Download psychoacoustics and the effects of hearing loss

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.