Hearing Problems

Central Auditory Processing Disorder (CAPD)

Central auditory processing disorder (CAPD) is said to affect about two percent of children. Children with CAPD display a number of behaviours similar to the symptoms associated with sensori-neural hearing loss. For example, they may complain that they find it difficult to hear when the classroom is noisy. These behaviours may become apparent in the early school years, or at a later stage of the child's life, due to changes in the acoustic environment, or to increased academic demands.

As a result of their difficulty hearing in noise, children with CAPD may suffer from "auditory fatigue". Have you ever been in a noisy restaurant, cutlery and crockery clanging and lots of people talking. After a while, the effort of trying to hear in this environment can make you very tired. Could you imagine if this is what it was like for you every day. Sometimes it is just easier to stop trying to listen. This may happen to children with CAPD. They may "give up" and be labelled as lazy or withdrawn. They could also "act out" in an effort to divert attention from their inability to hear and process speech in the classroom.

We don't know exactly what causes CAPD, however it is suggested that the neural pathways of the central auditory nervous system are involved. Children who have experienced repeated episodes of otitis media (glue ear) may be particularly susceptible to CAPD, perhaps due to the fact that their hearing levels fluctuate during periods of infection, affecting normal exposure to sound and compromising development of the auditory pathways. However, many children with CAPD have never had glue ear. Routine audiological tests will not diagnose CAPD and pure tone audiometry results in this population are typically normal. If CAPD is suspected assessment will involve a variety of specialised audiological tests.

Although children with CAPD generally have normal overall intelligence, if left untreated CAPD may lead to academic deficits in areas such as phonics, reading, and spelling. It may be the emergence of academic deficits that alerts a teacher, parent or other professional to suspect CAPD. However, it is vitally important to determine whether a child's difficulty comprehending speech in the classroom is actually related to CAPD or whether the child's difficulty is caused by another disorder altogether (such as attention, memory or speech-language problems, or even anxiety and motivation). It may also be that CAPD occurs along with another learning disorders (co-morbidity).

Also, if a child has CAPD he or she may be relying on the ability to use contextual cues to work out the bits of the signal they have missed. Think about "The ___ sat on the mat". Most of us could easily work out what the missing word is. This is called auditory-closure ability. As noise levels in the classroom fluctuate, all children, at some time or another will miss parts of the acoustic signal and will rely on auditory closure skills to work out what has been said. However, a child with CAPD may constantly have to use contextual cues to help them process speech - successfully or unsuccessfully depending on their knowledge of the topic. This uses up a lot of their mental resources just to work out what is being said, reducing their processing capacity for their school work (resource allocation theory).

So what specific auditory abilities are affected if a child has CAPD? The American Speech-Language-Hearing Association defined CAPD as a deficit in one or more of a number of skills, including difficulties knowing where a sound is coming from (sound localisation); the ability to detect changes in the duration of, and time intervals between auditory stimuli (temporal processing); and the ability to detect spectral variations in auditory stimuli (particularly those that differentiate sounds according to formant transitions between phonemes).

At NAL we are particularly interested in auditory stream segregation deficits as the basis to CAPD. Auditory stream segregation is the process by which a listener is able to differentiate the various auditory signals which arrive simultaneously at the ears and form meaningful representations of the incoming acoustic signals. Auditory cues such as the perceived spatial location of sounds, or the pitch of speakers' voices, help this process of segregating the total stream of sound. Our research has shown that a high percentage of children with suspected CAPD have an inability to differentiate sounds in the auditory environment because they have difficulty differentiating the physical location of the sounds, and therefore can't concentrate on a target sound and ignore distracting speech (spatial stream segregation deficit). All the incoming signals seem to be "mixed up together" and so the children with CAPD need the target signal to be much louder than the background noise compared to children who don't have this disorder. To see how we diagnose auditory stream segregation deficits see our research on the Listening in Spatialized Noise - Sentences Test (LISN-S; Cameron & Dillon, 2006).

The management program utilised to help a child with CAPD will depend on the type of ability that is diagnosed. If a child has an auditory stream segregation deficit, a personal FM device or sound-field amplification system can be used to improve the signal-to-noise ratio in the classroom. The child could also be trained in auditory closure skills and vocabulary building to improve their ability to use contextual cues to fill in missing sections of auditory information. The child is also taught to be aware of their listening environment, and to ask for repetition of missed auditory information. Finally we could offer specific auditory training in an attempt to train the brain to be able to differentiate the physical location of signals presented simultaneously at the ears. To see how we are remediating auditory stream segregation deficits see our research on the LISN & Learn auditory training software (Cameron & Dillon, 2007).

Whereas a lot of emphasis has been placed on children with normal hearing thresholds with CAPD in this overview, it is important to remember that CAPD can also affect adults, as well as both adults and children with peripheral hearing loss. We will be conducting research in these populations in the near future.

References:

1. Cameron, S., & Dillon, H. (2007). LISN & Learn Auditory Training Software (Research Version 1.1.0) [Computer software]. Sydney, NSW: National Acoustic Laboratories.

2. Cameron, S., & Dillon, H. (2006). Listening in Spatialized Noise (LISN®) - Sentences Test (Research Version 2.0.0) [Computer software]. Sydney, NSW: National Acoustic Laboratories.