Hearing Basics

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How do we hear?

Hearing is a transformation of physical sound vibrations into electrical impulses that are decoded in the brain as sound.  The actual transformation of pressure waves in all animals occurs with the help of hair cells in the inner ear. 

Sound from the environment or a voice enters the ear canal and is collected by the large surface area of the eardrum (tympanic membrane).  The eardrum is connected to the first of 3 hearing bones in the middle ear: they are called the malleus, incus and stapes (hammer, anvil and stirrup) because of their shapes.  The sound vibrating the large eardrum is concentrated to the very small surface of the footplate of the stapes.  This increases the power of the incoming vibrations so that they can enter the fluids of the cochlea (inner ear).

The cochlea is a long spiral snail shaped tube. The 3 semi-circular canals on the other end of the inner are for balance.  Resting on a membrane that rides up the spiral of the cochlea is the tiny organ of Corti which contains the hair cells of hearing.  Each inner ear contains approximately 10,000 hairs cells.  They are arranged from high to low frequency like the keys on a piano keyboard. While a piano keyboard has 88 keys, the organ of Corti has 3,400 separate keys (inner hair cells). When a sound enters the cochlea the hairs of the inner hair cells corresponding to the tones of that sound are moved.  Those inner hair cells secrete neurotransmitters to activate the nerves of hearing connected to them.

Attached to each inner hair cell are 10 to 20 auditory nerve fibers (30,000 in total).  Nerve signals travel in synchronized volleys down the auditory nerve fibers into the brainstem where they are first received in the cochlear nucleus in the brainstem.  Some nerve fibers cross to the opposite side so sounds on one side are transmitted up the brainstem to the auditory cortex on both sides. 

It is important to understand that sounds are collected by the outer ear, ear canal and tympanic membrane, are concentrated into the fluids of the inner ear by the middle ear mechanism consisting of the tympanic membrane and hearing bones, are changed to electrical impulses by hair cells in the inner ear, and are carried to the brain by the auditory nerve, but become actual sound perceptions  only when they reach the brain.  Any disruption of this progression of sounds from the environment to the cortex of the brain can result in a hearing loss.  Therefore, there are very many ways to be hearing impaired and many possible solutions depending on the cause and specific situation. 

When does hearing begin to develop?  When is language developed?  Does anything happen in my child’s brain before 2 years when he begins to speak?

 Your child’s hearing journey begins in utero.  While floating in the amniotic fluid of the womb, sound vibrations pass from your voice, your heart, your breathing and from the environment and directly to the child’s inner ear.  This may be an important part of pre-learning of language.  When the child is born he becomes more manifestly aware of environmental sounds and of speech.  The auditory cortex is densely programed with millions of connections to be ready to receive any kind of frequencies which tend to recur in the environment.  The most important of these sounds are phonemes, the parts of speech.  When these parts of speech recur in the child’s environment because they are around adults and other children who speak fluently, they begin the process of language learning.  Any neurons which correspond to the phonemes are strengthened and duplicated but neurons with interconnections which are never utilized (such as particular phoneme sounds not found in the language spoken at home) start to become pruned away.  This process of pruning in the hearing cortex continues aggressively in early development, and 70 to 80% of the job is done in the first two and one-half years of life.  In this way the child’s brain becomes tuned and strengthened to detect the sounds that are the building blocks of the particular language he is learning.  A child may not be able to speak before the age of 2, but the babbling he makes usually takes the form of repetition of some basic phonemes.  A speech pathologist can usually detect the early speech behavior as evidence that the child is hearing and beginning the process of language building.  At 2 years of age the child will typically begin to combine multiple phonemes to form words and to combine words to form short meaningful sentences. 

Even after speech emerges the child’s auditory system continues to develop and is not functioning in its fully adult state until the age of 15.  Children have a harder time focusing on a single sound in background noise than adults do, and need to hear sounds louder compared to the background noise level than adults do to recognize all of the words.

If a child has a hearing loss particular attention has to be directed to phoneme recognition development and deliberate speech and language development.  We cannot make assumptions, if the child has hearing impairment, that language will simply emerge naturally so long as hearing aids are used.  In early life, a very deliberate communication reinforcement system is used to ensure that auditory language listening skills are maximized.