www.elsevier.com/locate/heares

Hearing Research 196 (2004) 109–114

Auditory evoked magnetic fields in relation to interauralcross-correlation of band-pass noise

Yoshiharu Soeta a,*, Takuya Hotehama b, Seiji Nakagawa a,Mitsuo Tonoike a, Yoichi Ando b

a Institute for Human Science and Biomedical Engineering, National Institute of Advanced Industrial Science and Technology (AIST),

1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japanb Graduate School of Science and Technology, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

Received 3 March 2004; accepted 25 May 2004

Available online 21 August 2004

Abstract

Auditory evoked magnetic fields of the human brain were analyzed in relation to the magnitude of the inter-aural cross-corre-

lation (IACC). IACC of the stimuli was controlled by mixing diotic bandpass and dichotic independent bandpass noise in appro-

priate ratios. The auditory stimuli were binaurally delivered through plastic tubes and earpieces inserted into ear canals of the nine

volunteers with normal hearing who took part in this study. All source signals had the same sound pressure level. Auditory evoked

fields (AEFs) were recorded using a neuromagnetometer in a magnetically shielded room. Combinations of a reference stimulus

(IACC=1.0) and test stimuli (IACC=0.2, 0.6, 0.85) were presented alternately at a constant interstimulus interval of 0.5 s and

MEGs recorded. The results showed that the N1m latencies were not affected by IACC; however, the peak amplitude of N1m

significantly decreased with increasing IACC.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Magnetoencephalography (MEG); Auditory evoked response; N1m; Interaural crosscorrelation function

1. Introduction

The interaural cross-correlation of a dichotic stimu-

lus is a measure of the similarity of the waveforms pre-sented to left and right ears. The concept of interaural

cross-correlation is a fundamental one in the study of

binaural hearing; in which the width and distance of

0378-5955/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.heares.2004.07.002

* Corresponding author. Tel.: +81-72-751-8526; fax: +81-72-751-

8416.

E-mail address: y.soeta@aist.go.jp (Y. Soeta).

Abbreviations: ANOVA, analysis of variance; AEF, auditory evo-

ked field; AEP, auditory evoked potential; BMLD, binaural masking

level difference; ECD, equivalent current dipole; IC, inferior colliculus;

IACC, interaural cross-correlation coefficient; ITD, interaural time

difference; MEG, magnetoencephalography; MSO, medial superior

olive; RMS, root-mean-square

the sound image changes according to the interaural

cross-correlation coefficient (IACC) (Kurozumi and Oh-

gushi, 1983; Licklider, 1948). The IACC is also a signif-

icant factor in determining the perceived horizontaldirection of a sound and the degree of subjective diffuse-

ness of a sound field (Damaske and Ando, 1972). When

the IACC is high, a well-defined direction is perceived.

Additionally, numerous psychoacoustical studies have

shown that the IACC is the dominant factor in the

determination of the binaural masking level difference

(BMLD) (Durlach, 1964; Durlach et al., 1986; Licklider,

1948; Osman, 1971; Osman et al., 1975).In the field of architectural acoustics, it has been

shown that both the degree of subjective preference

and diffuseness for sound with a single echo depend

greatly on the IACC (Ando, 1977; Ando and Kurihara,

1986; Barron, 1971; Blauert and Lindemann, 1986a),

110 Y. Soeta et al. / Hearing Research 196 (2004) 109–114

leading to a recognition of the need for early lateral

reflections to decrease the IACC in concert halls. Subjec-

tive diffuseness, or no spatial impression of the sound

field, in a room is one of the fundamental attributes as-

cribed to good acoustics.

Electrophysiological recordings of the interaural timedifferences (ITDs)-sensitive neurons in the inferior colli-

culus (IC) of the guinea pig have shown that the re-

sponse to bandpass noise is influenced by the IACC of

the stimulus, that is, neurons� responses were modulated

with interaural delay for higher IACC and neurons were

insensitive to interaural delay for noise with zero IACC

(Palmer et al., 1999). It has also been shown that the

ITD sensitivity of low-frequency cells in medial superiorolive (MSO) and IC of the cat also reflects a process of

IACC (Yin et al., 1987). Compatible finding has also

been obtained in the optic tectum of the barn owl (the

avian homologue of the mammalian IC), where

the responsiveness of ITD-sensitive neurons declines as

the IACC is decreased (Saberi et al., 1998).

The psychological responses to IACC have largely

been obtained in humans, while the neurophysiologicalresponses are confined to animal studies. Further, since

neurophysiological responses to IACC has examined in

the auditory brainstem (Ando et al., 1991), there is little

evidence of the cortical responses to IACC. To obtain

such response, the present study used MEG to examine

whether or not there is an activity in the auditory cortex

corresponding to the IACC variations of band-pass

noise.

2. Method

2.1. Measurement of magnetic response

Bandpass noises were employed for acoustic signals.

To create bandpass noises, white noises were digitally fil-

tered between 200 and 3000 Hz (Chebychev bandpass:

order 18). The interaural cross-correlation function

between two sound signals at both ears fl(t) and fr(t) is

defined by

UlrðsÞ ¼1

2T

Z þT

�Tf 0l ðtÞf 0

r ðt þ sÞdt; ð1Þ

where f0l(t) and f 0r(t) are obtained after passing through

the A-weighted network, which approximately corre-sponds to ear sensitivity (Ando et al., 1987; Ando,

1998). The normalized interaural cross-correlation is

defined by

/lrðsÞ ¼UlrðsÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

Ullð0ÞUrrð0Þp ; ð2Þ

where Ull(0) and Urr(0) are the autocorrelation functions

at s=0 for the left and right ear, respectively. The mag-

nitude of the interaural cross-correlation function,

IACC, is defined by the possible maximum interaural

time delay, say, jsj61.0 ms.

The IACC of the stimuli was controlled by mixing in-

phase diotic bandpass and dichotic independent band-

pass noises in appropriate ratios (Blauert, 1983). Thefrequency range of these noises was always kept the

same. Fig. 1 shows examples of the power spectrum,

the temporal waveform, and the interaural cross-corre-

lation function of the stimulus used in this study. The

auditory stimuli were delivered to subjects through plas-

tic tubes and inserted earpieces at a comfortable listen-

ing level of approximately 68 dB, A-weighted, adjusted

separately for each subject. The sound pressure wasmeasured with an ear simulator, including a micro-

phone and a preamplifier, and an adaptor connected

to the earpiece. The interaural level difference was set

to 0 dB and there was no interaural delay between ear

signals, i.e., the maximum of the IACC was always at

s=0.

Nine volunteers with normal hearing (22–28 years; all

right-handed) took part in the study. Informed consentwas obtained from each subject after explaining to her/

him the nature of the study. The experiments were con-

ducted in accordance with the principles of the Declara-

tion of Helsinki. Brain activities evoked by sound

stimuli were recorded using a 122 channel whole-head

DC superconducting quantum interference device

(DC-SQUID) magnetometer (Neuromag-122TM; Neu-

romag Ltd., Helsinki, Finland) in a magneticallyshielded room (Knuutila et al., 1993). Combinations of

a reference stimulus (IACC=1.0) and test stimuli

(IACC=0.85, 0.6, 0.2) were presented alternately at a

constant 0.5 s interstimulus interval. During the experi-

ment, subjects were asked to close their eyes to fully con-

centrate on the stimuli. The magnetic data were sampled

at 0.4 kHz after being bandpass filtered between 0.03

and 100 Hz, and averaged more than 50 times. Any re-sponses coinciding with magnetic signals exceeding 3000

fT/cm were rejected from further analysis. The averaged

responses were digitally filtered between 1.0 and 30.0

Hz. The analysis time was 0.7 s from 0.2 s prior to the

stimulus onset. The average of the 0.2 s prestimulus per-

iod served as the baseline. The Neuromag-122TM has

two pick-up coils in each position, which measure two

tangential derivatives, oBz/ox and oBz/oy, of the fieldcomponent Bz. To evaluate the amplitude and the la-

tency of the response, the root-mean-square (RMS) of

oBz/ox and oBz/oy were determined as the amplitude

of the responses at each recording position. The N1m

peak amplitude and latency was defined as the RMS

peak and the latency in the latency range from 70–130

ms over the right and left hemisphere. In each subject,

we employed the N1m peak latency and amplitude witha channel that showed the maximum amplitude placed

at each hemisphere.

0 0.1 0.2 0.3 0.4 0.5

Time [s]

0 0.1 0.2 0.3 0.4 0.5

Time [s]

Rel

ativ

e am

plitu

de

100 1000-100

-50

0

Frequency [kHz]

Am

plitu

de [d

B]

100 1000-100

-50

0

5000

5000

50

50

(a)

(b)

(c)

(d)

Frequency [kHz]

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1-1

0

1

Delay time [ms]

lr

(e)

Frequency [Hz]

Frequency [Hz]

Time [s]

Time [s]

Fig. 1. Examples of the temporal waveform at the left (a) and right (b) ears, the power spectrum at the left (c) and right (d) ears, and the interaural

cross-correlation function (e) of the stimulus (IACC=0.6).

Y. Soeta et al. / Hearing Research 196 (2004) 109–114 111

To estimate the location and the strength of the

underlying neural activity of the N1m wave, a single

equivalent current dipole (ECD) was assumed as thesource of the magnetic field of the N1m wave. ECDs

were found by a least square search in each hemisphere

at the N1m peak latencies. Calculations were based on

the spherical conductor model, which takes into account

the volume current within a sphere (Kaukoranta et al.,

1986). All ECDs with goodness-of-fit values exceeding

80% were used in further analyses.

3. Results

Clear N1m responses were observed in both right and

left temporal regions in all subjects (Fig. 2). Fig. 3 shows

the response waveforms (RMS) in a subject for all kinds

of IACC, observed at the channel where the maximum

N1m amplitude was observed. The N1m latencies were

not systematically affected by the IACC as shown inFig. 4a. Two way analysis of variance (ANOVA) did

not show any significant effects of the IACC [F(2,

2)=0.43, P>0.05] or hemisphere [F(1, 2)=0.11,

P>0.05] on N1m latency. Fig. 4b depicts the mean

N1m amplitude (across nine subjects) as a function of

the IACC. The N1m amplitudes were normalized within

each subject with respect to the maximum value. Basi-

cally, a smaller IACC provided a larger N1m amplitude.Two way ANOVA (IACC·hemisphere) revealed a sig-

nificant effect of the IACC on N1m amplitude [F(2,

2)=9.79, P<0.001]. Variability in the left and right hem-

ispheres was not significantly different [F(1, 2)=0.60,

P>0.05]. Fig. 4c depicts the normalized mean N1m

ECD moment as a function of the IACC. A two way

Fig. 2. Typical waveforms of auditory evoked magnetic fields from 122 channels in a subject.

Fig. 3. Waveforms of brain magnetic field (RMS) observed in the right

temporal area in a subject.

112 Y. Soeta et al. / Hearing Research 196 (2004) 109–114

ANOVA (IACC·hemisphere) showed the significant

main effects were the IACC [F(2, 2)=34.42, P<0.001]

and the hemisphere [F(1, 2)=6.56, P<0.05]. There was

no significant interaction [F(2, 2)=2.99, P>0.05].

4. Discussion and conclusions

Using auditory evoked magnetic fields, the brain

activities corresponding to the IACC were analyzed in

this study. A previous study on auditory evoked poten-

tial (AEP) investigated the IACC using a one-third

octave bandpass-filtered noise with the center frequency

of 500 Hz as a sound signal (Ando et al., 1987). The re-

sults there showed that the peak-to-peak amplitude N1–

P2 decreases with increasing IACC. In the present study,

a higher IACC provided for a significantly smaller

amplitude of N1m, P2m, and the peak-to-peak ampli-

tude N1m–P2m, as shown in Figs. 4b and 5. A previousstudy on N1m response to dichotic tones of different fre-

quencies found that N1m increased with the interaural

frequency disparity (Yvert et al., 1998). Significantly

smaller AEPs or AEFs amplitudes were obtained with

binaural stimulation, compared to those obtained with

monaural contralateral stimulation; indicating some

kind of interference between ipsilateral and contralateral

pathways (�binaural interaction�) (Pantev et al., 1986;McPherson and Starr, 1993). It is considered that the

more sounds differed arriving at both ears, the larger

the N1m amplitude might be.

McEvoy et al. (1991) recorded AEPs to identical bin-

aural noise with an ITD of 0.8 ms. The noise was pre-

sented with intervening noise that was identical

between the two ears (IACC=1.0), or with uncorrelated

intervening noise (IACC=0.0). The noise with corre-lated intervening noise produced a larger N1 amplitude

than that with uncorrelated intervening noise. The result

does not seem to be consistent with the present finding.

In the study of McEvoy et al. (1991), the stimulus oc-

curred as a change in the ongoing ITD of the noise from

0 to 0.8 ms when presented with correlated intervening

noise, in contrast, the stimulus consisted of a change

from uncorrelated noise to correlated noise with an

0.25

0.50

0.75

1.00

1.25

Nor

mal

ized

EC

D m

agni

tude

0.00 0.25 0.50 0.75 1.00

IACC(c)

**

** **

50

10 0

15 0

N1m

Lat

ency

[ms]

0.00 0.25 0.50 0.75 1.00

IACC(a)

0.4

0.8

1.2

0.00 0.25 0.5 0 0.75 1.00

Nor

mal

ized

am

plitu

de o

f N1m

IACC(b)

*** *

Fig. 4. The mean latency (a), the mean amplitude (b), and the mean ECD moments (c) of the N1m from (s): left and (d): right hemispheres as a

function of IACC. Error bars are the 95% confidence interval. The N1m amplitudes and ECD moments are normalized within each subject with

respect to the maximum value. The asterisks indicate statistical significance (*P<0.05, **P<0.01; Bonferroni test).

Y. Soeta et al. / Hearing Research 196 (2004) 109–114 113

ITD of 0.8 ms when presented with uncorrelated inter-vening noise. Such differences of stimulus could influ-

ence the results of N1 amplitude.

Blauert and Lindemann (1986b) investigated the

broadening and splitting of auditory events in dichotic

listening condition with various degrees of the IACC

by using a psychoacoustical mapping method. The re-

sponse task of the subjects was to map the simple or

multiple auditory events that they perceived during thepresentation of each signal. The number of partial audi-

tory events decreased with increasing IACC. This could

result in larger magnetic field responses with a less inter-

aurally coherent sound.

A starting point for modeling efforts in binaural hear-

ing is the model of Jeffres (1948), which produces esti-

mates of cross-correlation functions of its two inputs.

Several theories regarding binaural systems rely on across-correlator to act as a comparator element for sig-

nals arriving at both the left and right ears (Blauert,

1983; Colburn, 1977; Lindemann, 1986; Osman, 1971;

Osman et al., 1975). It has been shown that the central

binaural neurons perform an operation very similar to

the cross-correlation of the inputs (Ando et al., 1991;Palmer et al., 1999; Saberi et al., 1998; Yin et al.,

1987). These inputs are transformed from the actual

acoustic signal by the peripheral auditory system; and

these transformations are reflected in the properties of

the cross-correlations. Among various approaches to

incorporating the evaluation of two interaural cues,

namely interaural level and interaural arrival time differ-

ences, into one consolidated model; a binaural cross-correlation model by contralateral inhibition was pro-

posed (Lindemann, 1986). A model using an interaural

cross-correlation analysis of the ear input signals should

be able to filter out the components of the ear input sig-

nals that are interaurally coherent (Blauert, 1983; Lind-

emann, 1986). Therefore, sounds that are interaurally

more coherent could lead to more binaural inhibition,

which causes the decrease of the strength of the N1mresponse.

In summary, an IACC change activates the human

auditory cortex; and the smaller the IACC, the larger

the N1m response. Latency of the N1m did not reveal

any systematic change relating to the IACC.

0.4

0.8

1.2

0.00 0.25 0.50 0.75 1.0 0

Nor

mal

ized

am

plitu

de o

f P2m

IACC

****

(a)

0. 4

0. 8

1. 2

0.0 0 0.2 5 0.50 0.7 5 1.00

Nor

mal

ized

am

plitu

de o

f N1m

-P2m

IACC

**** *

(b)

Fig. 5. The mean amplitude of the P2m (a) and the peak-to-peak

amplitude N1m–P2m (b) from (s): left and (d): right hemispheres as a

function of IACC. Error bars are the 95% confidence interval. The P2m

amplitudes and the peak-to-peak amplitude N1m–P2m are normalized

within each subject with respect to the maximum value. The asterisks

indicate statistical significance (*P<0.05, **P<0.01; Bonferroni test).

114 Y. Soeta et al. / Hearing Research 196 (2004) 109–114

Acknowledgements

This research was supported by a Grant-in-Aid for

Scientific Research from the Japan Society for the Pro-

motion of Science.

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