This is a three-year project led by Dr Jennifer Linden at the University College London (UCL) Ear Institute. She and her team started work in February 2016 and will finish in January 2019.
Auditory processing disorder (APD) covers a range of hearing disorders. What they have in common is that they are caused by problems with how the brain processes the information it receives from the ear.
A person with APD will be assessed as having 'normal' hearing when tested using standard tests. But they'll have problems with understanding and making sense of sounds, especially complicated and fast-changing sounds like speech.
It’s usually present from a young age, and there is some evidence linking APD with other conditions, such as autism spectrum disorder, and attention deficit hyperactivity disorder (ADHD) – some of the problems associated with these conditions may arise because of auditory processing difficulties. And auditory processing difficulties can often be seen in older people, even if their hearing appears to be normal – as the brain ages, its ability to process sounds diminishes.
APD is difficult to diagnose; there is no routine clinical test or definitive set of criteria. It's extremely likely that many children with auditory processing difficulties are not being identified and helped.
It’s thought that people with APD process sounds much more slowly than a normal-hearing person. Most people hear, process and perceive a sound (or combination of sounds) within a tiny fraction of a second; people with APD take significantly longer to do this. This means that, particularly when listening to complicated or quickly changing sounds, their brain can’t keep up and all the sounds become jumbled. A person’s capacity to process and perceive sounds is called their ‘auditory temporal acuity’ – this can be assessed by measuring someone’s ability to detect short gaps in a continuous sound. A normal-hearing person will be able to detect very small (millisecond) gaps in sound, but someone with APD will only be able to detect much longer gaps.
Recent work done at this Ear Institute lab suggests that this inability to detect gaps in sound may stem from a very specific defect in the brain’s ability to detect that a sound has ended – the gap isn’t detected because the brain takes longer to realise that the sound has ended than the gap lasts. Using a type of mouse in which approximately half of the mice develop a particular lesion (ectopia) in a specific region of the brain, the Ear Institute researchers have shown that the mice with an ectopia are far less able to detect brief gaps in noise – and are less able to detect that a sound has ended, compared to the mice without ectopias. It could be that APD is caused by a very specific defect in someone’s ability to detect when sounds end.
Dr Linden and her team are using the mice described above to further investigate this phenomenon. They are studying whether the ability of the brain to detect that a sound has ended (measured as the strength of the brain’s response to the end of the sound) is related to the length of time a gap has to last before it is detected. If the two are related, then the weaker the brain’s response to the end of a sound, the longer the gap will have to last before the mouse will detect it.
If Jennifer Linden and her colleagues show that the ability to detect brief gaps in sound is linked to the brain’s ability to detect that a sound has ended, this could have a big impact on the way researchers think about auditory processing disorders. Ultimately, these findings could lead to a better understanding of APD, a robust clinical test for APD and, possibly, strategies to improve hearing aids or develop new auditory training programmes to help to improve auditory processing.