Hearing

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MindWeavers has a particular interest in hearing - it is the main basis of communication. Research has shown that many children with a variety of learning problems, including language-based learning difficulties, autism, dyslexia, and attention deficit disorder, have signs of problems in the central auditory pathway of the brain. These problems are not detected through normal hearing tests, which detect problems in the ear. Instead, they are problems with the processing of sound by the brain. This processing includes aspects of the separation, grouping, spatial localization, timing and pitch of sounds. Thus, whereas normal hearing tests assess the function of the ear, many children with learning problems seem to have problems with the processing of sound by the brain.

The Ear

The ear consists of three parts, the outer, middle and inner ear. Sound travels through the outer ear, vibrates the eardrum, through the middle ear, into the inner ear or cochlea. There, the sound is converted into nerve activity that is conveyed to the brain.

The Brain

Nerve activity entering the brain from the ear arrives at the auditory cortex. The auditory system is particularly specialised for the processing of time-sensitive information. For example, we can detect differences in the arrival time of a sound at the two ears (part of spatial hearing) with an accuracy of better than 10 microseconds - that's just 1/100,000 of a second! Like other parts of the brain, the activity of an auditory neuron is subject to many diverse influences. In addiiton to sound, these influences include learning, attention, memory and emotion. Auditory learning experiments show that repeated sound stimulation can lead to a change in the effectiveness of a sound.

Measuring Hearing

To know how well someone hears, it is necessary to measure the sensitivity (or threshold) of hearing using an audiogram. The audiogram is a very useful measure of the function of the ear, but it is also influenced by brain processing. In the laboratory, hearing is usually measured by playing two sounds in succession. One contains a standard stimulus, the other contains a test stimulus. The listener has to indicate which of the two sounds is the test. By making the difference between the standard and the test smaller and smaller, and observing the listener's performance, we can measure the performance limits of any aspect of sound perception.

Auditory Training

When we listen to a sound repeatedly, our ability to hear it usually improves. This learning process can itself be improved by tracking the listener's ability to hear the sound and keeping the sound near their ability to hear - near their edge of competence. Auditory training can be used to improve the way the brain processes sounds, and this is particularly useful for learning to hear speech and to speak. People who have difficulties with these tasks often benefit most from this training, but MindWeavers experiments show that almost all individuals can benefit from auditory training.

Hearing Loss

The Dana Alliance for Brain Initiatives estimated, in 1996, that almost 30 million Americans have some degree of deafness, costing the nation over $56 billion annually. Pro-rata, this would translate in Britain to about 6 million affected individuals and a cost of £5 billion. However, we believe that even these impressive numbers greatly under-estimate the real incidence and cost of hearing loss. Hearing loss is traditionally considered to be of two types, conductive and sensorineural. But many people who complain of hearing loss appear not to have either of these problems.

Conductive Hearing Loss

Conductive hearing loss occurs when there is an interference with the normal passage of sound to the inner ear. Conductive hearing loss is often overlooked but can produce a considerable impairment. It is particularly common in young children who often have fluid in their ear ('glue ear'). Conductive hearing loss is usually reversible.

Sensorineural Hearing Loss

When sound reaches the inner ear, it is converted into neural activity via the hair cells of the cochlea. When these are missing or damaged, hearing loss is said to be 'sensorineural'. Damage to the auditory nerve or brainstem also results in sensorineural hearing loss. There is presently no cure, but hearing aids can sometimes help. Profoundly deaf people are sometimes treated with a cochlear implant, a device that bypasses the hair cells and presents electrical stimulation directly to the auditory nerve.

Central Auditory Processing Disorder (APD or 'Brain Deafness')

Many listeners report difficulty hearing, without an obvious problem with the ears. Research suggests that this may be due to problems with the way the brain processes nerve signals. An example is hearing with two ears (binaural hearing). People with normal audiograms in both ears can nevertheless have poor binaural hearing because of problems with brain processing. Others have problems with auditory attention (the ability to focus on one sound source in the presence of other sources) or auditory temporal processing (separating sounds in time). There seems to be a close link between APD, language disorders, and other developmental problems such as attention deficit, hyperactivity disorder (ADHD).