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Auditory Processing

The American Speech and Hearing Association (ASHA) guidelines identify that the auditory system undertakes auditory processing comprising sound localization and lateralization, auditory discrimination, auditory pattern recognition, and temporal aspects of audition—including temporal resolution, temporal masking, temporal integration, and temporal ordering. Additional auditory skills include processing auditory information in competing acoustic signals or degraded acoustic signals. This entry discusses testing and function of the auditory pathway.

How Do We Test a Complex Auditory Pathway?

A number of consensus conferences have worked to resolve the question of how to test auditory processing. In 2000, a minimal test battery was identified. The recommended test battery included assessing peripheral hearing and central auditory function using auditory evoked potentials, including the auditory brainstem response (ABR) and middle latency response (MLR). Tasks to measure frequency and temporal processing were also recommended. Despite these guidelines, neither researchers nor practicing audiologists administer tests that adhere to this recommended minimum battery. For instance, most clinicians do not use electrophysiological tasks because the equipment is not always available and there is limited normative data.

The clinical consensus is that the test battery should be tailored according to the case presentation and history. The ASHA guidelines recommend inclusion of tasks that evaluate different levels and processes within the auditory system. The motivation for the selection of a test battery also includes consideration of subject attributes such as age, fatigue, attention, languages spoken, and language ability. A number of linguistically based auditory processing tests use words or sentences; performance on these tasks is influenced by linguistic and auditory processing factors unless a correction or subtraction technique is used to adjust for the effect.

Where Does Auditory Processing Occur?

To understand the basis of the test battery, one needs to appreciate the “what” and “where” pathways of the auditory system including both afferent (outgoing motor fibers) and efferent (incoming sensory fibers) pathways. There are two main ways that the auditory system can be discussed: either by examining the processes at all levels within the auditory system (cochlea to brainstem to cortex) or by examining general processes of the auditory system without considering the neuroanatomical basis. The latter approach is adopted in most auditory processing assessment guidelines. There are, however, seminal works (lesion studies) by authors such as Frank Musiek and Jane Baran that provide an understanding of the auditory pathways involved in various auditory processes (see Further Readings).

Four salient features of auditory processing are described here: tonotopicity, temporal processing, binaural processing, and efferent processing. Understanding these aspects of auditory pathway function is important when considering disorders of the auditory system.

Tonotopicity

This is a systematic, orderly representation of frequency at each level of auditory system. The place-specific frequency decomposition of the incoming acoustic signal performed by the cochlea, with high frequencies coded at the base and low frequencies coded at the apex, is maintained throughout the auditory system. Frequency representation or coding is an important auditory process; it is critical for discriminating speech sounds in noise. Frequency encoding is measured using frequency selectivity or discrimination tasks that measure the ability to separate out the different frequency components. Numerous studies have investigated how frequency selectivity can be measured. This section focuses on the functionality of tonotopic organization.

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