Central Auditory Processing and Central Auditory Processing Disorder

Central Auditory Processing and Central Auditory Processing Disorder: Fundamental Questions and Considerations WAYNE J. WILSON Division of Audiology, School of Health and Rehabilitation Sciences, University of Queensland, Australia

CHYRISSE HEINE School of Human Communication Sciences, La Trobe University, Australia

LAUREN A. HARVEY Queensland Hearing, Brisbane, Australia

Despite several decades of research, neither clinicians nor academics can agree on a single definition of central auditory processing (CAP) or central auditory processing disorder (CAPD). This article considers why this is the case, and comments on the resulting implications for CAP assessment and CAPD rehabilitation in the clinic.

Deciding how best to assess central auditory processing (CAP) and to manage central auditory processing disorder (CAPD) can be confusing. The literature offers a vast array of options, all of which are sound in theory, few of which are sound in supporting data. As a result, clinical decisions about CAP and CAPD must often be made based on theory alone. It is in this theory, however, that much confusion lies. Central to this confusion is the lack of a universally agreed definition of CAP and CAPD. This four-part review paper will examine why this is the case. Part 1 will begin by offering a working definition of CAP, followed by a series of challenging considera-

tions. Part 2 will repeat this process for CAPD. Part 3 will consider the most widely cited definitions of CAP and CAPD, and the position they take with respect to the considerations discussed in parts 1 and 2. Finally, part 4 will consider the implications of all these theoretical arguments with respect to the assessment of CAP and the rehabilitation of CAPD in clinical practice. No parts of this paper will review issues surrounding terminology (see Chermak [2001] and Bellis [2003] for introductory reading on this issue) or specific CAP assessment or CAPD rehabilitation protocols (these will be the topics of future reviews by these authors). PART 1: DEFINING CENTRAL AUDITORY PROCESSING Definitions of CAP have been many and varied. Examples include ‘the manipulation and utilization of sound signals by the central nervous system’ (Lasky & Katz, 1983, p. 4), ‘the use we make of the auditory signal’ (Katz, Stecker, & Henderson, 1992, p. 4), and ‘the serial and parallel processing of the auditory system responsible for auditory

Parts of this paper have been previously published in Wilson, W.J. (2003). Confused about APD? Then consider the following questions. Acquiring Knowledge in Speech, Language and Hearing, 5, 123–126 (unrefereed). Correspondence and reprint requests: Dr Wayne Wilson, PhD, MAudSA CCP, Division of Audiology, School of Health & Rehabilitation Sciences, University of Queensland, Brisbane Qld 4072, Australia. E-mail: [email protected]

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CAP AND CAP DISORDER

attention, detection and identification of auditory signals, decoding of the neural message, as well as storage and retrieval of auditory-related information’ (modified from Katz et al., 1992, pp. 5–6). For the purposes of this article, however, we will begin our discussion by operationally defining CAP very simply as ‘what we do with what we hear’ (Katz et al., 1992, p. 5). Consideration 1a: What Processes Should be Included in a Definition of CAP?

If we define CAP as ‘what we do with what we hear’ (Katz et al., 1992, p. 5), then we must consider what individual processes should be included in this definition. It is no surprise to find that many different authors have listed many different processes under the umbrella of CAP. Possibly the most comprehensive list is that of Heasley (1974) who listed 14 processes: awareness of sound, auditory attention, auditory attention span, localization of sound, discrimination for sound, auditory memory, auditory memory span, auditory sequencing ability, auditory projection, auditory separation, auditory blending, auditory closure, subvocalisation and reauditorisation. Other examples, such as ASHA (1996; discussed in part 3 below), simply represent different authors’ beliefs as to what CAP is and how it should be approached. Consideration 1b: Is CAP Predominantly a Bottom-Up or a Top-Down Process?

One of the greatest influences on any definition of CAP is whether it should be defined as being a predominantly ‘bottomup’ or predominantly ‘top-down’ process. Defining CAP as being bottom-up defines it as being data (sound) driven, with the properties of the data being the primary determinants of higher-level representations and constructions (Chermak & Musiek, 1997). This is the approach taken by pathway models of auditory processing. Pathway models of auditory processing view the central auditory nervous system (CANS) as the pathway that processes the auditory information. They focus on the

evaluation of different levels of the CANS, and suggest that if testing is done in a controlled acoustic environment, then auditory processing can be separated from ‘higher, nonauditory’ factors such as attention, cognition, language, learning and memory (McFarland & Cacace, 1995). In contrast, defining CAP as being topdown defines it as being knowledge- or concept-driven, such that higher-level constraints guide the data processing, leading to data interpretation consistent with these constraints (Chermak & Musiek, 1997). This is the approach taken by network models of auditory processing. Network models of auditory processing emphasise the distributed nature of information processing in the nervous system, with the integration of sound, meaning and intention involving more than the auditory pathway (Friel-Patti, 1999). While bottom-up (data-driven/pathway model) and top-down (concept-driven/ network model) approaches are dissimilar, they are not contradictory (Friel-Patti, 1999; McFarland & Cacace, 1995). According to information processing theory, it is a combination of bottom-up and top-down factors that determines an individual’s ability to process auditory information (Bellis, 2003). In this way, top-down processes could ensure sounds are assimilated in a manner consistent with the listener’s experience and expectations, while bottom-up processes could ensure that the listener is alerted to novel information and information incompatible with ongoing hypotheses about the message (Neisser, 1976; Rumelhart, 1984). This approach would also allow for the relative contribution of bottomup and top-down processes to be driven by the demands of the listening environment; for example, the more degraded the signal, the greater the requirement for top-down processing (Chermak & Musiek, 1997). While the majority of recent research suggests CAP probably involves both bottom-up and top-down processes, such a conclusion poses problems for the assessment of CAP in the clinic, as it inhibits a 81

WAYNE J. WILSON, CHYRISSE HEINE AND LAUREN A. HARVEY

formal definition of exactly what processes should and should not be called CAP. Consideration 1c: Is CAP an Active or a Passive Process?

The contrast between active and passive processes is best demonstrated by considering their use in models of speech perception. Active theories postulate that the process of speech perception involves some aspects of speech production. In this way, the sounds are sensed and analysed for their phonetic properties by reference to how they are produced, and thus, recognised. In these theories, the recognition is based on the matching between perceived and peripherally processed units and internally generated representations (which can be based on speech production knowledge or phonemic categorisation of speech acoustic events). Therefore the listener would actively process the variable speech signal into constant perceptual units. Passive theories, on the other hand, emphasise the sensory, filtering mechanisms, and the direct mapping of the message onto the acoustic–phonetic features of the language. They regulate the role of speech production knowledge to a minor, secondary place, used only in difficult circumstances. For these theories, the listener does not make use of any internal representation, but rather simply derives linguistic information from the speech chain in a serial process (Kent, 1997). Which of these processes is dominant in CAP remains difficult to prove. Consideration 1d: Is CAP a General Auditory or a Speech Specific Process?

By borrowing on terminology from the Tallal versus Haskin’s Laboratories CAPD argument (reviewed in Friel-Patti, 1999, and in consideration 2h), a ‘general auditory’ approach to CAP argues that all sounds are processed similarly, whereas a ‘speech specific’ approach argues that nonspeech and speech sounds are processed differently. Evidence for the general auditory approach comes mostly from data that shows categorical perception is not 82

restricted to speech sounds only, with even nonhuman mammalian species demonstrating categorical perception. Evidence for the speech specific approach comes mostly from data that shows the rate of processing of speech sounds is much higher than nonspeech sounds, that context can have a significant effect on speech processing, that ‘duplex’ effects can occur with speech sounds, and that differences in perception of sinewave speech can occur following the cuing of a subject to expect ‘speech or nonspeech’ sounds. Despite much research, no universal agreement has been reached regarding the existence, or not, of a special mode of perception for processing speech (reviewed in Jusczyk & Luce, 2002). Consideration 1e: When Does CAP Become Language Processing?

By its very nature, CAP is usually thought to be associated with the processing of acoustic input (Bellis, 2003). While there appears to be agreement amongst most researchers that auditory processing and language processing are not interchangeable, the precise point at which auditory processing stops and language processing begins is still debated (Bellis, 2003). To better distinguish between auditory processing and language processing, Auerbach, Allard, Naeser, Alexander and Albert (1982) offer the concepts of prephonemic and phonemic processing. Prephonemic processing is thought to occur first, and to involve the initial perception of the acoustic aspects of speech sounds, that is, the basic sensory analysis of the spectrotemporal features of the acoustic signal. Phonemic processing is thought to occur next, and to involve the perception of the speech sounds themselves. The crossover is thought to occur at the point of categorical perception, that is, the point where speech sounds are categorised into psycholinguistically distinct percepts. The definition of a psycholinguistically distinct percept remains debated, however, with possibilities including demisyllables, context-sensitive allophones, syllables, and


context-sensitive spectra (Jusczyk & Luce, 2002). A purely acoustic feature theory of CAP would therefore state that CAP ends, and language processing begins, at the point of categorical perception where prephonemic processing becomes phonemic processing (Bellis, 2003). Such prephonemic versus phonemic processing boundaries have been criticized, however, due to their lack of ecological validity. Central to this criticism is the point that the suggested boundaries do not sufficiently consider both bottom-up and topdown processes, and therefore ignore the belief that acoustic and phonemic processing cannot be cleanly separated from each other, or from higher-order (top-down) linguistic influences (Bellis, 2003). Consideration 1f: Which Model of CAP Should I Use?

Many authors have tried to structure CAP by modelling it. The resultant models, however, range from simple to complex. An example of a simple model is that of Boothroyd (1997). He describes the sound source as containing sound patterns. The auditory system receives these sound patterns, converts them to sensory evidence, processes this sensory evidence, and finally, in combination with other systems, produces a percept. It is the middle step of ‘processes this sensory evidence’ that most readers will identify as CAP. An example of a complex model is that of Medwetsky (2002). He describes the processing of spoken language as consisting of sensory input, hydromechanical, neurochemical and electrical transduction, feature extraction, precategorical acoustic storage, extraction of slow-changing cues and fastchanging cues, synthesised auditory memory, comparison with long-term memory, pattern recognition, and short-term memory. He also postulates that the latter four of these processes are managed by a central executor, which is itself affected by an arousal system.

Examples of very complex models come mostly from models of spoken word recognition. Recent examples (all reviewed in Jusczyk & Luce, 2002) include the cohort theory (Marslen-Wilson & Welsh, 1978), the trace model (McClelland & Elman, 1986), the shortlist model (Norris, 1994), and the neighbourhood activation model (NAM) and PARSYN (Luce, Pisoni, & Goldinger, 1990). As per the different lists of CAP processes (consideration 1a), the different models of CAP tend to reflect the different authors’ beliefs as to what CAP is and how it should be approached. The simpler models tend to emphasise bottom-up, passive, general auditory processes, whereas many of the more complex and very complex models tend to emphasise (or at least involve to a greater extent) topdown, active, speech-specific processes. It is on these points that their differences are most intuitive. Consideration 1g: What About Anatomy and Physiology?

Paralleling the theoretical/model arguments surrounding CAP is a similarly complex anatomical and physiological argument. While beyond the scope of this article, the argument can be seen to begin with the need to decide whether CAP should directly include the peripheral auditory system, and end with the need to decide whether CAP should directly include nonauditory CNS structures. Between these two points lies the need to decide which CAP functions are performed by which structures and/or networks. Significantly influencing these questions is the recent trend in neuroscience away from the more traditional pathway models and towards the newer network models of signal processing (consideration 1b). This move lessens the importance of specific centre/single nerve cell activity, and emphasizes the importance of multiple serial, parallel and distributed brain network activity (Katz et al., 1992; Chermak & Musiek, 1997). Eggermont (2001) provides an excellent 83


review of elements of this argument from a neurophysiological viewpoint. PART 2: DEFINING CENTRAL AUDITORY PROCESSING DISORDER (CAPD) Definitions of CAPD, like definitions of CAP, are many and varied. Examples include ‘the inability or impaired ability to attend to, discriminate, recognise, or comprehend information presented auditorily even though the person has normal intelligence and hearing sensitivity’ (Keith, 1986, p. 3), ‘difficulty in processing and understanding both nonverbal and verbal auditory stimuli … believed to be the result of auditory stimuli reaching the brain with inadequate processing of the perceived stimuli’ (Boone, 1987, pp. 100–101), and ‘a decrease in auditory comprehension not necessarily accompanied by a decrease in auditory sensitivity’ (modified from Northern & Downs, 1991, p. 12). For the purposes of this article, however, let us continue our discussion by operationally defining CAPD very simply as ‘when something goes wrong with what we do with what we hear’ (modified from the definition of CAP provided by Katz et al., 1992, p. 5). Consideration 2a: The Origin of ‘CAPD’

One of the keys to understanding CAPD is to understand where it came from. Since the 1950s (Myklebust, 1954; Bocca, 1958), there have been regular literature reports of individuals who, despite having normal hearing, still have difficulty understanding speech and language in the presence of any type of acoustic distortion. To better describe these individuals, researchers attempted to list their common signs, symptoms and behaviours. Regularly cited examples include ‘being uncertain about what they hear, having difficulty listening in the presence of background noise, having difficulty following oral instructions, and having difficulty understanding rapid or degraded speech’ (modified from Jerger & Musiek, 2000, p. 467). To better label these individuals, researchers suggested terms such as central 84

deafness, auditory agnosia, dysacusis, central auditory imperception, central hearing loss, nonsensory hearing loss, obscure auditory dysfunction, CAPD, and the most recent term, APD (Keith, 1999). To better diagnose these individuals, researchers attempted to locate a common siteor site(s)-of-lesion. Despite almost 50 years of trying, no such site(s) have been found. In fact, many individuals who report CAPD-like signs, symptoms and behaviours present with no identifiable site-of-lesion at all. It then becomes clear that the concept of CAPD originated not from a common site(s)of-lesion resulting in a predictable pattern of signs, symptoms and behaviours, but from a pattern of signs, symptoms and behaviours that has yet to be attributed to a common site(s)-oflesion. This fact has a profound effect on the way we approach both auditory processing and CAPD, both as researchers and clinicians. Consideration 2b: Does CAPD exist?

Despite the apparent acceptance of CAP as a stand-alone entity, similar acceptance of CAPD cannot be assumed. The questions as to CAPD’s existence are generally threefold: is CAPD a stand-alone entity, does CAPD exists only as a subgroup of a larger disorder, and does CAPD exist at all? In support of the existence of CAPD as a stand-alone disorder are at least three lines of converging evidence (Jerger, 1998). First is the accumulation of over 30 years of audiological evidence linking identifiable site(s)-oflesion on the auditory pathway (e.g., tumours, vascular insults) with unique auditory processing consequences. It is not, therefore, unreasonable to infer that when similar symptomology is observed in individuals with CAPD, that a problem in the CANS can at least be suspected (Jerger, 1998). Second is the accumulation of over 10 years of in-depth studies of specific auditory perceptual deficits in adults and children who have one complaint only: the inability to hear in difficult listening environments (Jerger, 1998). Third is the growing evidence that the unique hearing deficits of the elderly may, at least in


some cases, be a result of age-related changes in the CANS as versus changes in the peripheral auditory system (Jerger, 1998). In support of the existence of CAPD as a subgroup of a larger disorder are at least two lines of evidence. First is the significant overlap between CAPD sequelae and those of other attention, communication, cognition, language, learning and memory disorders (a full discussion of which is beyond the scope of this paper and the reader is referred to Keller (1992), Sloan (1992), Keller (1998), and Chermak, Hall and Musiek (1999) for more detail). Second is the argument that the tools used to assess CAPD diagnose symptoms only, and not disease. This casts doubt over exactly what these tools are measuring (Rees, 1973; Northern & Downs, 1991; Friel-Patti, 1999). In support of the nonexistence of CAPD is the argument of Rees (1973). Rees (1973) argues that there is little, if any, need for CAP in speech understanding as very little can be gleaned from the ‘fuzzy and uninformative’ acoustic signal. This ‘extreme’ application of what is essentially a top-down argument posits that speech understanding results from higher cognitive processing only, with little or no dependence on the auditory signal for understanding the spoken word. Such a complete dependence on top-down, higher cognitive functions cannot hold, however, in light of the obvious effects of peripheral hearing impairment on speech understanding. While this again highlights the probable interdependence of both top-down and bottom-up processes, it does not completely justify the existence of CAPD as a stand-alone disorder. Consideration 2c: Central Auditory Processing Difficulty or Central Auditory Processing Disorder?

Contributing to the debate surrounding the existence of CAPD is the need to differentiate between CAP difficulty and CAP disorder. A disorder can be defined as ‘a derangement or abnormality of function; a morbid physical or mental state’ (Dorland’s Pocket Medical Dictionary, 1995, p. 251). With respect to CAPD, this highlights the need to

differentiate between subjects with various listening ‘difficulties’ and those with actual ‘disorders’. Of immediate relevance is the way in which we classify our test results, and how this classification relates to realworld performance. One way to classify an individual’s test results is to compare them to a range of test results obtained from ‘normal’ individuals. If the individual’s result falls outside the ‘normal’ range, then their performance could be deemed ‘abnormal’ (Jerger & Musiek, 2002). This is problematic, however, when two subjects’ test scores are very similar, but one lies just within the normal range, and the other just outside the normal range. It then becomes difficult to confidently conclude that the former is normal and the latter is abnormal (Jerger & Musiek, 2002), and alternative approaches to diagnosis need to be sought. Such alternatives could include using the individual as their own control by looking for intertest and interaural discrepancies (Jerger & Musiek, 2002). Of further importance is the need to decide how many tests must be failed before a difficulty can be deemed a disorder. Audiologists typically follow a cross-check principle (or the clinical application thereof) whereby one test result requires confirmation from another test result (Turner, 2003). This introduces a range of possible pass/fail criteria whereby a disorder could be concluded if one test only is failed (a ‘lax’ criterion), if two or more tests are failed (an ‘intermediate’ criterion), or if all tests are failed (a ‘strict’ criterion) (Bellis, 2003). The cross-check principle would argue that at least an intermediate criterion is needed to diagnose CAPD. Others have argued, however, that if each test within the battery assesses a different process under the CAP umbrella, then it is possible that one underlying process may be disordered while the others remain quite functional (Bellis, 2003). This could result in an abnormal performance on one test measure only. Possibly the most practical criterion for the diagnosis of a CAPD is that of Bellis (2003). 85


She states that in order for a CAP difficulty to be identified as a CAPD, then it must be established that one or more underlying CAPs are disordered or delayed, and that the disorder or delay, in all likelihood, has a significant impact on the individual’s ability to function or learn. Proving such impact, however, remains one of the greatest challenges to CAPD. Consideration 2d: What Processing Disorders Should CAPD Include?

If we define CAPD as ‘when something goes wrong with what we do with what we hear’ (modified from the definition of CAP provided by Katz et al., 1992, p. 5), then we must consider what individual processing disorders should be included in this definition. As with CAP, it is no surprise to find that many different authors have listed many different processing disorders under the umbrella of ‘CAPD’. These lists can typically be viewed as mirrors of the lists of CAP processes such as those cited in consideration 1a, that is, if CAP includes processes A, B and C, then CAPD is the disorder of processes A, B and/or C. As introduced in consideration 2d, it quickly becomes apparent that the availability of so many different and often contradictory lists of CAPD processes is of little use to the clinician trying to diagnose CAPD. In an attempt to alleviate some of this confusion, some authors have moved towards the concept of modality specificity. Consideration 2e: CAPD and Modality Specificity

Modality specificity refers to a deficit being critically dependent on the use of information presented to a specific sensory system. McFarland and Cacace (1995) argue that the existence of CAPD depends on our ability to show that CAPD is specific to the auditory modality. They elaborate by considering three groups of individuals, all of whom perform poorly on specific tests involving the processing of auditory information, and all of whom have the potential to be diagnosed with CAPD. 86

The first group have specific perceptual problems processing information presented in the auditory modality only. This group would be expected to perform poorly on auditory tasks, but within normal limits on nonauditory tasks (visual and tactile), and could be considered to have CAPD in its ‘purist’ form. The second group have auditory perceptual problems that coexist with other processing problems (which may exist in more than one sensory modality and may or may not be of a perceptual nature). This group would be expected to have mixed patterns of deficits, with modality-specific effects on some tests and global deficits on others. If the modality specific deficits were auditory, they could be considered to have CAPD co-occurring with other disorders. The third group perform poorly on tests of auditory processing not because they have auditory-specific perceptual problems, but because the test in question is sensitive to other processing demands, such as motivation, attention, memory, cognition and motor skills, which are necessary to perform any behavioural task (this would be considered a supramodal deficit). This group would be expected to have more global deficits across all modalities (auditory, visual and tactile), and as a result, could not be considered to have an auditory-specific disorder such as CAPD. McFarland and Cacace (1995) finally note that ‘thus the differential diagnosis of sensoryspecific perceptual deficits can be considered a form of hypothesis testing where the goal is to determine in which of the three groups described above a given individual belongs’ (p. 37). They also conclude that because research in the field of CAPD has been limited, almost exclusively, to investigations using acoustic stimuli, it is not yet possible to conclude that the observed effects of CAPD are indeed specific to the auditory system (McFarland and Cacace, 1995). Bellis (2003), while generally supporting the need to consider modality specificity, also warns that because higher level


functions (such as language, cognition, attention and executive function) and other sensory modalities influence auditory perception, and because of the auditoryresponsiveness of many ‘nonauditory’ areas of the brain, any definition of CAP and CAPD that insists upon modality specificity may be neurophysiologically untenable. Consideration 2f: Is CAPD Predominantly a Bottom-Up or a Top-Down Disorder?

As with CAP (consideration 1b), one of the greatest influences on any definition of CAPD is whether it should be defined as being predominantly a ‘bottom-up’ or a predominantly ‘top-down’ disorder. A bottom-up approach would consider CAPD as being a perceptual disorder, with the deficit occurring in the precursor behaviours, and the linguistic/cognitive factors being secondary to, or a result of, the original perceptual disorder. In contrast, a top-down approach would consider CAPD as being a conceptual disorder, with the deficit occurring in the representational–symbolic behaviours, and the linguistic/cognitive factors being primary to, or even independent of, the ‘lower level’ perceptual disorders. It certainly appears logical to assume that if the fundamental bottom-up encoding of an auditory signal was disrupted, then its final auditory percept would be affected. While such bottom-up processing disruptions have been readily demonstrated (consideration 2b), direct links to higher language, learning and communication disorders have been harder to prove. Similarly, it could be expected that these bottom-up factors would be affected in a top-down manner by higherorder factors (such as attention, cognition, language and memory) through the presence of complex feedback and feedforward mechanisms. Therefore, even if basic sensory encoding unfolded perfectly at all levels of the bottom-up pathway, top-down deficits could have a negative effect on the ability to process, and ultimately comprehend, spoken language (Bellis, 2003).

While efforts to promote the need for modality specificity in CAPD diagnosis have emphasised the bottom-up aspects of CAPD, the majority of recent research suggests CAPD probably involves both bottom-up and topdown processing disorders. As with CAP, this conclusion poses problems for the diagnosis of CAPD in the clinic, as it inhibits a formal definition of exactly what processing disorders should and should not be called CAPD. Consideration 2g: Is CAPD an Active or a Passive Disorder?

In a direct follow-on from consideration 1c, if CAPD is a disorder in an active process, then it could lie in the relationship between speech perception and speech production, and/or the matching between perceived and peripherally processed units and internally generated representations, and/or speech production knowledge or phonemic categorisation of speech acoustic events. If CAPD is a disorder in a passive process, then it could lie in the sensory, filtering mechanisms, and/or the direct mapping of the message onto the acoustic–phonetic features of the language (based on Kent, 1997). Neither of these arguments can proceed, however, until the original ‘active and/or passive CAP’ argument is decided. Consideration 2h: Is CAPD a General Auditory or a Speech-Specific Disorder?

The general auditory hypothesis of CAPD, driven mostly by Tallal and colleagues (Tallal, 1980, 1981; Tallal & Piercy, 1973, 1974, 1975; Tallal, Miller, & Finch, 1993; Tallal et al., 1996; Merzenich et al., 1996; Tallal, Miller, Jenkins, & Merzenich, 1997), argues that there is a deficit in the processing of acoustic signals entering the nervous system in rapid succession, and that this deficit occurs for all sounds, that is, it is not limited to speech sounds only. The speech-specific hypothesis of CAPD, driven mostly by researchers from Haskins Laboratories in the USA (Studdert-Kennedy & Mody, 1995; Mody, Studdert-Kennedy, & Brady, 1997), argues that the processing deficit is phonetic in origin, not auditory, that 87


is, the deficit is speech-sound specific and is not related to deficits in the perception of nonspeech sounds. Limiting both these hypotheses is an absence of data from subjects with CAPD. The general auditory hypothesis is drawn predominantly from subjects with specific language impairment, while the speechspecific hypothesis is drawn from subjects with reading impairment. Consideration 2i: When Does a CAPD Become a Speech–Language Disorder?

By its very nature, CAPD is usually considered to be a deficit associated with the processing of acoustic input (Bellis, 2003). Many manifestations of CAPD (consideration 2d), however, appear as language comprehension disorders. As with CAP (consideration 1e), CAPD manifestations could be seen to be either prephonemic or phonemic in origin (Bellis, 2003). A prephonemic deficit would affect the initial identification of the acoustic aspects of speech sounds, particularly those that change rapidly over time. This could then lead to a secondary impairment of spoken word recognition. This is part of the argument offered by Tallal and colleagues (consideration 2h). A phonemic deficit would only affect the identification of the speech sounds themselves. This could result in difficulties resolving the fine boundaries between phonemes, or difficulties grouping sounds with different acoustic properties into instances of the same phoneme. This is part of the argument offered by the Haskins Laboratories (consideration 2h). As with CAP (consideration 1e), a purely acoustic feature theory of CAPD would therefore state that CAPD ends, and language disorder begins, at the point of categorical perception where the prephonemic disorder becomes a phonemic disorder (Bellis, 2003). Also as per CAP (consideration 1e), however, the boundaries between prephonemic versus phonemic disorders have been 88

criticised due to their lack of ecological validity (Bellis, 2003). Consideration 2j: Does CAPD Have a Primary Deficit?

Our inability to identify a primary site(s)-oflesion for CAPD (consideration 2a) has not deterred efforts to identify a primary deficit. This has resulted in four distinct hypotheses; that CAPD is primarily a deficit in: • Auditory memory: Basic CAPs are performed successfully, but the information cannot be remembered long enough to adequately respond. • Auditory sequencing: Individual sounds are not perceived in their correct sequence. • Temporal resolution: The small temporal differences that differentiate sounds cannot be resolved. • Auditory figure–ground: Target and nontarget sounds cannot be separated (Jerger, 1998). Recent evidence favours the latter two (Jerger, 1998). Consideration 2k: Is CAPD a Unitary Disorder or a Series of Subprofiles?

Rather than thinking of CAPD as a single, primary deficit (a unitary disorder), other researchers have noted that, like all attention, communication, cognition, language, learning and memory disorders, it is probably a heterogeneous disorder. In an attempt to manage this heterogeneity, these researchers have divided CAPD into subgroups using a process known as CAPD subprofiling. The concept of CAPD subprofiling is not new. Older profiles, such as those offered by Musiek, Gollegly and Ross (1985), Musiek and Gollegly (1988), and Katz (1992), have been instrumental in the development of newer profiles, such as those offered by Bellis and colleagues (Bellis, 1996, 1999, 2003). The greatest potential offered by CAPD subprofiles is that of ‘differential diagnosis for specific rehabilitation’. Because of this, many of the CAPD subprofiles have been


rushed into clinical service. Such rapid implementations should be viewed with caution, however, as despite being strong in theory and practical usability, none of the subprofiles have been fully tested experimentally. This leaves their validity and reliability in doubt. Despite the many weaknesses of CAPD subprofiling, most researchers would agree that CAPD is a heterogeneous disorder. It is the nature of the subprofiles and their specific rehabilitation that remains the topic of significant debate. Consideration 2l: What about Anatomy and Physiology?

Paralleling the theoretical and modeling arguments surrounding CAPD is a similarly complex anatomical and physiological argument. While beyond the scope of this review, the argument can be seen to have similar elements to the CAP anatomy and physiology argument (consideration 1g), with the behaviours observed in CAPD being thought to correlate with underlying neurophysiological and/or neuromaturational deficits. By way of example, let us briefly consider the three subprofiles of child CAPD discussed in Musiek, Gollegly and Ross (1985) and Musiek and Gollegly (1988). The first subprofile was thought to result from neuromorphological disorder (consisting mostly of polymicrogyri and heterotopias in the left hemisphere and the auditory region of the corpus callosum), which could account for 65% to 70% of diagnosed CAPD in children. The second subprofile was thought to result from maturational delay of the central auditory nervous system, which could account for 25% to 30% of diagnosed CAPD in children. The last subprofile was thought to result from neurologic disorders, diseases and insults (including neurodegenerative disorders), which could account for less than 5% of diagnosed CAPD in children. Such classification systems are currently being investigated further using brain imaging and postmortem techniques.

PART 3: WHAT ARE THE MOST WIDELY CITED DEFINITIONS OF CAP AND CAPD, AND WHAT ARE THEIR POSITIONS WITH RESPECT TO THE ABOVE CONSIDERATIONS? Arguably the most cited definitions of CAP and CAPD in recent times are those provided by the ASHA Taskforce on Central Auditory Processing Consensus Development (ASHA, 1996). Rather than defining CAP per se, they defined central auditory processes as: The auditory system mechanisms and processes responsible for the following behavioral phenomena: Sound localisation and lateralisation; auditory discrimination; auditory pattern recognition; temporal aspects of audition, including temporal resolution, temporal masking, temporal integration and temporal ordering; auditory performance decrements with competing acoustic signals; and auditory performance decrements with degraded acoustic signals. (p. 41).

The ASHA Taskforce presumed these mechanisms and processes applied to nonverbal as well as verbal signals, and affected many areas of function including speech and language. They were also thought to have neurophysiological as well as behavioral correlates, and to engage many neurocognitive mechanisms and processes in recognition and discrimination tasks. Some of these were thought to be specifically dedicated to acoustic signals, whereas others (e.g., attentional processes, long-term language representations) were not. With respect to these nondedicated mechanisms and processes, the term central auditory processes referred particularly to their deployment in the service of acoustic signal processing (ASHA, 1996). After defining CAP, ASHA (1996) defined CAPD as a ‘deficiency in one or more of the above-listed behaviours’ (ASHA, 1996, p. 41) where the ‘above listed behaviours’ were those listed in their definition of CAP (see above). They also stated that ‘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 attention deficit 89


or neural timing deficit, that affects performance across modalities. It is also possible for CAPD to reflect coexisting dysfunctions of both sorts’ (ASHA, 1996, p. 41). The report of the Consensus Conference on the Diagnosis of Auditory Processing Disorders in School-Aged Children fine-tuned this definition by adding that CAPD is ‘a deficit in the processing of information that is specific to the auditory modality’ (Jerger & Musiek, 2000, p. 468). In comparing the ASHA (1996) definitions to the considerations discussed above, it can be seen that ASHA (1996): (1) supports the existence of both CAP and CAPD (considerations 1a, 2b, 2c and 2d); (2) supports the notion that CAPD is specific to the auditory modality (consideration 2e); (3) provides a specific list of processes that qualify as CAP, and in doing so, provides a list of disorders that qualify as CAPD (considerations 1a and 2d); (4) defines CAP as being predominantly bottom-up processing, although the possible influence of ‘top-down’ processes such as memory, attention or cognition is noted (considerations 1b and 2f); (5) infers that a disorder in the bottom-up CAP processes can affect higher-order processes including language (considerations 1b and 2f); (6) allows for both active and passive processing (considerations 1c and 2g); (7) allows for both general auditory and speech specific processing (considerations 1d and 2h); (8) infers that CAP and CAPD have more to do with ‘auditory’ processing than with ‘language’ processing (considerations 1e and 2i); (9) infers that CAPD does not have a primary deficit (consideration 2j), allowing for it to exist as a series of subprofiles (consideration 2k); and (10) allows for the involvement of a number of different anatomical structures and physiological processes (considerations 1g and 2l). While this ASHA Taskforce definition is probably the most widely cited, it has also been widely criticised. These criticisms include: (1) the use in the definition of the term being defined (i.e., processes); (2) the lack of clarity and illumination afforded by 90

the general phrase ‘auditory system mechanisms and processes’; (3) the overly restrictive nature of the list of specific auditory behavioural phenomena provided, with no acknowledgement of the possible interdependency of these behavioural abilities in auditory or spoken language processing, and no mention of additional auditory abilities not included in the list; (4) the lack of elaboration of how ‘higher-level neurocognitive processes’ relate to the ‘processing of acoustic signals’; (5) the specific referral to the auditory system only with higher-level neurocognitive processes (such as attention, memory, learning, long-term phonological representation and so on) considered only in their relation to the processing of the acoustic signal; and (6) the lack of exploration of the underlying mechanisms responsible for the auditory behaviours listed and how these relate to higher-level language, learning and communication (Bellis, 2003). PART 4: WHAT ARE THE CLINICAL IMPLICATIONS OF THE THEORETICAL ARGUMENTS SURROUNDING THE DEFINITIONS OF CAP AND CAPD? The most significant implication to be drawn from the above considerations is that, despite several decades of research and despite the effort of ASHA (1996), neither clinicians nor academics can agree upon a single definition of CAP or CAPD. Academically, this can only lead to one conclusion: the rational diagnosis and rehabilitation of CAPD cannot proceed until such definitions have been agreed upon. Clinically, however, such a conclusion is unworkable in the face of the rapidly growing demand in the community for CAPD (or CAPD-like) services. The only real option available to clinicians, therefore, is to continuously update their working definitions of CAP and CAPD by way of an iterative process based on clinically pragmatic approaches to the diagnosis and rehabilitation of CAPD in real subjects. To ensure that all the above controversies are adequately considered, this can only be done


within a multidisciplinary definition of CAP and CAPD. By defining CAP and CAPD from a multidisciplinary viewpoint, the clinician can allow himself or herself to assess CAP, and develop a plan to rehabilitate CAPD, beyond the strict limitations of academic definitions. We will elaborate on this by way of example, using the Bellis/Ferre model of CAPD as it is applied in the school classroom (Bellis, 2003). The Bellis/Ferre model of CAPD (Bellis, 2003) aligns itself with the ASHA (1996) definitions of CAP and CAPD. It argues that a schoolchild suspected of having CAPD should first be screened (by appropriate professionals) for peripheral hearing, classroom performance and behaviour, receptive and expressive language, intelligence (verbal and performance), and learning style. These screening results can then be used to estimate the child’s overall processing abilities across multiple modalities and environments. If they show repeated deficits in the auditory modality, then referral to an audiologist for a diagnostic CAPD assessment is appropriate. If they show no evidence of deficits in the auditory modality, or evidence of deficits across many modalities, then referral to another professional is more appropriate as the child may have a nonauditory deficit or a more global deficit (respectively). The immediate follow-on from such recommendations centre on basic clinical issues such as choosing which tests to use in the multidisciplinary assessment, determining if these tests actually assess the skills they purport to, determining the criteria for CAPD diagnosis, determining if a diagnosis of CAPD actually helps the individual being assessed, and choosing appropriate rehabilitation. While all of these issues are critical to the success or failure of CAPD management in the clinic, they will not be discussed here. Instead, they will be left for consideration in future review papers by this paper’s authors.

following conclusions. First, that there are many fundamental challenges facing clinicians and researchers trying to define CAP and CAPD. Second, that despite several decades of research, neither clinicians nor academics can agree upon a single definition of CAP or CAPD. Third, that the only practical option available to clinicians is to continuously update their working definitions of CAP and CAPD by way of an iterative process based on clinically pragmatic approaches to the diagnosis and rehabilitation of CAPD in real subjects. Finally, that this option is best achieved by following a multidisciplinary model of CAP and CAPD that assesses the child across multiple modalities with a conclusion of CAPD coming only after repeated demonstration of deficits in the auditory modality only. Arguably, three of the better multidisciplinary models of CAPD of recent times (in the audiology literature at least) are those of the ASHA Taskforce on Central Auditory Processing Consensus Development (ASHA, 1996) and the Concencus Conference on the Diagnosis of Auditory Processing Disorders in Schoolaged Children (Jerger & Musiek, 2000, 2002), Bellis and colleagues (Bellis, 1996, 1999, 2003), and Katz and colleagues (Katz et al., 1992; Medwetsky, 2002). Review of these models is recommended for any clinician interested in assessing CAP and rehabilitating CAPD in their clinic.

CONCLUSION It is hoped that by considering the above questions, the reader will have reached the

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