Hearing

head with music notes

Our body’s auditory system allows us to process external sound to better understand the world around us. In doing so, we utilize the parts of the ear to absorb sound and the steps of processing in the brain to interpret what we hear. 

Soundwaves 

We hear sounds (measured in decibels), through sound waves, whose height, length, and complexity determine what we hear. The amplitude, or the height of the sound wave, dictates the volume of the sound. A high amplitude means that the sound is loud, whereas a low amplitude causes a soft sound. Within the ear, loudness is determined by the number of activated hair cells in the cochlea. 

In contrast, the frequency, or the length of the soundwave, affects pitch: a low frequency is a low pitched sound and a high frequency is a high pitched sound. There are multiple theories used to help us understand pitch perception. Firstly, the place theory states that the perception of pitch is associated with vibration of different portions of cochlea. For instance, the base of the basilar membrane responds best to high frequencies while the tip of the basilar membrane is best suited to low frequencies. On the other hand, the frequency theory argues that the perception of pitch is associated with the frequency at which the entire basilar membrane vibrates.  

Ear Structure 

The ear’s structure allows us to understand how sound moves from the environment to our brains. Firstly, sound enters the ear through the outer ear (made up of the ear canal) and goes to the middle ear, which is the chamber between the eardrum and cochlea.

The middle ear contains the eardrum and three small bones (the hammer, anvil, and stirrup) that concentrate the vibrations of the eardrums on the cochlea’s window. Then, sound waves travel through the inner ear (made up of the cochlea, semicircular canals, and nerves). The cochlea, which is a coiled, bony, fluid-filled tube in the ear, contains cochlear fluid that triggers nerve impulses in the inner ear. Finally, these nerve impulses enter the brain where they are interpreted through processing. 

Processing

Processing occurs in three steps: transduction, sensation, and perception. These steps occur when processing information from all five senses, including hearing, taste, touch, sight, and smell. Firstly, transduction occurs when we convert one form of energy to another that can be used by the brain. For example, light waves are converted to sight, and sound waves to hearing. 

Next, sensation occurs when sensory organs (eyes, ears, skin, etc) and the nervous system receives stimuli from the environment. The stimulation of sensory receptor cells by energy leads to the delivery of neural information to the brain to be processed. This is known as bottom-up processing because information from sensory receptors work up to the brain to be interpreted.

Finally, perception occurs when the brain interprets sensations through top down processing. Utilizing prior experiences, ideas, and expectations, the brain interprets sensations to understand the stimuli and environment. 

Hearing Loss 

According to the CDC, there are four types of hearing loss. Firstly, conductive hearing loss is caused by something that stops sound from getting through the outer or middle ear to the cochlea. Oftentimes, this type of hearing loss is caused by ear infection and can be treated with medicine, surgery, or a hearing aid.

The second type of hearing loss is sensorineural hearing loss in which there are problems with the cochlea’s hair cell receptors or nerve damage. This type of hearing loss can be caused by disease, aging, or exposure to loud noise. For some nerve deafness, cochlear implants can restore hearing. Then, mixed hearing loss refers to hearing loss that includes both conductive and sensorineural hearing loss. 

Lastly, Auditory Neuropathy Spectrum Disorder is hearing loss that occurs when sound cannot be processed by the brain, despite normal entry into the ear. Due to some damage to the inner ear or the hearing nerves, sounds cannot be properly organized and understood in the brain. Hearing loss also ranges from mild (can hear some speech, but quiet sounds are hard to hear) to profound (cannot hear any speech or only very loud sounds). 

In summary, our ability to hear sounds can change throughout our lives. Nevertheless, hearing is a crucial sense that allows us to learn information, listen to others, and interpret the world around us. 

Sources: 

https://www.asha.org/public/hearing/how-we-hear/

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