A1) How We Hear

Acoustic Hearing 

For most individuals, acoustic hearing is the primary mechanism for processing sounds in an environment. Acoustic hearing is when the ears convert sound waves into neural signals to be processed by the auditory cortex, an area of the brain responsible for processing sound. 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.

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 (Figure 1).

Figure 1: Police car siren moving as sound waves. These waves vibrate the air, 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 (Figure 2).

Figure 2: Sound wave pathway through the ear.

3. Sound vibrations activate hair cells in the cochlea, sending signals to the brain

The cochlea is a fluid-filled, spiral-like structure that houses tiny hair cells. The sound causes vibrations in the 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 (Figure 3

Figure 3: : Hair cells convert sound wave vibrations to neural signals and into the brain.

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 (Figure 4)

Figure 4: Example of Tonotopic organization within the cochlea.

As the sound waves vibrate the hair cells, those hair cells send neural signals to the brain, allowing us to identify the sound as a police siren.

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 electrical 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.

Think of neural pathways like a hiking trail (Figure 5). 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 hike 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, and the 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.

Figure 5: Neural pathways explained through an analogy of a hiking trail: with repeated use of auditory pathways, the pathway is more efficient.

As infants, our brains are learning to process input of environmental sounds and determine what they mean. As infants 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)

It is important to nurture these auditory pathways to ensure proper development. Hearing devices like cochlear implants and hearing aids are great ways to restore levels of auditory input, stimulating the development of the pathways.

Hearing loss

Hearing loss can occur for many reasons, but a common cause is damage to the ear structures–specifically, the cochlea and its inner hair cells. Cochlear hair cell damage can happen for many reasons: exposure to loud noises, genetics, bacterial infection, injury, and natural deterioration from ageing. When the hair cells are damaged, the audibility of sounds decreases. As hearing loss progresses, a person is no longer able to access specific frequencies. In general, most people lose hearing in higher frequencies and later at lower frequencies. Contrary to popular belief, hearing loss can occur at all ages, not just the older population. Regardless of age, there are many devices that help with hearing loss. Our lab studies individuals with hearing loss who use cochlear implants and/or hearing aids.

Cochlear Implants:

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.

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. This information is converted into electric signals, which are sent to an electrode array implanted in the cochlea and further to the auditory nerve (Figure 7). The electrode array sends the electrical signals through the cochlea instead of the hair cells, allowing the user to interpret inputs like speech.

Figure 6: Cochlear implant model and function.

Implantation: Bilateral vs. Unilateral

Bilateral implants mean that cochlear implants are placed on both sides, while unilateral means a cochlear implant is placed in only one ear. One of the 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

Hearing aids are used to amplify sound waves in the environment. It allows the ear to detect frequencies it wouldn’t be able to access without the additional gain/amplification.

While there are many kinds of Hearing Aids, they generally work the same (Figure 7)

1. Sound waves are received by a set of microphones, which convert and process the sound waves into electrical signals.
2. These electrical signals are then sent to an amplifier, making the sounds louder.
3. The amplifier sends the sounds to your ear through the speaker, which is inserted into the ear.
4. 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.

Figure 7: Hearing Aid function.