Understanding Central Auditory Processing Disorders

Through the mid 1980's the evolution of Central Auditory Processing Disorder (CAPD) has been that anything originating as an auditory stimulus was called, "Auditory processing." Today the main heading is auditory processing with the sub categories of central auditory processing and language processing. There is still so much to be learned about how the brain processes information that it is a confusing and not a well understood area of study. Many just refer to the main heading of auditory processing disorder (APD) or refer only to language processing.

Definition:

To define central auditory processing disorder we can simply take the definition as stated by the American Speech-Language Hearing Association, in 1996 as, "An observed deficiency in one or more of a group of mechanisms and processes related to a variety of auditory behaviors." This definition left me with questions. I felt as if I was driving down my mountain in the middle of a dense fog searching for the road. The definition is vague to me. I found a definition that I felt explained the disorder better in the book, "Screening for Central Auditory Processing Difficulties," by Dorothy A. Kelly. Her definition is, "Difficulty in processing or interpreting verbal and /or nonverbal auditory stimuli, usually in the absence of a peripheral hearing loss." Central Auditory Processing Disorders/Difficulties is about how the ears and brain work together in taking in auditory information. Someone with normal hearing can listen to an acoustic stimulus and fill in distorted segments using their language knowledge. So, linguistic and acoustic information combine to attach meaning to the signal. Something happens to the auditory signal once within the auditory system. The central auditory system occurs after the ear receives the signal and starts to transfer the stimulus from the inner ear to the cortex via the eighth cranial nerve. (see fig. 3 on page 5) This nerve is considered the start of the central auditory nervous system. The eighth cranial or auditory nerve runs through Heschl's gyrus in the cortex of the brain. There can be a distinction drawn between language processing and central auditory processing. Language processing is mainly speaking of structures in the cortex that are above the brainstem, beginning post Heschl's gyrus.Auditory processing is from the eighth cranial nerve to Heschl's gyrus. To get a better understanding of this you can look at the book, (The Source for Processing Disorders) by Gail J. Richard pg. 39.
For language processing and its functions the main focus would be on the left temporal lobe.

Symptoms or Characteristics of CAPD:

*Difficulties with localizing a sound source. *Understanding the meaning of environmental sounds.
*Discriminating among words or sounds within words.
*Difficulty listening when there is background noise (auditory figure-ground.)
*Larger majority of people with CAPD disorder/difficulty are male.
*Being able to reproduce rhythm, pitch and melody of music may be a problem.
*"Combining syllables to form words and sentences." (Barr,1976)
*Many requests for verbal repeats or saying "huh" or "what" often.
*Normal tone hearing result, may also have a hearing loss(acuity) problem along with CAPD which would add to the difficulties they face with CAPD.
* "Difficulty with following oral directions and inconsistent response to auditory stimuli." (Gail J. Richard, 2001)
*Fatigues easily with auditory tasks and has a short auditory attention span.
*Both long and short term memory may be affected.
*May daydream often, or not seem to be paying attention to speaker even when looking at them.
*May do poorly in reading, spelling and phonics and have mild speech impairments.
*Display disruptive behaviors such as being easily distracted, impulsive and frustrated.
*History of ear infections.
*May have problems processing acoustic stimuli at the typical rate of verbal production. Therefore, they have a hard time with perception according to the rate and timing of incoming stimuli.

Central auditory processing disorder is looked at slightly differently depending on whether you are a speech-language pathologist or an audiologist. Meaning that speech language pathologists use a processing model called, "Top down" or the "Gestalt" approach. This view looks at the disorder with the main emphasis on the whole because we quickly map on meanings, or otherwise called, "Fast mapping ." We then gain or pull details to acoustically fine tune our processing. We are said to focus on the language and semantic knowledge whereas the audiologist looks at the processing model in a "Bottom up" fashion. Their emphasis is on the acoustic output and production as the critical component. Each individual phoneme is attained to continue onto the whole. Therefore, the focus of an audiologist is on the knowledge of the acoustic signal.

I learned from the book, "The Source for Processing Disorders, auditory, language," by Gail J. Richard c.2001 that the central auditory processing system (CAP), is an early part of a continuum model which is a part of the central nervous system(CANS). This processing accurately transfers a signal through the brainstem to the cortex. If the signal reaches the cortex undistorted, then the central auditory processing abilities are functioning within normal ranges. However, she feels that language processing occurs superior of the Heschl's gyrus once the language knowledge is used to attach meaning to a signal.

Gail J. Richard writes that the layers of interpretation are much like the layers of an onion and is processed on the signal during language processing. Richard believes that if there is a lack of auditory discrimination that it is probably a breakdown in the overlap region of the transition area of Heschl's gyrus in the cortex. She also believes one needs to have some knowledge of auditory neurology to understand the complex differences between central auditory processing and language processing.

The CAP pathways begin at the eighth cranial nerve and end within the cortex at Heschl's gyrus. There are also several crossover points along the CANS that add to neural redundancy or replications. The path of an acoustic stimuli will travel up the eighth cranial nerve after being stimulated by the organ of corti, exciting the fibers of the eighth nerve, these fibers are called spiral ganglion which end at the cochlear nuclei after the brainstem. From here the signal moves on the same or ipsilateral side through the cochlear nuclei and on to the superior olivary complex of the pons (reticular formation.)

If you look to, (The Source for Processing Disorders) by Gail J. Richard pg. 28 and (The Physiology Coloring Book) by Kapit Wynn pg. 97 you can gain a clearer understanding of this.

It is here at the superior olivary complex that the signal begins to have a bilateral progression and crossover happens. So, at the reticular formation there is ipsilateral and contralateral (Opposite side) routing of the incoming signal. The contralateral sides are the dominant pathways for the signal. It's here that sensory screening happens and it's determined which stimuli to send on up for further processing in the upper cortex. It also is the arousal area and alerts the brain of incoming signals. The signal continues on to the inferior colliculi and at this point there is another crossover. Some minor processing occurs here. Then it's on up to the medial geniculate bodies just before entering the cortex. Now the pathways move on to the Heschel's gyrus in the cortex of the temporal lobes. Sound in one ear will travel up and to both auditory cortices due to the duplication along the pathways. The information can be processed bilaterally. This duplication of the auditory signal is important in the assessment of central auditory processing. (See fig. 2 above.) Just when you think we are done there is more duplication that happens via the corpus callosum, which communicates information from one hemisphere of the brain to the other. The left hemisphere is the dominant side for language usually. The external and internal stimuli pathways and the duplications are important in understanding CAPD. Both ears will hear something at the same time (external) and the internal duplication happens at the crossovers to make sure the message is relayed bilaterally and multiple times on it's way to the cortex. Again, another look at the book, (The Souce for Processing Disorders) by Gail J. Richard pg 28 is recomended to better understand the crossover pathways.

The reticular activating system also known as the 1st functional unit, has several hats. The reticular activating system awakens the brain to keep it alert, it is the director of neural traffic, it decides which stimuli is needed and sends it up to the cortex and determines what to hold off. This is where there is a problem in Attention Deficit Disorder. The reticular formation isn't able to carry out it's job to effectively sort through the incoming stimuli and just throws up it's arms and says, "oh gee everybody up" The reticular formation is unable to sort and hold back certain unnecessary stimuli. You then have an individual who may be overwhelmed and confused by the abundant information that they are receiving.

The 2nd functional unit houses the parietal(tactile), occipital(visual), and temporal(auditory) lobes. It handles storage, organization, and coding. This is where we find Heschl's gyrus, the primary 2nd functional unit and Wernicke's in the secondary section of the 2nd functional unit. The angular gyrus lies within the tertiary area of the 2nd functional unit.

The 3rd functional unit is made up of the frontal lobes(Broca) and this area maintains active responses. It also handles the managing and planning of a person's behavior according to the knowledge and perceptions that are relayed to it by the 2nd functional unit.

In the book, (The Source for Processing Disorders -auditory -language,) she lays out a processing continuum model, as seen below, to show where the CAP occurs and where language processing takes over completely. The complex part is where we have a mixture of both, which is called the transition area where there is auditory and language processing transference. This takes place at Heschl's gyrus and comes before Wernicke's which lies in the secondary zone, falling into the language processing area. The entire model is part of the central nervous system. She goes on to say that the model is useful in looking at disorders in a neurologically objective way.

Assessment and Testing:

In assessing CAPD we need to look at what each area's main function is and remember that the brain is built with many redundancies so if there is a malfunction in one area, another area can gradually, in some cases, compensate for the loss. This is why auditory testing needs to degrade the auditory signal so that the normal abilities are not able to fill in. The pathways need to be challenged enough to compromise the normal function of the redundancies. Audiologists have been using the processing continuum model of assessment with instruments to test how the central nervous system (CNS) is functioning. They base their intervention according to the systems capabilities whereas she states, speech-language pathologists use evaluation subtests. When a child does poorly on a test then we drill them on that skill set. We could be drilling them on a skill they don't have the language ability or neurological maturation for yet for the task. She feels that this method fails to see the reason why the child performed poorly in that task. She states that our method is a "hit and miss" and we could possibly spend less time aiming if we were to look more at the neurological function with behavior as a second focus. First see what the neurological weaknesses are, then drill to see how those neurological weaknesses play out in responses. Testing prerequisites are that the child has normal/near normal hearing, and average receptive/expressive language skills, with normal/near normal intelligence levels. Usually the age of testing is about five years of age. There are exceptions of course. But to do the tests for an earlier age are much more difficult. Myelinization happens much later so that is why the tests are better done at a later age. There is even evidence that the maturation process for being able to adequately hear sounds with background noise can be as late as 13 to 15 years old.

Parent checklists look for nine or more behaviors noted to have a child at risk for CAPD There is a blending of audiologist and speech-language pathologists form of testing called SCAPD's Quick Screening Checklist. It observes responses to both nonverbal and verbal auditory stimuli. Referrals are done by checklists. One is The Children's Auditory Processing Performance Scale (Smoski, Brundt, and Tannahil, 1992.) Another is The Kindergarten Auditory Screening Test (Katz, 1971). For preschool, A Screening Test for Central Auditory Processing Disorders (Keith, 1986.) The more diagnostic evaluation would be done by an audiologist. Audiologist testing would be Electrophysiological tests called aural reflex test that measures ipsilateral vs. contralateral reflexes. Another one is Auditory Brainstem Response (ABR) it measures the elicit response of the early brainstem. Monotic, Dichotic and Binaural Interaction tests. An example of one is the Binaural fusion - the use of low and high frequency components of the same word are separated and presented at the same time. Low frequency to the left ear and high to the right ear with the results being lower for the child with CAPD. CAPD is said by Chermak and Musiek (1997) to affect 3% - 5% of school age children. These children are seen first by the speech-language pathologist and then on to an audiologist for further testing.

Prognosis:

At this time there is no known medical treatment available for CAPD, however there are behavioral treatments. Areas to work on are helping the individual with specific auditory skills and compensation strategies.

Types of Therapy:

When the brain is stimulated it will adapt. You can't repair an auditory malfunction but you can compensate or modify the environment for better learning outcomes. You can first try to enhance the signal by amplifying it. Try to keep the background noise down and allow optimal seating is a good strategy for better sound and visual stimuli. Simplify instructions and shorten them if possible. Gain the child's attention first before you speak with them and use their peers to share notes and assignments. Ask them more questions as you go to check for comprehension. Use multiple methods of relaying information such as verbal as well as visual props. Use good tone, speed and pitch to keep their attention and clarity. Highlight important information by repeating or having visuals as well as the verbal.

Always be open to new ideas and research because the brain is complex and we are only at the beginning of understanding. I found that I had so much more information to include. This is such an interesting area of research. In the next few years I hope to see more information available on this complex disorder. With our increased knowledge there should no longer be children out there that repeatedly hear the words, "you're not trying hard enough," and "I wish you would stop being lazy," which leaves a child that has a difficulty processing stimuli confused, sad or even angry. They are trying, they don't have the same tools as the rest of us. We need to help them work with what they do have. With encouragement these children have a much brighter future ahead of them.

Bibliography:

Gaulin, Cindy. 2000. Language Processing Problems: A Guide For Parents And Teachers.

USA: Xlibris Corporation. www.Xlibris.com.

Kapit, Wynn, & Macey, Robert & Meisami, Esmail. 1987. The Physiology Coloring Book.

Menlo Park, CA: HarperCollins College Publishers.

Kelly, Dorothy A. 2001. Screening for Central Auditory Processing Difficulties.

Oceanside, CA: Academic Communication Associates, Inc.

Richard, Gail J. 2001. The Source for Processing Disorders -auditory -language.

East Moline, IL: LinguiSystems, Inc.

Schwartz, Susan J. Internet 3/2007. NYU Child Study Center: Giving Children Back Their Childhood.

http://www.aboutourkids.org/aboutour/articles/capd.html