American Speech-Language-Hearing Association

Technical Report

Central Auditory Processing: Current Status of Research and Implications for Clinical Practice

Task Force on Central Auditory Processing Consensus Development


About this Document

This technical report was prepared by the American Speech-Language-Hearing Association (ASHA) Task Force on Central Auditory Processing Consensus Development and approved by the Executive Board in March 1995. This report is not an official policy of ASHA. Members of the task force are Hugh W. Catts, Gail D. Chermak, Chie Higuchi Craig, Judith R. Johnston, Robert W. Keith, Frank E. Musiek, Donald A. Robin, Christine Sloan, Diane Paul-Brown (coordinator), and Maureen E. Thompson (assistant coordinator). The following people participated as invited consultants to the task force: Thomas F. Campbell, Jeanane M. Ferre, Nina Kraus, Malcolm R. McNeil, Dennis P. Phillips, Rachel E. Stark, Paula Tallal, and Charles S. Watson.



Executive Summary

Consensus has been lacking on precisely how to define central auditory processing and its disorders and how the disorders can be identified and ameliorated through intervention.

To develop a technical report with “statements of consensus” on best practice related to the diagnosis and management of children and adults with central auditory processing disorders, the American Speech-Language-Hearing Association (ASHA) convened a Task Force on Central Auditory Processing in 1993. The task force arrived at consensus on a definition and on four of the major issues relative to central auditory processing: basic science, assessment, developmental and acquired communication problems, and clinical utility.

The task force reached consensus on the following definition of central auditory processes and central auditory processing disorders.

Central Auditory Processes are the auditory system mechanisms and processes responsible for the following behavioral phenomena:

  • Sound localization and lateralization

  • Auditory discrimination

  • Auditory pattern recognition

  • Temporal aspects of audition, including —

    • temporal resolution

    • temporal masking

    • temporal integration

    • temporal ordering

  • Auditory performance decrements with competing acoustic signals

  • Auditory performance decrements with degraded acoustic signals

These mechanisms and processes are presumed to apply to nonverbal as well as verbal signals and to affect many areas of function, including speech and language. They have neurophysiological as well as behavioral correlates.

Many neurocognitive mechanisms and processes are engaged in recognition and discrimination tasks. Some are specifically dedicated to acoustic signals, whereas others (e.g., attentional processes, long-term language representations) are not. With respect to these nondedicated mechanisms and processes, the term central auditory processes refers particularly to their deployment in the service of acoustic signal processing.

A Central Auditory Processing Disorder (CAPD) is an observed deficiency in one or more of the above-listed behaviors. For some persons, CAPD is presumed to result from the dysfunction of processes and mechanisms dedicated to audition; for others, CAPD may stem from some more general dysfunction, such as an attention deficit or neural timing deficit, that affects performance across modalities. It is also possible for CAPD to reflect co-existing dysfunctions of both sorts.

Central auditory processing and its disorders must be considered from a multidisciplinary perspective that takes into account

  • the physical structure of the acoustic stimulus,

  • the neural mechanism that encodes the stimulus,

  • the perceptual dimensions that arise from the encoding,

  • the interactions that occur between perceptual processes and the activation of the higher level resources, and

  • the nature of the pathological process.

The purpose of central auditory assessment is to determine if a central auditory processing disorder (CAPD) is present and, if so, to describe its parameters. Clinicians need to describe functional auditory performance deficits. A team approach to assessment is the best practice. With children, the neuromaturational status of the auditory nervous system should be considered. Further, a central auditory assessment should provide information about both developmental and acquired disorders of the central auditory system.

The diagnosis of a central auditory processing disorder is accomplished using a variety of indices, including:

  1. case history

  2. nonstandardized but systematic observation of auditory behavior

  3. audiologic test procedures

Categories of behavioral auditory measures include tests of —

    • temporal processes — ordering, discrimination, resolution (e.g., gap detection), and integration;

    • localization and lateralization;

    • low-redundancy monaural speech (time compressed, filtered, interrupted, competing, etc);

    • dichotic stimuli, including competing nonsense syllables, digits, words, and sentences; and

    • binaural interaction procedures (e.g., masking level difference). Electrophysiologic procedures may also be useful.

  • 4. speech-language pathology measures

CAPDs have been reported in persons manifesting a large and diverse set of clinical problems. CAPD may have a role in both language learning and language use difficulties of clients with and without clear evidence of neuropathology. However, clinicians certainly should not infer the existence of CAPD solely from evidence of learning disability or language impairment.

The impact of CAPD on language use is particularly evident in spoken language comprehension. Because spoken language comprehension is determined by a number of different factors, clinicians should be cautious in attributing spoken language comprehension difficulties to CAPD in any simple fashion. Audiologic considerations, as well, play a role. Thus, a diagnosis of CAPD requires comprehensive audiologic assessment; such a diagnosis cannot be made solely on the basis of poor comprehension of spoken language.

Management of CAPD entails two general and complementary approaches. The first approach is directed toward enhancing the client's own resources, whereas the second approach is directed towards enhancing the auditory signal and improving the listening environment. Therefore, individuals with CAPD can benefit from interventions that help them increase their knowledge of language and their ability to access that knowledge in a time-efficient manner for speech processing.

Given the current state of assessment tools in both language and audition, for many cases there may be no way to isolate the relative contributions of (a) language limitations and (b) signal processing limitations.

Regardless of the nature of the processing deficit, most persons who have difficulty with the comprehension of spoken language will profit both from procedures that enhance the acoustic signal and from procedures that increase the scope and control of central resources, particularly language resources. In fact, as audiologists increase their appreciation of linguistic and cognitive influences on perception, and as speech-language pathologists increase their attention to the acoustic properties of speech and hence to auditory processes, treatment strategies available to clients will tend to converge and to partake of both approaches. In other words, given our current understanding of language disorders and of central auditory processing, techniques that facilitate language competence are likely to improve the auditory processing of language and vice versa.

The task force identified three conceptual stumbling blocks for researchers and clinicians alike with respect to efficacy research:

  • heterogeneity of the population

  • treatment goals

  • evaluating change

Careful thinking about these issues should lead to improvements in efficacy research and to more reasonable claims about treatment efficacy on the part of clinicians.

The need for research to clarify a number of unresolved theoretical issues and clinical practice questions is evident. Answers to these questions will assist professionals, consumers, and policy-makers in reaching decisions.

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Introduction

After more than 20 years of dealing with issues related to central auditory processing, the American Speech-Language-Hearing Association (ASHA) still lacks statements of consensus on definition, identification procedures, or intervention practices. Controversy persists over central auditory processing and its disorders, and how it should be defined, identified, and ameliorated through intervention. For this reason, it was suggested that ASHA conduct a meeting to develop a technical report with “statements of consensus” that could be disseminated to ASHA members and other professionals and consumers.

Readers should note that the report on central auditory processing represents the considered, collective, professional opinion of a group of audiologists and speech language pathologists who have expertise in the area of central auditory processing. Their opinion is informed by consultation, study of the literature, and intensive discussion, although much of this background preparation is not explicitly reflected in the report. The task force acknowledges key sources with selective references in its Statement, but these citations are in no way comprehensive.

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Charge

In 1993, ASHA convened a Task Force on Central Auditory Processing to:

  • plan and conduct a consensus development conference to identify information and practices on which there is some agreement among experts and to develop a technical report with “statements of consensus” on best practice related to the diagnosis and management of children and adults with central auditory processing disorders, and

  • identify consultants with the knowledge and skills required to summarize current research and clinical practices in the area of central auditory processing disorders. The invited consultants were asked to prepare a resource paper summarizing a major issue related to central auditory processing on which consensus was needed.

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Issues

The ASHA Task Force on Central Auditory Processing arrived at consensus on four major issues and developed a working definition of central auditory processing. The four issues related to the diagnosis and management of children and adults with central auditory processing disorders for which consensus was reached follow:

  • Issue #1—What does basic science tell us about the nature of central auditory processing and its role in audition?

  • Issue #2—What constitutes an assessment of central auditory processing and its disorders?

  • Issue #3—What are the developmental and acquired communication problems associated with central auditory processing disorders?

  • Issue #4—What is the clinical utility of a diagnosis of central auditory processing disorders?

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Definition

The task force reached consensus on the following definition of central auditory processes and central auditory processing disorders. These definitions should be assumed for all of the remaining statements in this document.

Central Auditory Processes are the auditory system mechanisms and processes responsible for the following behavioral phenomena:

  • Sound localization and lateralization

  • Auditory discrimination

  • Auditory pattern recognition

  • Temporal aspects of audition, including —

    • temporal resolution

    • temporal masking

    • temporal integration

    • temporal ordering

  • Auditory performance decrements with competing acoustic signals

  • Auditory performance decrements with degraded acoustic signals

These mechanisms and processes are presumed to apply to nonverbal as well as verbal signals and to affect many areas of function, including speech and language. They have neurophysiological as well as behavioral correlates.

Many neurocognitive mechanisms and processes are engaged in recognition and discrimination tasks. Some are specifically dedicated to acoustic signals, whereas others (e.g., attentional processes, long-term language representations) are not. With respect to these nondedicated mechanisms and processes, the term central auditory processes refers particularly to their deployment in the service of acoustic signal processing.

A Central Auditory Processing Disorder (CAPD) is an observed deficiency in one or more of the above-listed behaviors. For some persons, CAPD is presumed to result from the dysfunction of processes and mechanisms dedicated to audition; for others, CAPD may stem from some more general dysfunction, such as an attention deficit or neural timing deficit, that affects performance across modalities. It is also possible for CAPD to reflect co-existing dysfunctions of both sorts.

The clinician should attempt to determine the factors that contribute to the disturbance of auditory behaviors (e.g., auditory, cognitive, linguistic), as these may influence clinical decision-making.

Issue #1: What does basic science tell us about the nature of central auditory processing and its role in audition?

Hearing is not a unitary sensory-perceptual skill. The central auditory nervous system (CANS) is a complex system with multiple components and levels of parallel and sequential but interactive organization. Anatomically it includes nuclei and pathways in the brainstem, subcortex, primary and association areas of the cortex and corpus collosum. Much of what constitutes central auditory processing is preconscious. That is, it occurs before a listener's conscious awareness. What is experienced by the listener is an auditory perceptual event (Plomp, 1976; Tyler, 1992; Warren, 1984).

Although the CANS is critical to auditory functions including spoken language processing and many other complex signals, several other factors are involved as well. Even the simplest auditory tasks are influenced by such higher-level, nonmodality-specific factors as attention, learning, motivation, memory, and decision processes. Higher level contextual information influences the perceptual analysis of the acoustic signal; and various knowledge sources interact and support the auditory processing of spoken language and other complex acoustic signals, such as music (Bregman, 1990; Duetch, 1975, 1982; Handel, 1984, 1989; Jones, 1987; McAdams & Bregman, 1979; McAdams & Saariaho, 1985).

Understanding spoken language ultimately depends on the initial sensory detection and perceptual analysis of the acoustic input to the central auditory nervous system. A critical behavioral science issue involves the nature of the interaction of the sensory/perceptual system information output with central, higher level information resources.

A goal of auditory neuroscience is to identify the aspects of hearing and their neurological substrates. The various aspects of hearing (e.g., laterality, location, pitch, and timing) may be mediated by different neurons and neural coding mechanisms, each of which may be differentially susceptible to pathologies. The representation of a given stimulus parameter is probably modified at every successive level of the auditory neural pathway.

Plasticity contributes to additional modification of neural representations of auditory stimuli. Plasticity characterizes those auditory mechanisms considered central, whereas stability seems to be a property of the auditory periphery. Plasticity refers to the observation that the organization of the central auditory system can be modified or reorganized by cochlear pathology, central lesion, maturation, experience, learning, or rehabilitation.

The CANS is sensitive to differences in the time structure of the acoustic stimulus (e.g., interaural time differences, click fusion, temporal ordering, and gap detection). Temporal information is used, for example, in localization, lateralization, and temporal ordering and sequencing. The CANS detects and analyzes rapidly changing spectra that characterize speech and music. The neurons involved in generating these percepts may be only partially overlapping.

The acoustic stimuli are of many different forms, and so too are the neural mechanisms that encode the forms (Forest & Green, 1987; Formby & Muir, 1988; Hirsch, 1959; Roth, Kochar, & Hind, 1980). These neural codes may be differentially susceptible to neural insults (Fedderson, Sandel, Teas, & Jeffress, 1957; Yost, 1974). The extent to which central auditory structure is involved in mediating perceptual responses to a stimulus characteristic may be related to the extent to which the relevant characteristic is represented in the brain (Sayers & Cherry, 1957).

The number of properties ascribed to the brain is large, as is the number of brain regions mediating those processes. As well, the mapping of representations in the brain relies on multiple serial, parallel, and distributed neural networks. Within well-defined limits, the individuality of brain organization and the pathologies that can affect the brain are manifest in different ways. Therefore, it is likely that the consequences of central pathology may vary from individual to individual.

Abnormalities of CANS have been shown to interfere with many auditory functions, such as sound localization, performance in competing acoustic signals, auditory discrimination, and so forth. The most penetrating understanding of central auditory processing and its disorders will be a multidisciplinary one, taking into account —

  • the physical structure of the acoustic stimulus,

  • the neural mechanism that encodes the stimulus,

  • the perceptual dimensions that arise from the encoding,

  • the interactions that occur between perceptual processes and the activation of the higher level resources, and

  • the nature of the pathological process.

Issue #2: What constitutes an assessment of central auditory processing and its disorders?

Because of the individuality of brain organization and the pathologies that affect such organization, CAPD can affect individuals differently. Hence, an individual approach must be taken to the selection of assessment measures and the interpretation of their results. Furthermore, factors such as a person's age, education level, social and cultural background, cognitive ability, use of medications, and so on, can influence how that person performs on behavioral tests. Clinicians also should be knowledgeable about the potential influence of these other variables on the test outcome. Clinicians also should be knowledgeable about the assessment procedures; consideration should be given to what modalities (other than auditory) are used and to the cognitive, linguistic, and attention demands required to perform the test — to name a few important variables.

A team approach to assessment is the best practice. This is particularly true for children, for adults with aphasia, and for individuals with other known communication disorders. At the very least, a collaborative assessment approach including audiology and speech-language pathology should be sought whenever possible.

The purpose of central auditory assessment is to determine the presence of a central auditory processing disorder and to describe its parameters. To do this, the clinician will need to describe functional auditory performance deficits. With children, the neuromaturational status of the auditory nervous system should be considered. Further, a central auditory assessment should provide information about both developmental and acquired disorders of the central auditory system.

The diagnosis of central auditory processing disorder is accomplished using a variety of indices, including:

  1. History: The history will include information on the subject's interuterine and birth experience, health status, speech and language behavior, family constellation and history, psychological factors, educational achievement, social development, cultural and linguistic background, and auditory behavior.

  2. Nonstandardized but systematic observation of auditory behavior: A number of questionnaires and checklists are available to identify individuals who are candidates for central auditory testing. Such questionnaires and checklists provide a framework for counseling and intervention. The available checklists have been developed primarily for use with children or persons with developmental disorders.

  3. Audiologic test procedures: Audiologic tests include both behavioral and electrophysiologic techniques. These procedures examine auditory function with rigorously specified acoustic stimuli under controlled listening conditions. Assessment of central auditory function always should be preceded by measures of peripheral function, including pure tone thresholds, speech recognition, acoustic immittance measures, and (when possible), otoacoustic emissions.

    Categories of behavioral auditory measures that can be used effectively to measure central auditory processing disorders include tests of —

    • temporal processes — ordering, discrimination, resolution (e.g., gap detection), and integration;

    • localization and lateralization;

    • low-redundancy monaural speech (time compressed, filtered, interrupted, competing, etc);

    • dichotic stimuli, including competing nonsense syllables, digits, words, and sentences; and

    • binaural interaction procedures (e.g., masking level difference).

    Electrophysiologic procedures can be used in the diagnosis of central auditory processing disorders. The brainstem evoked potential is well understood and applied routinely in the detection of lesions of the brainstem. The middle, late, and event-related auditory potentials are still in the developmental stage, but can be of considerable value in certain clinical situations (Kraus & McGee, 1993; Musiek, Baran, & Pinheiro, 1992).

  4. Speech-language pathology measures: The speech-language pathology assessment provides measures of speech and language ability and communicative function, and assists in the differential diagnosis of central auditory processing disorder.

    Some purported measures of auditory processing that are available for clinical use have questionable validity and test-retest reliability, suffer from poor control of acoustic stimuli, and/or have inadequate normative data. In addition to these concerns, when selecting assessment procedures the clinician should also consider availability of age-appropriate norms, ease of administration, test administration time, the acoustic quality of the recorded test materials, the calibration of equipment, and the acoustics of the test environment. Finally, it is important for the clinician to consider that some central auditory tests are designed for purposes of screening, whereas others are designed for diagnostic purposes.

The following principles should be applied when determining the composition of a central auditory test battery:

  1. The test battery process should not be test-driven but motivated by the referring complaint and the relevant information available to the clinician.

  2. A central auditory test battery should include measures that examine different central processes.

  3. Tests should generally include both nonverbal (e.g., tones, clicks, and complex waveforms) and verbal stimuli to examine different levels of auditory processing and the auditory nervous system. Until tests incorporating verbal stimuli are available in other languages, evaluation on the non-native listener may require reliance on nonverbal stimuli.

  4. Factors to consider in the selection of test procedures include information on the test sensitivity and specificity, test reliability and validity, and age appropriateness.

  5. The duration of the test session should be appropriate to the person's attention, motivation, and fatigue.

  6. It is important that the person who administers and interprets the central auditory test battery have both theoretical and practical knowledge. Typically, audiologists have such knowledge and are qualified to administer and interpret the central auditory test battery.

  7. Speech-language pathologists should collaborate in the assessment of central auditory processing disorders, particularly in cases in which there is evidence of speech and/or language deficits or other communication disorders.

  8. Central auditory processing disorders are most easily defined in the absence of peripheral hearing loss. Many central auditory tests are differentially affected by peripheral hearing loss. Unfortunately, little systematic research has been conducted examining central auditory test outcomes with persons with peripheral hearing loss, and few tests have been designed to separate central and peripheral effects. Tests that are minimally affected by peripheral hearing loss should be selected whenever possible (e.g., dichotic digits, competing sentences, MLR, P300). However, some strategies can be applied by the experienced clinician to estimate the influence of peripheral versus central factors on auditory perception.

For example, if there is a frequency with normal sensitivity, then behavioral tests such as the frequency or duration pattern tests should be administered at the frequency with normal sensitivity. In addition, when bilateral hearing loss and speech recognition scores are symmetrical, dichotic test laterality effects (dominant ear effects) may be interpreted as suggestive of a CAPD. Even when bilateral hearing loss or speech recognition scores are asymmetric, it still may be possible to deduce the presence of central auditory processing disorder. For instance, if the “better” ear presents poorer performance on central auditory measures relative to the “poorer” ear's performance, one may deduce the likelihood of a central auditory processing disorder. In any case, central auditory test results obtained from persons with hearing loss should be interpreted with caution. The validity of strategies used to separate peripheral from central effects must be ascertained through further research.

Issue #3: What are the developmental and acquired communication problems associated with central auditory processing disorders?

CAPDs have been reported for persons manifesting a large and diverse set of clinical problems. In some cases, CAPD is observed in patients who show clear evidence of central nervous system (CNS) pathology. The brain lesions associated with CAPD may be situated cortically in the left and right temporal and parietal lobes or subcortically in thalamus, basal ganglia, and brain stem structures. In certain disease states the lesions are distributed more broadly. A range of conditions, including cerebral vascular accident, traumatic brain injury, tumor, epilepsy, Alzheimer's, and multiple sclerosis, can lead to these various neuropathologies.

Cortical lesions can disrupt a variety of central auditory processes and typically do so bilaterally. Studies show, for example, that judgments of temporal order can be bilaterally impaired following lesion or neuropathology in the left and/or right temporal lobes (Colson, Robin, & Luschei, 1991; Robin, Tranel, & Damasio, 1989). Lesions of the right temporal-parietal area may result in bilateral difficulty with spectral analysis (Robin, Tranel, & Damasio, 1989). Contralateral deficits on dichotic listening and monaural low redundancy speech tasks have also been reported following cortical lesion. Electrophysiological deficits are commonly observed at the electrode nearest the lesion site for the middle and late potentials. Different behavioral and physiological profiles on tests of central auditory processing (CAP) are found when there is involvement of the brain stem and interhemispheric pathways (Musiek, Baran, & Pinheiro, 1994).

When there is clear evidence of CNS pathology, one may reasonably presume that the neuropath-ologies leading to the medical diagnoses are also responsible for the observed CAPD. The picture is more complex, however, when patients manifest speech or language disorders. Although these disorders are likely to stem from the neuropathology, the ways in which CAPD contribute to the speech or language problem is not well understood.

CAPD may also be associated with conditions where CNS pathology is uncertain (e.g., developmental language disorder or learning disability). Here again the relationships between CAPD and the co-existing disorder are not well understood. A number of theoretical frameworks posit links between CAP and the cognitive, linguistic, social, and affective functions that are involved in acquiring and using language. These frameworks vary in breadth from those that focus primarily on audition (e.g., Tallal, Miller, & Fitch, 1993) to those that view auditory processes as one component of a cognitive system (Campbell & McNeil, 1985). As a group these theories highlight the complexity of the relationships between CAPD and such disabilities as developmental language disorder or learning disability.

Although the links between them are complex, CAPD has a potential impact on both language learning and language use for clients with and without clear evidence of neuropathology.

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Language Learning

Acquisition of an oral language depends on the processing of acoustic information. Whatever the central mechanisms may be that allow children to learn oral language with rapidity and ease, they require auditory processing of spoken language input. It is not surprising then that central auditory processing deficits have been causally linked to developmental language disorders. The best evidence of such a link comes from studies that indicate that children with specific language impairment have difficulty in the temporal processing of auditory signals (Tallal, Miller & Fitch, 1993). Also, grammatical morphology is more vulnerable to deficits in languages where the signal properties of these forms make greater perceptual demands (Leonard, 1992). However, temporal processing difficulties alone may not be sufficient to produce specific language impairment (Ludlow, Cadahy, Bassick, & Brown, 1983), nor can they explain the entire range of phenomena associated with this syndrome. For example, deficits in early symbolic play (Terrell, Schwartz, Prelock, & Messick, 1984), poor immediate memory for spatial location (Doehring, 1960), impaired visual imagery (Johnston & Ellis Weismer, 1983), and deficiencies in hierarchical planning (Cromer, 1983) have no obvious connections to auditory temporal processing. There are also numerous other hypotheses for the causal basis of specific language impairment, including specific deficits in language mechanisms (Rice, 1993) and/or more general deficits in cognitive processing (Johnston, 1994).

Analogous arguments can be made for children with learning disabilities, many of whom have difficulty learning written language. A few studies have identified CAPD in children with learning disabilities through audiologic test batteries involving specific measures of auditory signal processing (e.g., Breedin, Martin, & Jerger, 1989; Elliott & Hammer, 1988; Jerger, Martin, & Jerger, 1987; Jerger, et al. 1991). This evidence does not mean, however, that CAPD is sufficient to create learning disabilities nor that all learning disabilities have an auditory source. Explanations posited for this syndrome include visual perceptual, attentional, cognitive, social, and linguistic factors (Wallach & Butler, 1994), as well as auditory processing.

Clinicians should be cautious in attributing language/learning difficulties to CAPD in any simple fashion. For most clients, the relative contribution of CAPD to a larger constellation of behavioral impairments will be difficult, if not impossible, to determine. Clinicians certainly should not infer the existence of CAPD solely from evidence of learning disability or language impairment.

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Language Use

The impact of CAPD on language use is particularly evident in spoken language comprehension. Adults with CNS pathologies and children with developmental language disorders or learning disabilities frequently have difficulty comprehending spoken language, even when they have the necessary language knowledge. If an individual with one of these conditions were to have a central auditory processing disorder, such a disorder would certainly contribute to the comprehension difficulties. For example, patients with right hemisphere lesions of temporal-parietal areas have difficulty analyzing spectral information and thus may lack the intonational information that assists in language understanding.

It is important to note, however, that in contemporary models of language use, the eventual comprehension of a spoken utterance depends upon much more than the processing of acoustic signals. The listener must not only identify, or estimate, the acoustic aspects of the signal, but also must interpret its linguistic value. This requires the activation of lexical representations, grammatical analysis, and judgments of meaning-in-context, to name just a few of the operations invoked in typical language processing models. Working with such models, aphasia researchers point to deficits in resource allocation (McNeil, Odel, & Tseng, 1991), attention (Robin & Rizzo, 1989), or computational inefficiency (Shapiro & Thompson, 1994), as well as temporal processing, in their explanations of language comprehension difficulties. A similar range of explanations can be found in the developmental language literature as well.

The relative importance of “bottom up” (i.e., signal-related) processes and “top down” (i.e., centrally emanating) processes undoubtedly varies among individuals and utterances, depending upon such factors as brain organization, content familiarity, and signal competition (Cole & Jakimik, 1979; Klatt, 1979; Lass, 1984; Marslen-Wilson, 1987; Marslen-Wilson & Welsh, 1978).

Because language comprehension is determined by a number of different factors, clinicians should be cautious in attributing language comprehension difficulties to CAPD in any simple fashion. Likewise, diagnoses of CAPD require comprehensive audiologic assessment and cannot be made solely on the basis of poor comprehension of spoken language.

Issue #4: What is the clinical utility of a diagnosis of central auditory processing disorder?

One goal of the assessment process is an intervention program that will improve the everyday function and life satisfaction of the people who come to us for care. Clients/patients and their families should certainly receive explanations for the difficulties they are having, but they should also be provided with a plan of action for alleviating or minimizing these difficulties. Issue #4 thus leads us to consider how the diagnosis of central auditory processing disorder can be translated into specific intervention strategies and whether these strategies are effective.

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Intervention Approaches

Given our current understanding of central auditory processes, comprehensive rehabilitation and management of CAPD may include interventions directed to acoustic signal enhancement, improvement of language and cognitive capacities, skills development, use of compensatory activities, employment of listening strategies, and/or improvement of the listening environment. These various interventions are examples of two general and complementary approaches to the management of CAPD. The first approach is directed towards enhancing the client's own resources, whereas the second approach is directed towards enhancing the auditory signal and improving the listening environment.

Approach #1: Enhancing Language Resources. Understanding spoken language depends not only on the acoustic signal and its properties, but also on what the listener brings to the listening situation. Listeners routinely use their knowledge of phonology, grammar, and vocabulary, as well as their world knowledge, to “fill in the blanks” of a speech signal. This capacity becomes even more crucial when signal properties are degraded because of auditory system deficiencies. For example, knowledge of comparatives, conjunctions, and other cohesion devices may be particularly useful in the processing of spoken discourse, and intervention could be directed to the learning (or recovery) of these forms (Wren, 1983). It may also prove useful to prepare clients for upcoming lessons, conferences, and the like by teaching any new vocabulary they will need. The information obtained from a speech-language assessment will help the clinician to determine what areas of language to target in intervention.

Once language forms are learned (or recovered), clinicians must also help clients use this knowledge, reliably and automatically, to interpret acoustic signals. Because much of what constitutes central auditory processing is preconscious, occurring without effort or awareness, this intervention goal cannot be met simply by encouraging clients to use what they have learned. Clients will also need opportunities for extended practice with newly learned (or recovered) grammatical patterns or words in order to improve their efficient use of this knowledge in speech processing (Casby, 1992; VanLehn, 1989).

There are some processing difficulties that cannot be addressed by increasing the availability of, or access to, language knowledge. Virtually all individuals with CAPD, in some or another situation, experience such signal degradation that their language resources are not automatically triggered. To assist with these occasions, clinicians can help clients with CAPD learn explicit comprehension strategies. By consciously focusing on crucial aspects of the spoken signal such clients can improve language processing. For example, they can learn to monitor their level of comprehension, pay greater attention to specific speech sounds, use prosody and sentence structure to predict degraded message elements, or deduce word meaning from context (Miller & Gildea, 1987).

Approach #2: Improving signal quality. The second approach to intervention for individuals with CAPD is directed towards improving the quality of the acoustic signal. One way this goal can be achieved is by reducing competing acoustic signals in the listening environment — that is, by reducing background noise and reverberation time. Another way signal quality can be improved is by boosting the intensity of the signal through preferential seating and the use of assistive devices such as FM systems or soundfield amplification. Devices such as these should be evaluated by an audiologist to ensure optimal fitting and to minimize possible detrimental effects (American Speech-Language-Hearing Association, 1994a).

Finally, the quality of linguistic signals can be improved by having communication partners, such as teachers or spouses, speak more slowly, pause more often, and emphasize key words (Ellis Weismer, & Hesketh, 1993; Keith, 1981). Visual aids such as gestures or graphic displays may also prove useful to speech understanding as long as the cues are readily interpretable. Although there has been some enthusiasm for sound control approaches (e.g., occlusion of the weaker ear), there is no theoretical or empirical support for their use.

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Distinctiveness of Intervention

Does identification of CAPD lead to distinctive management strategies? The distinctiveness of CAPD treatments can be considered from at least two vantage points: differential diagnosis and life span development.

Differential Diagnosis. In part, the historical disagreements surrounding CAPD reflect allegiances to different clinical taxonomies and an undue concern with diagnostic labels. Much of the controversy can be resolved by focusing on descriptions of function and the likely behavioral consequences of functional deficits or limitations. Take, for example, an 8-year-old child who has difficulty with spoken language comprehension and performs poorly on tests of auditory temporal resolution, an auditory function important to the processing of speech signals. Clinicians may argue over whether this child has a language disorder or a central auditory processing disorder, but these arguments will often prove impossible to resolve, even after thorough audiological and speech-language assessments are obtained. What is important is to obtain a good description of the child's functional capabilities in the auditory and language areas and to understand what the consequences of this functional description might be for that particular child.

Even with a functional perspective, however, certain clinical decisions will remain difficult. Take the example of a child who has difficulty comprehending spoken language in the presence of background noise. What might be the source of this difficulty? On the one hand, the child might lack automatic control of grammar and be unable to use language knowledge to enhance the degraded signal. From this point of view, the child's problem reflects inadequate language resources. On the other hand, the child may be so poor at registering the properties of the acoustic signal that even the full use of language knowledge cannot lead to successful comprehension. From this point of view, the child's problem reflects inadequate auditory signal processing capabilities.

Parallel difficulties in clinical decision making arise in the treatment of the adult with cognitive/linguistic disorders, (e.g., aphasia). Language comprehension problems in this population could result from memory and attention deficits, from impaired access to language knowledge, or from difficulties in registering the temporal properties of the speech signal. In fact, all of these factors may be combining to produce the behavioral outcome.

One might argue that if the client performs normally with nonverbal stimuli in tasks with competing signals, basic signal processing capabilities are not likely to be the problem. This argument assumes, however, that verbal and nonverbal assessment tasks are perceptually equivalent and are processed by the same auditory processing mechanisms—an assumption that is still a matter of debate. The situation is no better if the client has difficulty with the nonverbal stimuli. One cannot conclude that signal processing is therefore the primary explanation for failure to comprehend spoken language. Inadequate mastery of language could still be a major contributor.

Relative measures of auditory function, such as ear differences, monaural versus binaural differences, and message-to-competition ratio differences, may assist the clinician in determining the relative contributions of auditory-specific versus more generalized processes. For example, auditory dysfunction that is limited to one ear may manifest as an ipsilateral or contralateral ear deficit, whereas deficits in general processes would be likely to affect performance from both ears. In many cases, however, the evidence is not clear, and decisions about the relative contributions of auditory versus language or other general processes must remain uncertain.

Fortunately, the answer to this question is not crucial to much intervention planning. Regardless of the nature of the processing deficit, most persons who have difficulty with the comprehension of spoken language will profit both from procedures that enhance the acoustic signal and from procedures that increase the scope and control of central resources, particularly language resources. In fact, as audiologists increase their appreciation of linguistic and cognitive influences on perception, and speech-language pathologists increase their attention to the acoustic properties of speech and hence to auditory processes, there is an increasing convergence of treatment strategies available to clients. Given our current understanding of language disorders and of central auditory processing, techniques that facilitate language competence are likely to improve the auditory processing of language and vice versa.

Life Span Development. In adults with CAPD, auditory processes that were once intact have been disrupted. Moreover, these processes reside in a central nervous system that is inherently less plastic than that of the child. Hence, management strategies in adults will often be directed toward compensation rather than recovery of function. However, in those cases where acute brain insult provides some opportunity for spontaneous and/or stimulus-induced recovery of function, remedial approaches as well may be appropriate. Children present the complementary range of possibilities. Even in cases of neural insult, and certainly in the more frequent cases of developmental auditory dysfunction, intervention may take advantage of the inherent plasticity of the developing central nervous system and target the acquisition or recovery of normal function. In those cases where recovery is not achieved, or during the period of (re)learning, compensatory strategies may prove useful.

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Efficacy of Treatment

The ultimate merit of any approach to CAPD intervention depends on the efficacy of the treatment. Although theoretically based treatment approaches have been proposed (e.g., Chermak & Musiek, 1992; Sloan, 1986), there have been very few convincing demonstrations of the efficacy of CAPD treatments. The most promising research has made use of physiological and/or auditory behavioral change to measure outcomes. Jirsa (1992), for example, found a significant decrease in P300 latency and a significant increase in amplitude following a structured treatment program with a group of children with CAPD. Improvements were also noted on behavioral tests of auditory processing. These physiological and behavioral changes were not seen in a control group of children with CAPD who did not receive treatment. This study followed an earlier report that subjects with the shortest P300 latencies presented the best performance on an auditory processing behavioral test battery (Jirsa & Clontz, 1990). Kraus & McGee (1994) have recently reported a number of cases in which the absence of the mismatch negativity (MMN) event-related potential was correlated to auditory discrimination deficits in quiet and in competing backgrounds, but follow-up studies of treatment efficacy using this measure have not yet been done.

Physiological and auditory behavioral measures are not always adequate as indices of the desired, or actual, change. Other measures are needed when the anticipated effects of treatment include improvements in linguistic, educational, occupational, psychological, or social capabilities. Despite the need, few studies of CAPD treatment programs have looked at, much less demonstrated, these broad sorts of outcome.

Much of the existing evidence comes from investigations of remote microphone technology (FM). Clinical experience had suggested that clients/patients with CAPD benefit from the use of FM systems (Stach, Loiselle, & Jerger, 1991), and now researchers are attempting to document the impact of this technology on communication ability or quality of life. Project MARRS (Mainstream Amplification Resource Room Study — Ray, Sarff, & Glassford, 1984; Sarff, Ray, & Bagwell, 1981) reported that FM sound field amplification contributed to improved reading achievement in children with academic deficits. Blake, Field, Foster, Platt, and Wertz (1991) reported that FM auditory trainers increased attending behavior in children with learning disabilities, and a case study reported by Stach, Loiselle, and Jerger (1987) pointed to similar improvements in academic performance and behavior. Finally, Stach et al. (1987) reported academic gains for 11 children with CAPD who had been fitted with FM technology. Despite their promise, interpretation of these studies is difficult because only one of them used subjects who had undergone audiological assessment to confirm CAPD, and only one (a different one) used a control group.

The absence of evidence of efficacy does not necessarily mean that CAPD treatment is ineffective. Rather, the poor design, limited scope, and/or small size of most efficacy studies simply leaves us unable to judge. As the task force reflected on this body of research, we identified three conceptual stumbling blocks for researchers and clinicians alike:

  1. Heterogeneity of the Population. At present, CAPD cannot be thought of as a unitary disease entity. Rather, it is a description of function. CAPD can occur for different reasons and take different forms, each of which may have specific implications for treatment outcome. For example, an adult with aphasia and an otherwise normal adult with presbycusis would respond to CAPD treatment quite differently. Likewise, CAPD can occur in persons at various stages of CNS maturity, and with widely varying competencies in language, intellect, social skills, and the like. Again, these individual differences have implications for treatment outcome. For example, a preschooler at the early stages of language development and a lawyer with a long history of verbal experience would be expected to respond very differently to the use of an FM system. Meaningful efficacy research, and improved clinical practice, will require greater attention to the characteristics of the client/patient—those characteristics pertaining to the auditory processing deficit itself and those that pertain to linguistic, cognitive, social, and academic/vocational areas of function. Attention to the various resources and liabilities that will affect a client's response to CAPD treatment becomes increasingly important as treatment goals become broader.

  2. Treatment Goals. As noted above, clinicians often identify improvement in some broad function (e.g., academic performance or social skills) as the desired outcome of CAPD intervention. Such improvement will, of course, require much more than a change in central auditory processing, particularly for persons who are still developing, or recovering, their skills. Changes in central auditory processing may make some forms of learning/recovery easier, but the learning/recovery still must take place. Failure to find improvement in life functions may mean that the CAPD treatment was ineffective, but it may just as well mean that there are other impediments to learning/recovery. Likewise, clinicians should take care in attributing broad improvements in life function to specific CAPD interventions. Other factors could be responsible for the change.

  3. Evaluating Change. Humans of all ages and health states are constantly changing. We get sick or get well, learn new skills, make old skills automatic, grow older, acquire new interests or goals… the list is endless. Many of these changes can impinge on auditory performance or on performance in some function related to audition. To demonstrate treatment efficacy, researchers and clinicians must do more than show that a change in central auditory processing has occurred. They must show that treatment-related changes are greater/different than the changes that would have occurred anyway. Researchers have long been sensitive to the needfor “maturational” or “experimental” controls, but this perspective on efficacy needs wider acceptance in the clinical literature (Olswang & Bain, 1991). Some clinicians may be discouraged to learn that mere improvement is not enough to prove treatment efficacy. Others, however, will be excited to realize that there are some circumstances (e.g., neurodegenerative diseases) in which no-change-at-all can attest to treatment effectiveness.

Careful thinking about these three issues should lead to improvements in efficacy research and to more reasonable claims about treatment efficacy by clinicians.

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Implications for Professional Education

ASHA has repeatedly affirmed its belief that expert clinical practice requires knowledge of the scientific disciplines that have bearing on human communication (ASHA, 1994b). The typical professional training program includes coursework focused on such areas as the physics of sound, neuroanatomy, neurophysiology, articulatory kinematics, psychology, and linguistics. Too often, however, instruction in these areas is provided only in the undergraduate years, and teachers in subsequent clinical courses fail to draw the explicit connections between science and practice.

Members of the task force were impressed at the gulf between science and practice that seemed evident at the consensus conference. We heard much about central auditory processes and disorders from experts in neurophysiology, neuropsychology, hearing science, and cognitive science. We also heard much about the identification and management of central auditory processing disorders from clinicians who had special interest and experience in this field. Few of the scientists, however, commented on the clinical implications of their research, and few of the clinicians explained the scientific bases for their practice. These omissions may merely have reflected the occasion, but it is also likely that there is a true and growing rift between scientists and practitioners. Such a rift would make it difficult for speech-language pathologists and audiologists to function in a well-informed and flexible manner. It would also lessen the likelihood of clinically motivated research.

It is essential for scientists to convey to clinicians information about treatment efficacy research in the area of central auditory processing. This information will guide clinicians in selecting the most appropriate treatment protocol. Conversely, clinicians need to inform scientists about treatment outcomes with individuals who have central auditory processing disorders to assist with the development of relevant research designs.

Professional educators have a responsibility — to students, to their programs, and to themselves — to ensure that the practices they advocate are thoroughly motivated by the best of current science. This requires continuous scholarly effort, resources for faculty development, and ongoing curriculum/course revision. One such revision might be to create advanced graduate seminars, or continuing education programs, that revisit the basic sciences and examine the students'/professionals' clinical experiences from a scientific perspective. Other revisions might lead to seminars that cross professional boundaries and enroll students from both audiology and speech-language pathology, as well as from other related professional programs. Central auditory processing would be an excellent topic for either of these ventures.

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Implications for Research Priorities

The need for research to clarify a number of unresolved theoretical issues and clinical practice questions is evident throughout this technical report. Answers to these questions will assist professionals, consumers, and policy-makers in reaching decisions that will positively affect audiology and speech-language pathology. Future research needs are delineated below, aggregated under the following major headings: basic science, assessment, management, and professional practice.

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Basic Science

  • Elucidate the anatomical, physiological, and behavioral correlates of audition.

  • Determine the influence of peripheral hearing on central auditory processing and the influence of central auditory processing for peripheral processes.

  • Determine the neuropharmacological influences for diagnosis and treatment of central auditory processing disorders.

  • Explore the role of genetics and heredity as causal agents of central auditory processing disorders. Investigate the prevalence of central auditory processing disorders in multicultural populations.

  • Determine whether central auditory processing disorders in children result from neurologic abnormality, neuromaturational disorder, developmental delay, or some combination of factors.

  • Determine the interrelationships among central auditory processing, attention, cognition, and language.

  • Explore the influence of temporal characteristics of real-time speech signals and the real-world demands placed on on-line temporal processing mechanisms.

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Assessment

  • Establish efficiency (i.e., sensitivity and specificity) of behavioral and physiologic measures of central auditory processing. Psychometric rigor of tests purporting to assess central auditory processing must be established for both adults with known lesions and children with suspected diffuse neural dysfunction and documented listening problems.

  • Develop improved audiologic measures to ascertain the listener's use of both acoustic-phonetic and linguistic-contextual information for spoken language understanding.

  • Develop minimal test batteries of physiological and behavioral measures necessary and sufficient for identification and assessment of central auditory processing disorders.

  • Develop tests using verbal stimuli in other languages for assessment of central auditory processing on non-native listeners, as well as tests that evaluate the particular spoken language understanding problems facing non-native listeners.

  • Delineate the effects of neuromaturation on measures of central auditory processing.

  • Delineate the effects of aging on measures of central auditory processing.

  • Establish guidelines for the identification of children at risk for central auditory processing disorders.

  • Develop guidelines for knowledge and skill competencies in the administration and interpretation of tests of central auditory processing.

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Management

  • Establish guidelines for knowledge and skill competencies in the development and implementation of management strategies.

  • Develop guidelines for management strategies and techniques, including use of FM systems, in treating central auditory processing disorders.

  • Examine the efficacy of treatment methodology related to effectiveness and acceptability of treatments.

  • Ascertain the effects on central auditory processing of medication frequently prescribed for management of attention deficit hyperactivity disorder.

  • Determine the interrelationships among central auditory processing disorders, attention deficit hyperactivity disorder, and cognitive and language disorders.

  • Explore the applicability of new real-time speech rate conversation technology for the treatment of central auditory processing disorders.

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Professional Practice

  • Develop guidelines for a team approach to the identification, assessment, and management of children and adults with central auditory processing disorders focused on collaborative exchange with other disciplines and professionals.

  • Establish Common Procedural Terminology (CPT) codes to facilitate reimbursement for assessment and treatment services related to central auditory processing disorders.

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Appendix A. Members of the Task Force on Central Auditory Processing Disorders

Diane Paul-Brown (Coordinator)

Director, Speech-Language Pathology Division

American Speech-Language-Hearing Association

Rockville, Maryland

Maureen E. Thompson (Assistant Coordinator)

Director, Audiology Services Branch, ASHA

Hugh W. Catts

Associate Professor

Department of Speech-Language-Hearing

University of Kansas, Lawrence

Gail D. Chermak

Professor and Chair

Department of Speech and Hearing Sciences

Washington State University, Pullman

Chie Higuchi Craig

Associate Professor

Communication Sciences and Disorders

School of Allied Health Professions

University of Wisconsin, Madison

Judith R. Johnston

Professor and Director

School of Audiology and Speech Sciences

University of British Columbia, Vancouver

Robert W. Keith

Professor and Director

Division of Audiology and Vestibular Testing

University of Cincinnati Medical Center

Frank E. Musiek

Professor Otolaryngology and Neurology

Director of Audiology

Dartmouth-Hitchcock Medical Center

Lebanon, New Hampshire

Donald A. Robin

Department of Speech Pathology and Audiology

University of Iowa, Iowa City

Christine Sloan

Granville Ferry, Nova Scotia

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Appendix B. Invited Consultants to the Task Force on CAPD

Dennis P. Phillips

Dalhousie University

Halifax, Nova Scotia

Rachel E. Stark

Purdue University

West Lafayette, Indiana

Paula Tallal

Rutgers University

Newark, New Jersey

Charles S. Watson

Indiana University, Bloomington

Thomas F. Campbell

Children's Hospital of Pittsburgh

Jeanane M. Ferre

Private Practice

Oak Park, Illinois

Nina Kraus

Northwestern University

Evanston, Illinois

Malcolm R. McNeil

University of Pittsburgh

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Appendix C. Consensus Development Process

The consensus development process strives to enhance decisions made by ASHA members, consumers, policymakers, and other professionals as related to the practice of audiology and speech-language pathology by —

  • identifying experts' consensus on best practices,

  • promoting stronger evaluation of innovative practices,

  • synthesizing new evidence on efficacy with state-of-the-art knowledge, and

  • disseminating consensus information to ASHA members and others.

Once a topic was selected, the first activity was to convene a task force of ASHA members who are experts on the topic. Their objectives were to —

  • formulate those issues on which it is most important to arrive at consensus,

  • determine the most important topics on which resource papers (summarizing research and clinical practices) should be prepared for use by the task force, and

  • recommend other experts with the knowledge and skills needed for authoring resource papers and providing oral/written/audio/videotaped testimony at a public meeting of the task force.

Once the resource papers were commissioned and prepared, the task force reviewed them. Drawing from the resource papers and personal knowledge, task force members formulated brief, tentative statements on as many of the issues as they could. Each task force member reviewed the tentative positions of the other members and noted points of personal agreement or disagreement.

The task force met to —

  • hear oral testimony from the invited experts,

  • synthesize the full array of information presented on the issues, and

  • formulate final statements of consensus suitable for dissemination to ASHA members and others interested in the topic.

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Issues

The ASHA Task Force on Central Auditory Processing held a series of conference calls and one face-to-face meeting. They arrived at consensus on four major issues and drafted a working definition of central auditory processing. The four issues related to the diagnosis and management of children and adults with central auditory processing disorders for which consensus was sought follow:

  • Issue #1 - What does basic science tell us about the nature of central auditory processing and its role in audition?

  • Issue #2 - What constitutes an assessment of central auditory processing and its disorders?

  • Issue #3 - What are the developmental and acquired communication problems associated with central auditory processing disorders?

  • Issue #4 - What is the clinical utility of a diagnosis of central auditory processing disorders?

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Preparation for the Consensus Conference

Besides expert consultants, individuals with some expertise or an interest in the area of central auditory processing disorders were invited to attend the consensus development conference and to present information to the task force. Those unable to attend the conference were provided the option of submitting written or taped testimony. A general invitation to attend the consensus development conference was extended to all ASHA members through announcements in Asha magazine and the Audiology Update newsletter.

Before the conference, task force members sent the list of issues to the invited consultants and self-identified presenters. All of these speakers were asked to direct their comments during the conference to one or more of these issues. Task force members also sent the list of issues to other well known experts in the area of central auditory processing disorders and asked them to respond.

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The Consensus Development Conference

The Consensus Development Conference was held March 11–13, 1994 in Albuquerque, New Mexico. In addition to the 8 invited expert consultants, 20 individuals presented information to the task force on one of the four issues. Following a day and a half of presentations, task force members convened in closed session for a day and a half to prepare the technical report.

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Index terms: auditory processing

Reference this material as: American Speech-Language-Hearing Association. (1996). Central auditory processing: current status of research and implications for clinical practice [Technical Report]. Available from www.asha.org/policy.

© Copyright 1996 American Speech-Language-Hearing Association. All rights reserved.

Disclaimer: The American Speech-Language-Hearing Association disclaims any liability to any party for the accuracy, completeness, or availability of these documents, or for any damages arising out of the use of the documents and any information they contain.

doi:10.1044/policy.TR1996-00241

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