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

Independent or integrated processing of interaural time and level differences in human auditory cortex?

Sound localization in the horizontal plane is mainly determined by interaural time differences (ITD) and interaural level differences (ILD). Both cues result in an estimate of sound source location and in many real-life situations these two cues are roughly congruent. When stimulating listeners with headphones it is possible to counterbalance the two cues, so called ITD/ILD trading. This phenomenon speaks for integrated ITD/ILD processing at the behavioral level. However, it is unclear at what stages of the auditory processing stream ITD and ILD cues are integrated to provide a unified percept of sound lateralization. Therefore, we set out to test with human electroencephalography for integrated versus independent ITD/ILD processing at the level of preattentive cortical processing by measuring the mismatch negativity (MMN) to changes in sound lateralization. We presented a series of diotic standards (perceived at a midline position) that were interrupted by deviants that entailed either a change in a) ITD only, b) ILD only, c) congruent ITD and ILD, or d) counterbalanced ITD/ILD (ITD/ILD trading). The sound stimuli were either i) pure tones with a frequency of 500 Hz, or ii) amplitude modulated tones with a carrier frequency of 4000 Hz and a modulation frequency of 125 Hz. We observed significant MMN for the ITD/ILD traded deviants in case of the 500 Hz pure tones, and for the 4000 Hz amplitude-modulated tone. This speaks for independent processing of ITD and ILD at the level of the MMN within auditory cortex. However, the combined ITD/ILD cues elicited smaller MMN than the sum of the MMN induced in response to ITD and ILD cues presented in isolation for 500 Hz, but not 4000 Hz, suggesting independent processing for the higher frequency only. Thus, the two markers for independent processing - additivity and cue-conflict - resulted in contradicting conclusions with a dissociation between the lower (500 Hz) and higher frequency (4000 Hz) bands.

Assessment of the role of the cochlear latency effect in lateralization of click sounds in humans

Interaural time and intensity disparities (ITD and IID) are the two cues to sound lateralization. "Time-only" hypothesis claims that an IID is first converted to an interaural afferent delay (Delta t), and is then processed by the central ITD mechanism, rendering a separate IID processor unnecessary. We tested this hypothesis by assessing the contribution of the cochlear latency effect to the psychophysical ITD/IID trading ratio. Auditory brainstem responses (ABRs) were used to measure the interaural afferent delays (Delta ts) that developed with a 20/sec dichotic click train used in the trading experiment. Except for small IIDs at low loudness levels, the physiological Delta t delay produced by an IID was significantly smaller than the ITD psychophysically traded for the same IID. We concluded that the cochlear latency effect alone cannot explain the psychophysical ITD/IID trading ratios and a separate IID mechanism must be involved.

Right hemispheric dominance for echo suppression

When two sounds are presented sequentially within a short delay ( approximately 10ms), the listener perceives a single auditory event, the location of which is dominated by the directional information conveyed by the leading sound (the precedence effect, PE). The PE is not always instantaneous, but has been shown to build-up across repetitions of lead-lag pairs. Here, we investigated the contributions of lateralization cue (interaural time and intensity differences; ITD and IID, respectively) and the side of lateralization of the leading sound on the spatio-temporal activity associated with the PE. We applied electrical neuroimaging analyses to compare auditory evoked potentials (AEPs) in response to physically identical click pairs presented early and late within a stimulus train and perceived as two segregated events or as one fused auditory event. Significant topographic AEP modulations associated with the PE were observed over the 70-117ms post-stimulus period, with one topography characterizing fused perceptions and another segregated perceptions. The specific pattern of effects varied as a function of lateralization cue and the lateralization of the leading sound. The PE for ITD stimuli built-up during the stimulus train irrespective of the lateralization of the leading sound. The PE for IID stimuli did not exhibit build-up over the course of the stimulus train, but instead was generally affected by the lateralization of the leading sound. Source estimations further suggested that bilateral temporal networks were engaged when perceptions were segregated, whereas fused perceptions resulted in decreased activity in left temporal and increased activity in right temporo-parietal cortices.

Detection of combined changes in interaural time and intensity differences: Segregated mechanisms in cue type and in operating frequency range?

Although physiological studies have revealed segregated binaural pathways, namely the medial and lateral superior olives, it is unclear whether the human auditory system has separate mechanisms for different cue types (interaural time and intensity differences; ITD and IID, respectively) and for operating frequency ranges. This study hypothesized "channels" for ITD and IID processing, and examined channel interaction at low and high frequencies based on the signal detection theory. The stimuli were a 125- or 500-Hz tone and a 4-kHz tone amplitude-modulated with a half-wave-rectified 125-Hz sinusoid, presented dichotically with various baseline ITDs and IIDs. The detectability indices, d('), for ITD and IID changes, imposed individually or simultaneously in the same direction, were derived from the results of a forced-choice task. The degree of channel interaction was estimated by comparing d(') for combined cues with those for individual cues. The estimated interaction showed little effect of baseline ITD or IID. The results generally exhibited nonzero interaction, indicating that the cue processes are not completely independent. The interaction was stronger for high frequencies than for low frequencies. The results can be interpreted as indicating the involvement of different binaural mechanisms for different frequency regions.

The influence of externalization and spatial cues on the generation of auditory brainstem responses and middle latency responses

The effect of externalization and spatial cues on the generation of auditory brainstem responses (ABRs) and middle latency responses (MLRs) was investigated in this study. Most previous evoked potential studies used click stimuli with variations of interaural time (ITDs) and interaural level differences (ILDs) which merely led to a lateralization of sound inside the subject's head. In contrast, in the present study potentials were elicited by a virtual acoustics stimulus paradigm with 'natural' spatial cues and compared to responses to a diotic, non-externalized reference stimulus. Spatial sound directions were situated on the horizontal plane (corresponding to variations in ITD, ILD, and spectral cues) or the midsagittal plane (variation of spectral cues only). An optimized chirp was used which had proven to be advantageous over the click since it compensates for basilar membrane dispersion. ABRs and MLRs were recorded from 32 scalp electrodes and both binaural potentials (B) and binaural difference potentials (BD, i.e., the difference between binaural and summed monaural responses) were investigated. The amplitudes of B and BD to spatial stimuli were not higher than those to the diotic reference. ABR amplitudes decreased and latencies increased with increasing laterality of the sound source. A rotating dipole source exhibited characteristic patterns in dependence on the stimulus laterality. For the MLR data, stimulus laterality was reflected in the latency of component N(a). In addition, dipole source analysis revealed a systematic magnitude increase for the dipole contralateral to the azimuthal position of the sound source. For the variation of elevation, the right dipole source showed a stronger activation for stimuli away from the horizontal plane. The results indicate that at the level of the brainstem and primary auditory cortex binaural interaction is mostly affected by interaural cues (ITD, ILD). Potentials evoked by stimuli with natural combinations of ITD, ILD, and spectral cues were not larger than those elicited by diotic chirps.

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.

The spatio-temporal brain dynamics of processing and integrating sound localization cues in humans

Interaural intensity and time differences (IID and ITD) are two binaural auditory cues for localizing sounds in space. This study investigated the spatio-temporal brain mechanisms for processing and integrating IID and ITD cues in humans. Auditory-evoked potentials were recorded, while subjects passively listened to noise bursts lateralized with IID, ITD or both cues simultaneously, as well as a more frequent centrally presented noise. In a separate psychophysical experiment, subjects actively discriminated lateralized from centrally presented stimuli. IID and ITD cues elicited different electric field topographies starting at approximately 75 ms post-stimulus onset, indicative of the engagement of distinct cortical networks. By contrast, no performance differences were observed between IID and ITD cues during the psychophysical experiment. Subjects did, however, respond significantly faster and more accurately when both cues were presented simultaneously. This performance facilitation exceeded predictions from probability summation, suggestive of interactions in neural processing of IID and ITD cues. Supra-additive neural response interactions as well as topographic modulations were indeed observed approximately 200 ms post-stimulus for the comparison of responses to the simultaneous presentation of both cues with the mean of those to separate IID and ITD cues. Source estimations revealed differential processing of IID and ITD cues initially within superior temporal cortices and also at later stages within temporo-parietal and inferior frontal cortices. Differences were principally in terms of hemispheric lateralization. The collective psychophysical and electrophysiological results support the hypothesis that IID and ITD cues are processed by distinct, but interacting, cortical networks that can in turn facilitate auditory localization.

Better time–intensity trade revealed by bilateral giant magnetostrictive bone conduction

Sound lateralization tests were performed to compare the magnet coil bone-conduction headphone with the giant magnetostrictive bone-conduction headphone using 18 healthy participants. Although, no significant difference between these bone-conduction headphones was obtained for the interaural time difference and interaural intensity difference, a significant difference was obtained for the time-intensity trade. This revealed that the difference between the headphones is apparent in the integration of the heterogeneous sensations of the time and intensity difference at the cognitive level, but no difference is apparent between the homogeneous sensations of the discrimination of interaural time difference or interaural intensity difference at the sensory level. It was concluded that the difference at the cognitive level indicates the better performance of the giant magnetostrictive headphone.

Dipole source analysis of auditory brain stem responses evoked by lateralized clicks

The objective of this paper was to elucidate the relation between psychophysical lateralization and the neural generators of the corresponding auditory evoked potentials. Auditory brain stem responses to binaural click stimuli with different interaural time- and level differences were obtained in 12 subjects by means of multi-channel EEG recording. Data were modeled by equivalent current dipoles representing the generating sources in the brain. A generalized maximum-likelihood method was used to solve the inverse problem, taking into account the noise covariance matrix of the data. The quality of the fit was assessed by computing the goodness-of-fit as the outcome of a chi 2-test. This measure was advantageous compared to the conventionally employed residual variance. At the latency of Jewett wave V, there was a systematic variation of the moment of a rotating dipole with the lateralization of the stimulus. Dipole moment trajectories of stimuli with similar lateralization were similar. A sign reversal of the interaural differences resulted in a mirrored trajectory. Centrally-perceived stimuli corresponded to dipoles with the largest vertical components. With increasing lateralization, the vertical component of the moment decreased, while the horizontal components increased. The similarity of trajectories inducted by the same lateralization show that interaural time- and level differences are not processed independently. The present data support the notion that directional information is already extracted and represented at the level of the brain stem.

Characterization of the human superior olivary complex by calcium binding proteins and neurofilament H (SMI-32)

This study provides a morphologic characterization of the human superior olivary complex as revealed by immunohistochemistry by using antibodies against the calcium binding proteins parvalbumin, calbindin, calretinin, and the nonphosphorylated neurofilament H SMI-32. By combining these markers, it was possible to establish the neuronal architecture and details of the morphologic organization (including axonal terminals) of the different nuclei. The medial superior olivary nucleus is formed by a sheet of parallel-oriented cells. A clear segregation of axon terminals was noticed on the medially and laterally oriented dendrites of the mostly bipolar neurons. The lateral superior olivary nucleus lacked a distinct nuclear shape but was formed by several patches of rather irregularly arranged neurons. Calretinin or parvalbumin immunoreactive afferent terminals were observed which contacted somata or dendrites of these neurons. The immunolabeling also revealed the boundaries of the dorsal periolivary nucleus and morphologic detail of its neurons. A coherent nuclear structure that could be addressed as the medial nucleus of the trapezoid body was not identified by any single one or by combinations of the markers used. The data were also used to establish a three-dimensional-reconstruction of the three major subnuclei of the superior olivary complex. The results are discussed with respect to the possible role of the superior olivary complex in the processing of spatial acoustic information in the azimuthal plane.

Buildup and breakdown of echo suppression for stimuli presented over headphones-the effects of interaural time and level differences

The current study investigates buildup and breakdown of echo suppression for stimuli presented over headphones. The stimuli consisted of pairs of 120-micros clicks. The leading click (lead) and the lagging click (lag) in each pair were lateralized on opposite sides of the midline by means of interaural level differences (ILDs) of +/-10 dB or interaural time differences (ITDs) of +/-300 micros. Echo threshold was measured with an adaptive one-interval, two-alternative, forced-choice procedure with a subjective decision criterion, in which listeners had to report whether they heard a single, fused auditory event on one side of the midline, or two separate events on both sides. In the control conditions, referred to as the "single" conditions, echo threshold was measured for a single click pair, the test pair, presented in isolation. In addition to the control conditions, two kinds of test conditions were investigated, in which the test pair was preceded by 12 identical conditioning pairs: in the "same" conditions, the interaural configuration (ILDs or ITDs) of the conditioning pairs was identical to that of the test pair; in the "switch" conditions, the interaural configuration of lead and lag was reversed between the conditioning pairs and the test pair, in order to produce a switch in the lateralizations of the stimuli between the conditioning train and the test pair. No matter whether the lateralization of the clicks was produced by ILDs or by ITDs, most listeners experienced a buildup of echo suppression in the "same" conditions, manifested by a prolongation of echo threshold relative to the respective "single" conditions. However, the breakdown of echo suppression was much stronger in the ILD-switch than in the ITD-switch conditions. In five out of six listeners, the ITD switch had hardly any effect on echo threshold, although the ITDs (+/-300 micros) produced roughly the same degree of lateral displacement as the ILDs (+/-10 dB). These results suggest that the dynamic processes in echo suppression operate differentially in pathways responsible for the processing of interaural time and level differences.

Differences between the N1 waves of the responses to interaural time and intensity disparities: scalp topography and dipole sources

OBJECTIVES

Being the two complementary cues to directional hearing, interaural time and intensity disparities (ITD and IID, respectively), are known to be separately encoded in the brain stem. We address the question as to whether their codes are collapsed into a single lateralization code subcortically or they reach the cortex via separate channels and are processed there in different areas.

METHODS

Two continuous trains of 100/s clicks were dichotically presented. At 2 s intervals either an interaural time delay of 1ms or an interaural level difference of 20 dB (HL) was introduced for 50 ms, shifting the intracranial sound image laterally for this brief period of time. Long-latency responses to these directional stimuli, which had been tested to evoke no potentials under monotic or diotic conditions, as well as to sound pips of 50 ms duration were recorded from 124 scalp electrodes. Scalp potential and current density maps at N1 latency were obtained from thirteen normal subjects. A 4-sphere head model with bilaterally symmetrical dipoles was used for source analysis and a simplex algorithm preceded by a genetic algorithm was employed for solving the inverse problem.

RESULTS

Inter- and intra-subject comparisons showed that the N1 responses evoked by IID and ITD as well as by sound pip stimuli had significantly different scalp topographies and interhemispheric dominance patterns. Significant location and orientation differences between their estimated dipole sources were also noted.

CONCLUSIONS

We conclude that interaural time and intensity disparities (thus the lateral shifts of a sound image caused by these two cues) are processed in different ways and/or in different areas in auditory cortex.

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.