Binaural Interaction Component Research Articles

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.

Interaural delay-dependent changes in the binaural interaction component of the guinea pig brainstem responses

Auditory brainstem responses to monaural and binaural clicks with 23 different interaural time differences (ITDs) were recorded from ten guinea pigs without anesthesia. Binaural interaction component was obtained by subtracting the sum of the appropriately time-shifted left and right monaural responses from the binaural one. With increasing ITD, the most prominent peak of the binaural difference potential so obtained shifted to longer latencies and its amplitude gradually decreased. The way these changes depended on binaural delay was basically similar to that previously observed in a cat study [P. Ungan, S. Yagcioglu, B. Ozmen. Interaural delay-dependent changes in the binaural difference potential in cat auditory brainstem response: implications about the origin of the binaural interaction component. Hear. Res. 106 (1997) 66-82]. The data were successfully simulated by the model suggested in that report. We therefore concluded that the same model, which was based on the difference between the mean onset latencies of the ipsilateral excitation and contralateral inhibition in a typical neuron in the lateral superior olive, their standard deviations, and the duration of the contralateral inhibition, should also be valid for the binaural interaction in the guinea pig brainstem. The results, which were discussed in connection with sound lateralization models, supported a model based on population coding, where the lateral position of a sound source is coded by the ratio of the discharge intensity in the left and right lateral superior olives, rather than the models based on coincidence detection.

Electrophysiological measures of binaural interaction in cochlear implantees

Electrophysiological measures that can be used to objectively evaluate binaural interaction in cochlear implantees include the binaural interaction component (BIC) and an evoked potential analog of the ‘masking-level difference’ (MLD). Results from normal listeners show that cortical auditory-evoked responses obtained using these methods can be used to measure binaural processing. Preliminary data from three sequentially implanted adults are presented.

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 μs/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.

A time-frequency feature extraction scheme for the automated detection of binaural interaction in auditory brainstem responses

The binaural interaction component (BIC), the difference between the summed monaurally evoked potentials of each ear and the binaurally evoked brainstem potentials, has been shown to be related to directional hearing. However, the detection of the ß-peak as the most consistent part of the BIC is often difficult. Furthermore, there is no clearly defined signal feature characterizing the difference between the monaurally and the binaurally evoked brainstem responses. A closer look at the signals shows that amplitude differences as well as latency differences and variations in wave V slopes could be the reason for the formation of a ß-peak. Using a time-scale feature extraction scheme, we were able to define a signal feature (morphological local discriminant bases ( MLDB) coefficient 1) that accounts for the difference between the sum of the monaurally and binaurally evoked brainstem potentials. With use of this signal feature, reliable automated detection of differences between monaurally and binaurally evoked potentials is possible. As coefficient 1 replicates the behaviour of subjective measurements as well as of the BIC measurements, it can also be seen as a correlate of binaural interaction. With use of this signal feature, it is possible to judge from a given binaurally evoked potential whether it contains information on binaural interaction or not, without comparing it to the sum of the monaurally evoked brainstem responses. Consequently, binaural interaction can be assessed by one, instead of three, measurements by using the method described in this paper.

Analysis and detection of binaural interaction in auditory evoked brainstem responses by time-scale representations

The β-wave of the binaural interaction component (BIC) in auditory evoked brainstem responses has been shown to be an objective measure of binaural interaction. However, a reliable and automated detection of this component capable of clinical use still remains a challenge. In this study, wavelet based time-scale representations of auditory evoked brainstem responses were investigated for the analysis of binaural interaction and for an automated detection of the β-wave. Twenty normal hearing subjects with verified normal directional hearing and speech intelligibility in noise were included in our study. In all of these subjects, the BICs exhibited a characteristic concentration of energy in the time-scale domain which allowed for an automated detection of the β-wave. Moreover, our study provides an explanation why the β-wave is hard to detect for larger interaural time delays using time-scale entropy based arguments. It is concluded that time-scale representations of auditory brainstem responses are well suited for the analysis of binaural interaction and allow for an automated detection of the β-wave.

Effects of interaural frequency difference on binaural fusion evidenced by electrophysiological versus psychoacoustical measures.

The binaural interaction component (BIC=sum of monaural-true binaural) of the auditory brainstem response appears to reflect central binaural fusion/lateralization processes. Auditory middle-latency responses (AMLRs) are more robust and may reflect more completely such binaural processing. The AMLR also demonstrates such binaural interaction. The fusion of dichotically presented tones with an interaural frequency difference (IFD) offers another test of the extent to which electrophysiological and psychoacoustical measures agree. The effect of IFDs on both the BIC of the AMLR and a psychoacoustical measure of binaural fusion thus were examined. The perception of 20-ms tone bursts at/near 500 Hz with increasing IFDs showed, first, a deviated sound image from the center of the head, followed by clearly separate pitch percepts in each ear. Thresholds of detection of sound deviation and separation (i.e., nonfusion) were found to be 57 and 209 Hz, respectively. However, magnitudes of BICs of the AMLR were found to remain nearly. constant for IFDs up to the 400-Hz (limit of range tested), suggesting that the AMLR-BIC does not provide an objective index of this aspect of binaural processing, at least not under the conditions examined. The nature of lateralization due to IFDs and the concept of critical bands for binaural fusion are also discussed. Further research appears warranted to investigate the significance of the lack of effect of IFDs on the AMLR-BIC. Finally, the IFD paradigm itself would seem useful in that it permits determination of the limit for nonfusion of sounds presented binaurally, a limit not accessible via more conventional paradigms involving interaural time, phase, or intensity differences.

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 aids might be a feasible method to optimize binaural hearing and sound lateralization.

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

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. 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. 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. 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.

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

To study, in humans, the effects of sound source azimuth and elevation on primary auditory cortex binaural activity associated with sound localization. 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. 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. 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.

The cumulative effect of high click rate on monaural and binaural processing in the human auditory brainstem.

The objective of the present study was to compare the effects of high stimulus rate and click position in the train on monaurally and binaurally evoked activities in the human auditory brainstem and suggest their possible physiological mechanism. Auditory brainstem evoked potentials (ABEPs) were recorded from 15 normally and symmetrically hearing adults from 3 channels, in response to 50dB nHL, alternating polarity clicks, presented at a rate of 21/s as well as separately to each click in a train of 10 with an interstimulus interval of 11ms. Click trains were presented at a rate of 5.13/s. The binaural interaction components (BICs) of ABEPs were derived by subtracting the response to binaural clicks from the algebraic sum of monaural responses. Single, centrally located equivalent dipoles were estimated as concise measures of the surface-summated activity of ABEPs and BICs generators. A significant effect of click position in the train on equivalent dipole latency of ABEP component V and on equivalent dipole magnitude of III were found. Latency was prolonged and amplitude was increased the later the click's position in the train. A significant effect of click position in the train on equivalent dipole latencies of all components of BICs was found. Latencies were prolonged if the click's position occurred later in the train, with most of the latency shift achieved by the third click in the train for the first major BIC and by the seventh click for other BIC components. No significant effects on equivalent dipole magnitudes of BICs were found. No significant effect of click position in the train on orientation of any of the equivalent dipoles of ABEP or BIC was found. The progressive prolongation of latency of ABEP and BIC components with advancing position in the train may be attributed to cumulatively decreased synaptic efficacy at high stimulus rates, resulting in prolonged synaptic delays along the auditory pathway. The paradoxic enhancement of ABEP dipole III magnitude with advancing click position in the train may reflect higher sensitivity of inhibitory brainstem neurons to increased stimulus rate, resulting in disinhibition. The absence of significant effects on BIC dipole magnitudes may reflect the amplifying effect of divergence in the ascending auditory pathway, as has been observed for the monaurally evoked ABEP components from the upper pons.

Electrophysiologic correlates of direction and elevation cues for sound localization in the human brainstem: Correlaciones electrofisiológicas en las claves de dirección y elevación para la localizatión del sonido en el tallo cerebral humano

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 BICs 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.Nuestro objetivo fue estudiar los efectos de la dirección y la elevación de la fuente sonora sobre la actividad eléctrica del tallo cerebral humano asociados a la localización del sonido. Se estudiaron 15 adultos con audición normal y simétrica. Se registraron potenciales evocados auditivos del tallo cerebral (ABEPs) en tres canales, en respuestas a clicks de polaridad alternante, presentados a una tasa de 21.2/seg, en nueve localizaciones espaciales virtuales, con diferentes atributos de dirección y elevación. Se derivaron dipolos equivalentcs de los componentes de interacción binaural (BICs) de los ABEPs, restando la respuesta a los clicks binaurales en cada localización espacial, de la suma algebraica de las respuestas monoanrales a la estimulación de cada oido. Los BICs incluyeron dos componentes mayores, correspondientes en latencia a los componentes V y VI de los ABEPs registrados en la derivación vertex-cuello. Se observó una disminución significativa en la magnitud del primer dipolo equivalente de los BICs, cuando en el piano medio sagital se utilizaron clicks en la posición horizontal-frontal (sin elevación), y para clicks en el piano horizontal izquierdo (sin elevación) y en la posición derecha diagonal (elevación media), en el piano coronal, comparada con clicks colocados directamente por encima de la cabcza. Se encontraron efectos significativos sobre las latencies de los dipolos equivalentes dc este componente en las posiciones de-frentc-atrás en el piano medio-sagital y en las posiciones izquierda-derecha en el piano coronal, comparados con clicks presentados directamente por encima de la cabeza. El hallazgo más importante fue un cambio significativo en la orientación de los dipolos equivalentes en las diversas condiciones del estímulo. Concluimos que los cambios en latencia, amplitud y orientación en los dipolos equivalentes de los BICs, en las diferentes condiciones de estimulo, reflejan diferencias en la distribución de la actividad protuberancial binaural evocada por sonidos en diferentes localizaciones espaciales.

The binaural interaction component (BIC) in children with central auditory processing disorders (CAPD): El componente de interactión binaural (BIC) en niños con desórdenes del procesamiento central auditivo (CAPD)

The detection of binaural interaction is of diagnostic interest in patients with central auditory processing disorders (CAPDs), as binaural hearing tasks are frequently affected in these patients. Owing to the comorbidity associated with disorders such as an attention-deficit hyperactivity disorder, pathological results in subjective tests often show extra-auditory factors such as reduced attention rather than impaired central auditory function. Therefore, objective measures for auditory processing disorders are essential. The binaural interaction component (BIC), which is the arithmetical difference between the sum of the monaurally evoked auditory potentials of each ear and the binaurally evoked bramstem potentials, has been used as an objective measure of binaural interaction in humans. BIC measurements can therefore be considered its a possible diagnostic tool in CAPD patients. One aim of the present study was to examine whether and to what extent BIC measurements are capable of differentiating between normal children and children ‘at risk for CAPD’. BIC measurements were performed on 17 children at risk for CAPD and in a group of 25 children with normal results in the central audiometric tests used. Using the presence or absence of clearly demonstrable BIC waveforms as an indication of whether a CAPD is present or not, a sensitivity and specificity of 76% could be achieved. We conclude that BIC measurements might be of some diagnostic value in CAPD patients.

Origin of the binaural interaction component in wave P4 of the short-latency auditory evoked potentials in the cat: evaluation of serial depth recordings from the brainstem

There is no general agreement on the origin of the binaural interaction (BI) component in auditory brainstem responses (ABRs). To study this issue the ABRs to monaural and binaural clicks with various interaural time differences (ITDs) were simultaneously recorded from the vertex and from a recording electrode aiming at the superior olive (SO) in cats. Electrode path was along the fibers of the lateral lemniscus (LL). Binaural difference potentials (BDPs), which were computed by subtracting the sum of the two monaural responses from the binaural response, were obtained at systematic depths and across a range of ITD values. It was observed that only a specific BDP deflection recorded at the level at which lemniscal fibers terminate in the nuclei of LL coincided in time with the most prominent BDP in the cat’s vertex-recorded ABRs, the BDP in their wave P4. As ITD was increased, the latency shifts and amplitude decrements of the scalp-recorded far-field BDP wave exactly followed those recorded at this lemniscal near-field BDP locus. The data support our hypothesis that the BI component in wave P4 results from a binaural reduction in dischargings of axons ascending in the LL, with this reduction due to contralateral inhibition of the discharge activity of the inhibitory–excitatory units in the lateral nucleus of the SO. Furthermore, at the level of the SO, the BDP in the responses to contra-leading binaural clicks always had larger magnitudes than those evoked by ipsi-leading ones. This bilateral asymmetry is consistent with the view that the BDP in scalp-recorded ABRs is related to the function of sound lateralization.

Organization of the human superior olivary complex

The distinctive morphology of the human superior olivary complex reflects its primate origins, but functional evidence suggests that it plays a role in auditory spatial mapping which is similar to olivary function in other mammalian species. It seems likely that the well-developed human medial olivary nucleus is the basis for extraction of interaural time and phase differences. The much smaller human lateral olivary nucleus probably functions in analysis of interaural differences in frequency and intensity, but the absence of a human nucleus of the trapezoid body implies some difference in the mechanisms of this function. A window on human olivary function is provided by the evoked auditory brainstem response (ABR), including its binaural interaction component (BIC). Anatomical, electrophysiological, and histopathological studies suggest that ABR waves IV and V are generated by axonal pathways at the level of the superior olivary complex. Periolivary cell groups are prominent in the human olivary complex. The cell groups located medial, lateral, and dorsal are similar to periolivary nuclei of other mammals, but the periolivary nucleus at the rostral pole of the human olivary complex is very large by mammalian standards. Within the periolivary system, immunostaining for neurotransmitter-related substances allows us to identify populations of medial and lateral olivocochlear neurons. The human olivocochlear system is unique among mammals in the relatively small size of its lateral efferent component. Some consideration is given to the idea that the integration provided by periolivary cell groups, particularly modulation of the periphery by the olivocochlear system, is an extension of the spatial mapping function of the main olivary nuclei. Microsc. Res. Tech. 51:403–412, 2000. © 2000 Wiley-Liss, Inc.

Frequency-Following Response: Effects of Interaural Time and Intensity Differences

This research investigated whether brainstem neural mechanisms that mediate lateralization of sounds can be extracted from the frequency-following response (FFR). Monaural and binaural FFRs were obtained from normal-hearing subjects to low-frequency (500 Hz) linearly gated tone bursts (4-4-4 msec) at 40, 50, and 60 dB SL and four interaural time differences (ITDs) (0, 333, 500, and 667 microsec). FFRs were also recorded to ITDs and intensity presented in concert and in opposition (lateralization stimuli). The results show that overall intensity and interaural time differentially affect the FFR. The FFRs evoked by ITDs and intensity (in concert and in opposition) are strikingly different. The normalized amplitudes of the binaural interaction component (BIC) are minimally altered by ITDs and intensity. The study presents strong evidence that ITDs of 0, 333, 500, and 667 microsec and lateralization stimuli, easily discriminated perceptually, evoke clearly distinguishable FFR waveforms. These ITDs provide the cues that mammals use to localize sound in a freefield. The BIC is essentially unaffected by overall intensity, ITDs, and lateralization stimuli. Based on the findings of this study, the FFR has the potential to become a tool for identification of normal and abnormal binaural processing at lower brainstem levels.

Role of the efferent medial olivocochlear system in contralateral masking and binaural interactions: an electrophysiological study in guinea pigs.

Contralateral broadband noise (BBN) elevates ipsilateral auditory thresholds (central masking) and reduces the amplitude of ipsilateral brainstem auditory evoked potentials (BAEPs). Binaural interactions are complex psychophysical phenomena, but binaural interaction components are easily extracted from BAEPs to monaural versus binaural click stimulation. However, contralateral, or binaural, acoustical stimulation is known to activate simultaneously the crossed and uncrossed medial olivocochlear (MOC) efferent systems and decrease activity in both cochleas. Particularly, contralateral BBN stimulation suppresses in part ipsilateral peripheral activity. What is the role of such contralaterally induced peripheral suppression in the overall changes in central BAEPs observed during contralateral masking or binaural stimulation? Compound action potentials (CAPs) of the auditory nerve and BAEPs were recorded simultaneously in awake guinea pigs from electrodes chronically implanted on the round window of the cochlea and the surface of the brain. Peripheral and central measures of contralateral masking and binaural interactions were obtained from responses to monaural or binaural clicks, with or without contralateral BBN, recorded before, during, and after the reversible blockade of the MOC function following a single intramuscular injection of gentamicin. Contralateral BBN effectively reduced the amplitudes of CAP and of all BAEP peaks. CAP to ipsilateral click did not, however, change significantly from monaural to binaural click stimulation; still, normal binaural interaction components developed in the BAEPs. When the medial efferent function was blocked by gentamicin, the normal contralateral BBN suppression of CAP and of the earliest BAEP peak was lost; however, the later BAEP peaks were suppressed by contralateral BBN as before gentamicin, and the central binaural interaction components were unchanged. In these experimental conditions, the MOC efferent system seems to play little role in centrally recorded contralateral masking and binaural interactions.

Organization of the human superior olivary complex.

The distinctive morphology of the human superior olivary complex reflects its primate origins, but functional evidence suggests that it plays a role in auditory spatial mapping which is similar to olivary function in other mammalian species. It seems likely that the well-developed human medial olivary nucleus is the basis for extraction of interaural time and phase differences. The much smaller human lateral olivary nucleus probably functions in analysis of interaural differences in frequency and intensity, but the absence of a human nucleus of the trapezoid body implies some difference in the mechanisms of this function. A window on human olivary function is provided by the evoked auditory brainstem response (ABR), including its binaural interaction component (BIC). Anatomical, electrophysiological, and histopathological studies suggest that ABR waves IV and V are generated by axonal pathways at the level of the superior olivary complex. Periolivary cell groups are prominent in the human olivary complex. The cell groups located medial, lateral, and dorsal are similar to periolivary nuclei of other mammals, but the periolivary nucleus at the rostral pole of the human olivary complex is very large by mammalian standards. Within the periolivary system, immunostaining for neurotransmitter-related substances allows us to identify populations of medial and lateral olivocochlear neurons. The human olivocochlear system is unique among mammals in the relatively small size of its lateral efferent component. Some consideration is given to the idea that the integration provided by periolivary cell groups, particularly modulation of the periphery by the olivocochlear system, is an extension of the spatial mapping function of the main olivary nuclei.

Binaural interaction component and white-noise enhancement in middle latency responses: differential effects of anaesthesia in guinea pigs.

It is known that the response to binaural clicks is smaller in amplitude than the sum of two monaural responses. The difference is called the binaural interaction component (BIC). Also, an amplitude enlargement occurs in guinea pig middle latency responses (MLRs) to monaural clicks when white noise is applied to the other ear. This study was conducted to find out whether these two signs of binaural interaction result from the same mechanism. White noise enhancement (WNE) and BIC were computed from the evoked potential data simultaneously recorded in different phases of anaesthesia. WNE gradually decreased and disappeared with anaesthesia, but the relative amplitude of the BIC remained unchanged. This differential effect showed that different neural mechanisms must be responsible for BIC and WNE in guinea pig MLR.

The Binaural Interaction Component in Human ABR Is Stable within the 0- to 1-ms Range of Interaural Time Differences

The effect of unilaterally delayed acoustic stimuli on binaural interaction was studied in 12 subjects with normal hearing. Auditory brainstem response (ABR) was obtained in the midline between the forehead and the neck, and click stimuli were unilaterally delayed at 0.2-ms intervals in the 0- to 1-ms range. Binaural interaction was evaluated by measuring the β wave in the binaural difference waveform, i.e. the remainder after subtracting the binaurally evoked registration from the sum of the two monaural registrations. Computation of the binaural difference waveform showed the β wave amplitude to be relatively stable throughout the 0- to 1-ms range of interaural time differences. This finding suggests stable binaural interaction within the range of interaural time differences where binaural click stimuli induce a binaurally fused intracranial image.