What is CAPD?

Central Auditory Processing Disorder (CAPD) – also known as Auditory Processing Disorder (APD) – is an umbrella term for a variety of disorders that result in a breakdown in the hearing process. In short, our brain cannot make sense of what our ears hear because the auditory signal is distorted in some way. As a result, one of the biggest problems experienced by individuals with CAPD is difficulty listening in background noise.

CAPD is said to affect about two to five percent of children, but the percentage is not precisely known. Children with CAPD display a number of behaviours similar to the symptoms associated with sensorineural 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 They may "give up" and be labelled as lazy or withdrawn.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,but the neural pathways of the central auditory nervous system are involved in some way. 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 disorder (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 have have extensively investigated auditory stream segregation deficits as one important cause of 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 some 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 processing disorder, or SPD). All the incoming signals seem to be "mixed up together" and so the children with this form of 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 SPD see our section on the Listening in Spatialized Noise - Sentences Test (LISN-S).

The management program utilised to help a child with CAPD will depend on the type of deficit that is diagnosed. If a child has SPD, a personal frequency modulation (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. For more information on managing CAPD see the section on general CAPD management. Finally we can offer specific auditory training in an attempt to train the brain to be able to use the different directions of arrival of the wanted and unwanted sounds. To see how we are remediating SPD see our section on the LISN & Learn auditory training software.

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. To read about our work in the area of sensorineural hearing loss see the section on LiSN-S for hearing impaired people.

Our research at NAL into the various types of central auditory processing disorders ongoing. For further information see the section on current CAPD research. You can also volunteer to be in a study.