VV: How does the auditory system typically develop?
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stood by the brain as meaningful sounds, most importantly the sounds of speech. The cochlea is specially designed to interpret the loudness and pitch differences of each sound and those signals travel through the brainstem to the brain through a complex system of hair cells, neurons and nerve endings. The brain receives and interprets these sounds rapidly and efficiently, and this is why we often refer to the brain as the most important part of the auditory system. The anatomy of the human ear. Sound works its way through this complex system to travel to the brain, where it is interpreted as sound.
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Anu Sharma: The auditory system is processing sound all the way up to the highest levels of the brain by 28 weeks gestational age. At this point in time, the inner ear is fully matured, though the middle and outer ear will continue to develop, even after birth. The central auditory pathways develop and mature in response to sound traveling through the brainstem via the auditory nerve and up to the cortex of the brain. The cortex will only develop typically upon auditory input, as circuits within the cortex form to process auditory signals and transmit these signals to the language centers of the brain.
intervention services, the great majority of children who are born with hearing loss can gain listening and spoken language at a level commensurate their peers with typical hearing. VV: How do we hear? Estes: Simply put, sound pressure travels through the air, is collected by the outer ear and is funneled to the eardrum. The eardrum changes the sound energy to vibrations, which is then focused to a tiny spot on the cochlea so that the energy is strong enough to make vibrations in the inner ear. The cochlea’s job is to take those vibrations and turn them into signals that can be under-
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Estes: Because of biologically predetermined courses of development. The prenatal period is the time in which neurons are created (neurogenesis) and the nervous system determines where the neurons are going to be located (migration). The most important aspect of neurological development, the creation and cementing of connections among neurons (snyaptogenesis), happens as a result of the stimulation the brain receives through input from an individual’s senses. The human brain has a genetic predisposition to want to make sense of the world. This condition of forcing neurons to compete with each other to be selected for “hard wiring” is a necessary biological action that allows the VV: And how does this process affect individual to develop the skills necessary to children born with hearing loss? be successful in his or her environment. Which leads to the question, why don’t Sharma: If a child begins hearing within we wire all the necessary connections the womb, and development of the central together before birth? Because there are auditory system is dependent upon normal input of sound, one can imagine that hearing not enough genes in the human genome to predetermine these connections. A fully loss will have a definitive impact upon the developed brain has over 100 trillion conmaturation of the auditory cortex. Similarly, nections (pruned down from the quadriltypical development will not occur in the lions of connections in the 2-year-old brain), auditory language centers of the cortex due and there are only 80,000 genes. We use our to a lack of input from the primary auditory genes for lots of things (like neurogenesis cortex. However, our lab has found that if and migration). There just aren’t enough for appropriate and sufficient acoustic stimulasynaptogenesis too. Children during the first tion is provided with a cochlear implant when the child is age 3 or younger, the audi- two years of life create an overabundance of synaptic connections, which continues until tory cortex will develop similarly to that of a child who had typical hearing (Sharma et al., around age 8 as they learn new skills and behaviors. These connections are forced to 2009). This window of time during which compete for survival and are either pruned sound may be introduced to a deprived system and still promote typical development away or cemented based on the amount of electrical activity that occurs at that connecis termed a sensitive period. tion. This is an active process of pruning in addition to the active process of cementing. VV: Why are ages 0-5 critical to a child’s development, regardless of hearing ability? If a connection doesn’t get “traffic,” it’s cut.
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