The combined effects of forward masking by noise and high click rate on monaural and binaural human auditory nerve and brainstem potentials

OBJECTIVE: To study effects of forward masking and rapid stimulation on human monaurally- and binaurally-evoked brainstem potentials and suggest their relation to synaptic fatigue and recovery and to neuronal action potential refractoriness. METHODS: Auditory brainstem evoked potentials (ABEPs) were recorded from 12 normally- and symmetrically hearing adults, in response to each click (50 dB nHL, condensation and rarefaction) in a train of nine, with an inter-click interval of 11 ms, that followed a white noise burst of 100 ms duration (50 dB nHL). Sequences of white noise and click train were repeated at a rate of 2.89 s(-1). The interval between noise and first click in the train was 2, 11, 22, 44, 66 or 88 ms in different runs. ABEPs were averaged (8000 repetitions) using a dwell time of 25 micros/address/channel. The binaural interaction components (BICs) of ABEPs were derived and the single, centrally located equivalent dipoles of ABEP waves I and V and of the BIC major wave were estimated. RESULTS: The latencies of dipoles I and V of ABEP, their inter-dipole interval and the dipole magnitude of component V were significantly affected by the interval between noise and clicks and by the serial position of the click in the train. The latency and dipole magnitude of the major BIC component were significantly affected by the interval between noise and clicks. Interval from noise and the click's serial position in the train interacted to affect dipole V latency, dipole V magnitude, BIC latencies and the V-I inter-dipole latency difference. Most of the effects were fully apparent by the first few clicks in the train, and the trend (increase or decrease) was affected by the interval between noise and clicks. CONCLUSIONS: The changes in latency and magnitude of ABEP and BIC components with advancing position in the click train and the interactions of click position in the train with the intervals from noise indicate an interaction of fatigue and recovery, compatible with synaptic depletion and replenishing, respectively. With the 2 ms interval between noise and the first click in the train, neuronal action potential refractoriness may also be involved.     

Electrophysiologic correlates of direction and elevation cues for sound localization in the human brainstem

Our objective was to study the effects of sound source direction and elevation on human brainstem electrical activity associated with sound localization. The subjects comprised 15 normal-hearing and symmetrically hearing adults Auditory brainstem evoked potentials (ABEPs) were recorded from three channels, in response to alternating-polarity clicks, presented at a rate of 21.1/s, at nine virtual spatial locations with different direction and elevation attributes Equivalent dipoles of the binaural interaction components (BICs) of ABEPs were derived by subtracting the response to binaural clicks at each spatial location from the algebraic sum of monaural responses to stimulation of each ear in turn. The BICs included two major components corresponding in latency to the vertex-neck-recorded components V and VI of ABEP. A significant decrease of the first BIC's equivalent dipole magnitude was observed for clicks in the horizontal-frontal position (no elevation) in the midsagittal plane, and for clicks in the left-horizontal (no elevation) and right diagonally above the head (medium elevation) positions in the coronal plane, compared to clicks positioned directly above the head. Significant effects on equivalent dipole latencies of this component were found for front-back positions in the midsagittal plane and left-right positions in the coronal plane, compared to clicks positioned directly above the head. The most remarkable finding was a significant change in equivalent dipole orientations across stimulus conditions. We conclude that the changes in BIC equivalent dipole latency, amplitude and orientation across stimulus conditions reflect differences in the distribution of binaural pontine activity evoked by sounds in different spatial locations.     

Three-channel Lissajous' trajectories of auditory brainstem evoked potentials: contribution of fast and slow components to planar segment formation

Three-Channel Lissajous' Trajectories (3CLT) of Auditory Brainstem Evoked Potentials (ABEP) to clicks were obtained after finite impulse response filtering in three frequency bands. These bands were chosen to replicate the widely used passband (100-3000 Hz) and to selectively enhance the definition of the 'pedestal' (10-240 Hz) or the first, third and fifth components (240-483 Hz). Quantitative measures of 3CLT were calculated to describe apex latencies, planar segment orientations, durations, trajectory amplitude peaks and their latencies. In addition, dipole moments at the latencies of apical points along 3-CLT were calculated. The planarity of ABEP 3-CLT segments persisted after selective enhancement of the 'pedestal' or the first, third and fifth components. These results rule out the suggestion that planarity of ABEP segments results from the interaction of the 'pedestal' with the superimposed faster components. These results demonstrate summation of 3-CLT planar segments ('a' 'c' and 'e' with the 'pedestal') to form new segments (wide-band 'a', 'c' and 'e'). With the exception of 'c', planar segments and the equivalent dipole moments associated with apexes did not change orientations across passbands. The effects of passband on the orientation of planar segment 'c' and the dipole moment of its apex are explained by its superimposition on the 'pedestal' in the wide-band records. A similar analysis of ABEP to clicks as compared to low-frequency stimuli (high-pass masked clicks) revealed no change in planarity nor in plane parameters. These results are compatible with the suggestion that the generators of the first, third and fifth ABEP components are curved fiber tracts. The planarity of the slow 'pedestal' may be due to the summation of slow synaptic potentials in auditory brainstem nuclei. These findings indicate that the generators of ABEP are composites that may be separated by selective lesion studies.     

Comparison of binaural auditory brainstem responses and the binaural difference potential evoked by chirps and clicks

Rising chirps that compensate for the dispersion of the travelling wave on the basilar membrane evoke larger monaural brainstem responses than clicks. In order to test if a similar effect applies for the early processing stages of binaural information, monaurally and binaurally evoked auditory brainstem responses were recorded for clicks and chirps for levels from 10 to 60 dB nHL in steps of 10 dB. Ten thousand sweeps were collected for every stimulus condition from 10 normal hearing subjects. Wave V amplitudes are significantly larger for chirps than for clicks for all conditions. The amplitude of the binaural difference potential, DP1-DN1, is significantly larger for chirps at the levels 30 and 40 dB nHL. Both the binaurally evoked potential and the binaural difference potential exhibit steeper growth functions for chirps than for clicks for levels up to 40 dB nHL. For higher stimulation levels the chirp responses saturate approaching the click evoked amplitude. For both stimuli the latency of DP1 is shorter than the latency of the binaural wave V, which in turn is shorter than the latency of DN1. The amplitude ratio of the binaural difference potential to the binaural response is independent of stimulus level for clicks and chirps. A possible interpretation is that with click stimulation predominantly binaural interaction from high frequency regions is seen which is compatible with a processing by contralateral inhibitory and ipsilateral excitatory (IE) cells. Contributions from low frequencies are negligible since the responses from low frequencies are not synchronized for clicks. The improved synchronization at lower frequencies using chirp stimuli yields contributions from both low and high frequency neurons enlarging the amplitudes of the binaural responses as well as the binaural difference potential. Since the constant amplitude ratio of the binaural difference potential to the binaural response makes contralateral and ipsilateral excitatory interaction improbable, binaural interaction at low frequencies is presumably also of the IE type. Another conclusion of this study is that the chirp stimuli employed here are better suited for auditory brainstem responses and binaural difference potentials than click stimuli since they exhibit higher amplitudes and a better signal-to-noise ratio.     

Auditory Brain Stem Potentials Evoked by Clicks in Notch-Filtered Masking Noise: Audiological Relevance

Auditory brainstem-evoked potentials (ABEP) to clicks in notch-filtered masking noise were recorded from 10 adults with normal audiograms and from 10 adults with hearing losses. Masking noise had 0.5-octave-wide notches with central frequencies of 8 000, 2 000, 1 000, 500 and 250 Hz. For each notch frequency, decreasing-intensity series in 10-dB steps (for both the click and the noise) were derived until the detection threshold of ABEP was reached. From the detection thresholds of normal subjects, the normative detection threshold for each frequency was calculated. Deviations from the normative detection thresholds were considered ABEP estimates of hearing loss. The results of this study showed that while ABEP to clicks in notched noise were sensitive to hearing losses, correlations between audiometric hearing loss and ABEP estimates of hearing losses at specific frequencies were nonsignificant. At the present stage of the procedure, ABEP to clicks in notched noise cannot estimate hearing thresholds at specific frequencies.     

Three-channel Lissajous' trajectory of the binaural interaction components of human auditory middle-latency evoked potentials

Three-channel Lissajous' trajectories (3-CLTs) of the binaural interaction component (BI) of auditory middle latency evoked potentials (AMLEPs) were derived from 14 normally hearing adults by subtracting the response to binaural clicks from the algebraic sum of monaural responses. AMLEPs were recorded in response to 65 dB nHL, rarefraction clicks, presented at a rate of 3.3/s. A normative set of BI 3-CLT measures was calculated and compared with the corresponding measures of simultaneously recorded, single-channel vertex-left mastoid and vertex-neck derivations of BI and of AMLEP to binaural stimulation (B). 3-CLT measures included: apex latency, amplitude and orientation, as well as planar segment duration, orientation, size and shape.The results showed seven main apices and associated planar segments (‘Be’, ‘Bf’, ‘Bg’, ‘Bh’, ‘Bi₁’, ‘Bi₂’ and ‘Bj’) in the 3-CLT of BI. Apex latencies of the BI 3-CLT were comparable to peak latencies of the vertex-left mastoid and vertex-neck AMLEP and BI records, both in their absolute values and in intersubject variability. Durations of BI planar segments were approximately 5.0 ms. Apex amplitudes of BI 3-CLT were larger than the respective peak amplitudes of the vertex-mastoid and vertex-neck BI records, while their intersubject variabilities were comparable.The lateralization of BI components may indicate asymmetric processing of binaural auditory input, or may be connected with anatomical asymmetry such as skull thickness. Preliminary analyses did not reveal a clear correlation between the lateralization of the BI component ‘Bi₂’ and the handedness of the subject. We suggest that BI components of AMLEP may be associated with the primary auditory cortex and subcortical ascending structures.     

Frequency dependence of interear asymmetries and binaural interaction in the human ABR

In the present study derived auditory brain stem responses utilizing high pass masking were used to determine the frequency dependence of interear asymmetries and binaural interaction. Eight female adults with normal hearing participated in this study. The ABR was recorded for right and left ears and binaurally with clicks alone and in the presence of high pass masking noise with the following cutoff frequencies: 4000, 2000, 1000, and 500 Hz. Derived responses were obtained by successive waveform subtractions and were analyzed for significant right/left asymmetries, for latency and amplitude, utilizing a statistical method discussed by Spivak (unpublished doctoral dissertation, City University of New York, 1985). The binomial test revealed that the frequency of latency and amplitude asymmetries was not significantly above chance levels for the unmasked measures or for any of the derived bandwidth measures. The binaural interaction component (BIC) was obtained utilizing the method proposed by Dobie and Berlin (Arch Otolaryngol 1979; 105:391-398), and was expressed as a percentage of the true binaural. Analysis of variance revealed no significant differences in the ratio of the BIC amplitude to the true binaural amplitude as a function of the high pass masking cutoff frequency. The present data suggests that for the frequency bandwidths analyzed in this study, there are no specific frequency dependencies of interear asymmetries or binaural interaction in the human ABR.     

Electrophysiological correlates of azimuth and elevation cues for sound localization in human middle latency auditory evoked potentials

OBJECTIVE: To study, in humans, the effects of sound source azimuth and elevation on primary auditory cortex binaural activity associated with sound localization. DESIGN: Middle Latency Auditory Evoked Potentials (MLAEPs) were recorded from three channels, in response to alternating polarity clicks, presented at a rate of 5/sec, at nine virtual spatial locations with different azimuths and elevations. Equivalent dipoles of Binaural Interaction Components (BICs) of MLAEPs were derived from 15 normally and symmetrically hearing adults by subtracting the response to binaural clicks at each spatial location from the algebraic sum of responses to stimulation of each ear alone. The amplified potentials were averaged over 4000 repetitions using a dwell time of 78 micro sec/address/channel. Variations in magnitudes, latencies and orientations of the dipole equivalents of cortical activity were noted in response to the nine spatial locations. RESULTS: Middle-latency BICs included six major components corresponding in latency to the vertex-neck recorded components of MLAEP. A significant decrease of equivalent dipole magnitude was observed for two of the components: Pa2 in response to clicks in the backward positions (medium and no elevation); and Nb in response to clicks in the back and front positions (medium and no elevation) in the midsagittal plane. In the coronal plane, Pa2 equivalent dipole magnitude significantly decreased in response to right-horizontal (no elevation) clicks. Significant effects on equivalent dipole latencies of Pa2 were found for backward positions (no elevation) in the midsagittal plane. No significant effects on Pa2 and Nb equivalent dipole orientations were found across stimulus conditions. CONCLUSIONS: The changes in equivalent dipole magnitudes and latencies of MLAEP BICs across stimulus conditions may reflect spectral tuning in binaural primary auditory cortex neurons processing the frequency cues for sound localization.     

Binaural interactions of electrically and acoustically evoked responses recorded from the inferior colliculus of guinea pigs

Binaural interactions within the inferior colliculus (IC) elicited by electric and acoustic stimuli were investigated in this study. Using a guinea pig model, binaural acoustic stimuli were presented with different time delays, as were combinations of binaural electric and acoustic stimuli. Averaged evoked potentials were measured using electrodes inserted into the central nucleus of the IC to obtain the binaural interaction component (BIC), computed by subtracting the sum of the two monaural responses from the binaural response. The BICs to acoustic-acoustic stimulation and electric-acoustic stimulation were found to be similar. The BIC amplitude increased with stimulus intensity, but the shapes of the delay functions were similar across the levels tested. The gross-potential data are thus consistent with the thesis that the central auditory system processes binaural electric and acoustic stimuli in a similar manner. These results suggest that the binaural auditory system can process combinations of electric and acoustic stimulation presented across ears and that evoked gross potentials may be used to measure such interaction.     

Binaural interactions of electrically and acoustically evoked responses recorded from the inferior colliculus of guinea pigs

Binaural interactions within the inferior colliculus (IC) elicited by electric and acoustic stimuli were investigated in this study. Using a guinea pig model, binaural acoustic stimuli were presented with different time delays, as were combinations of binaural electric and acoustic stimuli. Averaged evoked potentials were measured using electrodes inserted into the central nucleus of the IC to obtain the binaural interaction component (BIC), computed by subtracting the sum of the two monaural responses from the binaural response. The BICs to acoustic-acoustic stimulation and electric-acoustic stimulation were found to be similar. The BIC amplitude increased with stimulus intensity, but the shapes of the delay functions were similar across the levels tested. The gross-potential data are thus consistent with the thesis that the central auditory system processes binaural electric and acoustic stimuli in a similar manner. These results suggest that the binaural auditory system can process combinations of electric and acoustic stimulation presented across ears and that evoked gross potentials may be used to measure such interaction.     

Binaural interaction in brainstem auditory evoked potentials elicited by frequency-specific stimuli

The frequency specificity of the binaural interaction in brainstem auditory evoked potentials (BAEP) was investigated in ten normal-hearing young adults. A novel stimulus paradigm was devised to reduce the influence of the acoustic reflex (middle ear muscle contraction) on the BAEP, and to minimize the effect of variations in noise level. Sequences of six stimuli (rarefaction clicks or Gaussian-shaped tone pulses with carrier frequencies of 1, 2, 4 and 6 kHz) were periodically presented in the following order: right monaural, left monaural, binaural, left monaural, right monaural, binaural, with an interstimulus interval of 22 ms. Since the sequence of monaural stimuli with binaural stimuli interposed produces a uniform loudness and since the acoustic reflex is a consensual reflex, the relative high stimulus repetition rate (approx. 45/s) causes a muscle contraction which is equal on both sides and rather constant in time. This paradigm turned out to be usable for stimulus intensities as high as 80 dB nHL. The binaural difference potential (BDP) was computed by subtracting the sum of the monaurally (ipsilateral and contralateral) evoked potentials from the binaurally evoked potential. The major binaural interaction occurred in the latency range of BAEP waves V and VI, and there was no evidence of interaction in the earlier portion of the BAEP. Both latency and amplitude of the BDP components were evaluated statistically. The latency of the BDP components - except of the lasted one - showed an almost linear dependence both on stimulus intensity and stimulus frequency. The amplitude grew larger with decreasing frequency, and the visual detection threshold elevated as the stimulus frequency increased. Click stimuli, however, produced the largest amplitudes with lowest visual detection threshold. This novel stimulus paradigm appears to be most suitable for routine clinical investigations since high stimulus intensities can be used.     

Equivalent dipoles of the binaural interaction components and their comparison with binaurally evoked human auditory 40 Hz steady-state evoked potentials

OBJECTIVE: The purpose of this study was to acquire the Binaural Interaction (BI) components of the auditory middle-latency steady-state 40 Hz potentials, compare them with those of the binaurally evoked 40 Hz response and with transient-evoked Auditory Middle Latency Evoked Potentials (AMEP) and suggest possible contributors and generators of the composite 40 Hz BI. METHODS: Potentials were recorded from 15 normal-hearing adults in response to 40/sec clicks. BI was derived by subtracting the binaurally evoked potentials from the algebraic sum of the evoked potentials to left and to right ear stimulation. Latencies, magnitudes and orientations of the dipole equivalents of 40 Hz components were compared with their BI counterparts, as estimated by three-channel Lissajous' trajectories. Comparison of the transient AMEP to binaural stimulation with the BI of the steady-state 40 Hz response was also conducted to elucidate the contributions of different levels along the auditory pathway to the 40 Hz BI responses. RESULTS: Each cycle of the BI of the steady-state 40 Hz AMEP included four components that corresponded in latency, amplitude, and dipole orientation to their counterparts in the binaurally evoked waveform. Amplitudes of BI components were 50 to 60% of the respective values in the binaurally evoked potentials. Orientations of BI components matched those of the cortical components in the transient-evoked AMEP. CONCLUSIONS: The results suggest that the main contribution to the 40 Hz BI is from rate resistant thalamo-cortical neurons. The results also suggest that the binaural cortical neurons contributing to the 40 Hz BI are less affected by increased rate than monaural neurons.     

Auditory evoked potentials and audiological follow-up of subjects developing noise-induced permanent threshold shift

Three examinations, including cochlear microphonics (CM) to 99/s 1-kHz tones and auditory brainstem evoked potentials (ABEP) to 10/s and 55/s clicks, as well as psychoacoustical tests, were performed on 31 normally hearing subjects, exposed to occupational noise for over a year. The results showed prolongation of the absolute latency of peaks I, III and V, without significant changes in interpeak latency differences (IPLD) or CM latency. The site affected by increasing stimulus rate, giving rise to increased IPLD, appears to be central rather than cochlear. The efficacy of increased stimulus rate in detecting noise-induced auditory changes was higher than achieved applying the 10/s click rate.     

Auditory brainstem evoked potentials peak identification by finite impulse response digital filters

Linear phase finite impulse response (FIR) filtering can be used to differentiate auditory brainstem evoked potentials (ABEP) components. The power spectrum of ABEP at high intensities indicates that they contain 3 frequency bands that can be distinguished by applying appropriate digital filters with the following characteristics: up to 240 Hz (revealing slow components), 240-483 Hz (resulting in medium components) and above 500 Hz (leaving only fast components). The results using these filters, indicate that the medium components coincide with peaks I, III and V and that the slow filter results in a 'pedestal' whose peak coincides with peak V. These findings were used for automatic identification of ABEP peaks. A coincidence of the 'pedestal' peak with a medium component was sought and labelled peak V. The preceding medium peaks were labelled, in order of decreasing latency, III and I. Validation of this procedure was conducted on ABEP from normal subjects, using different stimulus rates and intensities, as well as from selected neurological patients with lesions affecting the brainstem. Provided the waveform included a 'pedestal', the results proved this procedure to be reliable and in very good agreement with manual identification and measurement of ABEP peaks.     

Effects of click polarity on auditory brain-stem potentials: a three-channel Lissajous' trajectory study

Three-channel Lissajous' trajectories (3CLTs) of the auditory brain-stem evoked potentials (ABEP) were recorded from 16 adult subjects (28 ears) in response to rarefaction (R), condensation (C) and alternating polarity (A) clicks. 3CLTs were analysed and described in terms of their geometrical measures. All 3CLTs included 5 planar segments (named 'a', 'b', 'c', 'd' and 'e') whose latencies, durations, orientations, sizes and shapes were not affected by click polarity. Occasionally, planar segment 'd' was not defined, and its absence was parallelled by the absence of peak IV in the Vertex-Mastoid records. A small (0.03 microV), but significant increase was found in the trajectory amplitude of planar segment 'e' in C clicks. The effects of click polarity on 3CLT observed in this study suggest that some previously described ABEP changes with click polarity were the result of interactions between electrode positions and relative contributions of overlapping generators. The effects on the fourth and fifth ABEP components may be the results of changes in the temporal overlap of the activity of their generators.     

The effects of click and tone-burst stimulus parameters on the vestibular evoked myogenic potential (VEMP)

Vestibular evoked myogenic potentials (VEMP) are short latency electromyograms (EMG) evoked by high-level acoustic stimuli and recorded from surface electrodes over the tonically contracted sternocleidomastoid (SCM) muscle and are presumed to originate in the saccule. The present experiments examined the effects of click and tone-burst level and stimulus frequency on the latency, amplitude, and threshold of the VEMP in subjects with normal hearing sensitivity and no history of vestibular disease. VEMPs were recorded in all subjects using 100 dB nHL click stimuli. Most subjects had VEMPs present at 500, 750, and 1000 Hz, and few subjects had VEMPs present at 2000 Hz. The response amplitude of the VEMP increased with click and tone-burst level, whereas VEMP latency was not influenced by the stimulus level. The largest tone-burst-evoked VEMPs and lowest thresholds were obtained at 500 and 750 Hz. VEMP latency was independent of stimulus frequency when tone-burst duration was held constant.     

耳蜗电图诊断梅尼埃病的灵敏度研究

目的：探讨耳蜗电图在梅尼埃病（MD）诊断中的应用价值。方法：将测试对象分为确诊、可能和疑似MD组,将可能和疑似MD组合并为可疑MD组。依据纯音听阈测试结果对确诊MD组患耳按听力损失程度进行病情分级。采用短声（click）和1 000、2 000、4 000Hz 3种频率tone burst（TB）分别对测试耳进行耳蜗电图测试,并计算和电位（SP）与听神经复合动作电位（AP）的振幅比值（SP/AP）,同时采用由click刺激声引出的AP疏波和密波潜伏期的差值（AP shift）辅助诊断。分别计算不同刺激声不同测试方法的诊断阳性率,比较MD的诊断阳性率。结果：确诊MD组click SP/AP阳性率为41.2%,TB 1 000、2 000、4 000Hz阳性率分别为80.4%、72.5%、37.3%,AP shift阳性率为45.1%。配对χ^2检验显示,TB 1 000 Hz与click SP/AP阳性率比较、TB 2 000Hz与click SP/AP阳性率比较均差异有统计学意义（均P〈0.01）,其中TB 1 000Hz诊断阳性率最高,即灵敏度最高。确诊MD组与可疑MD组患耳click SP/AP阳性率分别为41.2%和12.0%（P〈0.05）,AP shift阳性率分别为45.1%和8.0%（P〈0.01）,χ^2检验显示2组间click SP/AP和AP shift阳性率差异有统计学意义。结论：耳蜗电图在MD诊断及鉴别诊断中作用显著,特别是采用1 000Hz和2 000HzTB刺激声,其诊断灵敏度高达80.4%和72.5%。AP shift也被证明是一种有效的测量方法,在辅助诊断MD中作用显著。     

The effects of intense click sounds on velocity storage in optokinetic after-nystagmus

Vestibular evoked myogenic potentials (VEMP) occurring after click stimulation in cervical muscles are thought to be a polysynaptic response of otolith-vestibular nerve origin. In optokinetic after-nystagmus (OKAN) the direction of after-nystagmus changes and slow-phase velocity decreases with head tilt. This phenomenon may be an otolith response to the direction of gravity. We assumed that intense clicks might have some influence on OKAN via the otolith-vestibular nerve. Twelve normal subjects who showed VEMP at 75 dB normal hearing level (nHL) clicks were examined. The OKAN was recorded under four conditions: right monaural, left monaural and binaural stimulation by 75 dB nHL clicks, and absence of click stimulation. Horizontal optokinetic stimulation was applied using stepwise increasing speeds from 30 deg/s to 90 deg/s. Two seconds before the stimulus ended, clicks were sounded. The slow-phase velocity of the recorded electro-nystagmography was manually measured. There was no effect on OKAN with unilateral stimulation but binaural stimulation suppressed it. These results suggest that a velocity storage integrator is influenced by intense clicks via the otolithic area. Received: 17 November 1999 / Accepted: 30 May 2000     

Binaural interaction of bone-conducted auditory brainstem responses in children with congenital atresia of the external auditory canal

Bilateral bone-conducted auditory brainstem responses (BC-ABRs) were recorded in children with atresia of the external auditory canal bilaterally (AECB) in order to compare the response characteristics to normal hearing adults. The binaural interaction component (BIC) of the ABR occurs when the sum of the monaural-evoked ABR amplitudes are different in amplitude when compared to the binaural-evoked ABR amplitude. Previous electrophysiological work from our lab has shown that children with AECB lateralize bone-conducted (BC) sound. Furthermore, we have found in normal-hearing adults that BICs exist using BC clicks. In adults, BC-BIC occurred in the latency region corresponding to waves IV-VI, whereas for children with AECB corresponding peak latencies occurred earlier. Same as normal-hearing adults, BC-ABR IV-V complex peak amplitudes for sum of the BC-monaural right and BC-monaural left ears were different from binaural response amplitude. Individual peak latencies were similar in children with AECB when compared to normal-hearing adults except for shorter latencies for BIC. These results indicate that: (1) BC-BI is present in children with AECB as well as normal-hearing adults; (2) the gross response properties of BIC are similar in children with AECB and normal-hearing adults; (3) fitting of a bilateral BC hearing aid might be a feasible method to optimize binaural hearing and sound lateralization.     

A comparison of auditory brain stem evoked potentials in hyperlipidemics and normolipemic subjects

Twenty-five hyperlipidemic, neurologically and audiologically asymptomatic patients were compared to 20 normolipemic control subjects regarding different interpeak latency differences (IPLD) of auditory brain stem evoked potentials (ABEP). ABEP were recorded in response to click stimuli presented at a rate of 10/sec and 55/sec. The net effect of increasing stimulus rate (ISR) on IPLD of ABEP was significantly greater in the hyperlipidemic patients for IPLD (V-I) and IPLD (III-I). ISR of ABEP indicates a trend of subclinical impairments of brain stem function in hyperlipidemic patients, probably due to ischemia accelerated by their condition.