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Transcript
PSYCHOACOUSTICS AND THE EFFECTS OF HEARING LOSS Psychacoustics is the study of the perceptual effects of the neuro-physiological activity of the cochlea, 8th (auditory) nerve and the auditory nervous system in the brain. Having an understanding of these effects helps in our understanding of the effects of hearing loss for children, and of the limitations of hearing aids. Auditory nervous system activity leads to sensations of loudness, pitch and location of sound in space. This same system also has to code the timing of rapid changes that occur between successive sounds such as in speech and music. Loudness The cochlea has a range of sensitivities to sound called a dynamic range – varying from the threshold for hearing faint sounds to the level which is uncomfortably loud (UCL). The dynamic range is the difference in decibels between the hearing threshold and the level which is uncomfortably loud at any frequency. The UCL does not increase at the same rate as the degree of hearing loss, except perhaps for a child with a purely conductive hearing loss. However, as the hearing thresholds become raised above the normal sensitivity range, a child’s dynamic range will start to reduce. For hearing losses below about 60dBHL, hearing for quiet sounds is mostly affected. For hearing losses above 50-60dBHL, the perception of loudness will increase more quickly as the sound entering the ear increases above the hearing threshold, due to the reduced dynamic range. Pitch The cochlea and auditory nerve code the frequency of sounds at any moment with the use of the hair cells on the basilar membrane in the cochlea as well as the activity of the auditory nerve. The ability of the auditory nervous system to code frequency sounds is called frequency resolution. A normal hearing person can distinguish between separately occurring sounds, and sometimes between simultaneously occurring sounds, that are close together in frequency. The cochlea and auditory nervous system allows this to occur by enhancing or sharpening the differences between the frequencies using a series of filters and amplifiers. In an ear with hearing loss, the filters become broadened and the amplifier effect may be reduced. The result is a reduced, or even lost ability to distinguish between sounds that are close together in frequency. This broadening of the filters becomes greater as the hearing loss increases, and the amplifier effect may be lost for hearing losses greater than 50-60 decibels. The effect is to make it harder for a child to understand speech due to certain speech phonemes that have similar frequency components being difficult to hear. The effect is greater in noise where the noise energy may be present at the same frequencies within a filter as the speech component of interest. Temporal Resolution is the ability to distinguish between two or more sounds that come in close succession. An example is hearing phonemes present in a spoken word. The first phoneme may create a temporary neural disturbance affecting the processing of the second sound, due partly to the time taken for the auditory nerve to recover from processing the first sound. The psychoacoustical term for this disturbance is temporal masking. The everyday equivalent of this effect is one sound casting a neural shadow over another in time because it is louder. The effect on speech is for a strong voiced sound such as the vowel /a/ to mask a following weak consonant unvoiced sound such as /th/ in the word /bath/. A loud sound can also mask an immediately preceding softer sound. Binaural Hearing Hearing with two ears helps us to locate sound sources and the movement of sounds, as well as to lock onto and listen to one talker in a crowd or noisy situation. This is enabled by the brain comparing sounds that reach each ear, and suppressing some unwanted sounds coming from a certain direction. If the brain is not receiving a complete neural signal of the environment from each ear, or it is unable to suppress some signals, it becomes more difficult to use binaural cues in the above manner. Neural Plasticity and Growth Children’s brains are organising themselves on the basis of sensory stimulation, using the neural pathways that are connected to each sensory reception system; such as the cochlea for hearing. The auditory cortex receives neural signals from the cochlea via the central auditory pathways. If these neural pathways are not used as much for transmitting signals from the cochlea, the brain may use them for other sensory systems, such as vision. Sensory stimulation with a hearing aid or cochlear implant may keep the auditory neural pathways in use for the hearing sense. Psychoacoustic Effects for Hearing Impaired Children Assessment of the audibility and clarity of the speech that children hear via their hearing aids is usually determined using at least three methods – observing reactions and spoken responses to spoken messages, formal speech perception tests and listening to their voluntary speech. It is important to remember, however, that hearing aids are amplifying sound into ears which have a reduced dynamic range and reduced sensitivity – sound will become louder above already raised hearing thresholds much more quickly than in a normally hearing ear. When compared to a normally hearing ear, ears with hearing loss may also have reduced frequency resolution ability (pitch discrimination) and reduced temporal resolution ability (eg, tracking the rapid changes in phonemes within words) while processing acoustically amplified sound. There may also be reduced ability to localise sounds and sound movement. As a result of these effects, understanding amplified speech while listening in noise will be especially difficult. Consequently, children may still not hear speech as clearly as someone with normal hearing even when wearing their hearing aids. This is why special recommendations are made regarding the communication activity of adults and siblings to maximise the opportunities for children to understand spoken communication (eg, moving to within each child’s best listening distance, and facing them when talking so they can lipread and see facial expressions. The closer one is to a hearing aid wearing person during conversation, the easier it is for them to hear speech.