Audiology: How We Hear
Acoustic Hearing: Turning Soundwaves into Brain Signals
People with typical hearing use acoustic hearing to listen to sounds in an environment. Acoustic hearing is when the ears convert sound waves into neural signals. These signals are sent to a part of the brain called the auditory cortex. The auditory cortex then sends the signal to different areas of the brain, allowing us to know the location, identity, and meaning of the sound.
Real World Example of Acoustic Hearing
1. A police siren moves through air as Sound Waves
A police car coming down the road switches on its siren. The sound of the siren travels as sound waves, or vibrations in the air molecules, eventually reaching our ears.
2. The sound waves send vibrations through the ear to the cochlea
As the sound waves from the siren enter the ear, they go through the ear canal and vibrate the eardrum. The eardrum vibrations move three tiny bones in the middle ear called the ossicles, sending vibrations into the cochlea.
3. The structure and function of the Cochlea
The cochlea is a fluid-filled, spiral-like structure that houses tiny hair cells. Each hair cell detects a specific frequency or pitch. An example of high-frequency sound would be a bell ringing, while low frequencies could be a kick to a drum.
The hair cells are organized in tonotopical order; cells closer to the entrance of the cochlea detect high frequencies, while the hair cells at the end of the cochlea detect lower frequencies.
4. Sound vibrations activate hair cells in the cochlea, sending signals to the brain
The sound causes vibrations in the cochlear fluid, causing hair cells to move. As the hair cells move, they send neural signals through the auditory nerve and into the auditory cortex of the brain allowing us to identify the sound as a police siren.
Why is this Important?
Acoustic hearing allows us to understand the location, identity, and meaning of the sound. By understanding how sound is processed, we can discover how damage to different parts of the auditory system can cause hearing loss or deafness. With that information, research and clinicians can discuss therapies and interventions that may help individuals regain access to sound.
Auditory Pathways: How the Brain Processes Sound
When the brain receives electrical signals from the cochlea, it uses auditory neural pathways – a series of neurons (brain cells) linked together that send the signals to different regions of the brain to process the sound’s meaning and location. As the brain listens to more inputs, the neural pathways strengthen, allowing the brain to process information with more efficiency. As infants, our brains are learning to process sounds and determine what they mean. As babies grow older, these pathways are used more and more, allowing for faster and more accurate processing of hearing. Studies have shown that around the age of 5 years, the majority of our hearing pathways are developed. However, our lab has found evidence that binaural hearing is refined from ages 5-13 (Abdi et al., 2024)
Think of neural pathways like a hiking trail. The first time you hike the trail, it is slow and chaotic: you have to use a map, you take a wrong turn, and the unused path means there are roots and rocks you have to remove to make progress. The first trial run is slow and inefficient. But you are learning and absorbing all kinds of new scenery and hiking techniques, finding the best way to walk the path.
As you hike the trail over and over again, you begin to remember the path and no longer have to look at a map. Any obstacles you once faced have either been removed or you know routes around them. Every time you hike the trail, you get faster and faster until it is effortless. Auditory neural pathways develop through repeated exposure to different sounds, enabling the brain to understand the input without much effort.
Why is this important?
It is important to nurture these auditory pathways to ensure proper development. Hearing devices like cochlear implants and hearing aids can help restore levels of auditory input, stimulating the development of the pathways.
Hearing Loss: Causes and Aids
Contrary to popular belief, hearing loss can occur at all ages, not just the older population. Hearing loss can happen for many reasons, but a common cause is damage to the cochlea and its hair cells. The damage can come from exposure to loud noises, genetics, bacterial infection, injury, and aging. When the hair cells are damaged, the audibility of sounds decreases. As hearing loss progresses, a person is no longer able to hear specific frequencies. However, devices like cochlear implants and hearing aids help with hearing loss.
Cochlear Implants (CI)
When the hair cells in the cochlea are damaged, they are unable to process sound vibrations. This means that information cannot be sent to the auditory nerves and beyond. When an individual experiences significant levels of hearing loss in higher frequencies or complete hearing loss, they can use cochlear implants to improve hearing.
Cochlear Implants
A cochlear implant is a device that stimulates the auditory nerve through electrical signals. The cochlear implant uses a microphone and processor outside of the head to pick up sound waves.
CI inside the ear
This information is converted into electric signals, which are sent to an electrode array implanted in the cochlea. The electrode array sends the electrical signals through the cochlea, bypassing the hair cells, to activating the auditory nerve. The signals then go to the auditory cortex, allowing the user to interpret inputs like speech.
Bilateral vs Unilateral implants
Bilateral implants mean that cochlear implants are placed on both sides, while unilateral means a cochlear implant is placed in only one ear. Unilateral CI users may have single sided deafness or prefer to implant one ear before implanting the other.
One of BHSL’s main research focuses is understanding how CIs alter auditory processing. We are also interested in understanding how synchronized input through bilateral CIs can influence localization.
Hearing Aids (HA)
Unlike individuals with cochlear implants, people with hearing aids process sound using acoustic hearing. The device amplifies sound waves in the environment, which allows the ear to detect frequencies they wouldn’t be able to access without the additional gain/amplification. While there are many kinds of hearing aids, generally, they work the same.
1) Sound waves Received by Microphone
Sound waves are received by a set of microphones, which convert and process the sound waves into electrical signals. These electrical signals are then sent to an amplifier, making the sounds louder. The amplifier then sends the sounds to your ear through the speaker, which is inserted into the ear.
HA: Electrical Signals Inside the Ear
The amplified sound is then sent into the ear and through the inner and middle ear into the cochlea, allowing the hair cells to pick up the amplified sound.
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