From Ear To Brain

The Miraculous Journey of Sound

Your ear is an amazing piece of engineering. It converts vibrations in the air into meaningful perception. As sound travels along the auditory pathways it is converted into three different forms of energy (mechanical, hydraulic and neuro-chemical) before it is actually heard. Let’s now follow sound on its miraculous journey towards perception!

The Nature of Sound

At its most basic level, sound is a cyclical agitation of molecules that spreads out in all directions, capable of travelling through gas, liquids and solids. Faster pulse rates are interpreted as high pitch, while slower pulse rates are interpreted as low pitch. Sound is the seed of hearing and signifies movement in our environment. It is capable of influencing our emotions and warning us of dangers and forms the basis of verbal communication. The process of hearing begins with collection of sound by the outer ear…

The Outer Ear

The outer ear (aurical) captures sounds associated with speech and funnels them into the ear canal. The convolutions of the aurical help us to identify where sounds come from and provide our brains with the necessary cues for focus.

Sounds are magnified slightly as they are funnelled down the relatively narrow ear canal before impinging on the eardrum. The eardrum is a very finely tuned membrane consisting of three primary layers. The middle fibrous layer has radial and concentric strands designed to maximize vibrating efficiency. Once the eardrum is set into motion, sound is converted from air-borne to mechanical energy as it travels into the middle ear.

The Middle Ear

The eardrum is pinched inward at its centre and held taut by its connection with the malleus bone. The malleus bone is the first of three tiny bones (malleus, incus and stapes) which make up the sound-bridge suspended in the middle ear space. The bones are hinged at their joints and suspended by ligaments in this air filled chamber known as the tympanum. A leveraging action of the bones helps to amplify mechanical vibrations as they travel toward a second smaller membrane known as the oval window. Because the eardrum is relatively large compared to the oval window, sound is concentrated by a factor of x22 as it travels through the middle ear. It is for this reason that the middle ear is sometimes likened to an amplifier. This added amplification is required for sound to make its next transformation as it enters the inner ear.

The Inner Ear

Beyond the oval window membrane lies a complex series of fluid filled chambers known as the labyrinth. The labyrinth contains the sensory receptors for both hearing and balance. The coiled end (cochlea) relates to hearing. It contains some 30 thousand sensory cells, each of which have dozens of microscopic “hairs” sprouting out their tops. These microscopic filaments are called cilia, and play an important role in pitch definition. The cochlea itself is divided into three parallel spiral chambers. The sensory receptors reside in the middle chamber (scala media) which is filled with an electrically charged type of spinal fluid.

The piston-like action of the stapes bone at the oval window sends a wave propagating through the spirals of the cochlea. It is at this point that mechanical vibrations are converted into hydraulic waves. As these waves travel, they agitate the sensory cells and their microscopic cilia. Different cells are activated by different pitches because there is a stiffness gradient along the cochlear spiral.

Central Pathways

As successive sound waves travel across the hair-like cilia, they sheer in a manner similar to reeds on an ocean floor. This sheering action opens up channels in the cilia, which permit an influx of chemicals into these sensory cell bodies. The mixing of chemicals inside the cell bodies produces an electrical discharge from the base of cells. In effect, a stream of electrical signals is transmitted along the auditory nerve and the central auditory pathways. Once the hydraulic wave motions are converted to an electrical signal, the final transformation of sound is now complete. On their way to the primary auditory cortices in the temporal lobes of the brain, these electrical signals are encoded by various waystations, including emotion and language centres. From ear to brain, the journey of sound is completed in less than a tenth of a second.

Auditory Perception

The almost incomprehensible connection between billions of brain cells permits the miraculous leap from neuro-chemical flux to auditory perception. Auditory perception and all the preceding events can be traced back to simple vibrations of molecules in the air. Through this amazing organ, we are connected with our environment in a way which enables us to “see” around corners, where our vision cannot.

Brain Power

Our brains are not simply passive receptors of sounds, they actually exert control over our ears. However, unlike our eyes, whose movements we can see, the inner workings of the ear are concealed from vision. It is therefore easy to forget that the cochlea is also under the command of the brain just as our eyes are.

Descending Pathways

The nerve fibres ascending from our ears to the brain are mirrored by an equally important descending array of fibres. The descending auditory pathways permit information from higher brain centres to be shared with lower brain centres and the cochlea. The hair cells in the cochlea possess microscopic muscles which allow them to enhance or dampen their movements in response to the needs of the listener. In this manner the brain is constantly filtering the type of information that is being gathered by the cochlea. Unfortunately the filtering action of the cochlear cells is compromised by sensori-neural hearing loss. A common complaint of people with this type of hearing loss, is that they cannot focus their hearing in background noise.

The Concept Of Attention

Further fine tuning of auditory signals is achieved by feedback loops within key brain regions associated with attention, emotion and language. An example of this automatic filtering occurs when our ears “prick up” in response to hearing our name mentioned at a party. Sounds of greater emotional importance tend to grab our attention more than irrelevant ambient noise. These processes come partly under the control of our autonomic nervous system, which means we can maintain a certain amount of auditory vigilance even when asleep. The sound of an infant’s crying is more likely to waken our senses than rain patter on a roof. As some wives might attest to, the concept of “selective deafness” stems more from central brain processes than the auditory periphery.

Two Ears Working Together

The benefits of binaural hearing are numerous. Not all sounds bend equally around the head. Short wave lengths associated with high pitch sounds tend to be reflected before reaching the other side, while long wavelengths associated with low pitch sounds may reach both ears more readily. Having two ears is like a blind spot detector for hearing. When one ear is in “auditory shadow”, the other is in “auditory light”. The brain also makes use of minute differences in signal strength, time of arrival and phase of soundwaves between the two ears to locate the direction of sounds. These differences between the ears also help our brains to suppress background noise and focus our hearing on the things that matter most. Individuals who lose hearing in one ear find that the auditory landscape becomes less three dimensional and have greater difficulty focusing in background noise or telling where sounds come.