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

Evidence for the origin of the binaural interaction component of the auditory brainstem response

The binaural interaction component (BIC) represents the mismatch between auditory brainstem responses (ABR) obtained with binaural stimulation and the sum of ABRs obtained with monaural left and right stimulation. It is generally assumed that the BIC reflects binaural integration. Its potential use as a diagnostic tool, however, is hampered by the lack of direct evidence about its origin. While an origin at the initial site of binaural integration seems likely, there is no general agreement on the contribution of the two primary candidate nuclei, the lateral and medial superior olives (LSO and MSO, respectively). Here, we recorded local field potentials (LFP) and responses of units in the LSO and MSO of Mongolian gerbils (Meriones unguiculatus), presenting clicks with an interaural time or level difference (ITD and ILD, respectively), while simultaneously recording ABR. We determined the BIC from the ABR and, importantly, from LFP and responses of units in the LSO and MSO. If stimulus-induced changes in the ABR-derived BIC have their source in the LSO and/or MSO, we expect coherent changes in the unit-derived and the ABR-derived BIC. We find that BIC obtained from LSO units exhibits the same ITD and ILD dependence as the ABR-derived BIC. Neither BIC obtained from MSO units nor LFP-derived BIC recorded in either LSO or MSO did. The data thus strongly suggest that it is the activity of LSO units in the gerbil that is decisive for the generation of the ABR-derived BIC, determining its properties.

The Physiological Basis and Clinical Use of the Binaural Interaction Component of the Auditory Brainstem Response

The auditory brainstem response (ABR) is a sound-evoked noninvasively measured electrical potential representing the sum of neuronal activity in the auditory brainstem and midbrain. ABR peak amplitudes and latencies are widely used in human and animal auditory research and for clinical screening. The binaural interaction component (BIC) of the ABR stands for the difference between the sum of the monaural ABRs and the ABR obtained with binaural stimulation. The BIC comprises a series of distinct waves, the largest of which (DN1) has been used for evaluating binaural hearing in both normal hearing and hearing-impaired listeners. Based on data from animal and human studies, the authors discuss the possible anatomical and physiological bases of the BIC (DN1 in particular). The effects of electrode placement and stimulus characteristics on the binaurally evoked ABR are evaluated. The authors review how interaural time and intensity differences affect the BIC and, analyzing these dependencies, draw conclusion about the mechanism underlying the generation of the BIC. Finally, the utility of the BIC for clinical diagnoses are summarized.

Signatures of Somatic Inhibition and Dendritic Excitation in Auditory Brainstem Field Potentials

Extracellular voltage recordings (V_e ; field potentials) provide an accessible view of in vivo neural activity, but proper interpretation of field potentials is a long-standing challenge. Computational modeling can aid in identifying neural generators of field potentials. In the auditory brainstem of cats, spatial patterns of sound-evoked V_e can resemble, strikingly, V_e generated by current dipoles. Previously, we developed a biophysically-based model of a binaural brainstem nucleus, the medial superior olive (MSO), that accounts qualitatively for observed dipole-like V_e patterns in sustained responses to monaural tones with frequencies >∼1000 Hz (Goldwyn et al., 2014). We have observed, however, that V_e patterns in cats of both sexes appear more monopole-like for lower-frequency tones. Here, we enhance our theory to accurately reproduce dipole and non-dipole features of V_e responses to monaural tones with frequencies ranging from 600 to 1800 Hz. By applying our model to data, we estimate time courses of paired input currents to MSO neurons. We interpret these inputs as dendrite-targeting excitation and soma-targeting inhibition (the latter contributes non-dipole-like features to V_e responses). Aspects of inferred inputs are consistent with synaptic inputs to MSO neurons including the tendencies of inhibitory inputs to attenuate in response to high-frequency tones and to precede excitatory inputs. Importantly, our updated theory can be tested experimentally by blocking synaptic inputs. MSO neurons perform a critical role in sound localization and binaural hearing. By solving an inverse problem to uncover synaptic inputs from V_e patterns we provide a new perspective on MSO physiology.SIGNIFICANCE STATEMENT Extracellular voltages (field potentials) are a common measure of brain activity. Ideally, one could infer from these data the activity of neurons and synapses that generate field potentials, but this "inverse problem" is not easily solved. We study brainstem field potentials in the region of the medial superior olive (MSO); a critical center in the auditory pathway. These field potentials exhibit distinctive spatial and temporal patterns in response to pure tone sounds. We use mathematical modeling in combination with physiological and anatomical knowledge of MSO neurons to plausibly explain how dendrite-targeting excitation and soma-targeting inhibition generate these field potentials. Inferring putative synaptic currents from field potentials advances our ability to study neural processing of sound in the MSO.

A model of the medial superior olive explains spatiotemporal features of local field potentials

Local field potentials are important indicators of in vivo neural activity. Sustained, phase-locked, sound-evoked extracellular fields in the mammalian auditory brainstem, known as the auditory neurophonic, reflect the activity of neurons in the medial superior olive (MSO). We develop a biophysically based model of the neurophonic that accounts for features of in vivo extracellular recordings in the cat auditory brainstem. By making plausible idealizations regarding the spatial symmetry of MSO neurons and the temporal synchrony of their afferent inputs, we reduce the challenging problem of computing extracellular potentials in a 3D volume conductor to a one-dimensional problem. We find that postsynaptic currents in bipolar MSO neuron models generate extracellular voltage responses that strikingly resemble in vivo recordings. Simulations reproduce distinctive spatiotemporal features of the in vivo neurophonic response to monaural pure tones: large oscillations (hundreds of microvolts to millivolts), broad spatial reach (millimeter scale), and a dipole-like spatial profile. We also explain how somatic inhibition and the relative timing of bilateral excitation may shape the spatial profile of the neurophonic. We observe in simulations, and find supporting evidence in in vivo data, that coincident excitatory inputs on both dendrites lead to a drastically reduced spatial reach of the neurophonic. This outcome surprises because coincident inputs are thought to evoke maximal firing rates in MSO neurons, and it reconciles previously puzzling evoked potential results in humans and animals. The success of our model, which has no axon or spike-generating sodium currents, suggests that MSO spikes do not contribute appreciably to the neurophonic.

Neural sensitivity to novel sounds in the rat's dorsal cortex of the inferior colliculus as revealed by evoked local field potentials

Evoked local field potentials in response to contralaterally presented tone bursts were recorded from the rat's dorsal cortex of the inferior colliculus (ICd). An oddball stimulus paradigm was used to study the sensitivity of ensembles of neurons in the ICd to novel sounds. Our recordings indicate that neuron ensembles in the ICd display stimulus-specific adaptation when a large contrast in both frequency and probability of occurrence exists between the two tone bursts used for generating an oddball paradigm. A local field potential evoked by a tone burst presented as a deviant stimulus has a larger amplitude than that evoked by the same sound presented as a standard stimulus. The difference between the two responses occurs after the initial rising phases of their predominant deflections. The degree of stimulus-specific adaptation increases with the rate of sound presentation up to 8/s, the highest rate used in this study. A comparison between our results and those from previous studies suggests that differences exist between responses to oddball paradigms in the ICd and those in the primary auditory cortex, a major source of projection to the ICd. These differences suggest that local mechanisms exist in the ICd for suppressing neural responses to frequently presented sounds and enhancing responses to rarely presented sounds. Thus, the ICd may serve as an important component of an integrative circuit in the brain for detecting novel sounds in the acoustic environment.

Using evoked potentials to match interaural electrode pairs with bilateral cochlear implants

Bilateral cochlear implantation seeks to restore the advantages of binaural hearing to the profoundly deaf by providing binaural cues normally important for accurate sound localization and speech reception in noise. Psychophysical observations suggest that a key issue for the implementation of a successful binaural prosthesis is the ability to match the cochlear positions of stimulation channels in each ear. We used a cat model of bilateral cochlear implants with eight-electrode arrays implanted in each cochlea to develop and test a noninvasive method based on evoked potentials for matching interaural electrodes. The arrays allowed the cochlear location of stimulation to be independently varied in each ear. The binaural interaction component (BIC) of the electrically evoked auditory brainstem response (EABR) was used as an assay of binaural processing. BIC amplitude peaked for interaural electrode pairs at the same relative cochlear position and dropped with increasing cochlear separation in either direction. To test the hypothesis that BIC amplitude peaks when electrodes from the two sides activate maximally overlapping neural populations, we measured multiunit neural activity along the tonotopic gradient of the inferior colliculus (IC) with 16-channel recording probes and determined the spatial pattern of IC activation for each stimulating electrode. We found that the interaural electrode pairings that produced the best aligned IC activation patterns were also those that yielded maximum BIC amplitude. These results suggest that EABR measurements may provide a method for assigning frequency-channel mappings in bilateral implant recipients, such as pediatric patients, for which psychophysical measures of pitch ranking or binaural fusion are unavailable.

Interaural delay-dependent changes in the binaural difference potential of the human auditory brain stem response

Binaural difference potentials (BDs) are thought to be generated by neural units in the brain stem responding specifically to binaural stimulation. They are computed by subtracting the sum of monaural responses from the binaural response, BD = B - (L + R). BDs in dependency on the interaural time difference (ITD) have been measured and compared to the Jeffress model in a number of studies with conflicting results. The classical Jeffress model assuming binaural coincidence detector cells innervated by bilateral excitatory cells via two delay lines predicts a BD latency increase of ITD/2. A modification of the model using only a single delay line as found in birds yields a BD latency increase of ITD. The objective of this study is to measure BDs with a high signal-to-noise ratio for a large range of ITDs and to compare the data with the predictions of some models in the literature including that of Jeffress. Chirp evoked BDs were recorded for 17 ITDs in the range from 0 to 2 ms at a level of 40 dB nHL for four channels (A1, A2, PO9, PO10) from 11 normal hearing subjects. For each binaural condition 10,000 epochs were collected while 40,000 epochs were recorded for each of the two monaural conditions. Significant BD components are observed for ITDs up to 2 ms. The peak-to-peak amplitude of the first components of the BD, DP1-DN1, is monotonically decreasing with ITD. This is in contrast with click studies which reported a constant BD-amplitude for ITDs up to 1 ms. The latency of the BD-component DN1 is monotonically, but nonlinearly increasing with ITD. In the current study, DN1 latency is found to increase faster than ITD/2 but slower than ITD incompatible with either variant of the Jeffress model. To describe BD waveforms, the computational model proposed by Ungan et al. [Hearing Research 106, 66-82, 1997] using ipsilateral excitatory and contralateral inhibitory inputs to the binaural cells was implemented with only four parameters and successfully fitted to the BD data. Despite its simplicity the model predicts features which can be physiologically tested: the inhibitory input must arrive slightly before the excitatory input, and the duration of the inhibition must be considerably longer than the standard deviations of excitatory and inhibitory arrival times to the binaural cells. With these characteristics, the model can accurately describe BD amplitude and latency as a function of the ITD.

Interaural time coincidence detectors are present at birth: evidence from binaural interaction

Binaural processing of sounds in mammals is presumably initiated within the auditory nuclei of the caudal pons. The binaural difference waveform (BD) can be derived from the sum of the waveforms evoked by right monaural clicks plus left monaural clicks minus the waveform evoked by binaural clicks. In adults, the BD's first positive peak (beta) is large only for stimuli with interaural time differences (ITDs) that produce a fused acoustic percept. Humans at birth can localize and discriminate sound sources, but their head circumference is about two-thirds of an adult head. In order to test whether beta is related to head circumference, we recorded beta in human neonates as a function of ITD. Binaural clicks with ITDs ranging between 0 and 1000 micros were used to derive BD waveforms in 34 neonates. For ITD=0, beta was detectable in 56% of newborns. The incidence of beta detection then decreased as ITD increased. Only 9% of the babies had detectable beta for all ITDs. No correlation was found between the existence of beta and other properties of the monaural or binaural auditory brainstem response. The finding that for some infants beta was present for all ITDs up to 1.0 ms suggests that there is no recalibration of brainstem delay lines with head growth. Our data suggest that the brainstem auditory pathway for detecting interaural time differences in the adult is probably present at birth. Maturational factors such as increased myelination and greater firing synchrony probably improve the detectability of beta with age. The second peak in the BD waveform (delta) was highly correlated with the existence of wave VI in the binaural and monaural waveforms.

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.

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.

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

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

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.

Contribution of click frequency bands to the human binaural interaction components

The purpose of this study was to determine the contribution of click frequency bands (broad-band, >2000 Hz, <2000 Hz and <1000 Hz) to binaural interaction components (BICs) of the human auditory brainstem evoked potentials (ABEPs). The human BICs were studied by subtracting the potentials to binaural clicks from the algebraic sum of monaurally evoked potentials to either ear. Effective frequency bands were derived using clicks alone or clicks with ipsilateral or binaural masking noise, high- or low-pass filtered at different cut-off frequencies. Analysis included single-channel vertex-cervical spinous process VII derivation of BIC and ABEP, as well as estimating the single, centrally located dipole equivalent of the surface activity from three orthogonally positioned electrode pairs, using the three-channel Lissajous' trajectory (3-CLT) analysis. All BIC 3-CLTs included three major components (labeled BdII, BeI, and BeII) approximately corresponding in latency to IIIn, V and VI ABEP peaks. All apex latencies of BIC 3-CLT, except BeI, were longer in response to <2000 Hz and <1000 Hz (low-frequency) effective clicks. Apex amplitude of components BeI and BeII of BIC 3-CLT were smaller with low-frequency effective clicks than with broad-band or high-frequency (>2000 Hz) clicks. We suggest that binaural interaction component BeI is mainly tuned to high frequencies, showing no frequency effect on latency, and decreasing in amplitude with decreasing click high frequency content. In contrast, BdII and BeII of the human BICs are evoked more synchronously by high-frequency binaural inputs, but are also sensitive to low frequencies, increasing in latency according to the cochleotopic activation pattern. These differences between BIC components may reflect their roles in sound localization.

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.

Binaural interaction in the human frequency-following response: effects of interaural intensity difference

The binaural interaction component (BIC) of the 500-Hz human frequency-following response (FFR) was evaluated as a function of interaural intensity difference (IID) using a lateralization paradigm. The robust FFR interaction component (FFR-BIC) was shown to decrease systematically with increasing IID with no discernible FFR-BIC for IID values larger than about 20 dB. These findings are similar to that observed for the high-frequency auditory brainstem response interaction component (ABR-BIC). Thus, like the ABR-BIC, the FFR-BIC may be correlated with binaural fusion and the perceived location of the fused image of the sound. These results taken together suggest that the binaural neurons in the brainstem are able to utilize IID cues presented in both low-frequency and high-frequency sounds.

Binaural masking level difference in human binaural interaction components

OBJECTIVE

The purpose of this study was to compare the effects of monaural and binaural broadband masking noise on binaural interaction components (BICs) of the human auditory brain stem evoked potentials (ABEPs).

DESIGN

The BICs of the human ABEPs were studied by subtracting the potentials to binaural clicks from the algebraic sum of monaurally evoked potentials to clicks alone or to clicks with ipsilateral monaural or binaural broadband masking noise. Alternating polarity, 11/sec clicks were presented at 65 dB nHL, and noise was presented at 45 dB nHL. Analysis included peak-to-prestimulus baseline amplitudes and latencies of BICs' peaks and troughs from the vertex-mastoid (A) and vertex-neck (Z) channels. In addition, 3-channel Lissajous' trajectory (3-CLT) analysis, estimating the single, centrally located dipole equivalent of surface activity, was performed on data recorded from three orthogonally positioned electrode pairs. 3-CLT measures included apex latency, amplitude, and orientation, as well as planar segment duration, size, shape, and orientation.

RESULTS

All BICs 3-CLTs included five main components (labeled BdI, BdII, BdIII, BeI, and BeII). In general, apex latencies were longer with masking noise. However, BdII and BeI apex latencies were shorter with binaural than with ipsilateral monaural masking noise. Apex amplitude and planar segment size of component BeI, as well as P1 peak amplitude in BICs of the Z-channel records, were larger with binaural than with monaural noise. No significant difference between the monaural and binaural noise conditions was found in durations, shapes, and orientations of planar segments of BICs 3-CLT, nor in peak latency of BICs in the A- and Z-channel records.

CONCLUSIONS

We suggest that these effects on the latency and amplitude of BICs reflect binaural processing in the human brain stem. In particular, the larger amplitudes and shorter latencies of P1 and BeI with binaural than with ipsilateral monaural masking may be associated with the psychophysical effect of binaural masking level difference.

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.

Interaural delay-dependent changes in the binaural difference potential in cat auditory brainstem response: implications about the origin of the binaural interaction component

Auditory brainstem responses (ABRs) evoked by dichotic clicks with 12 different interaural delays (ITDs) between 0 and 1500 microsecond(s) were recorded from the vertices of 10 cats under ketamine anesthesia. The so-called binaural difference potential (BDP), considered to be an indicator of binaural interaction (BI), was computed by subtracting the sum of the two monaural responses from the binaural one. The earliest and most prominent component of BDP was a negative deflection (DN1) at a latency between 4 and 4.8 ms. Like all the other components of BDP, DNI was also due to binaural reduction rather than enhancement of the corresponding ABR wave, P4 in this case. Furthermore, the way its latency increased as a function of ITD was also not compatible with what would be predicted by the delay-line coincidence detector models based on the excitatory-excitatory units in the medial superior olive (MSO). We therefore proposed an alternative hypothesis for the origin of this BI component based on the inhibitory-excitatory (IE) units in the lateral superior olive (LSO). The computational model designed closely simulated the ITD-dependent attenuation and latency shifts observed in DN1. It was therefore concluded that the origin of this BI component in the cat's vertex-ABR could be the lateral lemniscal output of the LSO, although the delay lines which have been shown to exist also in the mammalian brain may play an important role in encoding ITDs.

Cellular generators of the binaural difference potential in cat

In humans, lateralization and fusion of binaurally presented clicks are correlated with the latency and amplitude of the binaural difference potential (BDP) (e.g., Furst et al., 1985). The BDP is derived by subtracting the brainstem auditory evoked potential (BAEP) for binaural stimulation from the sum of the BAEPs for left and right monaural stimulation. Our aim in this work was to determine the cellular generators of the BDP and thus identify cells that may be crucial for specific types of binaural sound processing. To this end, we injected kainic acid into the superior olivary complex (SOC) or the cochlear nucleus (CN) in cats and examined the effects of the resulting lesions on the click-evoked BDP. Lesions confined to the anterior anteroventral CN (AVCNa) substantially reduced the BDP, while lesions primarily involving more posterior parts of the CN had little or no effect. BDP reductions occurred for lesions involving either high (> 10 kHz) or lower (< 10 kHz) characteristic frequency (CF) regions of the AVCNa (as well as the posterior CN). Lesions involving the SOC reduced the BDP and, in one case, eliminated the high-pass filtered (270 Hz cutoff) BDP. Combining these results with published information about the physiology and anatomy of auditory brainstem cells, we conclude that: (1) spherical cells in the AVCNa are essential for BDP production, (2) the earliest part of the BDP is generated by medial superior olive (MSO) principal cells which receive spherical cell inputs, (3) a later part is probably generated by the cellular targets of MSO principal cells and, (4) the cells involved in BDP generation have CFs above, as well as below, 10 kHz. Since humans, like cats, have a well-developed MSO, we suggest that the MSO may also be essential for BDP production in humans. Thus, perceptual correlates of the BDP, binaural fusion and click lateralization, apparently involve the MSO.

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.

Generators of the brainstem auditory evoked potential in cat. II. Correlating lesion sites with waveform changes

Brainstem regions involved in generating the brainstem auditory evoked potential (BAEP) were identified by examining the effects of lesions on the click-evoked BAEP in cats. An excitotoxin, kainic acid, was injected into various parts of the cochlear nucleus (CN) or into the superior olivary complex (SOC). The locations of the resulting lesions were correlated with the changes produced in the various extrema of the BAEP waveforms. The results indicate that: (1) the earliest BAEP extrema (P1, N1 (recorded between vertex and the earbar ipsilateral to the stimulus) and P1a, P1b, (vertex to contralateral earbar)) are generated by cells with somata peripheral to the CN; (2) P2 is primarily generated by posterior anteroventral CN (AVCNp) and anterior posteroventral CN (PVCNa) cells; (3) SOC, anterior anteroventral CN (AVCNa), AVCNp, and PVCNa cells are involved in generating P3; (4) AVCNa cells are the main CN cells involved in P4, N4, and P5 generation; (5) both ipsilateral and contralateral SOC cells have a role in generating monaurally evoked P4 and P5; and (6) P5 is generated by cells with characteristic frequencies below 10 kHz. From (2) and (4), it is clear that P2 and P4-P5 are generated by cells in distinct, parallel pathways.

Generators of the brainstem auditory evoked potential in cat. III: Identified cell populations

This paper examines the relationship between different brainstem cell populations and the brainstem auditory evoked potential (BAEP). First, we present a mathematical model relating the BAEP to underlying cellular activity. Then, we identify specific cellular generators of the click-evoked BAEP in cats by combining model-derived insights with key experimental data. These data include (a) a correspondence between particular brainstem regions and specific extrema in the BAEP waveform, determined from lesion experiments, and (b) values for model parameters derived from published physiological and anatomical information. Ultimately, we conclude (with varying degrees of confidence) that: (1) the earliest extrema in the BAEP are generated by spiral ganglion cells, (2) P2 is mainly generated by cochlear nucleus (CN) globular cells, (3) P3 is partly generated by CN spherical cells and partly by cells receiving inputs from globular cells, (4) P4 is predominantly generated by medial superior olive (MSO) principal cells, which are driven by spherical cells, (5) the generators of P5 are driven by MSO principal cells, and (6) the BAEP, as a whole, is generated mainly by cells with characteristic frequencies above 2 kHz. Thus, the BAEP in cats mainly reflects cellular activity in two parallel pathways, one originating with globular cells and the other with spherical cells. Since the globular cell pathway is poorly represented in humans, we suggest that the human BAEP is largely generated by brainstem cells in the spherical cell pathway. Given our conclusions, it should now be possible to relate activity in specific cell populations to psychophysical performance since the BAEP can be recorded in behaving humans and animals.

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, rarefaction 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.

The effect of broad-band noise on the binaural interaction components of human auditory brainstem-evoked potentials

Three-channel Lissajous trajectories (3-CLTs) of binaural interaction components (BI) of auditory brainstem potentials (ABEPs) were derived from 13 normally hearing adults by subtracting the response to binaural clicks from the algebraic sum of monaurally evoked responses to clicks. ABEPs were recorded in response to 65 dB nHL, alternating-polarity clicks, presented at a rate of 11/s. The procedure was repeated with clicks alone as well as with clicks with broad-band masking noise. Noise was presented at 25 and 45 dB nHL, producing a signal-to-noise ratio of +40 and +20 dB, respectively. All BI 3-CLTs included 6 planar segments (labeled BdI, BdII, BdIII, BeI, BeII and Bf) whose apex latencies, except Bf, increased with increasing noise level above 25 dB nHL, and whose durations, sizes, shapes and orientations did not change across noise levels. There were also significant increases in peak latencies of the BI from single channels vertex-mastoid and vertex-neck with increasing noise level. No significant change was found in the trajectory amplitude of apices, with the exception of apices BdIII and Bf whose amplitudes increased with increasing noise level. We suggest that the paradoxical increase in BI amplitude with masking noise may reflect a binaural enhancement of the effect of noise. The effects observed indicate that, whereas the response to clicks displays occlusion, the response to noise displays spatial facilitation at the brainstem level.

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.

Binaural interaction in the auditory brainstem response. Multichannel recordings

Binaural interaction (BI) waveforms were derived from multichannel recordings of auditory-evoked brainstem responses obtained at moderate and high intensity levels. The component latencies of all the BI responses derived from the contralateral channel were significantly prolonged compared with those derived from ipsilateral and non-cephalic channels. These channel differences were identified only at the moderate intensity level, indicating that BI cannot be isolated from the effects of stimulus interaction at higher intensities. The amplitudes were not significantly different across channels or intensities, indicating that ipsilateral, contralateral or non-cephalic recordings can be used to study BI. However, identification of channel differences on simultaneous multi-channel recordings may provide an index of true neural interaction.

Origin of the click-evoked binaural interaction potential, beta, of humans

The human click-evoked binaural difference waveform has as its most prominent feature the peak, beta, which has been shown to be related to binaural perception. In normal human subjects, we investigated the effect upon beta of (1) delivering the clicks in the presence of high passed masking noise (4000 Hz cut-off) and (2) reversing click polarity. In the presence of the masker, little activity occurs at the time the click-evoked beta would be expected. No significant change in beta latency occurs when the click polarity is inverted. We conclude that beta is principally due to the high-frequency components of the broad band click, so that it is through the activity in high characteristic frequency auditory nerve fibers that click-evoked beta is generated. Because the medial superior olive is the major nucleus of the human superior olivary complex, our results suggest that beta is possibly generated by the high-frequency cells of the medial superior olive.

The effect of brainstem lesions on brainstem auditory evoked potentials in the cat

Brainstem auditory evoked potentials (BAEPs) were recorded before and after cuts were made in either the midline trapezoid body (TB), the lateral lemniscus (LL), or the combined dorsal and intermediate acoustic striae (DAS/IAS) in 23 anesthetized cats. Monaural and binaural rarefaction clicks were presented at a rate of 10 per s, and the potentials recorded from a vertex electrode referenced to either earbar or to the neck. The potentials were filtered so that fast and slow components could be examined separately and special efforts were exerted to obtain stable conditions so that small changes in waveforms could be significant. Lesions of the DAS/IAS produced negligible changes in either the fast or slow waves. Lesions of the midline TB reduced the amplitudes of peaks P3 through P5, while greatly reducing the amplitude of the slow wave. Complete lesions of the LL always reduced the amplitude of the slow wave. Lesions of the ventral part of the LL were more likely to reduce the amplitude of P4-P5. Our interpretations of these lesion experiments are based on the idea that individual fast peaks of the BAEP represent compound action potentials of fiber pathways. According to this view, only synchronized activity generated in populations of neurons that are both favorably oriented in space and significant in number, will contribute to the fast peak.

Intra-ponto-mesencephalic recording of binaural interaction in human brain-stem auditory evoked potentials

Brain-stem auditory evoked potentials (BAEPs) were recorded at intraparenchymatous sites along a ponto-mesencephalic stereotactic penetration path in a patient with the rare condition of a ponto-medullary lesion which required biopsy but did not grossly alter scalp BAEPs. Click stimuli were applied either monaurally (with contralateral masking noise against acoustic cross-talk; conditions 'R,' 'L') or binaurally (condition 'RL'). A binaural interaction trace ('BI') was derived by subtracting the sum of the monaural from the binaural responses: BI = (RL)-(R + L). Despite failure to obtain significant BI components above noise level for scalp BAEPs, at the lower pons clearly discernible, multiphasic BI activity could be recorded beginning at the peak latency of scalp wave III and extending over approximately the next 4 msec. Its amplitude rapidly fell off with distance toward more rostral, mesencephalic recording sites. In relation to this positive finding, the equivocality among some of the previous studies on the detection of BI components in human scalp BAEPs is tentatively rephrased in terms mainly of a low signal-to-noise ratio and of functional peculiarities introduced by the respective stimulation protocols.

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.

Effects of electrolytic lesions of the superior olivary complex and trapezoid body on brainstem auditory-evoked potentials in the guinea pig. I. Vertex-tragus recordings

Brainstem auditory-evoked potentials (BAEP) were recorded between vertex and tragus in 22 guinea pigs, after destruction of the contralateral ear in order to produce monaural stimulation. Stimuli employed were 100 dB SPL clicks and tone bursts. Normative recordings showed five positive peaks, P1-P5 and four negative peaks, N1-N4. Electrolytic lesions were then made to the superior olivary complex (SOC) ipsilateral to the stimulation (6 animals), contralateral to the stimulation (8 animals), and trapezoid body (TB) (8 animals). In each group a statistical analysis of the results (amplitudes and latencies) was made (paired t test). After lesions to the ipsilateral SOC, there was an increase in the amplitude of P1 and a decrease in P3. After lesions to the contralateral SOC, there was a larger decrease in P3, which disappeared in 2 guinea pigs which had a large lesion involving many TB fibers. After TB lesions, there was a decrease in the P2-N2 composite wave, disappearance of P3, a decrease in P4, and an increase in the latency of P1 and P1-P2 and P2-P4 intervals. After ipsilateral SOC lesions, which certainly involved the uncrossed olivocochlear efferent tract, the increase in P1 suggested a disinhibition of the efferent fibers. The large decrease or the loss of P3 after TB lesions and SOC lesions involving many TB fibers suggested that P3 could be generated by the contralateral part of the TB fibers, in the vicinity of the contralateral SOC. These data are in agreement with the predominance of fiber tracts in the generation of BAEP.

Electrically elicited blink reflex and early acoustic evoked potentials in circumscribed and diffuse brain stem lesions

In the present paper, the function of the brain stem in patients with brain stem lesions of various aetiology is investigated with electrophysiological methods. The clinical observations are supplemented by experimental investigations on cats, in which the blink reflex and the early acoustic evoked potentials were registered during the acute elevation of intracranial pressure. The findings in patients with circumscribed space-occupying lesions in the posterior fossa document that the registration of the BR and the BAEP have a functional diagnostic significance above and beyond the neurological and radiological investigation. In the case of the cerebellar space occupations, specific alterations could not be observed. On the contrary, the alterations of BR and BAEP indicate a general disturbance of brain stem function, possibly as a result of a general increase of intracranial pressure. In cerebellopontine angle tumours, both BR and BAEP showed specific alterations which were usually asymmetrical. The BR changes ipsilateral to the tumour are of major topodiagnostic significance, whereas the alterations of the contralateral potential are especially informative in the registration of BAEP. The alterations of BR and BAEP also allow an appraisal of the localization and extent of the lesion in primary space occupations in the brain stem: A pathological R1 indicates a pontine lesion, whereas pathological R2 responses are found in medullary and in oral pontine and mesencephalic lesions. In contrast to cerebellopontine angle tumours, the BAEP tends to show symmetrical alterations in primary brain stem lesions. The prolongations of interpeak latencies correspond to the brain stem segment concerned, and the same also applies to pathological amplitude reduction and deformations of individual potentials. In patients with localized brain stem damage, the reflex pathway of R2 is discussed on the basis of the BR findings. In contrast to the view held up to now that only structures situated caudal of the facial nucleus area are responsible for the genesis of the R2 response, it is assumed on the basis of our own observations that pontomesencephalic structures rostral to the facial nuclei are also important for the genesis of R2. Registration of BR and BAEP in patients with acute diffuse brain stem damage shows that both methods have a high diagnostic and prognostic value. Isolated damage and combined brain stem lesion can be demonstrated and the course can be followed up. Normalization of pathological findings reflects clinical recovery, and conversely a secondary deterioration indicates the presence of complications.(ABSTRACT TRUNCATED AT 400 WORDS)