Introduction
Audiologists are faced daily with patients who report communication issues that they may attribute to hearing loss. These types of communication issues may include difficulty hearing in less than optimal listening situations, reliance on visual information to augment auditory information, a reduced appreciation of listening to music, and difficulty understanding speech when the speaker is unfamiliar. Patients report that these communication issues impact the quality of their lives- often the reason for seeking audiology services. For many of these patients, the results of an audiologic evaluation are consistent with a peripheral hearing loss, and the findings of the evaluation help to direct audiologic treatment to address these communication difficulties or to direct recommendations. However, some patients who present with concerns will demonstrate normal peripheral hearing acuity based on the results of standard audiometric testing. The assumption is often made that because the results of the audiogram are consistent with normal peripheral hearing acuity, the person's reported concerns are not validated. The purpose of this paper is to address the need to look beyond the audiogram to further validate the concerns raised by the patient.
Beyond the Audiogram
The concept of looking beyond the audiogram is not new. Karlin (1942) noted that tests of conventional auditory acuity had little value in predicting auditory behavior in more complex social situations. Despite this observation more than 50 years ago, audiologists often continue to hold the audiogram as the primary tool for addressing auditory behavior. The audiogram must certainly be a starting point. However, patients express frustration when told by the audiologist that communication issues do not exist based on the fact that the audiogram is "normal," as if to negate their presenting concerns. This is particularly problematic when the patient has a history of an etiology that may explain their communication deficits. In these cases, the role of auditory processing and its disorders must be explored to understand the patient's concerns and to develop a treatment plan to manage their communication difficulties.
As noted previously, auditory behaviors are complex. The auditory nervous system is designed to capitalize on redundancies in both the auditory nervous system and in language in order to utilize cues that enhance predictability in communication, particularly in less than optimal listening situations, such as when background noise is present. The auditory nervous system is also designed to process information quickly and flexibly, ranging from simple detection tasks to much more complex tasks such as listening to speech in a reverberant or noisy environment. When the auditory nervous system is compromised, the ability to listen effectively is negatively impacted. In order to assess this "real world" perspective, the auditory nervous system must be challenged or "taxed," making the auditory nervous system work beyond what is required for a detection task using pure-tone signals. Although the peripheral and central auditory nervous systems work together to process all types of auditory information, from simple pure tones to comprehending complex speech signals, in order to truly tax the auditory nervous system, pure tones are too simple to accomplish this task; thus the results on the audiogram tells very little about the more complex processing abilities of the auditory nervous system. Thus, there is a need to both assess the peripheral auditory system and the central auditory nervous system to effectively address patient complaints. However, the labels of "peripheral" and "central" are somewhat artificial differentiations, as both contribute to the behavioral or functional auditory deficits reported by the patient. Auditory processing skills may be considered to occur on a continuum of hearing and listening, and in some cases there is co-morbidity between peripheral hearing loss and deficits in auditory processing. The primary focus of this paper is on those patients with normal hearing acuity who report auditory deficits. However, the recent interest on aging of the auditory nervous system and the interaction between peripheral hearing loss and central auditory processing disorders ((C)APD) should not be overlooked (Gates, Anderson, Feeney, McCurry, & Larson, 2008).
Definitions
In order to address auditory processing disorder (APD) in adults, concepts that frame auditory processing must be defined. Bamiou, Musiek, & Luxon (2001) indicate that the central auditory nervous system (CANS) includes all the anatomical and processing mechanisms between the cochlear nuclei in the brainstem to the auditory cortex of the temporal region. They add that considerable activity in this region, including auditory memories stored in primary auditory cortex, Heschl's gyrus and Sylvian fissure as "auditory processing centers," and left planum temporale as controlling language processing, addresses the interaction between auditory and language processing. Central auditory processing may be defined as "...an umbrella term for all operations executed on peripheral auditory inputs, and which are required for the successful and timely generation of auditory precepts, their resolution, differentiation, and identification" (Phillips, 2002). From a functional perspective, roles of the central auditory nervous system would include handling rapid signals, gating or mediating the ear's response to incoming information, alerting to incoming information, communication between the two hemispheres of the brain, and coordinating or "teaming" between the two ears, with the recognition that the two ears are designed to work together as a unit. Phillips (1995) suggests that the role of the auditory nervous system is to "...establish a representation of the speech signal that is then available for perceptual or linguistic elaboration."
According to the ASHA (1996) statement, auditory processes are mechanisms responsible for the following behaviors: sound localization, auditory discrimination, temporal aspects of audition, including temporal resolution, masking, integration, ordering, auditory performance skills in the presence of competing acoustic signals, and auditory performance skills in the presence of degraded acoustic signals. Narrowly defined, auditory processing refers to the perceptual processing of auditory information in the central nervous system and the neurobiologic activity that underlies that processing and gives rise to electrophysiologic auditory potentials (ASHA, 2005). Auditory processing may be viewed as the efficiency and effectiveness with which the central auditory nervous system utilizes auditory information. The auditory nervous system is designed to capitalize on both the intrinsic redundancies built into the system itself and extrinsic redundancies built into the speech/language message. Intrinsic redundancies capitalize on the multiple representations built into the auditory nervous system, both peripherally and centrally. Examples of this include the multiple pathways that arise from the VIII nerve to carry signals to the cortex; some of these pathways may be considered superfluous or redundant but assure that auditory information can be transmitted from the ear to the brain. These multiple redundancies can be impacted by both the normal developmental pruning of the auditory nervous system or by disorders of the system, including demylinating diseases, injury, or neoplasm. Extrinsic redundancies are built into the speech signal, such as the ability to fill in information that is not heard by knowing the rules of linguistic construction in language, which enhance comprehension of speech. Extrinsic redundancy can be impacted by either environmental factors, such as the additional of competing noise signals, or by cognitive issues, such as Alzheimer's disease. Recent research in the neurosciences also supports the importance of neural plasticity and its role in the auditory nervous system. Neural plasticity can be referred to as "...the ability of the nervous system to undergo organizational changes in response to internal and external deviations" (Mendel, Danhauer, and Singh, 1999). This can be considered to result in altering of nerve cells to better conform to immediate environmental influences and often associated with behavioral changes, can be broken into three categories: developmental, compensatory, and learning related. All are important in terms of the assessment and management of auditory processing (Musiek & Berge, 1998).
In summary, the auditory nervous system is designed to be fast and flexible, to capitalize on its own redundancies, to support the development of other skills, such as language processing, to operate "automatically", and to utilize predictability and redundancy.
APD can be defined in a number of ways. Jerger and Musiek (2000) define APD as a deficit in the processing of information in the auditory modality. Moore (2007) describes an auditory processing disorder as a listening problem with hearing that cannot be explained by tests of peripheral auditory function. ASHA (2005) reports that auditory processing disorders may lead to or be associated with difficulties in higher order language, learning, and communication functions. Although auditory processing disorders may coexist with other disorders (e.g., attention deficit hyperactivity disorder [ADHD], language impairment, and learning disability), it is not the result of these other disorders (ASHA, 2005).
APD Characteristics
Auditory processing disorders are thought to be heterogeneous in nature, which reflects the complexity of the auditory nervous system, the interaction of auditory skills with other types of cognitive skills, and this complexity results in the ability to develop a consistent definition. A hallmark deficit often associated with APD is difficulty listening in the presence of background noise or reverberant environments; however, the deficit may actually reflect difficulties in one or more behaviors noted in the ASHA consensus statement (ASHA, 1996). In addition to these deficits, commonly reported issues in adults with APD include:
- Lack of music appreciation
- Difficulty following conversation on the telephone
- Difficulty following directions
- Difficulty following long conversations
- Difficulty taking notes
- Difficulty learning a foreign language or technical information where language is novel or unfamiliar
- Social issues—difficulty "reading" others/pragmatic communication issues
- Spelling, reading, writing issues
- Organizational problems (Baran, 1998)
Etiology, Prevalence and Incidence
Etiologies of APD in adults can range from trauma and tumors to auditory deprivation and periods of anoxia (Schminky & Baran, 1999). For some patients, the APD has been a long-standing issue, however, the impact of the disorder on their life has emerged or they have developed insight into the type of disorder and how to seek assessment and management. As noted previously in this paper, changes in the auditory nervous system resulting in auditory processing deficits are evident for some individuals as the auditory nervous system ages. The auditory nervous system appears to become less flexible with age. A flexible auditory system is designed to rise to the occasion of handling the extreme types of environments encountered by the individual (e.g. listening to whispered speech and hearing speech presented at a loud level in the same environment) or being able to address the , dynamic changes in the environment, such as listening speech in a less than optimal listening environment. Bregman (1980) supports this in his statement that "...the understanding of speech in daily life undergoes gradual change with increasing age because of a combination of peripheral and central alterations significantly affect the understanding of speech that is heard under less-than-optimal conditions." This observation is supported by recent research by Gates and colleagues (2008).
Pinpointing the incidence and prevalence of APD in adults is complex, as there is no common definition of the disorder and there is no agreed upon test battery. In addition, it is impossible to know how many patients have presented with APD only to fail to be identified by the standard audiologic assessment. An older estimate by Saunders & Haggard (1989) suggests that in the population of adult patients with normal peripheral hearing acuity across all age groups, the prevalence of APD is 5%. This is a significant underestimate in specific populations, such as those that experience APD following a traumatic brain injury. Bergemalm and Lyxell (2005) reported that 58% of the population of patients with closed-head injuries that are considered "well recovered" report having long-term APD issues.
Although the history of auditory processing assessment in adults has been related to "site of lesion" assessment, there is a need to look at functional issues in auditory processing. Interest in APD in adults has been longstanding (Jerger, Jerger, Oliver, & Pirozzollo, 1989; Jerger, Chmiel, Wilson, & Luchi, 1995); however, there is renewed interest with the return of veterans from Iraq and Afghanistan. In the past, interest in veterans was often related to noise-induced peripheral hearing from blast injury, but recent focus has been shifted to the population of soldiers returning who have experienced traumatic brain injuries with concomitant APD (Cohen et al, 2002; Hoge et al., 2008). Head injury has been described as the signature injury of the wars in Iraq and Afghanistan. Auditory symptoms most often associated with head injury or post concussive syndrome (PCS) are tinnitus, peripheral hearing loss, sound tolerance issues or increased sensitivity to sound also known as hyperacusis, and difficulty processing auditory information, often in areas of timing and hearing in less-than-optimal environments (Peterson, 2000; Barvarian et al., 1999).
Assessment
Audiologists are uniquely qualified to assess auditory processing issues in adults, due to the ability to control the listening environment, control the stimulus presentation and parameters, and having an expertise in hearing and listening. Hearing can be defined as perception or detection of sound, what audiologists may define as hearing acuity. Listening, as it is used in this paper, refers to the comprehension of auditory information, whether speech or environmental sounds. If viewed in terms of Erber's hierarchy (Erber, 1977) , hearing would be thought of as a "detection" task that provided a foundation for other types of listening skills, such as discrimination, identification, and comprehension of speech. Audiologists have the ability to address the continuum of hearing and listening skills.
Assessment of APD in adults begins with a comprehensive case history. Just as with a pediatric population, areas of interdisciplinary input should be considered for adults and will incorporate investigation of cognitive impairment, speech/language impairment, psychological/psychiatric issues, communication and vocational demands, and potential pending litigation. In addition to the standard areas, the case history obtained by the audiologist should address questions regarding history of childhood learning disabilities, otitis media, family history of communication disorders and/or learning disabilities, presence of tinnitus/sound sensitivities/balance issues, and reports of neurological "soft sign." Additional case history areas should include questions related to current medications (or those taken at the onset of symptoms), history of illness and injury, specific information about head injury and recovery, qualitative issues with hearing (changes in music perception, perception of speech), vestibular and/or visual issues, and questions regarding general communication including the question of how effective and how satisfied is the patient as a communicator/listener?
Tools to assess the patient's perceived communication difficulty and impact of communication issues on their life should be administered in an attempt to validate difficulties, using an authentic assessment approach. This approach helps to verify and pinpoint the issues that the patient is experiencing and to give the audiologist another type of assessment technique beyond the audiogram. This can be accomplished by using a diary or an outcome measure like the Client Oriented Scale of Improvement (COSI) (Dillon, James, & Ginnis, 1997) to document baseline concerns. This also helps to get a snapshot of the difficulties that the patient may be experiencing in "real world" situations that are less than optimal. Again, the audiologist should recall that auditory performance in quiet is a poor predictor of performance in the presence of noise or in more complex listening situations.
A comprehensive audiological assessment of peripheral hearing is also a critical component of this process. Included in this process should be an audiologic evaluation, which includes a measure of speech-in-noise performance, using a testing like the Quick-SIN or WIN, acoustic reflexes, and otoacoustic emissions (OAE). If the patient reports tinnitus and/or sound sensitivities, these should also be addressed. Administration of a tinnitus questionnaire, such as the Tinnitus Reaction Questionnaire (TRQ), should be completed to determine the impact of tinnitus and/or sound tolerance on the patient's life. Assessing additional frequencies as part of the conventional pure-tone assessment may also provide insight into reported tinnitus and help to direct treatment and management of this aspect of auditory function. Attempts to quantify tinnitus can be conducted using pitch matching, loudness matching, minimum masking level, and a measurement of residual inhibition. If sound tolerance issues are suspected, measurement of loudness discomfort levels (LDLs) may provide insight on this issue.
There is no standard test battery for assessment of auditory processing skills in adults. Thus, the audiologist working with this population will need to make decisions regarding which test battery may most effectively address the needs of their patients. All auditory processing assessments are based on the philosophy of taxing the auditory nervous system, whether behaviorally or electrophysiologically. Taxing the auditory nervous system with behavioral assessment involves reducing the redundancy of the stimulus to force the auditory nervous system to work harder. There is a need to consider varying linguistic loading in the test battery in order to address criticisms and concerns that auditory processing testing only taxes language processing skills. This can be done by incorporating stimuli that have varying stimulus lengths and complexity (e.g. consonant-vowel combinations, words, sentences) and may use more basic stimuli, such as tonal stimuli.
One of the assumptions that audiologists can make is that the listener has normal peripheral hearing acuity. This assumption is made for the scope of this paper, although protocols for addressing auditory processing skills in adults with peripheral hearing loss have been considered. A test battery approach, such as the Test for Auditory Processing Disorders in Adolescents and Adults (SCAN-A) (Keith, 1994) provides the ability to address a number of skills in a time effective manner. Tests from the Department of Veterans Affairs compact disc Tonal and Speech Materials for Auditory Perceptual Assessment (VA-CD) provide many options from which to select. The Multiple Auditory Processing Assessment (MAPA) (Domitz & Schow, 2000) also provides a test battery approach from which an audiologist could build their testing protocol. There are many other options available from which the audiologist can choose to address behavioral aspects of auditory processing in adults.
A controversy related to APD is the role electrophysiologic assessment plays in the diagnostic test battery. Some approaches of auditory processing assessment suggest that the only way to confirm an auditory processing disorder is to include electrophysiologic assessment, which may reflect levels of dysfunction (Jerger & Musiek, 2000). This is not a criticalcomponent of other approaches. Regardless, electrophysiologic testing may provide a unique measure of auditory nervous system function and improvement based on treatment; it also provides a method of isolating the auditory nervous system while minimizing linguistic influences. This approach would include basic tests such as auditory brainstem response (ABR) and distortion product otoacoustic emissions, both of which have a higher incidence of abnormal results in those with traumatic head injury who report listening issues than for those with normal pure tone audiograms without that history (Bergemalm & Lyxell, 2005; Hall, 2007). Some audiologists also incorporate middle latency and late evoked potentials as part of the auditory processing battery (Jerger & Musiek, 2000), however their diagnostic significance with this population is variable and thought to be influenced by other factors, such as formal musical training (Musacchia, Strait, and Kraus, 2008).. Cognitive evoked potentials such as P-300 and mismatched negativity also provide insight into auditory function.
Management
A management myth is that since there is no cure for APD in adults, there is no value in assessing these skills. Current research in long-term potentiation suggests that the auditory nervous system is plastic for a long time period, provided the stimulation is appropriate for making changes in the system and suggesting a framework for intervention (Whitelaw and Yuskow, 2005). The results of assessment should provide a guide to management and treatment. The hierarchy for treatment is based on presenting concerns and if the patient presents with sound tolerance issues, these should be addressed first. In addition, issues with tinnitus and hearing loss must be addressed prior to pursing issues of auditory processing.
Treatment may be considered as a three pronged approach: environmental modifications, compensatory strategies, and direct treatment. The goal is not a cure for the auditory processing disorder, but rather to develop strategies that minimize disability and maximize communication. As noted by Masters, Stecker, and Katz (1998), most adults with APD will live with these disorders throughout their lives. Therefore, a life-long approach with emphasis on self-management is critical. Environmental modifications may include addressing room acoustics and providing options for sound-enhancement technology. Personal FM systems have a long history of use with this population, and an ear-level device designed for people with normal hearing, such as the Phonak EduLink, may provide significant benefit to the patient (Stach, Loiselle, Jerger, Mintz, & Taylor, 1987). Audiologists may assist in developing compensatory skills that maximize communication in both the workplace and in social situations.
One of the most exciting areas of adult patients with APD is treatment. Recent research in brain plasticity has been incorporated into auditory training programs such as Listening and Communication Enhancement (LACE) (Sweetow & Henderson Sabes, 2006) and the Auditory Rehabilitation for Interaural Symmetry (ARIA) (Moncrieff & Wertz, 2008), which address specific auditory processing skills. LACE addresses listening in noise, and ARIA addresses dichotic listening skills, both of which are materials designed for adults. In addition, some auditory training programs have been linked to electrophysiologic measures that can be utilized to provide unique measures of improvement, such as Jirsa's (2002) approach to using P-300 measures and the BioMAP procedure (Johnson, Nicol, & Kraus, 2005). .
Summary
In summary, APD in adults can be an exciting and interesting population with which to work as an audiologist. This population requires an interdisciplinary perspective and capitalizes on a combination of assessment and treatment skills that define the profession of audiology. Currently, interest in this population has continued to grow and provides unique opportunities and challenges to the audiologist.
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