This neuroscience of music page is the product of my independent research project as a sophomore in high school.

How We Process Sound

Three Parts of the Ear

There are three parts of the ear: the outer ear, the middle ear, and the inner ear. The functions of the three parts of the ear and how they interact with each other to enable us to hear is explained below.

• First, there is the outer ear, otherwise known as the pinna, which represents the majority of the area that is perceivable to the human eye. The outer ear works to capture sound from outside the person and acts as a transmitter between the sound and the ear canal.

  • The pinna is essential due to the difference in pressure inside and outside the ear. The resistance of the air is higher inside the ear than outside because the air inside the ear is compressed and thus under greater pressure. In order for the sound waves to enter the ear in the best possible way the resistance must not be too high. This is where the pinna helps by overcoming the difference in pressure inside and outside the ear. The pinna functions as a kind of intermediate link which makes the transition smoother and less brutal allowing more sound to pass into the auditory canal.

  • Once the sound waves have passed the pinna, they move two to three centimeters into the auditory canal before hitting the eardrum, also known as the tympanic membrane. The function of the ear canal is to transmit sound from the pinna to the eardrum.

  • The eardrum (tympanic membrane), is a membrane at the end of the auditory canal and marks the beginning of the middle ear. The eardrum is extremely sensitive and pressure from sound waves makes the eardrum vibrate.

• Second, there is the middle ear, which is located between the eardrum and the oval window. The middle ear also works to transmit sound from the outer ear to the inner ear. The middle ear consists of three bones: the malleus, which resembles a hammer shape, the incus, which resembles an anvil shape, and the stapes, which resembles a stirrup shape. There is also the oval window, the round window, and the Eustrachian tube. Humans hear sound because the pressure from sound waves makes the eardrum vibrate. The vibrations are transmitted further into the ear via the three bones in the middle ear. These three bones form a kind of bridge, and the stapes (stirrup), which is the last bone that sounds reach, is connected to the oval window.

  • The oval window is a membrane covering the entrance to the cochlea in the inner ear. When the sound waves are transmitted from the eardrum to the oval window, the middle ear is functioning as an acoustic transformer amplifying the sound waves before they move on into the inner ear. The pressure of the sound waves on the oval window is some 20 times higher than on the eardrum. The pressure is increased due to the difference in size between the relatively large surface of the eardrum and the smaller surface of the oval window.

  • The round window in the middle ear vibrates in opposite phase to vibrations entering the inner ear through the oval window. In doing so, it allows fluid in the cochlea to move.

  • The Eustachian tube is also found in the middle ear, and connects the ear with the rearmost part of the palate. The Eustachian tube’s function is to equalise the air pressure on both sides of the eardrum, ensuring that pressure does not build up in the ear. The tube opens when you swallow, thus equalising the air pressure inside and outside the ear.

• Third, there is the inner ear which is the innermost part of the ear, which consist of the cochlea, the balance mechanism, the vestibular and the auditory nerve.

  • Once the vibrations of the eardrum have been transmitted to the oval window, the sound waves continue their journey into the inner ear. The inner ear is a maze of tubes and passages, referred to as the labyrinth. In the labyrinth can be found the vestibular and the cochlea.

  • In the cochlea, sound waves are transformed into electrical impulses which are sent on to the brain. The brain then translates the impulses into sounds that we know and understand. The cochlea resembles a snail shell and is filled with a fluid called perilymph and contains two closely positioned membranes. These membranes form a type of partition wall in the cochlea. However, in order for the fluid to move freely in the cochlea from one side of the partition wall to the other, the wall has a little hole in it (the helicotrema). This hole is necessary, in ensuring that the vibrations from the oval window are transmitted to all the fluid in the cochlea. When the fluid moves inside the cochlea, thousands of microscopic hair fibers inside the partition wall are put into motion. There are approximately 24,000 of these hair fibers, arranged in four long rows.

  • The auditory nerve is a bundle of nerve fibers that carry information between the cochlea in the inner ear and the brain. The function of the auditory nerve is to transmit signals from the inner ear to the brain. The hair fibers in the cochlea are all connected to the auditory nerve and, depending on the nature of the movements in the cochlear fluid, different hair fibers are put into motion. When the hair fibers move they send electrical signals to the auditory nerve which is connected to the auditory centre of the brain. In the brain the electrical impulses are translated into sounds which we recognize and understand. As a consequence, these hair fibers are essential to our hearing ability. Should these hair fibers become damaged, then our hearing ability will deteriorate.

  • The vestibular is another important part of the inner ear. The vestibular is the organ of equilibrium. The vestibular’s function is to register the body's movements, thus ensuring that we can keep our balance. The vestibular consists of three ring-shaped passages, oriented in three different planes. All three passages are filled with fluid that moves in accordance with the body's movements. In addition to the fluid, these passages also contain thousands of hair fibers which react to the movement of the fluid sending little impulses to the brain. The brain then decodes these impulses which are used to help the body keep its balance.

The Auditory Brain

The auditory brain is the part of the brain that processes sound and recognizes individual aspects of music, such as tone, pitch, or rhythm, and translates this feedback into a unified piece of music. In terms of how the parts of the ear and the brain are connected, the auditory nerve connects the cochlea of the inner ear directly to the auditory cortex on both sides of the brain, where sound is processed. In addition to this information, the auditory cortex is divided into the following three parts.

First, is the primary auditory cortex, which is primarily responsible for the ability to hear. Its purpose is to process sound along with its volume and pitch. Second, is the secondary auditory cortex, which processes harmonic, melodic and rhythmic patterns. Third, is the tertiary auditory cortex and researchers claim this area is where every aspect of sound is integrated into the overall experience of music.

Auditory messages are conveyed to the brain via two types of pathway: the primary auditory pathway which exclusively carries messages from the cochlea, and the non-primary pathway (also called the reticular sensory pathway) which carries all types of sensory messages.

• The primary auditory pathway

  • Schematically, this pathway is short (only 3 to 4 relays), fast (with large myelinated fibers), it ends in the primary auditory cortex. The pathway carries messages from the cochlea, and each relay nucleus does a specific work of decoding and integration.

    • The first relay of the primary auditory pathway occurs in the cochlear nuclei in the brain stem

    • The second major relay in the brain stem is in the superior olivary complex. After the second relay, there are two more relays that take place.

    • The final neuron of the primary auditory pathway links the thalamus to the auditory cortex, where the message, already largely decoded during its passage through the previous neurons in the pathway, is recognized, memorized and perhaps integrated into a voluntary response.

• The non-primary pathway

  • From the cochlear nuclei, small fibers connect with the reticular formation where the auditory message joins all other sensory messages. The next relay is in the non-specific thalamus nuclei before the pathway ends in the polysensory (associative) cortex. The main function of these pathways, also connected to wake and motivation centers as well as to vegetative and hormonal systems, is to select the type of sensory message to be treated first.

Auditory perception depends on our alertness. 

• Sound, which is transformed in the ear into a neural signal, is processed in the brain at a number of different levels. These levels are as following.

  • Other brain areas, which allow the perception to become conscious, recognize the sound by comparing it to those that have previously been memorized and determine an appropriate voluntary response.

  • The auditory cortex where the sound is perceived

  • A reflex where the arrival of the message causes us to jump or turn our head

  • When awake, all three levels above are activated.

    • Example: when we hear the sound of a voice, we start to listen (reflex), recognize a friend’s voice (memory) asking an important question (motivation, emotion) and then we answer.

  • When asleep, our ears are still working; sound enters the auditory pathway (and reflexes can therefore still occur) up to the auditory brain, but the other brain regions (involved in emotions, motivations, or memory) are inactive. There are therefore no voluntary responses or conscious perception.

    • Example: speaking to someone who is asleep (or a sound from the street) can make them move without waking them, and without them remembering it when they wake up.