THE BRAIN

HOW DO WE HEAR SOUND?

The ear is made up of three parts: the outer ear, middle ear and inner ear.  How these three parts work together enable us to receive and process sound waves that can be turned into electrical impulses that are transmitted to the brain.

The Outer Ear

The outer ear is made up of the visible part of the ear called the pinna, the ear canal (meatus) and the eardrum (tympanic membrane). The pinna is shaped a bit like a shell and gathers sound waves and directs them into the ear canal. When a sound wave strikes the eardrum, the eardrum vibrates.

The Middle Ear

The middle ear is an air-filled chamber that contains three tiny bones: the hammer (malleus) which is attached to the eardrum, anvil (incus) which is in the middle of the three bones and stirrup (stapes) which is attached to the oval window – a membrane covered opening to the inner ear. This chamber is connected to the upper part of your throat by a narrow channel called the eustachian tube. The tube opens and closes as you swallow and helps to drain any fluids that may have built up and equalise pressure in the ear (which is why swallowing in the pressurised cabin of an aeroplane makes that ‘blocked up’ feeling in your ears go away). Having equal pressure on both sides of the eardrum is important for normal vibration of the eardrum.

When your eardrum vibrates, it triggers a chain of vibrations through each of the three bones. Because of differences in the size of the ear drum and entry to the inner ear and size and shape of the bones, the force of the vibration is increased by nearly 20 times when it reaches the inner ear. This increase in force is necessary to transfer the energy of the sound waves to the fluid of the inner ear.

The Inner Ear

The inner ear contains a group of fluid-filled chambers that are all interconnected. The chamber that is shaped like a snail is called the cochlea and sound vibrations from the bones of the middle ear are transferred to the fluids of the cochlea. Tiny ‘hair cells’ that line the inside of the cochlea convert the vibrations into electrical impulses that are transmitted along the auditory nerve to your brain.

The other fluid-filled chambers of the inner ear include three tubes called the semi-circular canals and special sensors inside them help regulate your sense of balance.

THE AUDITORY SYSTEM

The auditory system is made up of two parts: the peripheral system (outer ear, middle ear, inner ear and auditory nerve) and the central system (auditory pathways in the stem and auditory cortex).

The Peripheral System

The outer ear is composed of the auricle (only visible part of the ear), and the external auditory canal.
The peripheral system allows sound to be transmitted from the outer ear to the first neurons of the auditory nerve.
The middle ear is composed of the ear drum, 3 ossicles (malleus, incus and stapes) and the eustachian tubes.
When a sound wave is sent through the external auditory canal, it vibrates the eardrum.

Central Auditory System

The two systems (peripheral and central auditory system) communicate through afferent nerve fibres, which run from the organ of Corti to the auditory cortex, and efferent nerve fibres, which run the opposite way.
The central auditory system allows sound to be transmitted from the first neurons of the auditory nerve to the brain.
The information from the auditory nerve is then sent to the brain via several relays in the brain stem.
These two types of cells are linked to nerve fibres, which form two auditory nerves.

The Temporal Lobe

The auditory cortex is the part of the temporal lobe that processes auditory information in humans and many other vertebrates.
This lobe is special because it makes sense of the all the different sounds and pitches (different types of sound) being transmitted from the sensory receptors of the ears.
This lobe is the location of the primary auditory cortex, which is important for interpreting the sounds and language we hear.
The temporal lobe is located at the bottom of the brain below the lateral fissure; there is one temporal lobe on each side of the brain, in close proximity to the ears.
The temporal lobes are involved with memory and hearing, and process information from our senses of smell, taste, and sound.
The primary function of the temporal lobes is to process auditory sounds.
In fact, we would not be able to understand someone talking to us, if it wasn’t for the temporal lobe.

CROSS-MODAL CORTICAL REORGANISATION

HOW DOES HEARING LOSS AFFECT THE BRAIN?

It has been found that when hearing loss occurs, areas of the brain devoted to other senses such as vision or touch will take over the areas of the brain which normally process hearing. This is called cross-modal cortical reorganization, which is reflective of the brain’s tendency to compensate for the loss of other senses.

Despite the long name, this is not a complicated process. When hearing loss occurs, the brain must overcompensate for this lost sense in some way. By placing additional emphasis on other senses like touch and vision, this process leads to fatigue and adversely affects concentration. While this can help hard of hearing people cope to some degree with the loss of their hearing, it can cause detrimental effects to brain function.

For example, when a person experiences hearing loss, the area of the brain that processes sound begins to deteriorate. When hearing loss occurs, the brain must overcompensate for this lost sense in some way. Because the brain must overcompensate for these weakening brain functions, higher-level thinking is forfeited for speech understanding. Early stages of hearing loss can lead to cognitive decline.

CROSS-MODAL CORTICAL REORGANISATION

hearing loss and dementia

Studies have found that people with untreated hearing loss have lower grey matter density of the auditory areas and less brain activity when listening to complex sentences.
However, researchers agree that people who wear hearing aids, especially for age related hearing loss have a decreased risk of developing dementia as hearing aids help maintain brain function over time.
There is a noticeable correlation between hearing loss and dementia, this could be due to many factors such as isolation, cognitive decline, and change in brain structure.
People with untreated hearing loss have less grey matter in the auditory cortex which may begin when hearing ability declines.
Protecting the ability to hear with hearing aids preserves a person’s perception, cognition, and how the brain processes sounds, including speech.
Studies have found a strong correlation between untreated hearing loss and the development of Alzheimer’s and Dementia.
In another study among hearing impaired adults, the researchers concluded that those with treated hearing loss had significant improvements in every dimension measured, such as relations at home, feelings about self, mental health, and social life).
Many people who develop hearing loss in their older years live the rest of their lives without experiencing dementia.
It is important to note that, hearing loss does not always lead to Alzheimer’s disease or other forms of dementia.
Untreated hearing loss also links to depression from the feeling of anxiety and social isolation from one’s family and friends.
Many symptoms of hearing loss are like symptoms of Alzheimer’s disease and depression, and result in difficulties in communication.
A study conducted by the University of Exeter and King’s College London indicated that people with hearing aids had faster reaction times and better concentration.
Hearing loss affects a person’s personality, cognition, and hearing acuity. It can cause daily frustration, irritability, and stress in a person’s life.
Hearing aids can not only improve hearing but can also preserve the brain.