Учебники / Otolaryngology - Basic Science and Clinical Review

patients tend to past-point, stagger, or fall toward the side of the lesion.

When the lesion is confined to the inner ear or CN VIII, the plasticity of the brainstem and cerebellar circuitry allows the vestibular system to adapt to the change in vestibular input, and the vertigo and nystagmus improve with time. If the loss of vestibular function is sufficiently slow, this adaptation may keep pace with it. Thus testing may reveal absent ipsilateral vestibular function in a patient with a large acoustic neuroma who had not experienced severe vestibular symptoms as the tumor slowly grew. Sacrifice of CN VIII during surgery causes no vestibular symptoms in such a patient. In contrast, patients with small acoustic neuromas may experience severe vertigo, nausea, and vomiting after surgery due to the abrupt loss of their previously relatively unimpaired vestibular input on the side of the tumor.

ADDITIONAL VESTIBULAR AND EYE

MOVEMENT–RELATED PROJECTIONS

The interstitial nucleus of Cajal, a group of cells located within the mesencephalic MLF, is also involved in vestibular processing. It receives input from the ipsilateral superior vestibular nucleus and the contralateral medial vestibular nucleus and sends projections to the cervical spinal cord; these projections travel through the descending MLF along with the fibers originating in the vestibular nuclei.

Excitatory ascending projections from the abducens nucleus, which decussate and then travel through the brainstem MLF to the contralateral oculomotor nucleus, serve to coordinate conjugate horizontal eye movements by causing simultaneous contraction of the lateral rectus (innervated by the abducens nerve) on one side and the medial rectus (innervated by the oculomotor nerve) on the opposite side. An MLF lesion blocks the projection to the oculomotor nucleus on that side, causing an “intranuclear ophthalmoplegia” with failure of adduction of the ipsilateral eye on attempted conjugate gaze to the opposite side. This is accompanied by nystagmus of the abducting eye; the pathophysiology of the nystagmus is unclear.Attempted ocular convergence can still cause the eyes to adduct, demonstrating that the oculomotor nuclei themselves and their connections to the medial rectus muscles are intact and that convergence is mediated by brainstem pathways distinct from the MLF.

Ascending projections from the vestibular nuclei to the thalamus terminate predominantly in the nucleus ventralis posterolateralis (VPL). In contrast to the

auditory and visual systems, which have separate thalamic relay nuclei (the medial and lateral geniculate nuclei, respectively), there does not appear to be a separate vestibular thalamic nucleus; thalamic areas that receive vestibular input also respond to somatosensory input. Similarly, some areas of cerebral cortex respond to both somatosensory and vestibular input. However, several lines of evidence (clinicalpathological correlations in patients with strokes, vestibular stimulation studies in human subjects, and homologies with the vestibular cortex of subhuman primates) identify the posterior insula as the predominant vestibular representation within human cerebral cortex.

EFFERENT VESTIBULAR PATHWAYS

The major projections to the vestibular nuclei from other brain regions arise from the cerebellum and from the vestibular nuclei on the opposite side, as already described. There are also descending pathways from vestibular cortex to the vestibular nuclei.

Like the auditory system, the vestibular system also contains an efferent pathway that synapses on the sensory hair cells within the inner ear. It is thought to modulate the dynamic range of the vestibular afferents to match expected acceleration. The efferent fibers arise from cell bodies located lateral to the abducens nuclei, travel within the vestibular nerves, and make excitatory synapses on the hair cells of the cristae and maculae. Like the olivocochlear bundle, the efferent vestibular projections include both crossed and uncrossed pathways. Also, the synapses on the hair cells are cholinergic for both the efferent auditory projections and the efferent vestibular projections to the inner ear.

SUGGESTED READINGS

Parent A. Carpenter’s Human Neuroanatomy. 9th ed. Baltimore: Williams & Wilkins; 1996

Patten J. Neurological Differential Diagnosis. 2nd ed. London: Springer; 1996

Strominger NL. The origins, course and distribution of the dorsal and intermediate acoustic striae in the rhesus monkey. J Comp Neurol 1973;147:209–234

Strominger NL, Nelson LR, DoughertyWJ. Second-order auditory pathways in the chimpanzee. J Comp Neurol 1977;172: 349–366

Strominger NL, Strominger AI.Ascending brain stem projections of the anteroventral cochlear nucleus in the rhesus monkey. J Comp Neurol 1971;143:217–242

SELF-TEST QUESTIONS

For each question select the correct answer from the lettered alternatives that follow.To check your answers, see Answers to Self-Tests on page 716.

1.Unilateral cochlear stimulation will result in afferent neuronal activity within

A.One auditory nerve, one superior olivary complex, one inferior colliculus, and one auditory cortex

B.One auditory nerve, one superior olivary complex, one inferior colliculus, and both auditory cortices

C.One auditory nerve, one superior olivary complex, both inferior colliculi, and both auditory cortices

D.One auditory nerve, both superior olivary complexes, both inferior colliculi, and both auditory cortices

2.Contraction of the stapedius muscle

A.Increases the acoustic impedance of the tympanic membrane

B.Is mediated by the fifth cranial nerve

C.Can result in hyperacusis

D.Increases the transmission of sound through the ossicles

3.A patient suddenly loses function in the right labyrinth.What are the symptoms and signs?

A.The patient will experience vertigo, with the feeling that he or she is rotating from right to left. The patient will have nystagmus, with the fast phase to the left, and will tend to fall to the right when walking.

B.The patient will experience vertigo, with the feeling that he or she is rotating from right to left. The patient will have nystagmus, with the fast phase to the right, and will tend to fall to the right when walking.

C.The patient will experience vertigo, with the feeling that he or she is rotating from left to right.The patient will have nystagmus, with the fast phase to the left, and will tend to fall to the left when walking.

D.The patient will experience vertigo, with the feeling that he or she is rotating from left to right.The patient will have nystagmus, with the fast phase to the right, and will tend to fall to the left when walking.

4.An unconscious patient whose labyrinth, cranial nerves, and brainstem are intact is positioned supine, with the head elevated 30 degrees.What will happen when ice water is instilled into the left external auditory canal?

A.Afferent activity in the left vestibular nerve will increase, and the eyes will deviate tonically to the left.

B.Afferent activity in the left vestibular nerve will increase, and the eyes will deviate tonically to the right.

C.Afferent activity in the left vestibular nerve will decrease, and the eyes will deviate tonically to the left.

D.Afferent activity in the left vestibular nerve will decrease, and the eyes will deviate tonically to the right.

Many practicing otolaryngologists are unfamiliar with central auditory processing and its abnormalities. The usual auditory function tests assess the peripheral organ (including both the middle ear and the inner ear) and the cochlear nerve. Some patients in whom the results of these tests, including a speech discrimination test, show normal hearing still complain of listening and understanding disabilities.These patients have conditions that have been called auditory perceptual disorders, auditory processing disorders, or, more commonly, central auditory processing disorders. Central auditory function testing is used to identify and characterize abnormalities of central auditory processing in these patients.

Central auditory function tests generally are used to characterize abnormalities of function rather than to diagnose macroscopic structural lesions affecting the central auditory pathways; lesions can be identified using sophisticated imaging techniques such as magnetic

resonance imaging (MRI) and computed tomographic (CT) scans. Brainstem auditory evoked potentials (BAEPs), also called auditory brainstem responses (ABRs), can help to localize abnormalities anatomically. In children presenting with delayed or abnormal language acquisition, central auditory function testing provides information about the level of maturation of the central auditory pathways. Documentation of a central auditory processing disorder allows caregivers to choose the best and most appropriate remediation strategy for the child.This chapter provides information about central auditory function testing and its results in several pathological conditions.

DEFINITIONS

Central auditory processes are the neuronal mechanisms responsible for the following behavioral phenomena: sound localization and lateralization, auditory

discrimination, auditory pattern recognition, temporal aspects of audition (including temporal resolution, temporal masking, temporal integration, and temporal ordering), auditory performance with competing acoustic signals, and auditory performance with degraded acoustic signals (American Speech Language Hearing Association, 1996). Central auditory processing disorders can be defined as abnormalities of the basic processes involved in understanding spoken language in the absence of dysfunction of the peripheral auditory system lesion (ear and cochlear nerve). They manifest as a deficit in information processing of the audible signal and/or as an impaired ability to discriminate, remember, recognize, and comprehend information presented to normal ears. The neuronal abnormalities that cause these disorders, therefore, must be localized between the cochlear nucleus and auditory areas of cerebral cortex.

NEUROPHYSIOLOGICAL BASIS OF CENTRAL AUDITORY FUNCTION

Several principles regarding the treatment of the auditory messages by the central nervous system (CNS) are summarized here.

•Channel separation: Information about an acoustic signal delivered to one ear is transferred to, and maintained in, the auditory cortex in a manner that keeps it distinct from information about a signal delivered to the contralateral ear.

•Binaural fusion: If a unique message is separated into two bands and if these two bands are delivered simultaneously to both ears, a fusion occurs at the level of the brainstem, and the subject will perceive only one message.

•Bilateral pathways: In normal subjects, information about an auditory signal delivered to one ear travels through both direct and crossed auditory pathways and reaches both temporal lobes. Ascending auditory fibers cross the midline at multiple levels within the brainstem. Furthermore, at the cerebral level, auditory information can cross from one hemisphere to the other through the corpus callosum, and perhaps through the anterior commissure as well.

•Cerebral dominance:The left cerebral hemisphere is “dominant” with respect to the perception of language in the majority of the population. However, some elements of the auditory message are processed in the “nondominant” right hemisphere.

AUDIOLOGICAL TESTS AND CENTRAL AUDITORY PROCESSING

PURE-TONE AUDIOMETRY

When testing central auditory function, pure-tone audiometry should be performed to identify any peripheral auditory dysfunction. Pure-tone audiometry usually will not detect a lesion of the central auditory pathways and will not be modified by a lesion above the inferior colliculus because intensity and frequency discrimination are performed at a level below the inferior colliculus.

SPEECH AUDIOMETRY

Speech audiometry assesses the ability of subjects to hear and understand the spoken word. A list of monosyllabic or disyllabic words is presented through prerecorded tapes to the subject at different intensities. The list consists of either phonetically balanced or isophonemic monosyllabic or spondaic (equally accented disyllabic) words. Phonetically balanced lists are those in which each phoneme in the list appears in proportion to its frequency of occurrence in natural language. Isophonemic lists are those in which a phoneme occurs once only in each list. The number of words in each list is usually 10. Two parameters are measured.The speech detection threshold (SDT) is the intensity at which 50% of the words are detected but not understood. The speech reception threshold (SRT) is the intensity at which 50% of the words are correctly reported.

The value of speech audiometry lies in the fact that many retrocochlear lesions have a greater effect on speech comprehension than on the pure-tone audiogram. In contrast to patients with a sensory hearing loss (in whom the predominant pathology is hair cell loss, with or without a loss of cochlear nerve fibers), the speech audiogram of a subject presenting with a retrocochlear lesion is often worse than anticipated from the mean hearing loss of the pure-tone audiogram. Speech audiometry frequently is severely abnormal in the affected ear with unilateral cochlear lesions, but results are usually not so asymmetrical in patients with central lesions.

MASKED SPEECH AUDIOMETRY

There are many ways of performing speech audiometry to attempt to detect lesions or abnormalities of auditory processing in the brainstem or cerebral cortex, including the use of distortions of spoken words and periodic interruptions of speech. To be useful in the clinic, however, a simple method is necessary. Masked speech audiometry is of some value. Basically, the technique

involves the presentation of words simultaneously with noise to one ear and comparing the results with those obtained when a list of words is presented in the absence of noise. Most patients with lesions of the central auditory pathways have a markedly reduced ability to understand the words in the presence of noise. Interestingly, the side of poorest performance is not always consistent with that of the lesion; in some patients, the abnormalities are bilateral. Furthermore, the rostrocaudal level of the lesion (e.g., pons or mesencephalon) does not correlate with the degree of abnormality of the masked speech audiogram.

MASKING LEVEL DIFFERENCES

The masking level differences (MLDs) test is a valuable test for the assessment of central auditory dysfunction. In essence, the test consists of the presentation of a pulsed 500 Hz tone to both ears (binaurally), in the presence of continuous broad-band noise presented at 60 dB.The intensity of the tone, which is pulsed at a rate of 200 msec on/200 msec off, is varied until the patient indicates that the tone is perceived.Two test conditions are evaluated. In the homophasic condition, the stimulus and the noise are presented to both ears in phase, and the detection threshold of the tone is determined. In the antiphasic condition, the signal is 180 out of phase between the two ears, whereas the noise remains in phase. The threshold obtained in antiphasic condition is subtracted from the threshold obtained in the homophasic condition to determine the amount of release from masking in dB. Masking level differences of less than 6 dB are considered abnormal in adult patients.The MLD test is sensitive to lesions in the lower brainstem, but it is largely unaffected by rostral brainstem or cortical lesions. Furthermore, there is a close correspondence between MLD and BAEP test results: patients with abnormalities in waves I, II, or III of the BAEP demonstrate little or no release from masking, whereas patients with abnormalities of waves IV and V show normal MLD results.

DICHOTIC SPEECH TESTING

Dichotic speech tests consist of the simultaneous presentation of different stimuli to each ear.The interpretation of the dichotic tests is based on the model developed by Kimura, which has the following premises: (1) the contralateral auditory pathways in humans are more numerous and robust than the ipsilateral pathways; (2) when monaural input is presented to the system, either pathway is capable of initiating and conducting the appropriate neural response; and (3) in

Figure 28-1 Central auditory pathways in dichotic hearing. (From Sparks R, Goodglass H, Nickel B. Ipsilateral versus contralateral extinction in dichotic listening resulting from hemisphere lesions. Cortex 1970;6:249–260. Reprinted with permission.)

dichotic situations, the weaker ipsilateral pathway is suppressed and the stronger contralateral pathways remain active. Hence, if one hemisphere is compromised, a deficit would be anticipated.

Two tests are widely used to evaluate the patients: the dichotic digits test and the staggered spondaic test. In the dichotic digits test, digits are presented to each ear in a dichotic fashion at a comfortable intensity level (usually 50 or 60 dB), and the patients are asked to repeat all digits heard. The digit dichotic test is fairly sensitive to intracranial lesions. In the staggered spondaic words test, developed by Katz (1977), 40 pairs of spondee words are presented to the patient in an overlapping but staggered fashion, in both competing and noncompeting conditions.These tests tend to reveal the site of the lesion along the central auditory pathways, in particular temporal lobe lesions or interhemispheric lesions (Fig. 28-1).

ELECTROPHYSIOLOGICAL TESTS AND CENTRAL AUDITORY PROCESSING

ACOUSTIC REFLEX

The efferent auditory system provides feedback control of the volume of sounds reaching the cochlea. Loud sounds cause reflexive contraction of the stapedius muscle, which limits ossicular vibration.

Because the tympanic membrane is connected to the ossicles, stapedius contraction modifies the acoustic impedance of the tympanic membrane and thus the acoustic impedance of the middle ear.This latter can be measured (“impedance audiometry”), and changes in it in response to sounds provide an assessment of the acoustic reflex.

The acoustic reflex is not considered a strong measure of central auditory function. However, the reflex may be disturbed by a lesion situated in the lower brainstem (the pons); the anatomical pathway subserving the acoustic reflex involves projections from the ventral cochlear nuclei to the superior olivary complexes and from there to the facial motor nuclei. Because of this, the acoustic reflex can be valuable in assessing central auditory integrity in children and adults.

EVOKED OTOACOUSTIC EMISSIONS

Otoacoustic emissions are low-level acoustic signals that are generated by the cochlea, both spontaneously and in response to auditory stimulation. The latter include (1) transiently evoked otoacoustic emissions (TEOAEs), which are produced in response to shortduration signals such as clicks and tone bursts; (2) distortion product otoacoustic emissions (DPOAEs), which are evoked when stimuli of two different frequencies are presented; and (3) stimulus-frequency otoacoustic emissions (SFOAEs), which are generated by continuous pure-tone stimuli that vary slowly in frequency. The TEOAEs and the DPOAEs are most widely used for clinical testing.

Although otoacoustic emissions should be strictly normal in central auditory processing disorders, they can be valuable in evaluating patients suspected of having such disorders to prove the normal function of the cochlea; it is known that cochlear abnormalities can be present in the face of a normal pure-tone audiogram.The measurement of otoacoustic emissions can provide an index of the integrity of the olivocochlear bundle, which originates in the region of the superior olivary complex and terminates at the base of the outer hair cells (medial system) and at the nerve fibers at the base of the inner hair cells (lateral system). Otoacoustic emissions are influenced by stimulating the olivocochlear bundle. When noise is presented to the contralateral ear, otoacoustic emissions are suppressed in amplitude by several decibels. Because the olivocochlear bundle appears to play a role in the ability to hear sounds in the presence of noise, it may be important to know whether the olivocochlear bundle is functioning normally in

patients presenting with central auditory processing disorders.

BRAINSTEM AUDITORY EVOKED POTENTIALS

BAEPs are the electrical signals produced by the infratentorial auditory system in response to transient auditory stimuli such as clicks or brief tone pips. Stimuli are presented monaurally, and masking noise is presented to the contralateral ear. The responses are recorded between the vertex and the ear or mastoid ipsilateral to the stimulated ear; additional recording channels are often used to clarify components or assist in their identification. The BAEP peaks, which have latencies of less than 10 msec, are typically labeled with roman numerals according to the convention of Jewett and Williston (Fig. 28-2); waves IV and V are often fused into a IV–V complex. Component amplitudes vary from subject to subject, but peak latencies are highly consistent across subjects.Thus the interpretation of BAEPs is predominantly based on measurements of absolute peak latencies, interpeak intervals, and the right-left differences of these measures; the IV–V: I amplitude ratio is also useful in identifying neurological abnormalities.

As the stimulus intensity is reduced, BAEP components increase in latency and decrease in amplitude, and eventually disappear. Thus BAEPs can be used to estimate hearing thresholds. Frequency-specific stimuli (e.g., tone pips) and frequency-specific masking (e.g., notched noise) can be used to measure thresholds at various frequencies and produce audiograms that are similar to behavioral audiograms obtained in the same subject. However, BAEPs are most useful for the assessment of retrocochlear abnormalities.

Wave I is generated in the distal (i.e., at the cochlear end) cranial nerve (CN) VIII, and may be preserved in lesions of the proximal CN VIII. It is identical to the electrocochleographic N1, the first peak of the CN VIII compound action potential. A delay in wave I indicates peripheral auditory dysfunction, such as a conductive or cochlear hearing loss. All of the subsequent BAEP components are the composites of contributions from multiple generators (Fig. 28-2). For example, wave II includes contributions both from the cochlear nucleus and from the second volley in the distal CN VIII (the electrocochleographic N2). However, in clinical-pathological correlations and localization of lesions with BAEPs, wave III predominantly reflects activity at the level of the lower pons, and wave V predominantly reflects activity at the level of the mesencephalon. Clinical

Figure 28-2 Diagram showing the probable generators of the human brainstem auditory evoked potentials (BAEPs). AC, auditory cortex; AR, auditory radiations;BIC,brachium of the inferior colliculus;CN,cochlear nucleus; IC, inferior colliculus; LL, lateral lemniscus; MGN, medial

interpretation of BAEPs is based predominantly on waves I, III, and V; the other components are variable and are occasionally not identifiable in normal subjects. BAEPs cannot be used to assess the auditory pathways rostral to the mesencephalon. For example, patients with bilateral temporal lobe infarctions involving auditory cortex may be deaf yet have completely normal BAEPs.

A unilateral cochlear, CN VIII, or cochlear nucleus lesion will cause unilateral BAEP abnormalities affecting the waveforms to stimulation of the ear ipsilateral to the lesion. The ascending auditory pathways become bilateral at the level of the superior olivary complex. Although more ascending fibers rostral to this level are crossed than are uncrossed, the subset of ascending auditory neurons generating the BAEPs are predominantly ipsilaterally driven because unilateral brainstem lesions that produce unilateral abnormalities involving either the I–III or the III–V interpeak interval usually do so upon stimulation of the ear ipsilateral to the lesion.

BAEPs are highly sensitive ( 95% sensitivity) in the detection of tumors of CN VIII. Small tumors may

geniculate nucleus;SN,slow negativity after waveV;SOC,superior olivary complex. (From Legatt AD,Arezzo JC,Vaughan HG Jr.The anatomic and physiologic bases of brain stem auditory evoked potentials. Neurol Clin 1988;6:681–704, Fig. 12, p. 698. Reprinted with permission.)

prolong the I–III interpeak interval or cause loss of waves III and V. As the tumors grow, they can compress the internal auditory artery, which originates in the intracranial circulation (usually as a branch of the anterior inferior cerebellar artery) and courses through the internal auditory canal next to CN VIII to supply blood to the cochlea. This can prolong the latency of wave I and even cause the loss of wave I and all subsequent BAEP components, due to cochlear ischemia or infarction.With very large tumors, the III–V interpeak interval to stimulation of the contralateral ear may become prolonged, reflecting brainstem compression. BAEPs are used for intraoperative monitoring during resection of CN VIII tumors, as well as during other temporal bone or posterior fossa operations in which the ear, CNVIII, or brainstem are at risk.

BAEPs are used as a diagnostic test in patients suspected of having multiple sclerosis, though their sensitivity is less than that of visual evoked potentials. Demyelination results in a slowing of neural conductions and thus prolongation of BAEP peak latencies and interpeak intervals, which can be recognized at a point

when the myelin damage is subclinical. BAEPs are also highly sensitive in the detection of intrinsic brainstem tumors, such as gliomas.

BAEPs can detect unilateral brainstem lesions affecting the auditory pathways in patients with normal audiograms; these patients have no hearing loss because the ascending auditory pathways are bilateral. BAEPs can also detect central auditory processing abnormalities in patients with bilateral brainstem disease that slows but does not interrupt neural conductions; these patients may have normal hearing because the information that the sound has occurred, though delayed, still reaches the auditory cortex.

CENTRAL AUDITORY

PROCESSING DISORDERS

Central auditory processing disorders can be defined as a dysfunction of the basic processes involved in understanding the spoken language in the absence of a peripheral auditory system lesion. Many clinicians believe that children with learning disabilities are the only population in whom central auditory testing is appropriate. However, it may be of value in many other clinical situations.

Patients with degenerative neurological diseases that may affect the auditory pathways are another population that deserves central auditory evaluation.The most common of these diseases that has an auditory correlate is multiple sclerosis, in which the pathophysiology involves demyelination of axons within the central nervous system. Abundant evidence indicates that individuals with multiple sclerosis can have auditory deficits, primarily when the auditory pathways are involved. Disorders of central auditory processing, such as impaired sound localization, have been found in multiple sclerosis patients who have normal audiograms. BAEPs demonstrate the central dysfunction in these subjects, and the degree of BAEP abnormality correlates with the degree of impairment of sound localization.

The absolute latency of wave I of the BAEP is sometimes called the “peripheral transmission time,” and the I–V interpeak interval is labeled the “central transmission time.” The latter might seem inappropriate because wave I originates in the most distal portion of CN VIII, and conduction along the cranial nerve is thus included within the “central transmission time.” However, the cochlear nerve axons are ensheathed by CNS-type myelin, produced by oligodendrogliocytes, along most of CN VIII; the transition to peripheral nervous system–type myelin, produced by Schwann cells,

occurs close to the cochlea. CN VIII is therefore vulnerable to diseases that affect CNS myelin, such as multiple sclerosis.

Neurological degenerative diseases other than multiple sclerosis also have been shown to involve the auditory system; for example: Charcot-Marie-Tooth disease, Alzheimer’s disease, olivopontocerebellar degeneration, Friedreich’s ataxia, Parkinson’s disease, and various leukodystrophies. However, these diseases are not as common as multiple sclerosis, nor have they been studied as much from an auditory perspective.

Patients with seizure disorders with epileptogenic foci at or near the auditory areas of the cerebrum also are candidates for central auditory assessment. Epilepsy or the causative lesions can cause dysfunction of the central auditory nervous system. In addition, two surgical primary treatments for intractable epilepsy, corpus callosotomy (split brain) and resection of the seizure focus, may affect central auditory processing. The split brain procedure and its effect on auditory processing have been well studied. Also well studied from an audiological point of view is the effect of anterior temporal lobectomy, which is a common surgical treatment for intractable focal epilepsy. Preservation of the temporal lobe speech centers is important during temporal lobectomy in the language dominant hemisphere.

Patients with mass lesions of the central auditory nervous system also are candidates for central auditory evaluation. By using central auditory tests, insight may be gained as to how much the auditory system is compromised and what may be the functional consequences of these lesions or their surgical removal.

Patients who have suffered head trauma often have damage to the central and/or peripheral auditory systems. Sometimes central auditory function tests can provide insight into the nature of the recovery or the lack thereof in these patients. Unfortunately, central auditory function tests are seldom employed in these patients, even though extensive neuropsychological testing is often performed in them.

Patients who wear, or are candidates for, hearing aids and who do not do well with amplification may require central auditory assessment. In many cases, hearing aids are not effective because of some compromise of central auditory processing. Often, these patients have a history of CNS pathology, which has affected their central auditory processing. Also, the great majority of hearing aids users are older adults, who are at risk for central auditory processing abnormalities. Management of patients who do poorly with amplification can be improved if a central deficit is

defined. Peripheral auditory function may be symmetrical, or one ear may be better, but a central evaluation may indicate an entirely different result in regard to ear symmetry. This information may have implications for the management of the patient. Some patients suffer from a binaural interference phenomenon, in which extremely poor speech recognition on one side contaminates good performance from the other side. In this condition, binaural amplification may be worse than a single hearing aid.

Patients who complain about difficulty hearing in noisy environments and have normal audiograms suffer from obscure auditory dysfunction, and their central auditory function should be evaluated. These patients are most often adults without underlying disorders. Obscure auditory function represents a relatively large population of patients who, until

SELF-TEST QUESTIONS

For each question select the correct answer from the lettered alternatives that follow.To check your answers, see Answers to Self-Tests on page 716.

1.A patient is felt to have a purely central auditory processing disorder. Which of the following findings would be inconsistent with that diagnosis?

A.Impaired ability to lateralize sounds in space

B.A high-frequency hearing loss

C.A masking level difference of less than 6 dB

D.An elevated speech reception threshold in the presence of a normal pure-tone audiogram

2.Otoacoustic emissions

A.Can be recorded following a continuous puretone stimulus but not following a click stimulus

B.Are usually abnormal in patients with central auditory processing disorders

C.Are affected by lesions of the inferior colliculus

D.Are affected by stimulation of the olivocochlear bundle

recently, were dismissed by clinicians as having no hearing problems.

SUGGESTED READINGS

American Speech Language Hearing Association. Central auditory processing: current status of research and implications for clinical practice. Am J Audiol 1996;5:41–54

Katz J. The staggered spondaic word test. In: Keith R, ed. Central Auditory Dysfunction. NewYork: Grune & Stratton; 1977

Legatt AD. Brainstem auditory evoked potentials: methodology, interpretation, and clinical application. In: Aminoff MJ, ed. Electrodiagnosis in Clinical Neurology. 4th ed. New York: Churchill Livingstone; 1999:451–484

Probst R, Harris FP. Otoacoustic emissions. In:Alford BR, Jerger J, Jenkins HA, eds. Electrophysiologic Evaluation in Otolaryngology. Basel: Karger; 1997:182–204.Advances in Otorhinolaryngology; vol 53

3.Brainstem auditory evoked potentials (BAEPs) can detect abnormalities

A.Within cranial nerve (CN) VIII, but not within the pons, medial geniculate, or auditory cortex

B.Within CN VIII or the pons, but not within the medial geniculate or auditory cortex

C.Within CN VIII, the pons, or the medial geniculate, but not within the auditory cortex

D.Within CN VIII, the pons, the medial geniculate, or the auditory cortex

4.In multiple sclerosis, demyelination

A.Can involve CN VIII

B.Prolongs BAEP peak latencies but not interpeak intervals

C.Can increase conduction velocities within the central auditory pathways

D.Affects the audiogram and sound localization abilities equally

There is but one area of evolutionary specialization that has enabled the Darwinian success of humans: language. The foundation of our present evolutionary success is the ability to increase our knowledge through our use of language. All histories, archives, and knowledge bases are a result of our linguistic abilities, and there appears to be no other life form that is so well endowed with our particular linguistic traits.What are the biological characteristics that underlie this remarkable evolutionary product?

The conceptualization of language has undergone its own development. The ancients saw language as a fixed human characteristic and felt that it was an inherent function, and furthermore that there was a “primary” language. Herodotus, in the 5th century BC, referred to a language experiment of sorts conducted by Pharaoh Psammetichus, who had placed two children who were being nursed by goats in isolation for 2 years.The children’s vocalizations were noted at the end of those 2 years. Their utterance was the sound “Becos,” which was interpreted as being the Phrygian word for bread. Psammetichus was reportedly disappointed because he had hoped that Egyptian would have been the children’s “primary” language. More likely the children were using the sounds that they had heard, which was the bleating of their “nurses,” the goats. One could characterize the pharaoh’s interpretation of the study as the extreme example that

language was all nature—or an intrinsic human ability; for him, the bleating of the goats had no effect on the children.

The intrinsic nature of language remained the dominant idea until the beginning of the 19th century. It is surprising that awareness of the lack of language in the congenitally deaf and their occasional habilitation, beginning in the 17th century, did not result in consideration of the nurture/extrinsic contribution to the establishment of a person’s language.Two case reports, that of Itard in 1801 and Wardrop in 1813, concern themselves with a few years of life and its effect on the linguistic abilities of the child.Wardrop’s patient, James, illustrate the effect of deprivation of auditory linguistic input on a child who had adventitious linguistic exposure—sight, touch, and so on.Although deaf from birth (in retrospect, James was probably a child with congenital rubella), James was able to have emotion and rudimentary communication skills. Wardrop’s study illustrates the tenet of an intrinsic linguistic mechanism that needs very little extrinsic input to achieve a modicum of language. Itard’s patient, Victor, had normal hearing but was probably a victim of social deprivation: he was found in a forest and was assumed to have been raised by animals. After much work, the boy attained little more than rudimentary linguistic ability. Itard’s study illustrates that there is a need for extrinsic linguistic input for the development