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Hearing involves the conversion of sound waves that enter the ears into nerve signals that are received by the brain (Mayo Clinic, 2007).  The ear is divided into three areas: the outer ear, middle ear and inner ear.  The outer ear receives sound waves and transmits these to the eardrum as vibrations.  Three small bones of the middle ear made up of the hammer, anvil and stirrup, together with the eardrum, intensify the vibrations as these are transmitted along the inner ear and into the cochlea, a winding structure of the inner ear.  The cochlea harbor nerve cells that are equipped with cilia that convert sound vibrations into electrical signal that can be recognized by the brain.  The cilia have the ability to distinguish different sounds from each other.  The human ear can discern soft sound waves (0 to 20 decibels (dB)) as well as withstand loud sounds (?110 dB) (NADP, 2003).

  The brain is responsible for sensing sound through the use of approximately 10,000 fibers that are present in the auditory nerve.  These fibers transmit sound frequencies to the sub-cortex of the temporal lobe of the brain at a rate of 1/3 second.The temporal lobe is responsible for the body’s sense of hearing during consciousness (Lawrence, 1968).  Background noise that exists in the environment are filtered out by the central auditory system.  Examples of background noise include the ticking of the clock or the humming of the air-conditioner.

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  The sound of the particular background noise may be enhanced through a modification of electrical resistances between the nerve cells associated with its signal transmission to the brain (NADP, 2003).The mechanism is analogous to the telephone exchanges that take place on the switchboard, wherein a particularly enhanced signal is received by the sub-cortex as a type of conscious perception, and a corresponding electrical impulse is immediately generated that would be equivalent to the pattern that has been created in the hearing memory.  Should the corresponding electrical impulse be weak, the perception of the sound is also weak.

  Loud sounds are associated with strong electrical impulses, which in turn cause to generate a perception of an intrusive sound.  The limbic system, also known as the center of learning and emotion, and the prefrontal cortex, or the section of the brain responsible for behavior, generally control the body’s capability to perform pattern matching and sound perception.  This tightly-coupled feature of sound perception must have evolved through the need to detect potential threats in the environment.An individual’s perception of loudness is simply based on the strength or intensity of the sound itself, but also on the association and/or implication of that particular sound.  An example would be the sound of water dripping in the middle of the night.  The individual that is sensitive enough to hear this sound will be worried that he might not be able to sleep because of such persistent sound, of which his brain tends to intensify the sound because of the individual’s attention has been turned to it.

  There are also times when over-sensitivity to sound is triggered by unexplainable fear or distress.  Examples of this situation include individuals whose lives have been severely affected by noise from neighboring factories or generators.  Unfortunately, the central auditory processing capability of the brain is very influential that it can instruct itself to continuously perceive small insignificant background noise which is are translated into extremely noisy and disturbing sounds that remain audible to an individual’s perception.  An individual’s current emotional state influences his ability to hear sounds of different intensities in his immediate environment.  During stressful conditions, an individual may be hypersensitive not only to sound, but also to sight, smell, touch and pain, because their emotional state has triggered the perception of its general senses in response to a threatened state of condition.Continuous loud noise generally bothers people, yet there are certain individuals who are very sensitive to loud sounds and are incapable of enduring the levels of noise.

  Such discomfort emanating from loudness or recruitment is described as hearing loss or impairment of hearing (NADP, 2003).  Such condition is associated with the loss of the cilia in the inner ear, which are usually employed for perception of sounds that are of high frequencies.  While cilia and nerve cell endings collect sounds and transmits these to the inner ear, these structures are significantly reduced or bent in individuals with hearing impairment, hence their ability of classify different sound intensities is affected.  In addition, the nerve cells involved in hearing perception may have degenerated, resulting in impaired transmission of sound to the brain (Mayo Clinic, 2007).

  At this setting, sound intensity is higher than the threshold of hearing, and the reduced nerve fibers in the cochlea generate a loud sound that is almost at its highest intensity.  At some instances, sounds with high pitches are subdued that it becomes difficult to understand any words amidst background noise.  Therefore, people with hearing impairment tend to react to normal intensity sounds because they perceive these as irritably loud.Hearing loss may also be caused by a severe accumulation of earwax along the ear canal, which blocks and prevents transmission of sound waves to the inner ear (Mayo Clinic, 2007).  Infections or tumors in particular areas of the ear as well as physical damage to the eardrum may also result in loss of hearing.Individuals diagnosed with hearing loss or impairment are advised to use hearing aids, which facilitates their perception of different sound intensities from their environment.

  Hearing aid fitting is performed in order to avoid overloading the ear with amplified sound.  Hearing aids have the ability to prevent irritating sounds from being further enhanced in the brain.  Such compression capability is very helpful, because it controls the volume of the sound and avoids further damage to the hearing of the individual.  Moulds are features in hearing aids which facilitate transmission of sound waves along the ear canal.  If an individual has a loss his ability to hear sounds of high frequency, it is best keep the mould open so that vibrations can pass through continuously.

  The mould may also be adjusted to allow unwanted low frequency sounds to be released, so that amplification of the necessary sounds is easier.Maskers, or white noise generators, may also be employed to reduce discomfort in the perception of irritatingly loud sounds, which is usually observed in individuals with normal to near normal levels of sound perception.  White noise is generally administered in a subtle manner and adjusted in gradual intervals starting from the lowest levels until the desired volume of sound is set.  This technology is provided by audiologists and is usually combined with counseling, as well as behavioral therapy that would teach individuals to manage stress and to learn how to induce relaxation.

It is also possible to redirect or teach the auditory system at both conscious and unconscious levels to normally respond to outside noise that an individual has initially categorized as irritating or damaging.  This behavior modification treatment employs evaluation and discussion of the causes for the individual’s extreme sensitivity to sound, as well as psychological concerns about noises that are strongly correlated with their emotional states.  This desensitization therapy is very tedious and usually last from several months a to year, but is generally effective.Auditory processing disorder (APD) is usually diagnosed in children who find difficulty in identifying subtle differences between sounds and words, even when these are delivered in good audible levels (NIDCD, 2004).  This disorder usually occurs when a child is brought up in a noisy environment or when a child is listening to composite information.  APD is also known by the terms central auditory processing disorder (CAPD), auditory perception problem, auditory comprehension deficit, central auditory dysfunction, central deafness or word deafness.  To date, research has not indicated the exact mechanism of how this disorder is generated.  It has been associated with dyslexia, attention deficit disorder, autism, autism spectrum disorder, specific language impairment, pervasive developmental disorder or developmental delay.

  Unfortunately, APD is misdiagnosed in normal-hearing or normal-language children but yet present with learning disabilities.Children with APD usually have normal hearing sensitivities as well as intelligence.  Symptoms of APD include difficulty in paying attention as well as memorizing information, poor capabilities in following multi-step directions, reduced listening skills, need for extended duration to process information and poor academic performance.  These children also have behavioral problems, difficulty in understanding language, and suffer from problems with reading, spelling, vocabulary and comprehension.

APD in children is usually initially noticed by any person who spends time with the child.  This may be a parent, teacher or daycare provider, who may have observed that the child is experiencing difficulty in processing auditory information and this affect the child’s performance in school.  A healthcare professional may confirm the diagnosis by reviewing the child’s growth and development and by referring the child to an otolaryngologist.  Disorders associated with hearing functions may be detected through an audiologic evaluation, which tests sound perception as well as word and sentence comprehension.  Problems with language comprehension may also be evaluated by a speech-language pathologist.  In addition, cognitive and behavioral assessments may be performed by a psychiatrist.There is currently no identified treatment for APD, yet several strategies are available that will help children cope with their difficulties in auditory processing.

  These include the employment of auditory trainers which are electronic gadgets that helps an individual concentrate on a speaker and decrease the intervention of background noise.  Auditory trainers are frequently employed in classrooms where teachers use microphones to transmit a sound, and a headset is used by an individual to hear the broadcasted sound.  Strategic modifications in the child’s immediate environment may also help, such as the installation of speakers, and premeditated seating arrangements for children.  Learning curves may also be positively influenced by the employment of exercises on improving language-building skills.  Auditory memory enhancement, which converts specific information to more basic versions, may also reduce the burden of experiencing difficulty in auditory memory.  The auditory system may also be directly taught to consciously decrease hearing distortion through auditory integration training.  However, further research has to be performed for further validation of this treatment.

There are also instances when a child is born with a hearing loss condition.  Congenital hearing loss may be caused by cytomegalovirus infection or premature birth, affecting 1 in every 1,000 newborns.  There are approximately 6,000 to 8,000 newborns presenting with hearing loss symptoms every year (Kenna, 2003).

  Hearing loss in infants may be classified as conductive, sensorineural, mixed and central (CDCP, 2007).  Conductive hearing loss is caused by a defect in the outer or middle ear and results in hearing loss at all sound frequencies.  Sensorineural hearing loss is due to inner ear or auditory nerve defects.  A combination of conductive and sensorineural symptoms is diagnosed as mixed hearing loss.  Auditory pathway defects caused central hearing impairment, which is usually rare (DOE, 2002).  Unfortunately, approximately 42% of hearing-impaired children will not be diagnosed due to poor screening criteria (Wrightson, 2007).

  The creation of a universal newborn hearing screening program has facilitated diagnosis of hearing-impaired infants through definitive testing and intervention services, that may improve language and communication skills that have not be established earlier.Hearing loss is a sensory disorder that is of significant importance to the health condition in the general human population.  A better understanding of the causes of hearing impairment is needed, as well as a more comprehensive and efficient screening program in both newborn and adults, in order to implement early intervention programs and provide technical and behavioral treatments to diagnosed patients.  Future research endeavors may provide a better understanding of the mechanics of sound perception, as well as provide information on possible routes in prevention and ultimately, treatment. 

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