Guest Editorial : Auditory Processing Disorder

This article provides a review of recent developments in our understanding of how cochlear nonlinearity affects sound perception and how a loss of the nonlinearity associated with cochlear hearing impairment changes the way sounds are perceived. The response of the healthy mammalian basilar membrane (BM) to sound is sharply tuned, highly nonlinear, and compressive. Damage to the outer hair cells (OHCs) results in changes to all three attributes: in the case of total OHC loss, the response of the BM becomes broadly tuned and linear. Many of the differences in auditory perception and performance between normal-hearing and hearing-impaired listeners can be explained in terms of these changes in BM response. Effects that can be accounted for in this way include poorer audiometric thresholds, loudness recruitment, reduced frequency selectivity, and changes in apparent temporal processing. All these effects can influence the ability of hearing-impaired listeners to perceive speech, especially in complex acoustic backgrounds. A number of behavioral methods have been proposed to estimate cochlear nonlinearity in individual listeners. By separating the effects of cochlear nonlinearity from other aspects of hearing impairment, such methods may contribute towards identifying the different physiological mechanisms responsible for hearing loss in individual patients. This in turn may lead to more accurate diagnoses and more effective hearing-aid fitting for individual patients. A remaining challenge is to devise a behavioral measure that is sufficiently accurate and efficient to be used in a clinical setting.

Children with listening difficulties, but normal audiometry, may be diagnosed with APD. The diagnosis is typically based on poor performance on tests of perception of both non-speech and speech stimuli. However, non-speech test results correlate only weakly with evaluations of speech-in-noise processing, cognitive skills, and caregiver evaluations of listening ability. The interpretation of speech test results is confounded by the involvement of language processing mechanisms. Overall, listening ability is associated more with higher-level, cognitive and analytic processing than with lower-level sensory processing. Current diagnosis of a child with APD, rather than another problem (e.g. language impairment, LI), is determined more by the referral route than by the symptoms. Co-occurrence with other learning problems suggests that APD may be a symptom of a more varied neurodevelopmental disorder. Alternately, APD has been proposed as a cause of language-based disorders, but there is no one-to-one mapping between listening and language among individuals. Screening for APD may be most appropriately based on a well-validated, caregiver questionnaire that captures the fundamental problem of listening difficulties and identifies areas for further assessment and management. This approach has proved successful for LI, and may in future serve as a metric to help assess other, objective testing methods.

We need to rethink how we assess auditory processing disorder (APD). The current use of test batteries, while necessary and well accepted, is at risk of failing as the size of these batteries increases. To counter the statistical, fatigue, and clinical efficiency problems of large test batteries, we propose a hierarchical approach to APD assessment. This begins with an overall test of listening difficulty in which performance is measurably affected for anyone with an impaired ability to understand speech in difficult listening conditions. It proceeds with a master test battery containing a small number of single tests, each of which assesses a different group of skills necessary for understanding speech in difficult listening conditions. It ends with a detailed test battery, where the individual tests administered from this battery are only those that differentiate the skills assessed by the failed test(s) from the master test battery, so that the specific form of APD can be diagnosed. An example of how hierarchical interpretation of test results could be performed is illustrated using the Listening in Spatialized Noise-Sentences test (LiSN-S). Although consideration of what abilities fall within the realm of auditory processing should remain an important issue for research, we argue that patients will be best served by focusing on whether they have difficulty understanding speech, identifying the specific characteristics of this difficulty, and specifically remediating and/or managing those characteristics.