How Sound Travels to the Brain

We may hear with our ears, but we also hear with our brain. Our ears detect sound, while our brain processes what is being heard. It is important to understand how sound travels to the brain before fully understanding the different types of hearing loss.
There are three main components of the ear:

  • Outer ear: consists of the outer part of the ear that you can see (called the pinna) and the ear canal
  • Middle ear: comprised of the eardrum and three tiny bones called the ossicles (the malleus, incus and stapes) 
  • Inner ear: contains the organ of hearing called the cochlea, 3 semicircular canals, and the hearing nerve, called the vestibulocochlear nerve.

For normal hearing, these three components of the ear must be healthy and functioning well for sound to effectively travel to the brain. Any issue along this pathway can cause hearing loss. 

Sound enters the ear via the outer ear. The pinna collects environmental sounds and the sound enters the ear canal. This sound causes the eardrum to vibrate, which in turn causes movement of the three ossicles. Movement of the stapes (the third ossicle) then causes movement of fluid inside the cochlea, which causes the tiny hair cells to sway back and forth. Movement of those tiny hair cells then sends an electrical impulse up the vestibulocochlear nerve and onto the brain which signals to us that we have heard something. 
 

Types of Hearing Loss

While there are many different causes of hearing loss, there are a variety of types of hearing loss as well:

  • Sensorineural 
  • Conductive
  • Mixed
  • Central

The type of hearing loss an individual has determines their treatment plan for better hearing. Depending on the type of hearing loss, referrals to a family physician or otolaryngologist (ear, nose and throat specialist) may need to occur prior to rehabilitation with hearing devices.

SENSORINEURAL HEARING LOSS

This is the most common type of hearing loss we see at our clinics. Sensorineural hearing loss is most often permanent. It occurs in the inner ear when tiny hair cells are damaged or missing inside the cochlea. Once the hair cells are damaged, they cannot regenerate. This is when amplification with hearing devices comes into play. 

Sensorineural hearing loss can be caused by a variety of factors such as:

  • Excessive noise exposure
  • Ageing (presbycusis)
  • Family history of hearing loss (heredity)
  • Viruses
  • Autoimmune inner ear diseases
  • Meniere’s disease
  • Medications
  • Disease
  • Head trauma
  • Tumors

CENTRAL HEARING LOSS

Central hearing loss occurs when there is a problem in the vestibulocochlear nerve, the brainstem and/or the hearing centres of the brain in the central nervous system. 
When an individual suffers from central hearing loss, this results from an issue with the brain’s interpretation of sound. The outer, middle and inner ear may be working correctly, and sound is being detected; however, the brain is unable to process the sound correctly. 

Central auditory processing disorder is one of the most common forms of central hearing loss. With central auditory processing disorder, or CAPD, there is a disconnect between what is heard and what is understood. People with CAPD often report significant difficulty understanding conversations in noisy environments and difficulty localizing where sound is coming from. There is some research to suggest that amplification with hearing aids may be beneficial to help treat central auditory processing disorder and other causes of central hearing loss. 

CONDUCTIVE HEARING LOSS

Conductive hearing loss can be permanent but is also often temporary, depending on its cause. When hearing loss is conductive in nature, the hair cells in the cochlea are not necessarily damaged.  Conductive hearing loss occurs when sound is unable to successfully travel to the inner ear to the cochlea. Somewhere along the pathway, the sound is dampened. Technically, excessive wax build-up or the use of hearing protection causes conductive hearing loss. 

Often medical issues can result in conductive hearing loss. Some examples include:

  • Infection of the ear (outer, middle or inner)
  • Impacted cerumen (earwax)
  • Eustachian tube dysfunction
  • Allergies
  • Perforated eardrum
  • Foreign body in the ear canal 
  • Otosclerosis
  • Malformation of the outer ear, ear canal, or middle ear structure

During an initial hearing evaluation, if a conductive hearing loss is identified, your audiologist will need to ensure medical clearance before recommending hearing aids. This is because in some cases, depending on the cause of the conductive hearing loss, surgery alone is all that is required to address the hearing loss. Sometimes, surgery in conjunction with amplification with hearing aids is recommended, and other times, surgery is not recommended, and the treatment plan will be amplification alone. 

MIXED HEARING LOSS

Mixed hearing loss occurs when conductive and sensorineural hearing loss occur at the same time. Depending on the cause of the conductive component, your audiologist will also want to obtain medical clearance before proceeding with hearing aids. Again, sometimes surgery alone or in conjunction with hearing aids is what is recommended.