From Ear To Brain
The Miraculous Journey of Sound
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.
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.
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.
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.
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.
The concept of hemispheric specialization suggests that one side of the brain is better equipped to perform certain functions that the other. In reality the division of labour between the two hemispheres is rarely as clear cut as we would like to imagine. Like most perceptual skills, auditory abilities are supported differently across brain hemispheres, but they are by no means mutually exclusive. Auditory signals from each ear are transmitted to both hemispheres, but preference is given to the opposite side. Information arriving at both cortices is eventually shared with the opposite hemisphere through a large tract of nerve fibres known as the corpus callosum. In effect both hemispheres receive and share similar neural information, although differences in signal strength and time of arrival play a role in creating so-called regional specializations. Strangely enough, the corpus callosum may actually enhance single-sided processing by suppressing information from the opposite side when needed.
Language And Handedness
For most individuals the left side of the brain (right ear) is better equipped to process the elementary units of language. Although the right hemisphere (left ear) may be more adept at perceiving overarching intention of speech conveyed via intonation. In simple terms, speech comprehension is reliant on both mathematical analyses from the left hemisphere and holistic interpretation from the right hemisphere. Similarly, music appreciation is achieved through the integration of analytical and emotional brain responses.
Modern imaging techniques not only support the notion of hemispheric specialization, but also reveal sub-specializations within different regions of each hemisphere. For ten percent of the population, hemispheric specialization is reversed. Handedness is a poor predictor of actual specialization, as most left- and right-handed individuals exhibit the same brain mapping for language processing. It should be remembered that the brain has the ability to reorganize itself to a certain extent, as is often seen following traumatic brain injury.
Like the rest of our body, our ears and hearing undergo maturational changes. Throughout childhood the auditory system is acquiring the skills necessary for focus and comprehension. Memory and emotional centres of the brain also learn to classify sounds for later recall. In this way, a child learns about the relevance and emotional value of a myriad of different sounds (think: lolly wrapper versus a hornet’s buzz!) The time taken for neural impulses to transmit from ear to brain also decreases through puberty and reaches peak performance by one’s late 20s. Networking between key brain regions also strengthens due to ongoing exposure to different auditory stimuli. Paradoxically, cochlear functioning has been demonstrated to decrease from the onset of puberty. It is unclear whether these changes are a consequence of natural development or environmental factors. It is possible to hone one’s auditory skills too. Children who take up musical instruments early in life may develop and maintain better pitch perception throughout life than those who do not. Remedial speech and language therapy also depends on the plasticity of the brain’s auditory structures, as does learning a foreign language.
Fortunately degenerative processes occur very slowly in the auditory system. Most changes in hearing happen so gradually that we may not be fully aware of the extent of hearing decline. There is also a certain amount of redundancy built into speech and hearing, which tends to mask early changes in hearing ability. Many people first become aware of changes in their hearing in their mid 50s, but may not perceive any real disability until later. It is typically in ones 60s or 70s that hearing loss begins to interfere with communication in a way that is noticeable to the listener and others.
The aging process occurs along a number of fronts in the ear. Most noticeably we begin to lose clarity of speech and focus in noise due to the cumulative loss of hair cells in the cochlea. The high frequencies needed for speech definition tend to be affected first. Because the low frequencies remain relatively intact, loss of overall volume is less evident to the listener. There are effects higher up the system too. As communication between brain centres slows, so too does processing of auditory signals. This slowing down of auditory processing is most noticeable under challenging conditions, such as in background noise or when people speak quickly.
Lifestyle And Diet
It has long been understood that noise, particularly high pitched noise associated with industry, has the potential to permanently damage hearing. What is becoming increasingly evident however, is that hearing acuity is linked to other lifestyle factors such as diet, cardio-vascular health and smoking. It should come as no surprise that maintaining good overall health can minimize the risk of early onset hearing loss. Like all body cells, the inner ear hair cells as well as brain cells associated with hearing, rely on blood-borne nutrients for maintenance and survival. It follows that lifestyle factors that restrict blood circulation can also affect hearing in the longer term. A balanced diet rich in fruit, vegetables and wholegrains combined with regular exercise remains good advice, also where hearing is concerned. Alcohol under moderate intake may be beneficial, although overdoing it can easily have the opposite effect.
While industry has become well versed at protecting its workers’ hearing, there is a growing incidence of noise induced hearing loss related to leisure activities. Most young people today will have spent many hours connected to earphones during their growing years. Without proper output limiting, some devices can produce dangerous levels of volume, especially combined with the duration of the exposure. Community education about the dangers of leisure noise has improved, but ultimately law-makers may need to step in to enforce safe limits for portable devices and public arenas.
There is a mounting body of evident to show that reduction in cochlea output can drive or accelerate changes further up the auditory system. There is an established link between (untreated) hearing loss and brain atrophy (loss of brain mass). Hearing loss may also hasten the loss of brain cells in regions outside the primary auditory cortex associated with language and social skills. Prescribed amplification from hearing aids may prevent or slow the effects of auditory deprivation as they partly restore the cochlear output needed to stimulate higher auditory regions. Moreover, imaging studies show re-structuring of auditory brains centres in response to regular use of prescribed amplification.
Hearing Aids And Cognitive Health
The ultimate goal of hearing aid therapy is to maintain social participation, which increasingly appears to be important for maintaining cognitive health in later years. Current research suggests that individuals with significant untreated hearing impairment are at increased risk for cognitive decline, including dementia. These findings reinforce the notion that hearing, communication and social participation are intricately entwined and are the very processes which invigorate our human brains.