilar membrane. Lower-frequency sounds cause maximum vibrations at the opposite end.

Sound Localization Involves interpretation by the brain of sound waves entering both ears in order to determine the direction the noise is coming from. Possible because the sound waves arrive at one ear faster than they reach the other ear, and this information about timing is then interpreted by the brain.

Sound Localization Sounds that originate directly above, below, in front of, or behind a person are the most difficult to locate because they reach each ear at the same time and with the same intensity. In order to determine the location of these sounds, humans also utilize their sense of vision or move their heads to cause the messages to arrive at different times.

Deafness Conduction Deafness The result of problems with funneling and amplifying sound waves to the inner ear Typically the result of damage to the eardrum or the bones of the middle ear A person will have equal difficulty hearing both high- and low- pitched sounds (sounds have become softer). Treatments include medication, surgery, or the use of a hearing aid to amplify the sound waves to the cochlea

Deafness Sensorineural (nerve) Deafness The result of damage to the aspects of the auditory system related to the transduction of sound waves, or the transmission or neural messages. Typically the result of damage to the cilia caused by prolonged exposure to loud noise, aging, or disease. Hair cells (cilia) do not regenerate so the damage is permanent. A person will have a much harder time hearing high- pitched sounds than low ones. Treatment is most likely hearing aids but severe damage can be treated with cochlear implants.