Auditory processing is ''what the brain does with what the ear hears'' (Katz, 1992). Although the ear is responsible for picking up sound and directing it to our auditory system, auditory processing allows us to differentiate and interpret this signal. Auditory processing is responsible for:
Sound localization and lateralization
Auditory pattern recognition
Temporal aspects of audition
Auditory performance with competing acoustic signals
Auditory performance decrements with degraded acoustic signals.
Auditory processing disorder (APD) refers to a deficiency in one or more of the auditory behaviors listed above. An individual with APD is unable to attend to, discriminate, recognize, or understand auditory information and therefore, has trouble making sense out of what is heard. APD does not describe a single deficit, but encompasses a variety of functions. It is important to point out that individuals with APD may have normal hearing sensitivity, i.e. sounds are loud enough to hear, but the individuals are unable to successfully use auditory information.
Over the years, APD has been the center of controversy as some professionals do not recognize APD as a unique functional deficit. This controversy has revolved around three basic issues.
First, other types of disorders, such as attention deficit hyperactivity disorder (ADHD), language impairment, learning disability, autistic spectrum disorders, and reduced intellectual function, may produce similar behaviors, so auditory deficits have sometimes been deemed a function of these deficits. Second, audiologic assessment procedures do not adequately differentiate APD from other disorders. Finally, other processes and functions, such as motivation, attention, cooperation, and understanding, may confound test interpretation and lead to misdiagnosis of a related disorder. Skepticism, however, is diminishing. The development of reliable and valid behavioral and electrophysiological assessment procedures has legitimized APD as a recognized auditory deficit.
Individuals with APD form an extremely heterogeneous group and do not present with a single, identifying profile. They may exhibit one or more of the following 15 signs:
- They appear hearing impaired, but in fact have normal hearing sensitivity.
- They say ''huh'' or ''what'' a lot, and often ask the speaker to repeat.
- They experience difficulties hearing in the presence of noise.
- They respond inconsistently or inappropriately to auditory information.
- They take longer to respond when participating in oral communication situations.
- They have trouble following oral directions or instructions.
- They experience difficulties with phonics and speech sound discrimination.
- They are often unable to locate where sounds are coming from.
- They cannot remember information presented auditorally.
- They are unable to concentrate, or stay on task, and are easily distracted.
- They have difficulty directing, sustaining, or dividing attention.
- They have chronic otitis media, or a history of chronic otitis media.
- They exhibit poor reading and spelling skills.
- They are poorly organized.
- They are unable to learn songs or nursery rhymes, and have poor music and singing skills.
Prevalence of APD is difficult to determine but the identification of APD generally occurs in two distinct populations: children and the elderly. For children, a prevalence of 2-3% has been estimated, although some professionals believe these figures underestimate the true extent of the disorder since many children are undiagnosed or misdiagnosed. For older adults, the estimate is 10-20%; for individuals over 60 years of age the estimated prevalence is 70%. APD has often been linked or found to coexist with other disorders or deficits, such as learning disability, attention deficit hyperactivity disorder (ADHD), dyslexia, speech disorder, and language impairment. However, it has not been determined if one these disorders is causal to the other.
Individuals with known neurological disorders, insults, or diseases, such as aphasia, Alzheimer's disease, traumatic brain injury, stroke, tumor, epilepsy and multiple sclerosis often exhibit APD. However, in the majority of cases, APD occurs in the absence of any documented neuropathological condition. Because of the similarity in the profiles of documented and undocumented APD, it is concluded that APD is associated with some sort of dysfunction within the central auditory nervous system (CANS).
The source of CANS dysfunction has been explored, and causes of APD have been suggested. For children, APD has been related to 3 general factors. First, some children may be genetically predisposed for APD, since a parent often reports similar auditory difficulties as a child. Second, some children may experience delays in the neuromaturation of the CANS, which reaches adult values at approximately 12-13 years of age. For this reason, it is suggested that children not be evaluated for APD until at least 7 years of age. Finally, APD may be the result of auditory deprivation. The most common source of deprivation is hearing loss related to chronic otitis media, which results in fluctuating conductive hearing loss and, therefore, inconsistent exposure to auditory stimuli. For older adults and the elderly, APD is probably a natural part of the aging process. Structural degeneration has been observed in the inner ear and CANS pathways. Neural degeneration has been found in the auditory nerve, brainstem, and cortex. It is speculated that this degeneration causes the functional deficits associated with APD.
If APD is suspected, the individual should undergo a complete audiological evaluation to ensure normal hearing sensitivity, or to determine the type and degree of hearing loss. APD behaviors and presentations are often similar to those of peripheral hearing loss.
The evaluation should include air and bone conduction thresholds, Speech Reception Threshold (SRT), word recognition in quiet and noise and acoustic immittance with ipsilateral and contralateral acoustic reflex testing. Importantly, many APD test instruments have only been normed on normal hearing listeners and must be interpreted cautiously in the presence of hearing loss!
Unfortunately, there is no standard APD Battery. Often, the clinic or the professionals create a test battery that best meets their needs, based on clinical and professional experience, and the needs of their patients and clients. APD batteries should include behavioral and electrophysiological tests to ensure the assessment of peripheral and central auditory processes and pathways.
Behavioral tests should include verbal and nonverbal stimuli, and linguistically loaded and non-linguistically loaded tests. Tests should be age-appropriate and the duration of the test session should be appropriate for the individual's attention span and motivation. Electrophysiological tests measure the brain's response to sound and assess brainstem processes as well as responses of the auditory cortex. Electrophysiological tests measure the brain's response to sound and provide information regarding the neuromaturation of the central auditory pathways. To perform these tests, electrodes are placed on the earlobes and head of the listener and an auditory stimulus in the form of a click or tone pip is presented. Tests of the lower brain, or brainstem, may be performed while the listener is relaxed or sleeping. Tests of the auditory cortex require the listener's attention.
I believe at least one test should be included from each of the categories listed below:
Dichotic Tests. Dichotic tests present a different auditory stimulus to each ear simultaneously. The listener must either repeat everything heard, assessing binaural integration, or only the information presented to one ear while ignoring the information presented to the opposite ear, assessing binaural separation.
Monaural Low-Redundancy Speech (MLRS) Tests. MLRS tests assess auditory closure, the listener's ability to fill in and recognize an acoustic signal when parts of the signal are missing or modified by changing frequency, temporal or intensity characteristics. The stimuli are presented to one ear at a time and the listener must repeat the correct word or sentence that has been modified. Distortion occurs because high frequency information has been filtered out of the signal, acoustic information has been speeded up, or acoustic information has been compressed and reverberation added.
Temporal Processing Tests. Temporal processing tests measure the listener's ability to recognize the order or pattern of nonverbal auditory signals. Tones are presented to each ear using different time or pitch patterns, and the listener must either ''hum'' or verbally describe the pattern.
- Binaural Interaction Tests. Binaural interaction tests assess binaural fusion, the listener's ability to take incomplete information presented to each ear and fuse the information into an understandable signal. Two different parts of an acoustical signal or presented simultaneously to each ear and the listener must repeat the complete signal. Information presented to either ear alone is unrecognizable, and understanding depends on the brain's ability to integrate the information.
Management of APD:
If an APD is diagnosed, it is appropriate to refer the individual to other professionals to evaluate for related disabilities. A speech-language pathologist can determine the presence of speech or language deficits or delays, and may be the professional most involved in the management of the person with APD. A psychologist can assess cognitive function, and learn about the social and emotional impact of the disorder. With this additional information, a treatment plan addressing the unique deficits of the individual can be outlined.
An interactive intervention and management plan based on the unique listener profile should be developed to address the individual's strengths and weaknesses. Therapy may be recommended to improve listening skills, decrease distractibility or inattention, and improve auditory memory and listening comprehension. Specific deficit areas, such as poor reading and spelling skills, may be addressed. Coping strategies, such as using a tape recorder or note-taker, writing lists, or using calendars/day planners, may be recommended to ensure the correct reception of auditory information. Environmental modifications, such as preferential seating and decreasing background noise may be discussed. Communication aids in the form of assistive listening devices, sound field systems and/or FM systems may be recommended.
American Speech-Language-Hearing Association (1996). Central auditory processing: Current status of research and implications for clinical practice. American Journal of Audiology, 5 (2), 41-54.
Baran, J. (1996). Audiologic evaluation and management of adults with auditory processing disorders. Seminars in Speech-Language, 17 (3), 233-244.
Bellis, T. (2002). When the brain cant hear: Unraveling the mystery of auditory processing disorder. New York, NY: Simon & Schuster.
Bellis, T. & Ferre, J. (1996). Assessment and management of central auditory processing disorders in children. Educational Audiology Monograph, 4, 23-27.
Cacace, A. & McFarland, D. (1998). Central auditory processing disorder in school-aged children: A critical review. Journal of Speech, Language, and Hearing Research, 41, 355-373.
Chermak, G. & Musiek, F. (1997). Central auditory processing disorders: New perspectives. San Diego: Singular Publishing Group.
Hall, J., Baer, J., Byrn, A., Wurm, F., Henry, M., Wilson, D. & Prentice, C. (1993). Audiologic assessment and management of central auditory processing disorder (CAPD). Seminars in Hearing, 14 (3), 254-264.
Jerger, J. (1998). Controversial issues in central auditory processing disorders. Seminars in Hearing, 19, 393-398.
Jerger, J. & Musiek, F. (2002). On the diagnosis of auditory processing disorder. Audiology Today, 14 (2), 19-21.
Jerger, J. & Musiek, F. (2000). Report of the Consensus Conference on the Diagnosis of Auditory Processing Disorders in School-Aged Children. Journal of the American Academy of Audiology, 11 (9), 467-474.
Katz, J., Stecker, N. & Henderson, D. (Eds.) (1992). Central auditory processing: A transdisciplinary view. St. Louis, MO: Mosby-Year Book.
Keith, R. (1996). Understanding central auditory processing disorders. The Hearing Journal, 49 (11), 20-28.
Kraus, N. & McGee, T. (1994). Mismatch negativity in the assessment of central auditory function. American Journal of Audiology, 3 (2), 39-51.
Rosenberg, G. (1999). Contemporary research and viewpoints on CAP and CAPD. Educational Audiology Review, 16 (2), 39-40.
Wynne, M. & Mehta, Z. (2001). CAPD in older adults: What is the role of central auditory function in the evaluation and selection of hearing aids in older adults? Advance for Speech-Language Pathologists & Audiologists, 11 (39), 10.
This article was originally published on HealthyHearing.com on 3/15/04 and is reprinted here, based on popular request.