Auditory Processing Disorder and Specific Language Impairment

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Reprinted with permission from ENT & audiology news, NOVEMBER/DECEMBER 2013 98, VOL 22 NO 5

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Auditory Processing Disorder and Specific Language Impairment

  1. 1. Auditory Processing Problems and Specific Language Impairment Prof Dorothy VM Bishop, MA, MPhil,DPhil Wellcome Principal Research Fellow / Professor of Developmental Neuropsychology, Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford, OX1 3UD, UK. Correspondence E: dorothy.bishop@ psy.ox.ac.uk Declaration of Competing Interests None declared. pecific language impairment (SLI) is identified when a child has problems understanding or producing spoken language for no obvious reason [1]. Some children catch up after a slow start, but others have more persistent problems. By school age, the language difficulties may be less obvious to outsiders, but formal assessment can reveal that the child has a limited vocabulary, uses only simple sentence structures, and may have difficulty understanding complex instructions. These language difficulties typically have a knock-on effect on educational progress, because children with SLI have difficulty learning to decode print and understand written texts. Many of these children will be referred to an audiologist with the aim of excluding hearing loss as a cause of language learning problems. High frequency hearing loss can produce a clinical picture that resembles SLI [2]. However, in children with SLI, sensorineural hearing is normal. Fluctuating conductive loss associated with middle ear disease is a common problem in young children, and was once thought to be a risk factor for SLI. However, more recent epidemiological studies suggest that middle ear disease is unlikely to be a major cause of persisting language problems in children, unless other risk factors are present [3]. S There is, however, much more to auditory processing than peripheral hearing, and for several decades now there has been interest in the idea that SLI may be caused by an impairment in the higher auditory pathways going from the auditory nerve to the brain. However, whereas there are well-established methods for assessing integrity of the peripheral auditory system, there is much less consensus about how to test for higher-level auditory dysfunction. There are some test batteries for ‘auditory processing disorder’ (APD), but they are problematic for several reasons [4]. We can distinguish between three kinds of processing that are involved in translating heard speech into meaning (Figure 1) [5]. First, the sound must be detected: problems with this stage will be picked up on a conventional hearing test. Next, key features of the sound must be discriminated. It has been suggested that, just as some people are colour-blind, and so cannot see the difference between red and green, there may be children who are unable to distinguish between aspects of sound such as frequency, duration or intensity. This might leave them unable to tell the difference between sounds in their language, such as ‘t’ and ‘k’. Another kind of discrimination problem is difficulty in separating important aspects of sound, such as a speech signal, from background noise. An Detection: Which interval contains a stimulus? Discrimination: Are the two stimuli the same or different? Identification: Which interval contains an A? Figure 1: Illustration of the distinction between detection, discrimination and identification using a visual analogy. The identification stage involves matching what is perceived to stored knowledge in the brain. Identification of speech sounds will be influenced by knowledge of language. 98 ENT & audiology news | NOVEMBER/DECEMBER 2013 | VOL 22 NO 5
  2. 2. feature Many children who present with listening difficulties meet criteria for a neurodevelopmental disorder such as SLI, developmental dyslexia, autism spectrum disorder, or attention deficit-hyperactivity disorder. important aspect of language comprehension is the next step, that of identification. This involves matching what has been heard against existing stored templates in the brain. This final step is completely dependent on prior learning. If somebody talks to me in Chinese, I won’t be able to identify the individual speech sounds, let alone make any sense of what was said, even if my hearing and auditory discrimination are entirely normal. The reason why tests of auditory processing are often controversial is because they typically assess identification of sounds, often using real words. For instance, the task may be to repeat words that have been acoustically modified by filtering, or to listen to words that are presented dichotically. Failure on this kind of task is difficult to interpret, because it could reflect either a bottom-up problem in discriminating certain sound features, or a lack of the top-down knowledge that is needed for speech identification. If a child with language difficulties fails one of these tests, there is a tendency to conclude that they have an auditory processing problem that has caused the language difficulty. However, there’s another possibility, which is that they may have a language difficulty that was caused by something else, but which impairs their ability to do tasks that involve word identification. So the APD test might just be picking up the consequences of having a language problem. We therefore need to be very careful in how we interpret the results from APD tests with children. Just because the test says it is measuring auditory processing does not mean it is valid for that purpose. A useful armchair exercise when evaluating an APD test is to consider how you would perform on such a test if it were administered in a language you didn’t know well. If language knowledge has an impact on test performance, then poor performance could reflect language impairment rather than auditory processing difficulties [6]. This is not the only issue affecting interpretation. Many tests of auditory discrimination and identification also place demands on sustained attention and memory skills – two areas where children with language problems are often impaired. Anyone who has tried establishing an auditory discrimination threshold by asking a rambunctious six-year-old to judge if pairs of sounds are same or different will know it can be well-nigh impossible to get reliable results. Typically the child will start to ask how much more they have to do after about six trials. For reasons like this, a team at the MRC Institute of Hearing Research in Nottingham developed a new battery of tests for APD that had two important features. First, they did not involve language: instead they used meaningless sounds. Second, for each test, there were two versions: one that taxed the auditory system, and another that adopted exactly the same format, but did not require the child to discriminate auditory stimuli on a key dimension. By subtracting the thresholds obtained for the two tests, one could get an estimate of ability to discriminate that dimension, after taking into account ‘task variables’ – e.g. ability to attend to the task, to remember the stimuli, to make appropriate responses and so on [7]. However, the resulting measures were generally not very reliable and did not relate closely to parents’ reports of listening or communication problems. It’s often thought that neurophysiological methods can solve the problems inherent in behavioural APD tests, because you can directly measure the brain’s response to sounds, without requiring a behavioural response from the child. However, the more such methods are used, the more it becomes clear that they too are influenced by top-down knowledge. To take one example, the mismatch negativity – an event-related potential (ERP) component that is recorded when a rare sound occurs in a sequence of standard sounds – varies depending on whether the sounds are used meaningfully in the person’s native language [8]. So it’s complicated! It’s entirely possible that there are children who have genuine problems in the central auditory system that affect discrimination and identifica- tion of sounds. However, in practice it can be very hard to distinguish such problems from higher-level difficulties with language, attention and motivation. In some children, poor performance on auditory processing tests may be the consequence of having a specific learning disability, rather than the cause. Many children who present with listening difficulties meet criteria for a neurodevelopmental disorder such as SLI, developmental dyslexia, autism spectrum disorder, or attention deficit-hyperactivity disorder [9, 10]. Presence of one of these conditions does not preclude the possibility that auditory processing problems are part of the clinical picture, but attempting to unravel causation is complex. It is important to work closely with other professionals who can assess language, cognitive ability and attentional skills when assessing children who present with possible APD. References 1. Bishop D, Norbury CF. In Rutter M et al. (Editors), Rutter's Child and Adolescent Psychiatry. Oxford, Blackwell Publishing, 2008: 782-801. 2. Stelmachowicz P, Pittman AL, Hoover BM, Lewis DE. The importance of high-frequency audibility in the speech and language development of children with hearing loss. Archives of Otolaryngology–Head & Neck Surgery 2004;130:556. 3. Roberts JE, Rosenfeld RM, Zeisel SA. Otitis media and speech and language: A meta-analysis of prospective studies. Pediatrics 2004;113:E238. 4. Moore DR. Auditory processing disorder (APD): Definition, diagnosis, neural basis, and intervention. Audiological Medicine 2006;4:4. 5. Bishop DVM. Uncommon Understanding: Development and Disorders of Language Comprehension in Children. Hove, Psychology Press; 1997. 6. Crandell CC, Smaldino JJ. Speech perception in noise by children for whom English is a second language. American Journal of Audiology 1996;5:47. 7. Moore DR, Ferguson MA, Edmondson-Jones AM, Ratib S, Riley A. Nature of auditory processing disorder in children. Pediatrics 2010;126:e382. 8. Näätänen R, et al. Language-specific phoneme representations revealed by electric and magnetic brain responses. Nature 1997;385:432. 9. Dawes P, Bishop D. Psychometric profile of children with auditory processing disorder (APD) and children with dyslexia. Arch Dis Child 2010;95:432. 10. Ferguson MA, Hall RL, Riley A, Moore DR. Communication, listening, cognitive and speech perception skills in children with auditory processing disorder (APD) or specific language impairment (SLI). J Speech Lang Hear Res 2011;54:211. Further reading Dawes P, Bishop D. Auditory processing disorder in relation to developmental disorders of language, communication and attention: a review and critique. International Journal of Language and Communication Disorders 2009;44:440-65. Website with short videos explaining aspects of Specific Language Impairment http://www.youtube.com/RALLIcampaign ENT & audiology news | NOVEMBER/DECEMBER 2013 | VOL 22 NO 5 99

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