Human auditory cortical mechanisms of sound lateralization: I. Interaural time differences within sound

Cortical Processing of Binaural Cues as Shown by EEG Responses to Random-Chord Stereograms

Spatial hearing facilitates the perceptual organization of complex soundscapes into accurate mental representations of sound sources in the environment. Yet, the role of binaural cues in auditory scene analysis (ASA) has received relatively little attention in recent neuroscientific studies employing novel, spectro-temporally complex stimuli. This may be because a stimulation paradigm that provides binaurally derived grouping cues of sufficient spectro-temporal complexity has not yet been established for neuroscientific ASA experiments. Random-chord stereograms (RCS) are a class of auditory stimuli that exploit spectro-temporal variations in the interaural envelope correlation of noise-like sounds with interaurally coherent fine structure; they evoke salient auditory percepts that emerge only under binaural listening. Here, our aim was to assess the usability of the RCS paradigm for indexing binaural processing in the human brain. To this end, we recorded EEG responses to RCS stimuli from 12 normal-hearing subjects. The stimuli consisted of an initial 3-s noise segment with interaurally uncorrelated envelopes, followed by another 3-s segment, where envelope correlation was modulated periodically according to the RCS paradigm. Modulations were applied either across the entire stimulus bandwidth (wideband stimuli) or in temporally shifting frequency bands (ripple stimulus). Event-related potentials and inter-trial phase coherence analyses of the EEG responses showed that the introduction of the 3- or 5-Hz wideband modulations produced a prominent change-onset complex and ongoing synchronized responses to the RCS modulations. In contrast, the ripple stimulus elicited a change-onset response but no response to ongoing RCS modulation. Frequency-domain analyses revealed increased spectral power at the fundamental frequency and the first harmonic of wideband RCS modulations. RCS stimulation yields robust EEG measures of binaurally driven auditory reorganization and has potential to provide a flexible stimulation paradigm suitable for isolating binaural effects in ASA experiments.

Ear-Specific Hemispheric Asymmetry in Unilateral Deafness Revealed by Auditory Cortical Activity

Profound unilateral deafness reduces the ability to localize sounds achieved via binaural hearing. Furthermore, unilateral deafness promotes a substantial change in cortical processing to binaural stimulation, thereby leading to reorganization over the whole brain. Although distinct patterns in the hemispheric laterality depending on the side and duration of deafness have been suggested, the neurological mechanisms underlying the difference in relation to behavioral performance when detecting spatially varied cues remain unknown. To elucidate the mechanism, we compared N1/P2 auditory cortical activities and the pattern of hemispheric asymmetry of normal hearing, unilaterally deaf (UD), and simulated acute unilateral hearing loss groups while passively listening to speech sounds delivered from different locations under open free field condition. The behavioral performances of the participants concerning sound localization were measured by detecting sound sources in the azimuth plane. The results reveal a delayed reaction time in the right-sided UD (RUD) group for the sound localization task and prolonged P2 latency compared to the left-sided UD (LUD) group. Moreover, the RUD group showed adaptive cortical reorganization evidenced by increased responses in the hemisphere ipsilateral to the intact ear for individuals with better sound localization whereas left-sided unilateral deafness caused contralateral dominance in activity from the hearing ear. The brain dynamics of right-sided unilateral deafness indicate greater capability of adaptive change to compensate for impairment in spatial hearing. In addition, cortical N1 responses to spatially varied speech sounds in unilateral deaf people were inversely related to the duration of deafness in the area encompassing the right auditory cortex, indicating that early intervention would be needed to protect from maladaptation of the central auditory system following unilateral deafness.

Assessment of haptic memory using somatosensory change-related cortical responses

Haptic memory briefly retains somatosensory information for later use; however, how and which cortical areas are affected by haptic memory remain unclear. We used change-related cortical responses to investigate the relationship between the somatosensory cortex and haptic memory objectively. Electrical pulses, at 50 Hz with a duration of 500 ms, were randomly applied to the second, third, and fourth fingers of the right and left hands at an even probability every 800 ms. Each stimulus was labeled as D (preceded by a different side) or S (preceded by the same side). The D stimuli were further classified into 1D, 2D, and 3D, according to the number of different preceding stimuli. The S stimuli were similarly divided into 1S and 2S. The somatosensory-evoked magnetic fields obtained were divided into four components via a dipole analysis, and each component's amplitudes were measured using the source strength waveform. The results showed that the preceding event did not affect the amplitude of the earliest 20-30 ms response in the primary somatosensory cortex. However, in the subsequent three components, the cortical activity amplitude was largest in 3D, followed by 2D, 1D, and S. These results indicate that such modulatory effects occurred somewhere in the somatosensory processing pathway higher than Brodmann's area 3b. To the best of our knowledge, this is the first study to demonstrate the existence of haptic memory for somatosensory laterality and its impact on the somatosensory cortex using change-related cortical responses without contamination from peripheral effects.

Event-related potentials to single-cycle binaural beats of a pure tone, a click train, and a noise

There are only few electrophysiological studies on a phenomenon called "binaural beats" (BBs), which is experienced when two tones with frequencies close to each other are dichotically presented to the ears. And, there is no study in which the electrical responses of the brain to BBs of complex sounds are recorded and analyzed. Owing to a recent method based on single-cycle BB stimulation with sub-threshold temporary monaural frequency shifts, we could record the event-related potentials (ERPs) to BBs of a 250-Hz tone as well as those to the BBs of a 250/s click train and to the BBs of a recurrent 4-ms Gaussian noise. Although fundamental components of the click train and noise stimuli were lower in intensity than the tonal stimuli in our experiments, the N1 responses to the BBs of the former two wide-spectrum sounds were recorded with significantly larger amplitudes and shorter latencies than those to the BBs of a tone, suggesting an across-frequency integration of directional information. During a BB cycle of a complex sound, the interaural time differences (ITDs) of the spectral components are all equal to each other at any time; whereas their interaural phase differences (IPDs) are all different. The ITD rather than the IPD should, therefore, be the cue that is relied upon by the binaural mechanism coding the perceived lateral shifts of the sound caused by BBs. This is in line with across-frequency models of human auditory lateralization based on a common ITD, fulfilling a straightness criterion.

Properties of echoic memory revealed by auditory-evoked magnetic fields

We used auditory-evoked magnetic fields to investigate the properties of echoic memory. The sound stimulus was a repeated 1-ms click at 100 Hz for 500 ms, presented every 800 ms. The phase of the sound was shifted by inserting an interaural time delay of 0.49 ms to each side. Therefore, there were two sounds, lateralized to the left and right. According to the preceding sound, each sound was labeled as D (preceded by a different sound) or S (by the same sound). The D sounds were further grouped into 1D, 2D, and 3D, according to the number of preceding different sounds. The S sounds were similarly grouped to 1S and 2S. The results showed that the preceding event significantly affected the amplitude of the cortical response; although there was no difference between 1S and 2S, the amplitudes for D sounds were greater than those for S sounds. Most importantly, there was a significant amplitude difference between 1S and 1D. These results suggested that sensory memory was formed by a single sound, and was immediately replaced by new information. The constantly-updating nature of sensory memory is considered to enable it to act as a real-time monitor for new information.

Auditory cortex responses to interaural time differences in the envelope of low-frequency sound, recorded with MEG in young and older listeners

Interaural time and intensity differences (ITD and IID) are important cues in binaural hearing and allow for sound localization, improving speech understanding in noise and reverberation, and integrating sound sources in the auditory scene. Whereas previous research showed that the upper-frequency limit for ITD detection in the fine structure of sound declines in aging, the processing of envelope ITD in low-frequency amplitude modulated (AM) sound and the related brain responses are less understood. This study investigated the cortical processing of envelope ITD and compared the results with previous findings about the fine-structure ITD. In two experiments, participants listened to 40-Hz AM tones containing sudden changes in the envelope ITD. Multiple MEG responses were analyzed, including the auditory evoked N1 responses, elicited both by sound onsets and ITD changes, and 40-Hz responses, elicited by the AM. The first experiment with healthy young adults revealed a substantial decline in the magnitudes of the ITD change N1 response, and the 40-Hz phase resets at higher carrier frequencies, suggesting a similar frequency characteristic as observed for fine structure ITD. The amplitude of the 40-Hz ASSR declined only gradually with increasing carrier frequency, and it was excluded as a confounding factor in the decline in the ITD response. Larger responses to outward ITD changes than inward changes, here first reported for envelope ITD, were another characteristics that were similar to fine-structure ITD. A second experiment with groups of young and older listeners examined the effects of aging and concurrent noise on the cortical envelope ITD responses. One important research question was, whether binaural cues are accessible in noise. Behavioural tests showed an age-related hearing loss in the older group and decreased performance in envelope ITD detection and speech-in-noise (SIN) understanding. Binaural hearing and SIN performance were correlated with one other, but not with hearing loss. The frequency limit for envelope ITD was reduced in older listeners similarly as previously found for fine structure ITD, and older listeners were more susceptible to concurrent multi-talker noise. The similarities between responses to envelope ITD and to fine structure ITD suggest that a common cortical code exists for the envelope and fine structure ITD. The dependency on the carrier frequency suggests that envelope ITD processing at the subcortical level requires stimulus phase locking, which might be reduced in aging.

Human cortical sensitivity to interaural time difference in high-frequency sounds

Human sound source localization relies on various acoustical cues one of the most important being the interaural time difference (ITD). ITD is best detected in the fine structure of low-frequency sounds but it may also contribute to spatial hearing at higher frequencies if extracted from the sound envelope. The human brain mechanisms related to this envelope ITD cue remain unexplored. Here, we tested the sensitivity of the human auditory cortex to envelope ITD in magnetoencephalography (MEG) recordings. We found two types of sensitivity to envelope ITD. First, the amplitude of the auditory cortical N1m response was smaller for zero envelope ITD than for long envelope ITDs corresponding to the sound being in opposite phase in the two ears. Second, the N1m response amplitude showed ITD-specific adaptation for both fine-structure and for envelope ITD. The auditory cortical sensitivity was weaker for envelope ITD in high-frequency sounds than for fine-structure ITD in low-frequency sounds but occurred within a range of ITDs that are encountered in natural conditions. Finally, the participants were briefly tested for their behavioral ability to detect envelope ITD. Interestingly, we found a correlation between the behavioral performance and the neural sensitivity to envelope ITD. In conclusion, our findings show that the human auditory cortex is sensitive to ITD in the envelope of high-frequency sounds and this sensitivity may have behavioral relevance.

Visual-induced expectations modulate auditory cortical responses

Active sensing has important consequences on multisensory processing (Schroeder et al., 2010). Here, we asked whether in the absence of saccades, the position of the eyes and the timing of transient color changes of visual stimuli could selectively affect the excitability of auditory cortex by predicting the “where” and the “when” of a sound, respectively. Human participants were recorded with magnetoencephalography (MEG) while maintaining the position of their eyes on the left, right, or center of the screen. Participants counted color changes of the fixation cross while neglecting sounds which could be presented to the left, right, or both ears. First, clear alpha power increases were observed in auditory cortices, consistent with participants' attention directed to visual inputs. Second, color changes elicited robust modulations of auditory cortex responses (“when” prediction) seen as ramping activity, early alpha phase-locked responses, and enhanced high-gamma band responses in the contralateral side of sound presentation. Third, no modulations of auditory evoked or oscillatory activity were found to be specific to eye position. Altogether, our results suggest that visual transience can automatically elicit a prediction of “when” a sound will occur by changing the excitability of auditory cortices irrespective of the attended modality, eye position or spatial congruency of auditory and visual events. To the contrary, auditory cortical responses were not significantly affected by eye position suggesting that “where” predictions may require active sensing or saccadic reset to modulate auditory cortex responses, notably in the absence of spatial orientation to sounds.

Emphasis of spatial cues in the temporal fine structure during the rising segments of amplitude-modulated sounds

The ability to locate the direction of a target sound in a background of competing sources is critical to the survival of many species and important for human communication. Nevertheless, brain mechanisms that provide for such accurate localization abilities remain poorly understood. In particular, it remains unclear how the auditory brain is able to extract reliable spatial information directly from the source when competing sounds and reflections dominate all but the earliest moments of the sound wave reaching each ear. We developed a stimulus mimicking the mutual relationship of sound amplitude and binaural cues, characteristic to reverberant speech. This stimulus, named amplitude modulated binaural beat, allows for a parametric and isolated change of modulation frequency and phase relations. Employing magnetoencephalography and psychoacoustics it is demonstrated that the auditory brain uses binaural information in the stimulus fine structure only during the rising portion of each modulation cycle, rendering spatial information recoverable in an otherwise unlocalizable sound. The data suggest that amplitude modulation provides a means of "glimpsing" low-frequency spatial cues in a manner that benefits listening in noisy or reverberant environments.

Psychophysics and neuronal bases of sound localization in humans

Localization of sound sources is a considerable computational challenge for the human brain. Whereas the visual system can process basic spatial information in parallel, the auditory system lacks a straightforward correspondence between external spatial locations and sensory receptive fields. Consequently, the question how different acoustic features supporting spatial hearing are represented in the central nervous system is still open. Functional neuroimaging studies in humans have provided evidence for a posterior auditory "where" pathway that encompasses non-primary auditory cortex areas, including the planum temporale (PT) and posterior superior temporal gyrus (STG), which are strongly activated by horizontal sound direction changes, distance changes, and movement. However, these areas are also activated by a wide variety of other stimulus features, posing a challenge for the interpretation that the underlying areas are purely spatial. This review discusses behavioral and neuroimaging studies on sound localization, and some of the competing models of representation of auditory space in humans. This article is part of a Special Issue entitled Human Auditory Neuroimaging.

Habituation of visual evoked responses in neonates and fetuses: a MEG study

In this study we aimed to develop a habituation paradigm that allows the investigation of response decrement and response recovery and examine its applicability for measuring the habituation of the visually evoked responses (VERs) in neonatal and fetal magnetoencephalographic recordings. Two paradigms, one with a long and one with a short inter-train interval (ITI), were developed and tested in separate studies. Both paradigms consisted of a train of four light flashes; each train being followed by a 500Hz burst tone. Healthy pregnant women underwent two prenatal measurements and returned with their babies for a neonatal investigation. The amplitudes of the neonatal VERs in the long-ITI condition showed within-train response decrement. An increased response to the auditory dishabituator was found confirming response recovery. In the short-ITI condition, neonatal amplitude decrement could not be demonstrated while response recovery was present. In both ITI conditions, the response rate of the cortical responses was much lower in the fetuses than in the neonates. Fetal VERs in the long-ITI condition indicate amplitude decline from the first to the second flash with no further decrease. The long-ITI paradigm might be useful to investigate habituation of the VERs in neonates and fetuses, although the latter requires precaution.

The analysis of simple and complex auditory signals in human auditory cortex: magnetoencephalographic evidence from M100 modulation

OBJECTIVE

Ecologically valid signals (e.g., vowels) have multiple components of substantially different frequencies and amplitudes that may not be equally cortically represented. In this study, we investigate a relatively simple signal at an intermediate level of complexity, two-frequency composite tones, a stimulus lying between simple sinusoids and ecologically valid signals such as speech. We aim to characterize the cortical response properties to better understand how complex signals may be represented in auditory cortex.

DESIGN

Using magnetoencephalography, we assessed the sensitivity of the M100/N100m auditory-evoked component to manipulations of the power ratio of the individual frequency components of the two-frequency complexes. Fourteen right-handed subjects with normal hearing were scanned while passively listening to 10 complex and 12 simple signals. The complex signals were composed of one higher frequency and one lower frequency sinusoid; the lower frequency sinusoidal component was at one of the five loudness levels relative to the higher frequency one: -20, -10, 0, +10, +20 dB. The simple signals comprised all the complex signal components presented in isolation.

RESULTS

The data replicate and extend several previous findings: (1) the systematic dependence of the M100 latency on signal intensity and (2) the dependence of the M100 latency on signal frequency, with lower frequency signals ( approximately 100 Hz) exhibiting longer latencies than higher frequency signals ( approximately 1000 Hz) even at matched loudness levels. (3) Importantly, we observe that, relative to simple signals, complex signals show increased response amplitude-as one might predict-but decreased M100 latencies.

CONCLUSION

: The data suggest that by the time the M100 is generated in auditory cortex ( approximately 70 to 80 msecs after stimulus onset), integrative processing across frequency channels has taken place which is observable in the M100 modulation. In light of these data models that attribute more time and processing resources to a complex stimulus merit reevaluation, in that our data show that acoustically more complex signals are associated with robust temporal facilitation, across frequencies and signal amplitude level.

Topography of syllable change-detection electrophysiological indices in children and adults with reading disabilities

INTRODUCTION

Developmental dyslexia (DD) is a frequent language-based learning disorder. The predominant etiological view postulates that reading problems originate from a phonological impairment.

METHOD

We studied mismatch negativity (MMN) and Late Discriminative Negativity (LDN) to syllables change in both children (n=12; 8-12 years) and young adults (n=15; 14-23 years) with DD compared with controls.

RESULTS/DISCUSSION

The present study confirmed abnormal automatic discrimination of syllable changes in both children and adults with developmental dyslexia. MMN topographic, amplitude and latency group differences were evidenced, suggesting different brain mechanisms involved in elementary auditory stimulus change-detection in DD, especially in the left hemisphere. The LDN results demonstrated that the auditory disorder of temporal processing in DD children becomes more serious at late stages of information processing and that the apparent cerebral hypo reactivity to speech changes in DD actually may correspond to additional processes. The age-related differences observed in both MMN and LDN topographies, amplitudes and latency between subjects with DD and controls could indicate different developmental courses in the neural representation of basic speech sounds in good and poor readers, with a tendency to normalization with increasing age.

CONCLUSION

Our results showing atypical electrophysiological concomitants of speech auditory perception in DD strongly support the hypothesis of deviant cortical organization in DD.

Asymmetric lateral inhibitory neural activity in the auditory system: a magnetoencephalographic study

BACKGROUND

Decrements of auditory evoked responses elicited by repeatedly presented sounds with similar frequencies have been well investigated by means of electroencephalography and magnetoencephalography (MEG). However the possible inhibitory interactions between different neuronal populations remains poorly understood. In the present study, we investigated the effect of proceeding notch-filtered noises (NFNs) with different frequency spectra on a following test tone using MEG.

RESULTS

Three-second exposure to the NFNs resulted in significantly different N1m responses to a 1000 Hz test tone presented 500 ms after the offset of the NFNs. The NFN with a lower spectral edge closest to the test tone mostly decreased the N1m amplitude.

CONCLUSION

The decrement of the N1m component after exposure to the NFNs could be explained partly in terms of lateral inhibition. The results demonstrated that the amplitude of the N1m was more effectively influenced by inhibitory lateral connections originating from neurons corresponding to lower rather than higher frequencies. We interpret this effect of asymmetric lateral inhibition in the auditory system as an important contribution to reduce the asymmetric neural activity profiles originating from the cochlea.

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.

Cortical response to auditory motion suggests an asymmetry in the reliance on inter-hemispheric connections between the left and right auditory cortices

The aim of the current study was to measure the brain's response to auditory motion using electroencephalography (EEG) to gain insight into the mechanisms by which hemispheric lateralization for auditory spatial processing is established in the human brain. The onset of left- or rightward motion in an otherwise continuous sound was found to elicit a large response, which appeared to arise from higher-level nonprimary auditory areas. This motion onset response was strongly lateralized to the hemisphere contralateral to the direction of motion. The response latencies suggest that the ipsilateral response to the leftward motion was produced by indirect callosal projections from the opposite hemisphere, whereas the ipsilateral response to the rightward motion seemed to receive contributions from direct thalamocortical projections. These results suggest an asymmetry in the reliance on inter-hemispheric projections between the left and right auditory cortices for auditory spatial processing.

Auditory evoked magnetic fields in relation to interaural time delay and interaural correlation

The detection of interaural time differences (ITD) for sound localization depends on the similarity between the left and right ear signals, namely interaural correlation (IAC). Human localization performance deteriorates with decreasing IACs. In order to examine activity related to localization performance in the human cortex, auditory evoked magnetic fields to the ITD of bandpass noises with different IACs were analyzed. When the IAC was 0.95, the N1m amplitudes, i.e., the estimated equivalent current dipole moments, increased with increasing ITD. However the effect of ITD on the N1m amplitudes was not significant when the IAC was 0.5. When the ITD was 0.7 ms, the N1m amplitudes decreased with decreasing IACs. There were no systematic changes in the source location of N1m in the auditory cortex related to changes in ITD or IAC. The results suggest that localization performance is reflected in N1m amplitudes.

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.

Effects of the frequency of interaural time difference in the human brain

The two cues to the horizontal location sound sources are interaural time differences and interaural level differences. For low-frequency tones, interaural time differences provide effective and unambiguous information. For higher frequency sounds, however, interaural time differences provide ambiguous cues. In order to evaluate the effect of frequency of interaural time differences in the human auditory cortex, the auditory evoked fields to different interaural time differences of pure tone were examined. The results showed that the N1m magnitudes varied with the interaural time differences when the frequency of the pure tone was 800 Hz. The N1m magnitudes, however, did not vary with the interaural time differences when the frequency of the pure tone was 1600 Hz. These results indicate that localization performance might be reflected in N1m magnitudes.

Auditory evoked fields to variations of interaural time delay

Auditory motion can be simulated by presenting binaural sounds with time-varying interaural time delays. Human cortical responses to the rate of auditory motion were studied by recording auditory evoked magnetic fields with a 122-channel whole-head magnetometer. Auditory motion from central to right and then to central was produced by varying interaural time differences between ears. The results showed that the N1m latencies and amplitudes were not affected by the fluctuation of interaural time delay; however, the peak amplitude of P2m significantly increased as a function of fluctuation of the interaural time delay.

Predicting EEG responses using MEG sources in superior temporal gyrus reveals source asynchrony in patients with schizophrenia

OBJECTIVE

An integrated analysis using Electroencephalography (EEG) and magnetoencephalography (MEG) is introduced to study abnormalities in early cortical responses to auditory stimuli in schizophrenia.

METHODS

Auditory responses were recorded simultaneously using EEG and MEG from 20 patients with schizophrenia and 19 control subjects. Bilateral superior temporal gyrus (STG) sources and their time courses were obtained using MEG for the 30-100 ms post-stimulus interval. The MEG STG source time courses were used to predict the EEG signal at electrode Cz.

RESULTS

In control subjects, the STG sources predicted the EEG Cz recording very well (97% variance explained). In schizophrenia patients, the STG sources accounted for substantially (86%) and significantly (P<0.0002) less variance. After MEG-derived STG activity was removed from the EEG Cz signal, the residual signal was dominated by 40 Hz activity, an indication that the remaining variance in EEG is probably contributed by other brain generators, rather than by random noise.

CONCLUSIONS

Integrated MEG and EEG analysis can differentiate patients and controls, and suggests a basis for a well established abnormality in the cortical auditory response in schizophrenia, implicating a disorder of functional connectivity in the relationship between STG sources and other brain generators.

Human cortical representation of virtual auditory space: differences between sound azimuth and elevation

Sounds convolved with individual head-related transfer functions and presented through headphones can give very natural percepts of the three-dimensional auditory space. We recorded whole-scalp neuromagnetic responses to such stimuli to compare reactivity of the human auditory cortex to sound azimuth and elevation. The results suggest that the human auditory cortex analyses sound azimuth, based on both binaural and monaural localization cues, mainly in the hemisphere contralateral to the sound, whereas elevation in the anterior space and in the lateral auditory space in general, both strongly relying on monaural spectral cues, are analyzed in more detail in the right auditory cortex. The binaural interaural time and interaural intensity difference cues were processed in the auditory cortex around 100-150 ms and the monaural spectral cues later around 200-250 ms.

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.

Right-hemisphere dominance for the processing of sound-source lateralization

Cortical processing of change in direction of a perceived sound source was investigated in 12 human subjects using whole-head magnetoencephalography. The German word "da" was presented either with or without 0.7 msec interaural time delays to create the impression of right- or left-lateralized or midline sources, respectively. Midline stimuli served as standards, and lateralized stimuli served as deviants in a mismatch paradigm. Two symmetrically linked dipoles fitted to the mismatch fields showed stronger moments in the hemisphere contralateral to the side of the deviant. The right dipole displayed equal latencies to both left and right deviants, whereas left dipole latencies were longer for ipsilateral than contralateral deviants. Frequency analysis between 20-70 Hz and statistical probability mapping revealed increased induced gamma-band activity at 53+/-2.5 Hz to both types of deviants. Right deviants elicited spectral amplitude enhancements in this frequency range, peaking at latencies of 160 and 240 msec. These effects were localized bilaterally over the angular gyri and posterior temporal regions. Coherence analysis suggested the existence of two separate interhemispheric networks. For left-lateralized deviants, both spectral amplitude enhancements at 110 and 220 msec and coherence increases were restricted to the right hemisphere. In conclusion, both mismatch dipole latencies at the supratemporal plane and gamma-band activity in posterior parietotemporal areas suggested a right hemisphere engagement in the processing of bidirectional sound-source shifts. In contrast, left-hemisphere regions responded predominantly to contralateral events. These findings may help to elucidate phenomena such as unilateral auditory neglect.

Combined mapping of human auditory EEG and MEG responses

Auditory electric and magnetic P50(m), N1(m) and MMN(m) responses to standard, deviant and novel sounds were studied by recording brain electrical activity with 25 EEG electrodes simultaneously with the corresponding magnetic signals measured with 122 MEG gradiometer coils. The sources of these responses were located on the basis of the MEG responses; all were found to be in the supratemporal plane. The goal of the present paper was to investigate to what degree the source locations and orientations determined from the magnetic data account for the measured EEG signals. It was found that the electric P50, N1 and MMN responses can to a considerable degree be explained by the sources of the corresponding magnetic responses. In addition, source-current components not detectable by MEG were shown to contribute to the measured EEG signals.

Human long-latency responses to brief interaural disparities of intensity

The electroencephalographic responses to abrupt changes in interaural differences of time (ITD) and intensity (IID) should provide important information on the dynamic characteristics and integrity of the binaural mechanisms detecting the azimuthal shifts of a sound image. However, a change in either or both of these cues to sound lateralization would stimulate not only the binaural mechanisms but also the monaural ones. There are several reports evidencing that in the case of ITD changes this problem can be overcome by using time-shifted noise or repetitive clicks. Any change in IID, however, will inevitably have a stimulating effect also on purely monaural mechanisms. Therefore, the stimulation techniques described in the literature so far for recording the long-latency responses related to IID mechanism cannot be regarded as being specific for binaural mechanisms. We used dichotically presented 100/s click trains which were amplitude modulated with a random sequence of 50 or 100 ms square wave-intervals, so that the sound intensities at the two ears simultaneously alternated between 60 dB and 80 dB levels except during brief periods of time (50 ms) in which the interaural intensity balance was impaired, leading to an IID of 20 dB every 2 s. Owing to the fact that the cortical mechanisms remain unresponsive to repetitive stimuli presented with intervals shorter than a certain recovery period, this stimulus did not evoke any significant potential when it was presented monotically or diotically, yet it could produce lateral sound image shifts and therefore evoke pronounced long-latency responses when presented dichotically. The main components N1 and P2 of these shift responses and those of the pip responses, also recorded from the same subjects, were compared with respect to their midline distributions and hemispheric or bilateral asymmetries. The significant differences found between the shift and pip responses indicated that those evoked by the IID stimulation we designed should not be considered simply as a non-specific vertex potential.

Human auditory cortical mechanisms of sound lateralisation: III. Monaural and binaural shift responses

Neuromagnetic responses were recorded over the whole head with a 122-channel gradiometer. A pair of 150-ms 1-kHz tones separated by an interval of 150 ms was presented to one ear every 2 s. The other ear received either no input, an identical pair simultaneous to the first, an identical pair alternating with the first or a continuous 600-ms tone. The 'monaural shift' condition in which stimuli alternated between ears produced a clear perception of changing lateralisation, but the evoked response could be explained as merely the sum of simple monaural onset and offset responses; thus we found no evidence for a separate response to interaural intensity difference in this condition. The 'binaural shift' condition, in which intensity changed in one ear while the other received a continuous tone, evoked a transient response (N130m) at a latency of about 130 ms. N130m was larger over the hemisphere contralateral to the direction of shift, and larger than the corresponding monaural response, whether to an onset or an offset. We concluded that N130m also was not a separate directional response, but was analogous to a simple monaural response, the prolonged latency being due to masking and the enhanced amplitude to facilitation by the sustained response to the continuous tone.

Effect of interaural time differences on middle-latency and late auditory evoked magnetic fields

To determine if interaural time differences (ITDs) in binaural stimuli affect the middle-latency auditory evoked fields (AEFs) in the same manner as they affect the N100m deflection, neuromagnetic responses were recorded over the whole head using a 122-channel SQUID magnetometer. Binaural stimuli were lateralized to three positions, left, midline, and right, on the basis of ITDs. The N100m was significantly larger to stimuli with contralaterally-leading ITDs than to stimuli with no, or with ipsilaterally-leading ITDs. Neither the P30m nor the P50m deflections of the middle-latency response were significantly affected by ITD, although the P30m showed a tendency, similar to but smaller than that of N100m, to be larger to stimuli with contralaterally-leading ITDs. In some subjects, the source location of the P50m was anterior and inferior to the sources of the P30m and N100m, which are generated in the superior surface of the temporal lobe. Sound-related muscular artifacts were seen in the posterior recording channels of one subject, and the contribution of this activity to the signals over the temporal area was determined.

Human auditory cortical mechanisms of sound lateralization: II. Interaural time differences at sound onset

Neuromagnetic responses were recorded over the right temporal cortex using a 24-channel gradiometer. Stimuli were binaural click trains, presented with six separate interaural time differences (ITDs). N100m to sound onset was larger and earlier for stimuli presented with left- than with right-leading ITDs. With stimulus lateralization taken into account, monaural and binaural stimuli evoked responses of roughly equal amplitude. In selective adaptation and oddball experiments, stimuli presented with different ITDs excited overlapping neuronal populations, but the amount of overlap decreased as the ITD between the stimuli increased. There were no systematic differences in the cortical source locations of the N100m as a function of ITD, however. Thus it appears that ITD-sensitive neurons in the human auditory cortex are not organized into a large-scale, orderly representation, which could be resolved by MEG.