Background

Evaluation of the central auditory nervous system (CANS) is essential in order to obtain information on its anatomical and functional integrity. Both, children and adults may suffer from central auditory processing disorders (CAPD). This fact has been underestimated but as research in this field progresses, it shows that specific mental disorders may be the outcome of a CAPD or that CAPD can co-exist with a neurological or mental disorder [1].

Assessment of the CANS begun at the mid-1950s with the confirmation by Bocca and his colleagues [2] that CANS disorders do exist and that there are sensitive tests to reveal them. However, at that time acceptance of the new diagnostic methods by the audiologists, who were the first to be interested in this field was limited. This can be attributed to the slow acceptance of each new method before it is fully validated. Better understanding of the anatomy and physiology of the CANS was gained by advances concerning the presence and physiology of neurotransmitters and the accumulation of data on the psychoacoustic and electrophysiologic tests [3]. As a result audiologists started applying the new diagnostic tests more often and appreciated their contribution. Other medical specialties became aware and interested in the disorders of the CANS. These were mainly psychiatry and neurology. The assessment of the CANS is also of great value concerning neuropsychology and special education [4–6].

Anatomy and physiology of the CANS

Clinical evaluation of central auditory function requires understanding of the anatomy and physiology of the CANS and appreciation of its complexity. The CANS extends from the anterior and posterior cochlear nuclei which are situated on the surface of the inferior cerebellar peduncle to the auditory cortex. In between important structures through which nerve fibers pass are: the trapezoid body, the lateral lemniscus, the inferior colliculus, the medial geniculate body and the acoustic radiation of the internal capsule. The auditory cortex includes the gyrus of Heschl on the upper surface of the superior temporal gyrus, the planum temporale and the Silvian fissure.

It is essential to point out that nerve impulses from each ear proceed along auditory pathways on both sides of the brainstem. Both ipsilateral and contralateral pathways are important in ensuring interchange of auditory information. The contralateral pathway exhibits dominance as opposed to the ipsilateral one [7]. Thirty thousand afferent auditory nerve fibers with different range of frequency response are responsible for conveying auditory information to the cortex [8]. Many components of the stimulus are analyzed separately. There is an increasing complexity of the whole process in the auditory cortex. One should keep in mind that, understanding of the exact way of processing the auditory information at the level of the auditory cortex, is still incomplete. It is in this understanding that Phychoacoustics helps as it is the science concerned with the evaluation of the sensation of hearing as an outcome of the sound or speech stimulus.

Components of central auditory processing

Central auditory processing occurs prior to language comprehension [9]. It consists firstly of auditory discrimination, which is responsible for the ability to group sounds according to how similarly or differently they are heard. Auditory memory is the component responsible for storing and recalling auditory information. Auditory perception concerns the reception and understanding of sounds and words. It plays a significant part in reading skills, managing verbal information, communication and social relationships. Auditory-vocal association consists of the interaction between what is heard and verbal response. Auditory synthesis is responsible for combining sounds or syllables to formulate comprehensible patterns (words) and de-combining words into separate sounds. Auditory-vocal automaticity is the ability to predict how future linguistic events will be heard by utilizing past experience. Auditory figure-ground plays a role in diminishing sounds which are not important while focusing on others [10]. It is due to this component that someone can listen to another person talking in a railway station, where a lot of environmental noise exists.

Material and methods

The medline research revealed 564 papers when using the keywords 'auditory deficits' and 'mental disorders'. 79 papers were referring specifically to CAPD in connection with mental disorders, as this is a new term for auditory deficits and one mostly used by audiologists. Auditory deficit is a more general term used mostly by psychiatrists. Both terms refer to the same disorder. It is essential to point out that 25 of the 79 papers are published between 2000 – 2003.

Schizophrenia is found related to CAPD in 175 papers, 49 of them are published between 2000 – 2003 showing the research focus of the last three years. Learning disabilities were found related to CAPD in 126 papers. Parkinson's disease was related to CAPD in 29 papers. Dyslexia is related to CAPD in 88 papers, 37 of them are between 2000–2003. Alzheimer's disease and auditory deficits are connected in 39 papers. The remaining articles are on depression, alcoholism, anorexia and childhood mental retardation, all being related to some extend to CAPD.

Assessment of the CANS is carried through a great variety of tests that fall into two main categories: psychoacoustic and electrophysiologic testing. Psychoacoustic tests are considered more subjective. Electrophysiologic ones are more objective with the exception of P300 component.

Results

psychoacoustic tests

Learning disabilities, attention deficit disorders and dyslexia are assessed through a great variety of psychoacoustic tests. Age limitations have to be considered [11] and specially designed tests are used for different age groups. When evaluating children who are less than 12 years old an important step is the Pediatric Speech Intelligibility (PSI) Test. This consists of single words and sentences presented with a competing message at varying levels of difficulty [12]. In this test it is essential that performance is adjusted for language age according to previously determined normative data [13]. Evaluation of this test may provide the cause of learning disabilities including dyslexia [14, 15].

Children older than 12 years old are assessed through a more complex test battery that contains several tests. These tests are based on the stimulation of the auditory system with tones, numbers, syllables, words and sentences. Evaluation is made according to the different components of the auditory processing. One widely used test is that of the dichotic digits which consists of different pairs of numbers presented simultaneously to each ear [16]. The person under examination has to repeat all four numbers regardless of order. This test is easy to use in order to detect the auditory deficit of dyslexia particularly since it does not contain language and phonological parameters [17].

The Staggered Spondaic Word Test (SSW) consists of two-syllable spondaic words that are presented simultaneously to each ear [18]. This involves the diagnosis of auditory deficits in attention disorders, autism, learning disabilities and chronic alcoholism [19, 20].

A series of experiments were planned by Nielzen and Olsson on the basis of psychoacoustic handling of auditory stimulation. The results of these psychacoustic experiments show significant differences between a group of schizophrenic patients and a group of reference subjects thus indicating central auditory processing disorders even in a phase of illness remission or during treatment with neuroleptics [21].

electrophysiologic tests

In all mental disorders assessed with the suspicion of CAPD an objective measure of the peripheral auditory system is mandatory. The Auditory Brainstem Responses (ABR), measure the electrophysiologic activity from the 8th cranial nerve to the medial geniculate body of the brainstem [22]. A very important element of ABR evaluation is the morphology and synchronization of the waveform. One should always begin his evaluation while observing waveform changes on real time [23].

The Auditory Middle Latency Responses (AMLRs) provide an electrophysiologic measure of the primary auditory cortex function [24]. The AMLRs can diagnose central auditory processing disorders in children with learning disabilities [25], patients with Alzheimer's disease [26], adult autistic subjects [27, 28] patients with Schizophrenia [29] The Auditory P300 Response, which consists of the measure of the hippocampal and auditory cortex function again from an electrophysiological point of view [30]. The P300 response has been considered an endogenous event-related potential. Endogenous responses depend both on the context within which the auditory stimuli are presented and the psychologic condition and attention of the subject. P300 has been used in diagnosing CAPD in patients with dementia of the Alzheimer type [31], in monitoring long-term effects of donepezil in patients with Alzheimer's disease [32], in anorexic patients [33], in children with mental retardation during a selective attention task to auditory stimuli [34] and in first episode and chronic schizophrenia [35]. Mismatch Negativity Response (MMN) is an event-related evoked potential that measures the electrophysiologic activity of the auditory cortex function [36]. The MMN is always elicited 100–250 msecs from stimulus change onset. Its application is in detecting CAPD in alcoholism [37], in Schizophrenia [38–43], in attention deficit and in developmental dyslexia [44].

psychoacoustic and electrophysiologic testing according to type of lesion

In the selection of tests for the evaluation of brainstem lesions the examiner should keep in mind that all psychoacoustic tests have been reported to aid in the diagnosis. According to the studies of Kartz [45] the Staggered Spondaic Words Test may help differentiating brainstem from cortical lesions and upper from lower brainstem lesions. Musiek et al [46] concluded that Auditory Brainstem Responses in combination with either Masking Level Differences or Dichotic Digits Test may be as sensitive in evaluating a group of patients suffering from multiple sclerosis as a seven test battery. Jerger et al [47] reported that for patients suffering from multiple sclerosis the best test battery was a combination of stapedial reflex measures and speech audiometry.

The usual finding in central auditory tests regarding cortical lesions is a deficit or impairment in the ear contralateral to the side of lesion. Psychoacoustic tests such as Dichotic Digits and SSW in patients with well documented cortical and hemispheric lesions demonstrate primarily contralateral ear deficits and impairments [48]. Two exceptions that the examiner should always keep in mind are when frequency and duration tests are applied and when compromise of auditory fibers of the corpus callosum has occurred [49].

Regarding interhemispheric dysfunction, test results may be difficult to evaluate. Representation of auditory information at the cortical level is mostly contralateral as is clearly depicted in dichotic listening situations. When speech responses are required by the subject auditory information from the right ear are projected through to the left hemisphere without the participation of the opposite hemisphere for the production of a speech response. On the contrary auditory stimuli from the left ear must cross the midline through the corpus callosum for the production of a speech response. Patients with split brain disorders subjected to dichotic testing have interestingly demonstrated decreased scores regarding the left ear and enhanced scores in the right ear [50, 51].

Considerable evidence has been reported that indicates a relation between various learning disabilities, including dyslexia, attention deficit hyperactivity disorder and poor performance scores on central auditory tests Learning disabilities in children might be the expression of various underlying central auditory disorders such as maturational, developmental or neurological as depicted by abnormal CAPD test results [52].

Conclusions

CANS assessment represents a fascinating field. Cooperation of professionals in psychiatry, neurology, neuropsychology and pediatric psychology, with the otolaryngologist-audiologist is a prerequisite. Central auditory processing disorders may co-exist with various mental disorders such as: learning disabilities, attention deficit hyperactivity disorder, dyslexia, autism, chronic alcoholism, Alzheimer's disease, adult autistic disorder, Schizophrenia, anorexia and mental retardation. Assessing these disorders is difficult due to the complex anatomy and physiology of the CANS. This explains the great variety of existing methods of testing with two main categories: those of psychoacoustic methodology and those based on electrophysiologic measures. Physiology of CANS is still not completely understood and further research is needed on development of new tests and validation of their clinical applicability.

Conflict of interest

none declared