The process of hearing involving a complex and remarkable pathway beginning with the ear and ending in the brain. The brain uses both the left and right ears to process and interpret sound, especially when localizing the direction sound is coming from and understanding speech in the presence of background noise.
Sound is collected and processed using three parts of the ear (the outer ear, the middle ear and the inner ear), central auditory pathways and finally the brain. The ear converts the physical characteristics of sound into a neural or biological response. That biological response is then sent along central auditory pathways that relay, process and modulate sound, with the brain finally interpreting sound.
The Outer Ear
Even before sound enters the ear, it is processed by the Pinna, the part of the ear that is seen by others. Sound of frequencies (tones ranging from bass to treble pitches) from 20-20,000 Hz is partially attenuated/selected and then funneled by the shape, size and grooves of the pinna down into the ear canal. Ear canals are unique like fingerprints but generally each ear canal twists a bit like an “S”, tapers like a funnel and is mildly up-sloped. The skin lining the ear canal is home to glands that secrete wax (cerumen) that keep the skin lubricated and is believed to aid in the prevention of foreign substances advancing down the canal. Because of this, a modest amount of cerumen in the ear canal is healthy. Although cerumen can occasionally accumulate in the ear canal, most ears naturally self clean themselves through a process called epithelial migration in which the skin cells lining the ear canal migrate outwards approximately every 90 days. Cerumen can build up inside the ear canal when this process is disrupted. Examples would include repeated use of Q-tips which can actually push some of the cerumen deeper into the canal rather than remove it. Although rarely does this affect hearing, it can feel uncomfortable and water can more easily become trapped in the ear canal because of this. When this occurs a hearing healthcare professional is sometimes needed to manually remove the cerumen.
The Middle Ear
At the end of the ear canal is a ring of frosted skin called the tympanic membrane (ear drum). Contrary to popular belief, the tympanic membrane is not delicate. It requires significant force to pierce or damage it. Sound strikes the drum, converting it to unique vibrations representing the pitch of the sound. Attached to the ear drum are the three smallest bones in the human body; the malleus, the incuse and the stapes. The three bones are housed in an air filled cavity and transfer sound vibrations to the inner ear.
The middle ear is ventilated by the Eustachian tube, which opens and closes through musculature contractions to allow air from the environment to rush in or out, equalizing the pressure on both sides of the ear drum. The tube is typically closed, which is why when ascending or descending during airplane flights one’s ears feel plugged until one exerts effort (through chewing, yawning etc) to unplug them. Fluid build-up in the middle ear space will also cause one’s ears to feel plugged. Children with chronic fluid buildup will often have “tubes” placed in the ear drum to allow the fluid to drain.
Medical conditions occurring in the outer and middle ear are generally temporary and can be treated by a physician. Hearing loss in this part of the ear is called conductive hearing loss because only the “mechanical portion” of the ear that processes sound is affected, not the hearing “nerves”. Approximately 15% of those with hearing impairmenthave hearing loss due an issue with this part of their ear.
The Inner Ear
The inner ear houses the nerves and nerve receptors for hearing and balance. The organ for hearing is called the cochlea (it is snail-shaped in the diagram). The cochlea contains approximately 15, 000 sensory hearing receptors called “hair cells” that convert sound vibrations from the stapes in the middle ear into a biological response for further processing by the auditory nerve and brain. Damage to these hair cells (typically from aging, noise exposure and/or unlucky genetics) results in sensorineural hearing less, often referred to as nerve hearing loss. Approximately 85% of persons diagnosed with hearing impairment have this type of hearing loss.
The hair cells in the cochlea are selectively responsive to the pitch and loudness of sound. Once activated, a response is generated and relayed along the VIIIth cranial nerve (hearing nerve) into the brain for further analysis.
The vestibular system is the organ for balance which resides with the cochlea. Similar sensory receptors as the cochlea respond to movement and orientation of the head rather than sound vibrations. There are three half-circled outcroppings along the inner ear named semi-circular canals. Vestibular receptors in these structures assist the brain in maintaining balance with regards to head orientation. Two nearby bulbous structures respond to acceleration and movement. Information from the vestibular system sensory receptors are converted to a biological response and relayed along the VIIth cranial nerve (vestibular nerve) into the brain for further analysis.
Central Auditory Pathways
Information transmitted to the hearing nerve in the cochlea is quickly sent to multiple “relay stations” across various parts of the brain. Sound information from both the left and right ears is sent for processing to various sides of the brain(see diagram). It is not clearly understood what happens at these different relay stations but there is evidence that the different relay stations analyze various aspects of sound like timing, rate, pitch and loudness as well as other critical attributes of sound. Furthermore, sound can be enhanced and processed as well as compared to sound from the opposite ear or combined with sound from the other ear for a single perception.
Eventually, sound makes its way to the auditory cortex of the brain, the highest center of the brain. Although sound is processed by the left and right side of the auditory cortex, it is believed that the left side is more critical for speech and language processing and the right side more dedicated to emotional meaning and basic tonal qualities of sound for things like music and voice recognition of family, friends versus a stranger’s voice.
Damage to any of these central auditory processing pathways due to trauma, disease, noise exposure or even simple aging can disrupt the ability to “hear”. For example, listening to speech in a noisy restaurant becomes more difficult for most as we age, even for those with no measurable hearing loss. It is believed this is due to the slower processing abilities of the hearing system which might be occurring somewhere at one or more of the relay stations. Hearing research is still needed in the area of central auditory processing to further understand how it all works, how to more reliably evaluate and diagnose disorders and ultimately how to treat them when they are identified.
A video of the journey sound makes from the ear canal to the brain can be viewed on the website of Med EL, a company that designs and manufactures a cochlear implant for individuals with severe to profound hearing loss. This can also be viewed on YouTube.