The auditory brainstem response to binaural delayed stimuli in man

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.

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.

Evidence for efferent effects on early components of the human auditory brain-stem evoked potentials

OBJECTIVES AND METHODS

Auditory brain-stem evoked potentials (ABEPs) were recorded from 10 normal hearing subjects in response to rarefaction clicks, presented at a rate of 11/s. Stimuli were binaurally symmetrical and isochronic at 75 dB peSPL or with interaural time disparities (ITDs) of +/-0.4 ms, or intensity disparities (IIDs) of +/-10 dB. Potentials were recorded from vertex-neck, as well as from 3 orthonormally positioned differential derivations. The amplified potentials were averaged over 8000 repetitions using a dwell time of 20 micros/address/channel. The effects of contralateral stimulation on neural responses of the peripheral auditory system were obtained by subtracting the binaural response from the algebraic sum of responses to left and right monaural stimuli. From the 3 orthonormal derivations, 3-channel Lissajous' trajectories (3-CLTs) to the various stimulus conditions and difference waveforms were derived.

RESULTS

The results corroborated earlier studies on binaural interaction components (BICs), which include 3 major components corresponding in latency to the vertex-mastoid peaks IV-VI of ABEP. In addition, the binaural difference waveforms included 3 earlier, low-amplitude components. Latency correspondence and comparison of difference waveform and ABEP 3-CLTs indicated that the first and third early difference waveform components corresponded to the negative peaks following I and III, respectively, of the vertex-neck ABEP to binaural clicks.

CONCLUSIONS

These results indicate that early ABEP peaks, generated peripheral to binaural convergence, may be affected by contralateral stimulation. These contralateral effects were in a pattern compatible with suppression. most probably by efferents of the olivo-cochlear bundle.

Effects of localized pontine lesions on auditory brain-stem evoked potentials and binaural processing in humans

OBJECTIVES AND METHODS

Four sets of measurements were obtained from 11 patients (44-80 years old) with small, localized pontine lesions due to vascular disease: (1) Monaural auditory brain-stem evoked potentials (ABEPs; peaks I to VI); (2) Binaural ABEPs processed for their binaural interaction components (BICs) in the latency range of peaks IV to VI; (3) magnetic resonance imaging (MRI) of the brain-stem; and (4) psychoacoustics of interaural time disparity measures of binaural localization. ABEPs and BICs were analyzed for peak latencies and interpeak latency differences. Three-channel Lissajous' trajectories (3-CLTs) were derived for ABEPs and BICs and the latencies and orientations of the equivalent dipoles of ABEP and BICs were inferred from them.

RESULTS

Intercomponent latency measures of monaurally evoked ABEPs were abnormal in only 3 of the 11 patients. Consistent correlations between sites of lesion and neurophysiological abnormality were obtained in 9 of the 11 patients using 3-CLT measures of BICs. Six of the 11 patients had absence of one or more BIC components. Seven of the 11 had BICs orientation abnormality and 3 had latency abnormalities. Trapezoid body (TB) lesions (6 patients) were associated with an absent (two patients with ventral-caudal lesions) or abnormal (one patient with ventral-rostral lesions) dipole orientation of the first component (at the time of ABEPs IV), and sparing of this component with midline ventral TB lesions (two patients). A deviant orientation of the second BICs component (at the time of ABEPs V) was observed with ventral TB lesions. Psychoacoustic lateralization in these patients was biased toward the center. Rostral lateral lemniscus (LL) lesions (3 patients) were associated with absent (one patient) or abnormal (two patients) orientation of the third BICs component (at the time of ABEPs VI); and a side-biased lateralization with behavioral testing.

CONCLUSIONS

These results indicate that: (1) the BICs component occurring at the time of ABEPs peak IV is dependent on ventral-caudal TB integrity; (2) the ventral TB contributes to the BICs component at the time of ABEPs peak V; and (3) the rostral LL is a contributing generator of the BICs component occurring at the time of ABEP peak VI.

Evidence for separate processing in the human brainstem of interaural intensity and temporal disparities for sound lateralization

Sound lateralization can be induced by interaural intensity disparities (IIDs) or by interaural temporal disparities (ITDs). The purpose of this study was to indicate whether IIDs and ITDs are processed by the same central units that detect interaural disparity in timing of afferent activity. If sound lateralization to intensity and time cues was determined by the same afferent latency disparity detectors in the brainstem, lateralization would be the same, regardless of whether latency disparity was induced by IIDs or ITDs. Moreover, the disparity detectors, and thus their dipole equivalents, would be the same for equal lateralizations, whether induced by IIDs or ITDs. Auditory brainstem evoked potentials (ABEPs) were recorded in response to monaural and binaural clicks, with a variety of IIDs and ITDs. Peak II (proximal auditory nerve activity), peak III (input to the superior olivary complex), and binaural interaction components (BICs) BeI and BeII (binaurally activated upper pons) were identified and their latencies measured. The psychophysical lateralization of the clicks (in cm from vertex) was also measured in response to the same binaural stimuli. The correlations between interaural afferent latency disparities (difference in corresponding peak latencies originating in each ear) and psychophysical click lateralization were calculated. Similarly, the correlations with click lateralization of the BICs equivalent dipole latency as well as orientation change (relative to symmetrical clicks) were determined. A strong correlation with lateralization was found for peaks II and III latency disparities, with steeper slopes for IIDs than for ITDs. Moreover, binaural activity across the same lateralizations differed between IIDs and ITDs. These results, therefore, indicate that interaural time and intensity cues are processed by separate systems in the brainstem, both at the afferent convergence level and after interaural disparities are determined.

Evidence for spatio-topic organization of binaural processing in the human brainstem

Three-channel Lissajous' trajectories (3-CLT) of the binaural interaction (BI) in auditory brainstem evoked potentials (ABEP) were derived from 13 normally and symmetrically hearing adults by subtracting the response to binaural clicks from the algebraic sum of monaural responses. ABEPs were recorded from four channels, three of them orthonormal to each other, in response to alternating polarity clicks, presented at a rate of 11/s with interaural time differences (ITD) of 0.2, 0.4 and 1.0 ms and an intensity of 65 dB nHL, or isochronic to both ears with interaural intensity differences (IIDs) of 5, 10 and 15 dB (65 dB nHL +/- 2.5, 5.0 and 7.5 dB, respectively). All 3-CLTs included 6 planar segments (labeled BdI, BdII, BdIII, BeI, BeII and Bf). Amplitudes of 3-CLT BI components were not significantly affected by increasing ITDs and IIDs, but latencies of all components increased significantly. The most remarkable finding was a significant change in apex orientations of BeI and BeII of the BI 3-CLT across stimulus conditions. The changes in BeI and BeII apex orientations, across stimulus conditions, may reflect differences in the anatomical representation of activity evoked by differently lateralized sounds. We suggest that this may indicate spatio-topic organization in the human brainstem.

Three-channel Lissajous' trajectory of the binaural interaction components in human auditory brain-stem evoked potentials

The 3-channel Lissajous' trajectory (3-CLT) of the binaural interaction components (BI) in auditory brain-stem evoked potentials (ABEPs) was derived from 17 normally hearing adults by subtracting the response to binaural clicks (B) from the algebraic sum of monaural responses (L + R). ABEPs were recorded in response to 65 dB nHL, alternating polarity clicks, presented at a rate of 11/sec. 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 ABEP L + R and B. 3-CLT measures included: apex latency, amplitude and orientation, as well as planar segment duration and orientation. The results showed 3 apices and associated planar segments ("BdII," "Be" and "Bf") in the 3-CLT of BI which corresponded in latency to the vertex-mastoid and vertex-neck peaks IIIn, V and VI of ABEP L + R and B. These apices corresponded in latency and orientation to apices of the 3-CLT of ABEP L + R and ABEP B. This correspondence suggests generators of the BI components between the trapezoid body and the inferior colliculus output. Durations of BI planar segments were approximately 1.0 msec. Apex amplitudes of BI 3-CLT were larger than the respective peak amplitudes of the vertex-mastoid and vertex-neck recorded BI, while their intersubject variabilities were comparable.

Cross-correlation function in the analysis of auditory brainstem response in spinocerebellar degeneration

Cross-correlation functions were derived from the analysis of auditory-evoked brainstem response (ABR) and compared with measurements of wave latency and computed tomography findings in the assessment of ABR findings in spinocerebellar degeneration (SCD). Gender-specific normative ABR templates were produced from 30 normal males and 30 normal females separately. The cross-correlation indices used were the correlation coefficient at time 0, the maximal correlation coefficient and the latency delay in milli-seconds. The technique was applied to 33 patients with SCD. The incidence of abnormal cross-correlation functions (81.8%) was greater than the incidence of abnormal ABR peak latencies assessed according to gender (75.8%) which, in turn, was more common than the abnormal peak latencies assessed conventionally (69.7%). Moreover, the incidence of abnormal cross-correlations and latencies in Menière's disease was much lower (less than 8%). These results suggest that the evaluation of ABR waveform characteristics with cross-correlation functions using normative ABR templates of the same gender contributes to the precise detection of abnormality in the brainstem auditory pathway.

Anatomical bases of binaural interaction in auditory brain-stem responses from guinea pig

There is a non-linear interaction of binaural stimulation on auditory brain-stem potentials in both human and animals. The interaction takes the form of the binaurally evoked ABR being of smaller amplitude than the sum of the monaurally evoked ABRs. In the guinea pig this interaction occurs at the time of components P4, N4 and P5. In order to investigate the generator sites of binaural interaction in the ABR, various lesions were made in the brain-stem auditory system in 29 guinea pigs. The effects of those lesions on binaural interaction were as follows: (1) unilateral lesion of lateral lemniscus or bilateral lesions of the inferior colliculi had no significant effect on binaural interaction; (2) transection of the lateral lemnisci bilaterally was associated with a loss of the component of binaural interaction associated in time with N4; (3) a lesion just lateral to the lateral superior olivary complex resulted in an attenuation of the component of binaural interaction associated in time with P4; (4) complete section of the decussating fibers of the trapezoid body or a complete unilateral lesion of the superior olivary complex led to a loss of all components of binaural interaction. These results suggest that binaural interaction in the guinea pig ABR requires the integrity of several distinct portions of the brain-stem auditory pathway, i.e., both lateral lemnisci are required for the interaction occurring at the time of N4; the brain-stem just lateral to the lateral superior olive participates in the interaction at the time of P4. The trapezoid body and superior olivary nucleus are required for binaural interaction at P4, N4 and P5.

Application of fast Fourier transform to auditory evoked brainstem response

The current application of fast Fourier transform (FFT) to the analysis of auditory evoked brainstem response (ABR) is reviewed under four categories: (1) digital filtering, which facilitates isolation of fast and slow components from the same ABR wave, is the most common use of FFT; (2) power spectral analysis: this seems significant in ABR for isolating and analysing slow, middle and fast components from the Fourier components around each peak of the power spectrum with a three-peak pattern by inverse fast Fourier transform; (3) cross correlation function shows the relationship between two signals being analysed from the viewpoint of their phase. Clinical applications are used in the diagnosis of multiple sclerosis and for automatic detection of ABR; and (4) phase spectral analysis: the synchrony measure method (Fridman, 1984) is a type of phase spectral analysis. In this method, the phase variances of selected Fourier components are calculated, from among 10 averaging groups of 200 sweeps in the same stimulating conditions, to determine the presence or absence of a response. The clinical application of this method to the automatic evaluation of ABR is discussed.

Intra- and extracranially recorded auditory evoked potentials in the cat. II. Effects of interaural time and intensity differences

The effects of interaural time differences and interaural intensity differences on binaural interactions in the brain-stem evoked response (BSER) and auditory field potentials (AFPs) in superior olive and inferior colliculus were studied. Interaural time differences of up to +/- 2048 microseconds and interaural intensity differences of up to +/- 30 dB were used. Binaural interactions were studied for waves P4 and P5 of the BSER and the corresponding AFP components. When binaural interactions were present (wave P4 and subsequent waves), the dichotic potential was less than the sum of left and right evoked potentials. At zero interaural intensity difference the maximum binaural interaction was seen at zero interaural time difference. When an interaural intensity difference was present, maximum interaction was shifted away from zero interaural time difference such that left louder gave maximal interaction at right lead and vice versa. The time intensity trading values for this shift were between 9 and 20 microseconds/dB. The trading ratios for the superior olive wave P4 component and BSER P4/P5 were in the same range, i.e., no extra effects could be seen in the BSER postsynaptic to the superior olive. These time intensity trading ratios correspond to those of medial superior olive cells but not to those of lateral superior olive cells (Caird and Klinke 1983). We suggest that these binaural effects are produced by binaural mechanisms in the medial superior olive and that the lateral superior olive does not significantly contribute to the BSER. The inferior colliculus AFP slow wave binaural interactions do not correspond to those of the BSER.

Brain stem potentials evoked by binaural click stimuli with differences in interaural time and intensity

Auditory-evoked brain stem potentials were recorded from 12 adults with normal hearing using click stimuli with differences in interaural time and intensity. Almost independent superimposed Jewett V peaks were produced, whose latency and amplitude depended on the parameters of the stimulus applied to either ear. This indicates that separate binaural information for the evaluation of sound source direction is still available at the brain stem level where wave V originates. We demonstrate that the normal nonlinear latency/intensity function may be responsible for the subjective compensation of time and intensity differences, since the well-known trading functions show similar intensity-dependent gradients.

Latencies of ABR (waves III and V) to binaural clicks: effects of interaural time and intensity differences

Auditory brainstem responses to monaural clicks and to binaural clicks delivered with interaural time differences of 0.5 and 1 ms (delayed clicks in left ear) and interaural intensity differences (right ear minus left ear) of 0, +/- 10, +/- 20, and +/- 30 dB were recorded bilaterally in 7 normal subjects. Latencies of wave III and wave V were studied as functions of click intensity difference for each of the two time-of-onset differences. As the intensity difference was gradually varied from +30 to -30 dB, the latencies were seen to shift (with constant III--V interval) from those of a monaural right-ear (non-delayed clicks) response to those of a monaural left-ear (delayed clicks) response by 0.5 and 1 ms. In all subjects this shift occurred in the 20-dB interval between equal intensity and 20-dB lagging-click dominance, and almost always most of the shift took place in either of the two 10-dB subintervals. Occasionally double-peaked waves appeared in the 20 dB-interval. Binaural ABRs may become useful for diagnosis in patients with signs of brainstem disorder but with normal-hearing and normal audiometric findings including monaural ABR, as such patients have been found to shift their latencies more slowly with varying interaural intensity difference.