Relationships among auditory brain stem responses, masking level differences and the acoustic reflex in multiple sclerosis

Acute vestibular syndrome and hearing loss mimicking labyrinthitis as initial presentation of multiple sclerosis

Acute vestibular syndrome is most often caused by vestibular neuritis or stroke, although demyelinating diseases may be responsible for 4% of all AVS episodes. The authors present the case of a previously healthy 17-year-old female patient complaining of spontaneous vertigo and right-sided hearing loss. Otoneurological examination suggested a peripheral vestibular cause and video head impulse test revealed a reduced vestibulo-ocular reflex gain. The presence of sensorineural hearing loss raised the suspicion of a central cause and prompted imaging evaluation. A brain MRI evidenced demyelinating lesions in the right middle cerebellar peduncle and the patient was ultimately diagnosed with Multiple Sclerosis.

Acquired Central Auditory Processing Disorder in Service Members and Veterans

Purpose A growing body of evidence suggests that military service members and military veterans are at risk for deficits in central auditory processing. Risk factors include exposure to blast, neurotrauma, hazardous noise, and ototoxicants. We overview these risk factors and comorbidities, address implications for clinical assessment and care of central auditory processing deficits in service members and veterans, and specify knowledge gaps that warrant research. Method We reviewed the literature to identify studies of risk factors, assessment, and care of central auditory processing deficits in service members and veterans. We also assessed the current state of the science for knowledge gaps that warrant additional study. This literature review describes key findings relating to military risk factors and clinical considerations for the assessment and care of those exposed. Conclusions Central auditory processing deficits are associated with exposure to known military risk factors. Research is needed to characterize mechanisms, sources of variance, and differential diagnosis in this population. Existing best practices do not explicitly consider confounds faced by military personnel. Assessment and rehabilitation strategies that account for these challenges are needed. Finally, investment is critical to ensure that Veterans Affairs and Department of Defense clinical staff are informed, trained, and equipped to implement effective patient care.

Electrophysiological Evidence for the Sources of the Masking Level Difference

PURPOSE

The purpose of this review article is to review evidence from auditory evoked potential studies to describe the contributions of the auditory brainstem and cortex to the generation of the masking level difference (MLD).

METHOD

A literature review was performed, focusing on the auditory brainstem, middle, and late latency responses used in protocols similar to those used to generate the behavioral MLD.

RESULTS

Temporal coding of the signals necessary for generating the MLD occurs in the auditory periphery and brainstem. Brainstem disorders up to wave III of the auditory brainstem response (ABR) can disrupt the MLD. The full MLD requires input to the generators of the auditory late latency potentials to produce all characteristics of the MLD; these characteristics include threshold differences for various binaural signal and noise conditions. Studies using central auditory lesions are beginning to identify the cortical effects on the MLD.

CONCLUSIONS

The MLD requires auditory processing from the periphery to cortical areas. A healthy auditory periphery and brainstem codes temporal synchrony, which is essential for the ABR. Threshold differences require engaging cortical function beyond the primary auditory cortex. More studies using cortical lesions and evoked potentials or imaging should clarify the specific cortical areas involved in the MLD.

Hearing disorders in multiple sclerosis

Multiple sclerosis (MS) is a disease that is both a focal inflammatory and a chronic neurodegenerative disease. The focal inflammatory component is characterized by destruction of central nervous system myelin, including the spinal cord; as such it can impair any central neural system, including the auditory system. While on the one hand auditory complaints in MS patients are rare compared to other senses, such as vision and proprioception, on the other hand auditory tests of precise neural timing are never "silent." Whenever focal MS lesions are detected involving the pontine auditory pathway, auditory tests requiring precise neural timing are always abnormal, while auditory functions not requiring such precise timing are often normal. Azimuth sound localization is accomplished by comparing the timing and loudness of the sound at the two ears. Hence tests of azimuth sound localization must obligatorily involve the central nervous system and particularly the brainstem. Whenever a focal lesion was localized to the pontine auditory pathway, timing tests were always abnormal, but loudness tests were not. Moreover, a timing test that included only high-frequency sounds was very often abnormal, even when there was no detectable focal MS lesion involving the pontine auditory pathway. This test may be a marker for the chronic neurodegenerative aspect of MS, and, as such could be used to complement the magnetic resonance imaging scan in monitoring the neurodegenerative aspect of MS. Studies of MS brainstem lesion location and auditory function have led to advances in understanding how the human brain processes sound. The brain processes binaural sounds independently for time and level in a two-stage process. The first stage is at the level of the superior olivary complex (SOC) and the second at a level rostral to the SOC.

Does the 40-Hz auditory steady-state response show the binaural masking level difference?

In contrast to the transient (N1-P2) and steady-state (<20 Hz) cortical responses, neither the transient (auditory brain stem response) nor the steady-state (80 Hz) brain stem responses show the binaural masking level difference (BMLD). This study determined behavioral and 40-Hz auditory steady-state response (ASSR) BMLDs for both signal and noise interaural inversions. Results showed clear behavioral BMLD but no 40-Hz ASSR BMLD. However, ASSR amplitudes were significantly smaller in dichotic compared with diotic conditions. Thus, although 40-Hz ASSR thresholds do not reflect the BMLD, the amplitude suppression under dichotic conditions may be a precursor to the subsequent <20-Hz ASSR and behavioral BMLDs.

Auditory function in adrenomyeloneuropathy

Auditory brainstem responses (ABR), ipsilateral and contralateral acoustic reflexes and the masking level difference for speech (MLD) were studied in 29 patients with adrenomyeloneuropathy (AMN). Abnormalities were seen for all ABR components with Waves V and III affected to the greatest degree. For male patients with AMN, the I-III, III-V and I-V interpeak latency intervals were abnormal for a majority of patients. For female patients with AMN, the I-V and III-V interpeak latency intervals were abnormal for a majority of patients with the I-III interval less affected. Contralateral acoustic reflexes were elevated or absent for approximately 50% of ears. Ipsilateral acoustic reflexes were abnormal for 25% of ears. MLDs were significantly reduced in 72% of patients. When considered in terms of the earliest ABR wave abnormality, the earlier components of the ABR (i.e., Waves III and I) were the initial components impaired for the majority of ears. Word recognition in quiet was relatively unimpaired for all subjects. Despite the presence of marked ABR abnormalities, patients with AMN denied the presence of significant difficulty hearing.

Auditory Efferent Function Is Affected in Multiple Sclerosis

OBJECTIVE

This study evaluated efferent medial olivocochlear (MOC) function in patients with multiple sclerosis (MS). Various afferent auditory abnormalities have been described in MS, but there is a paucity of data on efferent function. The brain stem is a site of predilection for MS plaques and the efferent MOC pathway may be affected at this level.

METHODS

The study included 30 patients who had normal hearing. According to MRI findings, they were divided into two groups: those with an identifiable brain stem lesion (n = 10) and those with MS lesions in other parts of the central nervous system but without demonstrable MS plaques in the brain stem (n = 20). MOC function was evaluated by the olivocochlear suppression test, using transient evoked otoacoustic emissions. All subjects underwent standard auditory tests, including pure-tone audiometry and recording of auditory brain stem evoked responses. Twenty-two healthy subjects with normal hearing, matched for age and gender, served as a reference group for the auditory data.

RESULTS

The results showed that 66.6% of all patients had reduced MOC function, particularly those (90%) with identified lesions of the brain stem on MRI. Furthermore, abnormal MOC function was found in 55% patients without evidence of a brain stem lesion on MRI.

CONCLUSIONS

This study provides the evidence for a deficit of efferent auditory function in the majority of patients with MS. Taking into consideration the possible roles of the MOC system in processing of auditory information, abnormal MOC suppression in patients with MS may explain a variety of auditory presentations that are currently largely overlooked. This study also highlights the diagnostic value of the MOC suppression test as a site-of-lesion diagnostic test in MS and in identifying subtle brain stem lesions undetected by MRI, suggesting that subtle brain stem lesions may exist and that the MOC suppression test is sufficiently sensitive to detect them. Accordingly, the MOC suppression test may provide a tool for an early diagnosis of MS.

Odds of demonstrating auditory processing abnormality in the average older adult: the Blue Mountains Hearing Study

OBJECTIVE

To determine, for the average older adult, the odds of demonstrating an auditory processing abnormality for each of seven speech-based measures of auditory processing and how these odds vary based on a number of independent subject variables.

DESIGN

Using a cross-sectional design, 1576 adults aged 55 years and older were assessed with speech measures of central auditory processing and questionnaires pertaining to health status, cognitive and perceived auditory function. The speech-based measures from which abnormal/normal outcomes were derived were (a) right ear Macquarie Synthetic Sentence Identification (MSSI) test maximum performance score (Rt MSSImax), (b) left ear MSSI test maximum performance score (Lt MSSImax), (c) right ear Macquarie Dichotic Sentence Identification (MDSI) test score (Rt MDSI), (d) left ear MDSI test score (Lt MDSI), (e) difference score for the right and left ear MDSI test (MDSI Diff score), (f) right ear MSSI test maximum performance score subtracted from the maximum performance score for monosyllabic word list materials in the same ear (Rt PB-MSSImax), and (g) left ear MSSI test maximum performance score subtracted from the maximum performance score for monosyllabic word list materials in the same ear (Lt PB-MSSImax).

RESULTS

The odds of demonstrating auditory processing abnormality for average older participants, increased by 4 to 9% per year of age. Men were approximately twice as likely as women to demonstrate this abnormality, but the gender difference was only evident with dichotic measures. With increasing hearing handicap, the odds of demonstrating auditory processing abnormality increased, but this was only evident for speech-in-noise measures. With subtle cognitive decline, the odds of demonstrating auditory processing abnormality also increased.

CONCLUSIONS

This population-based study provides evidence of a link between perceived hearing handicap and outcomes on speech-in-noise measures as well as evidence of a gender difference that became apparent using dichotic tests. The contribution of central auditory processing abnormality to hearing health should therefore not be overlooked in the provision of auditory rehabilitation programs to older adults.

Brain stem and cortical mechanisms underlying the binaural masking level difference in humans: an auditory steady-state response study

OBJECTIVE

The behavioral binaural masking level difference (BMLD) is believed to reflect brain stem processing. However, this conflicts with transient auditory evoked potential research that indicates the auditory brain stem and middle latency responses do not demonstrate the BMLD. The objective of the present study is to investigate the brain stem and cortical mechanisms underlying the BMLD in humans using the brain stem and cortical auditory steady-state responses (ASSRs).

DESIGN

A 500-Hz pure tone, amplitude-modulated (AM) at 80 Hz and 7 (or 13) Hz, was used to elicit brain stem and cortical ASSRs, respectively. The masker was a 200-Hz-wide noise centered on 500 Hz. Eleven adult subjects with normal hearing were tested. Both ASSR (brain stem and cortical) and behavioral thresholds for diotic AM stimuli (when the signal and noise are in phase binaurally: SoNo) and dichotic AM stimuli (when either the signal or noise is 180 degrees out-of-phase between the two ears: SpiNo, SoNpi) were investigated. ASSR and behavioral BMLDs were obtained by subtracting the threshold for the dichotic stimuli from that for the diotic stimuli, respectively. Effects for modulation rate, signal versus noise phase changes, and behavioral versus ASSR measure on the BMLD were investigated.

RESULTS

Behavioral BMLDs (mean = 8.5 to 10.5 dB) obtained are consistent with results from past research. The ASSR results are similar to the pattern of results previously found for the transient auditory brain stem responses and the N1-P2 cortical auditory evoked potential, in that only the cortical ASSRs (7 or 13 Hz) demonstrate BMLDs (mean = 5.8 dB); the brain stem ASSRs (80 Hz) (mean = 1.5 dB) do not. The ASSR results differ from the previous transient N1-P2 studies, however, in that the cortical ASSRs show a BMLD only when there is a change in the signal interaural phase, but not for changes of noise interaural phase.

CONCLUSIONS

Results suggest that brain processes underlying the BMLD occur either in a different pathway or beyond the brain stem auditory processing underlying the 80-Hz ASSR. Results also suggest that the cortical ASSRs have somewhat different neural sources than the transient N1-P2 responses, and that they may reflect the output of neural populations that previous research has shown to be insensitive to binaural differences in noise.

Introversion and individual differences in middle ear acoustic reflex function

A growing body of psychophysiological evidence points to the possibility that individual differences in early auditory processing may contribute to social withdrawal and introverted tendencies. The present study assessed the response characteristics of the acoustic reflex arc of introverted-withdrawn and extraverted-sociable individuals. Introverts displayed a greater incidence of abnormal middle ear acoustic reflexes and lower acoustic reflex amplitudes than extraverts. These findings were strongest for stimuli presented at a frequency of 2000 Hz. Results are discussed in light of the controversy concerning the anatomic loci (peripheral vs. central neuronal activity) of the individual differences between introverts and extraverts in early auditory processing.

The effect of midline petrous apex lesions on tests of afferent and efferent auditory function

OBJECTIVES

Historically, audiological procedures have focused on the assessment of the afferent (ascending) cochlear-VIIIth nerve system and have, for the most part, ignored the efferent (descending) auditory system. We report afferent and efferent auditory test results for two cases with a cholesterol cyst of the right petrous apex; one lesion involves the afferent segment of the auditory system, and the second lesion involves both the afferent and efferent segments of the auditory system. These "natural experiments" provide a unique opportunity to study the effect of a space-occupying lesion of the petrous apex on afferent and efferent function of the auditory system.

DESIGN

Transient evoked otoacoustic emission (TEOAE) suppression studies were performed to assess the effect of the cholesterol cyst on the efferent system of the two cases. In addition, three complementary afferent tests of brain stem auditory function were administered: 1) acoustic reflex thresholds (ARTs); 2) masking level difference (MLD); and 3) auditory brain stem response (ABR). These tests are complementary because the superior olivary complex (SOC) is involved not only in the mediation of the sound evoked efferent reflex assessed in TEOAE suppression, but in the mediation of the ARTs, the MLD, and the ABR.

RESULTS

The two cases with midline petrous apex lesions, one not involving the VIII-cochlear efferent auditory system, differed from each other with regards to TEOAEs suppression, and ARTs. Specifically, the case with only afferent involvement produced normal TEOAE suppression, a normal MLD, normal ARTs, and abnormal waves III and V of the ABR, whereas the case with both afferent and efferent involvement produced abnormal TEOAE suppression, a normal MLD, abnormal ARTs, and abnormal waves III and V of the ABR.

CONCLUSIONS

These cases illustrate that although several auditory tests can be mediated within the same or adjacent anatomical structures, i.e., the SOC, they may not be equally affected by the same lesion due to different physiology. Further, the TEOAE suppression paradigm is a clinically relevant test to assay the sound evoked efferent reflex that is mediated by the medial olivocochlear system of the SOC.

The brain stem evoked response and medial nucleus of the trapezoid body

Single-unit responses of cat superior olivary complex neurons to acoustic stimuli were examined to determine whether the units' action potentials were sufficiently synchronized to contribute to the brain stem evoked response. The medial nucleus of the trapezoid body and lateral superior olive are two major nuclei within the cat superior olivary complex. The first-spike discharge latencies of medial nucleus of the trapezoid body and lateral superior olivary neurons to monaural presentations of tone burst stimuli were measured as a function of stimulus level. Evidence is provided to support the hypotheses that in cat the medial nucleus of the trapezoid body may contribute directly to the monaural brain stem evoked response by producing action potentials synchronized to stimulus onset and may also contribute indirectly to the brain stem evoked response binaural difference wave bc by inhibiting the lateral superior olive unit excitatory responses synchronized to stimulus onset.

Brainstem auditory evoked responses from birth to adulthood: development of wave amplitude

Brainstem auditory evoked response (BAER) was recorded in children from birth to 6 years and adults to study the development of wave amplitude. The amplitudes of all BAER waves increased with age, the greatest changes occurring during early infancy. Adult values were reached at 6 months of age for wave I and 2 years for wave V. The two waves continued to increase above the adult values until the highest amplitude value was reached at 3 years for wave I and 5 years for wave V. Subsequently, the amplitudes decreased towards the values in adults. The V/I amplitude ratio, which was slightly than the adult value shortly after birth, decreased during the first year of life and reached the minimum value between 1 and 4 years. Thereafter, it increased towards the adult value. Throughout the maturational stages the ratio was smaller than in adults. The amplitude of wave V was relatively stable and its variation was much smaller than those of wave I and V/I amplitude ratio. Lower normative limits of these amplitude measures were given to provide a reference baseline for further study under the similar experimental conditions. We suggest that, in light of age-related differences of the V/I ratio, the normative criteria for the amplitude ratio should be established in relation to age. The amplitude of wave V is recommended to be useful in some clinical situations.

Does type I afferent neuron dysfunction reveal itself through lack of efferent suppression?

We present here two patients and three control subjects to demonstrate the clinical utility of studying evoked otoacoustic emissions and their contralateral suppression, as an aid to the delineation of afferent neuron dysfunction and possible lack of efferent suppression. The key patients here who fail to show contralateral suppression of their very robust otoacoustic emissions, concomitantly show paradoxically absent auditory brainstem responses (ABRs) and absence of middle ear muscle reflexes despite normal audiograms in the 2 kHz region and normal tympanograms. One of these patients has nearly normal pure tone sensitivity up to 3 kHz. The other has normal sensitivity in the 2 kHz region, but poor sensitivity on either side of that frequency. In addition, the two patients of interest show absent masking level differences and inordinately poor speech discrimination. Three 'foils' are presented: one patient with poor hearing on either side of 2 kHz, one with Bell's Palsy, and the third with bilateral temporal lobe disease. These patients show respectively that (1) isolated normal hearing at 2 kHz, (2) absence of middle ear muscle reflexes and (3) conscious cortical awareness of sound do not contribute directly to this intriguing clinical state. We propose that these patients with absent ABRs suffer from an auditory nervous system dysfunction which disrupts access to the efferent system. This condition also disables whatever systems contribute to the neural synchrony inherent in recording compound far-field action potentials such as the ABR. There are a number of hypotheses to be considered here. One suggests that the key patients are deficient in synchronous activation of Type I afferent fibers to the degree that they cannot activate efferent feedback, or they may be able to use only so-called Type II afferent neurons to support their normal zones of pure tone sensitivity. A less likely consideration focuses on the notion that discharge of primary neurons might be in some way synchronized by an efferent system which in these patients is the primary source of deficit.

Effects of noise bandwidth on the late-potential masking level difference

The masking level difference (MLD) is a psychoacoustic phenomenon derived from the subtraction of S pi No thresholds (signals pi radians out of phase and noise in phase at the two ears) from SoNo thresholds (signals and noise in phase at the two ears). The purpose of this study was to determine if the MLD derived from the late components (P1, N1, P2, N2) of the auditory evoked potentials was a physiological correlate of the behavioral MLD. Subjects were 15 young adults with normal hearing. Comparisons were made between behavioral and late potential thresholds to 500 Hz stimuli in So and S pi conditions in quiet, and to 500 Hz stimuli in SoNo and S pi No conditions in narrow band (50 Hz) and wide band (600 Hz) noise. No significant differences were seen for behavioral or late-potential thresholds to So and S pi conditions. The S pi No thresholds was significantly lower than the SoNo threshold, yielding an MLD for both the behavioral and physiological responses. The magnitudes of both the behavioral and late-potential MLD were larger with the narrow band noise than with the wide band noise. Evidence, therefore, is provided that the late-potential MLD reflects similar processes as are responsible for the behavioral MLD.

Prediction of binaural click lateralization by brainstem auditory evoked potentials

A previous study by Furst et al. (1985) has shown that in healthy subjects brainstem responses evoked by binaural auditory stimuli with interaural time difference (ITD) and interaural level difference (ILD) include information about the integration of data received by both ears. A correlation was found between the first major peak of the binaural difference waveform and perception of click lateralization and fusion. We have now tested whether a similar correlation exists in patients with multiple sclerosis (MS). The ability to lateralize dichotic clicks was tested in MS patients with normal audiograms. Two kinds of psychoacoustical experiments were employed: (1) A matching experiment in which the subject was asked to match the perceived positions of two click trains, one of which consisted of dichotic clicks with ILD and the other dichotic clicks with ITD; and (2) A positional JND experiment in which the subject was asked to determine the difference in perceived position of two successive click trains. Two reference positions were tested, the head center and the side of the head near the ear, while the control was either on ITD or on ILD. According to the psychoacoustical performances, three groups of patients were identified. Group I consisted of patients who performed normally in all the psychoacoustical experiments. Group II patients were able to lateralize binaural clicks but performed abnormally in the matching experiment and in the position discrimination experiment when the control was on ITD and the reference position was the head center. The patients in Group II performed normally in the discrimination experiments when the control was on ILD, and when the control was on ITD but the reference position was the head side. Group III consisted of those who were not able to perform either one of the psychoacoustical experiments. They perceived the same binaural clicks in different positions in different times. Brainstem auditory potentials evoked by dichotic clicks with different ILDs and ITDs were measured in all the MS patients, and the corresponding binaural difference (BD) waveforms were calculated. Whenever beta, the first major peak of BD, was identified it was used to obtain a physiological matching curve. It was derived by matching an ILD on the basis of similar beta latencies. For every patient, in either Group I or II, the physiological matching curve was very similar to his psychoacoustical matching curve.

Homo- and anti-phasic stimulation in ABR

Effects of homo-phasic and anti-phasic stimulation on the binaurally evoked auditory brain-stem responses (ABRs) were investigated in 10 normal subjects. Clicks were presented at 47 dB SL with two in-phase (binaural condensation and binaural rarefaction) and two out-of-phase (left ear condensation, right ear rarefaction and right ear condensation, left ear rarefaction) stimulus configurations. The earlier of the latencies obtained with homo-phasic stimulation were compared with the earlier of the latencies obtained with anti-phasic stimulation. Repeated measures ANOVAs on the peaks (P) and troughs (N) of each component, revealed significantly earlier latencies for the anti-phasic condition, for components II N (p less than 0.049), III P (p less than 0.012) and VIN (p less than 0.025). Similar trends were apparent for components IVP (p less than 0.075), VP (p less than 0.099) and VIP (p less than 0.12). The results are discussed in terms of the binaural-level difference phenomenon and possible clinical applications.

Auditory brainstem response and audiologic findings in adrenoleukodystrophy: its variant and carrier

Auditory brain stem response, auditory sensitivity, speech discrimination function, acoustic reflex threshold, and reflex decay were studied in seven children with adrenoleukodystrophy (ALD), nine adults with adrenomyeloneuropathy (AMN), one child with neonatal ALD, and two carriers of ALD. Significant auditory dysfunctions were found in only two patients; a retrocochlear type auditory disorder in one child with ALD, and a profound deafness in a child with neonatal ALD. None of the patients showed significant abnormality in acoustic reflex threshold and reflex decay except for one patient with ALD who showed a slightly elevated reflex threshold. All but two ALD patients showed significantly prolonged wave latencies and interwave intervals. The latency values revealed the highest incidence of abnormality in the I to III interval; this was followed by the III to V interval, then by wave I. Significantly, one child with asymptomatic ALD and both heterozygous female relatives showed abnormal ABRs, demonstrating the high sensitivity of ABR in detection of the existence of pathophysiological condition in subclinical or presymptomatic ALD.

The value of combining auditory brainstem responses and acoustic reflex threshold measurements in neuro-otological diagnosis

Acoustic reflex thresholds (ART) and auditory brainstem responses (ABR) were measured in 69 patients referred to the Neuro-otological clinic on the suspicion of a retrocochlear lesion. The pure-tone selection criterion was an average hearing loss of less than 60 dB at 0.5, 1, 2, and 4 kHz, with no thresholds exceeding 70 dB and only one threshold exceeding 60 dB. Both tests proved independently to be most effective in the diagnosis of cerebello-pontine angle (CPA) tumours and to a lesser extent for brainstem lesions. The ABR was positive in 100% of CPA group and 90% in the brainstem group compared with 93% and 70% respectively for the ART. The combination of the two tests provides patterns of abnormality which are more disease specific. In the CPA cases the most common and consistent patterns of abnormality were the unilateral delay of Wave V or III and V of the ABR associated with a vertical ART pattern indicative of an abnormality of the ipsi and contralateral reflexes on stimulating the affected side. Intrinsic brainstem lesions most commonly resulted in bilateral ABR abnormalities combined with the full-house ART pattern indicative of bilateral abnormalities of both ipsi and contralateral reflexes.

Interaction of click polarity, stimulus level, and repetition rate on the auditory brainstem response

The present study investigated the interaction of click polarity (compression, rarefaction, alternating), repetition rate (2.3 and 9.2 clicks per second), and stimulus level (60, 75, and 90 dB nHL) on auditory brainstem responses. Two tracings (trials) were obtained for each condition using 45 normally hearing subjects. Although no systematic Wave I, III, or V latency or amplitude differences were observed among polarities or repetition rates at the three intensities, statistically significant differences were observed for the following conditions: (1) Wave III latency at 90 dB nHL was longer for the 9.2 repetition rate than for the 2.3 rate, and latencies for compression clicks were shorter than rarefaction clicks; (2) Wave V latencies at 75 dB nHL were longer with compression clicks than with rarefaction clicks; (3) mean Wave V latencies at 60 and 75 dB nHL were slightly longer for the 2.3 click rate than for the 9.2 rate; and (4) Wave V amplitudes at 90 dB nHL were larger for rarefaction clicks than for compression clicks. Because latency and amplitude differences were small and only found in a few conditions, the results indicate that polarity is not a significant variable in normally hearing subjects when using slow repetition rates (less than or equal to 10 clicks per second) at moderate to high intensities (60 to 90 dB nHL). Because repetition rates of less than 10 clicks per second increases examination time but does not improve wave morphology, rates of 10 per second or faster are recommended for gathering normative data. Absolute amplitudes showed considerable intersubject variability, especially for Waves I and III. Intrasubject variability (test-retest) also was substantial. These findings suggest that absolute amplitudes may be of little use for distinguishing normal from pathologic populations.

The brainstem auditory evoked potential asymmetry is replicable and reliable

Lateralization of neural function is generally thought to occur only at the level of the cerebral cortex and perhaps the thalamus. Levine and McGaffigan [EEG Clin. Neurophysiol. 55, 532-537, 1983] challenged this view by identifying a neural asymmetry at the level of the brainstem. They analyzed brainstem auditory evoked potentials (BAEPs) and found that peak III(+) amplitude (baseline-to-peak) was significantly larger in response to right than to left ear stimulation. The current paper demonstrates that this BAEP asymmetry is (a) reliable within and between subjects, (b) present for 33/sec and 10/sec click rates, and (c) more reliable when amplitude is measured peak-to-peak. This brainstem asymmetry may reflect the general tendency of humans to orient rightward and/or may be a precursor of higher level asymmetries.

Masking level difference: an electrophysiological approach

Auditory brainstem responses (ABRs), middle latency responses (MLRs), and slow cortical potentials (SCPs) were registered in normal-hearing adults to trains of low-frequency signals delivered binaurally on a background of a continuous masking noise. Two stimulus conditions, labelled as S0M0 and S pi M0 paradigms, respectively, were systematically compared. In the S0M0 paradigm, both the signals and the masker were in-phase at two ears. In the S pi M0 paradigm, the signals were out-of-phase at two ears, while the masker was in-phase. The psychoacoustic release from masking in S pi M0 vs. S0M0 paradigms was regularly accompanied by an increase in amplitudes and a shortening in peak latencies of the SCPs. In contrast, no differences were evidenced between the S0M0 and the S pi M0 paradigms with respect to the ABRs and the MLRs. Considering the generation loci of the studied electric responses, it is concluded that the binaural psychoacoustic phenomenon, referred to as the masking level difference, is operated primarily at the cortical level.

Abnormalities of the acoustic reflex in multiple sclerosis

Acoustic reflect morphology was examined in 122 patients with a diagnosis of definite multiple sclerosis. Abnormality in some dimension of acoustic reflex morphology was observed in 75 per cent of the study population. The most commonly observed abnormality was an alteration in one or more of the three relative amplitude indices (afferent, efferent, or central pathway index). Other abnormalities included delayed onset latency, delayed offset latency, reduction in absolute amplitude, and threshold elevation.

Central auditory tests and auditory nerve-brainstem evoked responses in multiple sclerosis

In 43 multiple sclerosis (MS) patients and an equal number of age- and sex-matched normal controls, auditory nerve-brainstem evoked responses (ABR), audiometric tests, central auditory tests (CAT) and clinical-neurological examinations were conducted. The CAT included rapid alternating speech perception, masking level differences to tones and to words and lateralization based on interaural time and intensity differences. The results in the MS group on the ABR and each of the CAT differed significantly from those in the control group. The ABR results and the lateralization test results were each abnormal in 50% of the MS patients. The ABR, CAT and clinical examination results were all mutually correlated. These findings are probably related to the fact that the demyelination in MS induces a decrease in impulse velocity, to temporal dispersion and to desynchrony of impulses in groups of affected axons and to the fact that the ABR and the CAT tests chosen are dependent on synchronous impulse firing and on the precise timing of the arrival of impulses from each ear at some brain centre. These tests may therefore contribute to neurological and to audiological diagnosis.

Brain-stem involvement in multiple sclerosis: a comparison between brain-stem auditory evoked potentials and the acoustic stapedius reflex

Brain-stem auditory evoked potentials (BAEPs) and the acoustic stapedius reflex (ASR) were recorded in 68 patients with definite, probable and possible multiple sclerosis (using the definitions of McAlpine). The high incidence of abnormal results, 68% and 60%, respectively, pointed to the diagnostic value of these two measures in detecting brain-stem dysfunction. Combination of the methods increased the diagnostic yield to 85%. Since in part the same brain-stem generator sites underlie BAEPs and the ASR, it was considered that a study of their correlation might serve to increase the reliability and validity of these techniques. There was 71% agreement overall between results from the two measures. Furthermore, 72% of the joint BAEP and ASR abnormalities corresponded in detection of the brain-stem lesion site. It was concluded that the combined approach may supply powerful, complementary information on brain-stem dysfunction, which may aid in establishing the diagnosis of multiple sclerosis.

Binaural interaction measured behaviorally and electrophysiologically in young and old adults

12 young men (mean age = 39.1 years) and 12 older men (mean age = 69.4 years) presenting some degree of sensorineural hearing loss were chosen so that the two groups were audiometrically matched. Behavioral tests of binaural interaction (BI) included binaural fusion, rapidly alternating speech perception, and masking level difference (at 500 Hz). There was a tendency toward better performance by the younger subjects for each of these tests, but these comparisons fell short of statistical significance. Auditory brainstem responses (ABRs) and middle-latency responses (MLRs) to clicks were recorded in both monaural and binaural modes, with off-line derivation of BI components. Latencies of wave V (in ABR) and waves Pa and Nb (in MLR) were prolonged in the older subjects for all conditions; this effect was independent of degree of hearing loss. BI components did not differ substantially between the two groups, except for a 0.45-ms latency difference for the 'N1' peak, as expected from the raw ABR data. MLRs for both groups showed a pattern of BI which was different from that seen in young subjects with normal hearing, in that latency for wave Pa in MLR was not reduced in the binaural condition. The young subjects showed the expected reduction of binaural wave Pa amplitude compared to monaural sum responses, while the older subjects, as a group, did not. No significant correlations were found between behavioral and electrophysiological tests.

Stapedius reflex and brainstem auditory evoked responses in multiple sclerosis patients

The stapedius reflex (SR), its adaptation and the brainstem auditory evoked potential (ABR) were recorded in a group of 53 multiple sclerosis patients. All cases were classified as definite according to Schumacher's criteria, and their grade of disability in Hyllested's system was 1-4: no cases of the gravest (grades 5 and 6) disability were included in the series. The peripheral hearing (pure-tone audiogram and speech threshold) was normal in 44 and slightly impaired in 8 cases. SR was abnormal in 6 (11%) and borderline in 11 (21%), whereas the ABR was considered abnormal in 21 (40%) patients. Deviant SR and ABR findings had only slight or no correlation to clinical data. Intercorrelation between SR and ABR abnormality was better but not implicit and a few cases with deviant ABR showed normal SR: The frequency of abnormal and borderline SR findings was clearly higher in bilateral than in unilateral ABR abnormality. Subclassification of ABR into an upper and a lower type of pathology did not correlate with SR abnormality.