Neuromagnetic Responses to Binaural Beat in Human Cerebral Cortex

Neuromagnetic Responses to Binaural Beat in Human Cerebral Cortex

Shotaro Karino,1 Masato Yumoto,2 Kenji Itoh,3 Akira Uno,4 Keiko Yamakawa,3 Sotaro Sekimoto,3 andKimitaka Kaga1

1Departments of Otolaryngology Head and Neck Surgery,2Laboratory Medicine, and 3Cognitive Neuroscience, Faculty of Medicine,University of Tokyo, Tokyo; and 4Graduate School of Comprehensive Sciences, University of Tsukuba, Tsukuba, Japan

Submitted 16 August 2005; accepted in final form 18 June 2006

Karino, Shotaro, Masato Yumoto, Kenji Itoh, Akira Uno, KeikoYamakawa, Sotaro Sekimoto, and Kimitaka Kaga. Neuromagneticresponses to binaural beat in human cerebral cortex. J Neurophysiol 96:1927–1938, 2006. First published June 21, 2006; doi:10.1152/jn.00859.2005.The dichotic presentation of two sinusoids with a slight difference infrequency elicits subjective fluctuations called binaural beat (BB).BBs provide a classic example of binaural interaction considered toresult from neural interaction in the central auditory pathway thatreceives input from both ears. To explore the cortical representation ofthe fluctuation of BB, we recorded magnetic fields evoked by slow BBof 4.00 or 6.66 Hz in nine normal subjects. The fields showed smallamplitudes; however, they were strong enough to be distinguishedfrom the noise accompanying the recordings. Spectral analyses of themagnetic fields recorded on single channels revealed that the re-sponses evoked by BBs contained a specific spectral component ofBB frequency, and the magnetic fields were confirmed to represent anauditory steady-state response (ASSR) to BB. The analyses of spatialdistribution of BB-synchronized responses and minimum-norm cur-rent estimates revealed multiple BB ASSR sources in the parietal andfrontal cortices in addition to the temporal areas, including auditorycortices. The phase of synchronized waveforms showed great vari-ability, suggesting that BB ASSR does not represent changing inter-aural phase differences (IPD) per se, but instead it reflects a higher-order cognitive process corresponding to subjective fluctuations ofBB. Our findings confirm that the activity of the human cerebralcortex can be synchronized with slow BB by using information onthe IPD.

I N T R O D U C T I O N

The presentation of a sinusoid with a small interaural fre-quency difference (IFD) in each ear elicits various perceptionsresulting from neural interactions in the central auditory path-way, which receives input from both ears. When the IFD iszero, a single fused image is heard. Furthermore, the image iscentered in the listener’s head when both the interaural differ-ences in phase and level are zero. If the IFD is sufficientlylarge, two discrete images are heard (Perrott and Barry 1969).In the transitional range from a single stationary fused image totwo nonfused images, a continuum of subjective effects isperceived (Perrott and Musicant 1977). These sensations arecalled “binaural beats” (BBs). Licklider et al. (1950) suggestedthat in a broad sense, BBs could be described as a triad ofsubjective effects. When the IFD is small, a single auditoryimage moves between the two ears, depending on the phase ofthe two signals (Kuwada et al. 1979). This movement percep-tion has been called the “rotating tone” (Perrott and Musicant

1977). As the IFD increases, the localization effect is replacedby a beating tone with a rate equal to the IFD. This perceptioncorresponds to “BBs in the narrow sense” and has beendescribed as “a periodic fluctuation in loudness” (Perrott andMusicant 1977) or “a fluctuation in amplitude” (Yin andKuwada 1983b). Further increases in the IFD produce a fast-beating tone (roughness), which gives way to a smooth bitonalexperience (Licklider et al. 1950).

In the cochlea, neural spikes tend to occur at a particularphase of the sinusoidal waveform (phase locking), and infor-mation on the phase of the acoustic stimuli is preserved in theauditory nerve fibers (Palmer and Russell 1986) and the spher-ical bushy cells of the antero-ventral cochlear nucleus (Gold-berg and Brownell 1973), the axons of which project to themedial superior olivary (MSO) nucleus (Goldberg and Brown1969; Smith et al. 1993). In the central auditory system, theinteraural relative phase is detected by integrating informationfrom each ear. BB reflects an orderly and continuously chang-ing interaural phase difference (IPD) through one IFD cycle.Electrophysiological studies showed that when BBs are pre-sented to mammals, the central auditory system responds to acontinuously changing IPD. Kuwada et al. (1979) found thatthe responses of cat inferior colliculus neurons are phase-locked to the frequency of BBs. Moreover the discharges arehighly periodic and synchronize to a particular phase of the BBcycle (Yin and Kuwada 1983a). Reale and Brugge (1990)found that some neurons in the primary auditory cortex (PAC)of anesthetized cats are sensitive to the dynamically changingIPD created by BBs.

To detect noninvasively responses synchronized to BB in thehuman brain, we must record the corresponding neuronalresponses in a steady state because BB is a periodical hearingphenomenon caused by continuously changing IPD. A steady-state response is an evoked potential the constituent discretefrequency components of which remain constant in amplitudeand phase and is considered stable over a temporal windowmuch longer than the duration of a single stimulus cycle.Steady-state responses are recorded when stimuli are presentedperiodically, and they demonstrate how the brain follows astimulus or how the stimulus drives a response (Picton et al.2003; Regan 1989). In previous studies using click trains(Forss et al. 1993; Galambos et al. 1981; Gutschalk et al. 1999;Makela and Hari 1987) or amplitude-modulated (AM) tones(Engelien et al. 2000; Herdman et al. 2002; Pantev et al. 1996;Ross et al. 2002), conventional auditory steady-state responses(ASSR) were recorded as brain activity synchronized to peri-

Address for reprint requests and other correspondence: S. Karino, Dept. ofOtolaryngology Head and Neck Surgery, Faculty of Medicine, University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan, 113-8655 (E-mail:[email protected]).

The costs of publication of this article were defrayed in part by the paymentof page charges. The article must therefore be hereby marked “advertisement”in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

J Neurophysiol 96: 1927–1938, 2006.First published June 21, 2006; doi:10.1152/jn.00859.2005.

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odic acoustic stimulations. In human, ASSR to BB with IFD of40 Hz was recorded by electroencephalography (EEG)(Schwarz and Taylor 2005). The IFD of 40 Hz was adoptedbecause ASSR at this rate was easily evoked in precedingstudies using click train or AM tones. However, BB with thisIFD does not correspond to BB in the narrow sense withsubjective fluctuations.

Two points should be stressed about the importance ofinvestigating ASSR to BB. First, a precise coding of tonefrequency by phase locking in the peripheral auditory systemon each side is indispensable for the detection of BB. The BBASSR assesses this ability of temporal coding, which is alsorelated to other auditory functions, such as speech and musicalpitch recognition. Various diseases of the peripheral (Karino etal. 2005) and central auditory systems would be expected toperturb the temporal coding. Thus the binaural beat ASSR mayenrich the diagnostic repertoire of the EEG and magnetoen-cephalography (MEG). Second, brain activities related to BBcontain information induced by integrating information fromboth ears. Therefore even if neuronal responses to BB areanalyzed in only one hemisphere, the results reflect the infor-mation from one ear to which a higher frequency tone ispresented and the other ear to which a lower tone is presented.Furthermore, whether the higher tone is presented to the earipsilaterally or contralaterally from the viewpoint of the ob-served hemisphere may influence the findings in one hemi-sphere.

The periodic loudness fluctuations of BBs are based not onlyon the properties of acoustic stimulation itself but also on aproduct of binaural integration in the brain. The goal of thisstudy was to verify whether the fluctuations of BBs are repre-sented in ASSR by using MEG with high temporal resolution.

M E T H O D S

Subjects

Nine normal-hearing subjects [6 males, 3 females; age, 23–57 yr;36.1 � 11.2 (SD) yr] participated in this study. The subjects had nohistory of otological or neurotological disorders and had normalaudiological status. Handedness was established with the Edinburghhandedness questionnaire (Oldfield 1971). Laterality quotients rangedfrom 89.5 to 100, except for one subject whose value was 0. Informedconsents were obtained from all subjects after a full explanation of the

aim and methods. The procedure used in this study was approved bythe Ethics Committee of the University of Tokyo and conducted inaccordance with the principles of the Declaration of Helsinki.

Stimulation

Continuous pure tones were played on an Apple personal computervia MOTU 828 (Mark of the Unicorn, MA) audio interface and led toER-3A (Etymotic Research, Elk Grove Village, IL) foam insertearphones with extended plastic tubes. The transmission delay of �6ms was compensated by an appropriate shift of the trigger signal. Theearphones had almost a flat frequency response in the range from 100to 1,000 Hz. First, hearing thresholds for continuous pure tones of 240and 480 Hz were determined in both ears of all subjects seated in thesame conditions as with a MEG recording in a shielded room. Theintensity of pure tone was set at 40 dB above each subject’s sensationthreshold to prevent cross-hearing. In healthy subjects with intactexternal and middle ears, bone conduction plays a negligible role atthis level because the conduction between ears is attenuated at 50–60dB (Zwislocki 1951, 1953). Therefore the addition of a contralateraltone 40 dB above the ipsilateral threshold would not perturb neuronalphase locking sufficiently to affect the sound source localization(Blauert 1996) or BB patterns. In our experiments, 4.00 and 6.66 Hzwere employed as IFD. We realized 4-Hz BB by the combination of240.00 and 244.00 Hz and that of 480.00 and 484.00 Hz. Moreover,6.66-Hz BB was realized by the combination of 240.00 and 246.66 Hzand that of 480.00 and 486.66 Hz. Measurement under controlconditions with a binaural presentation of the same pure tones of240.00 or 480.00 Hz with the same starting phase (no-BB condition)was also executed to elucidate the characteristics of responses evokedby BB. One run of BB or no-BB presentation lasted for 5–10 min. Intotal, averaged steady-state responses in eight types of BB conditionsand two kinds of no-BB conditions were recorded for each subject(Table 1). The subjects were instructed to watch a silent video movieon a screen to maintain vigilance level during recording.

Recording

Neuromagnetic cortical signals were recorded with a whole scalpneuromagnetometer (Vectorview; Neuromag, Helsinki, Finland),which has 204 first-order planar gradiometers. However, we found achannel that was noisy in the left antero-temporal zone, which wasthen inactivated. Thus a total of 203 channels were used for actualmeasurements. During the recordings, the subjects were seated underthe helmet-shaped dewar in a magnetically shielded room. The posi-tion of the head under the helmet was determined by attaching fourcoils to the head surface and measuring the coil positions with respect

TABLE 1. Combination of frequency to the left ear (FL) and to the right ear (FR) and intensity of steady-state responses in eachcondition

FL, Hz FR, HzIFD

(� FL � FR)Left 39

ChannelsCenter 124Channels

Right 40Channels

4-Hz BB 240.00 244.00 �4 2.44 � 0.23 1.95 � 0.27 2.36 � 0.36244.00 240.00 �4 2.52 � 0.27 2.12 � 0.32 2.63 � 0.49480.00 484.00 �4 2.62 � 0.40 2.23 � 0.37 2.78 � 0.51484.00 480.00 �4 3.05 � 0.68 2.61 � 0.52 3.33 � 0.80240.00 246.66 �6.66 2.70 � 0.36 2.23 � 0.22 2.50 � 0.23

6.66-Hz BB 246.66 240.00 �6.66 2.72 � 0.34 2.33 � 0.26 2.73 � 0.31480.00 486.66 �6.66 2.78 � 0.42 2.49 � 0.34 2.79 � 0.36486.66 480.00 �6.66 2.58 � 0.35 2.29 � 0.29 2.66 � 0.33

No-BB 240.00 240.00 0 2.16 � 0.27 1.82 � 0.25 2.35 � 0.47480.00 480.00 0 2.08 � 0.26 1.83 � 0.25 2.29 � 0.45

In each of the eight types of binaural beat (BE) conditions [interaural frequency difference (IFD) � FL � FR � �6.66, �4, �4, or �6.66] and the two typesof no-BB conditions (IFD � 0), the mean � SE values (n � 9) of root mean square (RMS) intensity of the magnetic field (fT/cm) are shown separately for thethree areas.

1928 KARINO, YUMOTO, ITOH, UNO, YAMAKAWA, SEKIMOTO, AND KAGA

J Neurophysiol • VOL 96 • OCTOBER 2006 • www.jn.org

to landmarks on the skull with a three-dimensional (3-D) digitizer; thecoil locations in the magnetometer coordinate system were deter-mined by leading current through the coils and measuring the corre-sponding magnetic fields. The recording passband was 1.0–200 Hz,and the data were digitized at 600 Hz. A vertical electrooculogram(EOG) was recorded simultaneously, and all traces of EOG activity�150 �V or signals in planar gradiometers �3,000 fT/cm wereexcluded from the on-line averages. The averaged signals werelow-pass filtered at 40 Hz.

In single measurements using each combination for BB, the mag-netic fields were recorded while 1,000–2,000 cycles of BB werepresented to obtain steady-state responses. The analysis time of fourBB cycles was used, and consequently 250–500 responses wereaveraged on-line. Figure 1 demonstrates the arrangement of triggersfor averaging. Each trigger was transmitted from the tone generatingPC to the neuromagnetometer at the moment the two sinusoidscrossed the zero line and IPD was zero (Fig. 1, A and B). However,fewer triggers with an interval of four BB cycles were employed foractual averaging to display a periodic fluctuation of responses evokedby four cycles of BBs (Fig. 1, C and D). The mean amplitude of thefour BB cycles was used as the baseline.

To explore the effect of probable noise, measurements without asubject were performed. The earphones were attached to the portionscorresponding to ears in the helmet-shaped dewar, and the samemeasuring procedure was conducted with each of the eight kinds ofBB and two types of no-BB stimulations.

Spectral analysis on each channel

Fast Fourier transform (FFT) spectra were calculated on eachchannel across 8,192 samples of the continuously recorded magneticfield signals, and the FFT window was moved in steps of 4,096samples; this procedure resulted in a frequency resolution of 0.074Hz. Approximately 70 spectra were averaged to improve the signal-to-noise ratio. On each channel of each subject, spectra in BBconditions were compared with those in a corresponding no-BBcondition within a spectral range of 1–20 Hz.

Minimum-norm current estimate

Source localization was performed using L1 minimum-norm esti-mation. The L1 estimation results in a current distribution with thesmallest integral of the absolute value of the current density that couldgenerate the measured magnetic field and provides a minimum-normcurrent estimate (MCE) (Uutela et al. 1999) with location and strengthinformation for the current sources at each time point. The origin ofthe spherical model was determined for each subject based on his/heranatomical magnetic resonance image (MRI) by fitting a sphere to thecurvature of the outer surface of the brain. For the source localizationof BB ASSR, MCEs were calculated separately in each condition foreach subject in the period of one BB cycle displayed in Fig. 1.

R E S U L T S

Wave configurations of steady-state responses

All the participants in our study reported that both 4 and 6.66Hz BBs containing 240- or 480-Hz tones allowed them toperceive BB in the narrow sense. Steady-state responses withdominant amplitudes in the temporal channels were recordedfrom both hemispheres in all subjects. Figure 2A shows atypical example of averaged responses in a subject, evoked in4-Hz BB and corresponding no-BB condition as control. In thisexample, four peaks were clearly recognized under BB stim-ulation, mainly in the temporal channels in the time windowthat corresponds to four BB cycles, but no such peaks wereidentified under control conditions. Figure 2, F and G, displaysenlarged wave configurations on demonstrative channels se-lected from the left and right temporal channels, respectively.In most subjects and in most BB conditions, isofield contourmaps showed a clear dipolar pattern over the temporal area inboth hemispheres. Figure 2, B and C, displays examples ofisofield contour maps in the left and right hemisphere, respec-tively. These field patterns imply a dominant current source ineach temporal area. However, the time point at which suchdipolar patterns were observed varied across subjects, condi-tions, and hemispheres, even in a single subject. The timepoints of magnetic fields shown in Fig. 2, B and C, are not thesame because they were selected independently within one BBcycle to demonstrate a typical dipolar pattern in each hemi-sphere.

Intensities of steady-state responses

First, to present the time course of magnetic fields, the rootmean square (RMS) of the magnetic field amplitude wascalculated separately in the left (Fig. 2D) and right (E) tempo-ral channels and also in the remaining 124 channels (center).The selected temporal channels covered the perisylvian areas,including the auditory cortices. Next, to present the meanintensity of the magnetic fields, we calculated the temporal

FIG. 1. A: 1 cycle of binaural beat (BB). In this example, the frequency ofthe sinusoid delivered to the left ear (FL: thin line) was lower than thatdelivered to the right ear (FR: thick line). For a schematic illustration, theamplitudes of the 2 sine waves are different, and the frequencies of the tonesare much lower than those actually used. This illustration shows the procedurein which the interaural phase (IPD � right phase – left phase) slowly shiftedfrom the left ear leading (�� IPD � 0) to the right ear leading (0 � IPD ��) through 1 complete cycle of the beat frequency. B: each trigger (arrow) wastransmitted to the neuromagnetometer at a moment when the 2 sinusoidscrossed the 0 line and IPD was 0. C: fewer triggers with analysis timeequivalent to 4 BB cycles were employed for actual averaging to display theperiodic fluctuation of responses evoked by 4 cycles of BB. D: example of anaveraged wave of magnetic field on a single channel.

1929NEUROMAGNETIC RESPONSES TO BINAURAL BEAT


mean of the RMS in one BB cycle at the center of the analysisperiod. Figure 3 shows the grand mean (�SE) values (n � 9)of the temporal mean of RMS intensity. The RMS measuredwithout a subject (8 types of BB and 2 types of no-BBconditions) were much smaller in each of the three areas (left,center, and right). The intensity of magnetic fields under BBcondition was �2.5 times as large as the noise intensitydemonstrated under no-body condition, and the intensity ofmagnetic fields under no-BB condition was about twice aslarge as the control. To examine the statistical significance ofthis finding, we first applied two-way ANOVA adopting threeconditions (BB vs. no-BB vs. no-body) and three areas (left vs.center vs. right) as factors, although the correspondence ofsubjects was neglected. There were 72 data (8 types of BBstimulations * 9 subjects), 18 data (2 types of no-BB stimula-tions * 9 subjects), and 10 data (8 types of BB and 2 types ofno-BB stimulations) under BB, no-BB, and no-body condi-

tions, respectively. No significant main effect of areas wasfound; however, the main effect of conditions [F(2,4) � 26.38,P � 0.001] was significant. A post hoc test using Bonferroni’smultiple comparison procedure revealed that RMS intensityunder BB condition was significantly greater than those underboth no-BB (P � 0.013) and no-body (P � 0.001) conditions.The RMS intensity under the no-BB condition was signifi-cantly greater than that under the no-body condition (P �0.001). There was no significant condition-area interaction.These results indicate that magnetic fields under BB and no-BBwere large enough to be distinguishable from the noise level.

Table 1 shows the details of RMS intensity under BB andno-BB conditions. To evaluate precisely the RMS intensityunder BB and no-BB conditions using the correspondent dataacross subjects, we applied two-way repeated-measuresANOVA adopting 10 types of stimuli (including 8 types of BBand 2 types of no-BB stimulations) and three areas (left, center,

ALeft Right

1000 ms1000 ms

0

-20 fT/cm

+20-20

0

+20

L: 240 HzR: 244 Hz(4Hz BB)

L: 240 HzR: 246.66 Hz(6.66Hz BBas control)

L: 240 HzR: 240 Hz

(No BBas control)

B

D

-125 12500

1

2

3

4

5

Mean

(fT

/cm

)

(ms) -125 12500

1

2

3

4

5

Mean(f

T/c

m)

(ms)

C

(fT

/cm

)Hz

Frequency (Hz)

1 10 200

5

10

15

20

25

3 4 55

10

15

20

25

1 10 200

5

10

15

20

25

3 4 55

10

15

20

25

1 10 200

5

10

15

20

25

3 4 55

10

15

20

25

H

1 10 200

5

10

15

20

25

3 4 55

10

15

20

25

1 10 200

5

10

15

20

25

3 4 55

10

15

20

25

1 10 200

5

10

15

20

25

3 4 55

10

15

20

25

Frequency (Hz)

I

(fT

/cm

)Hz

4Hz BB

No BB

4Hz BB

No BB

E

F G



or right) as within-participant factors. No significant maineffect of stimuli was found, though the RMS intensities underthe two types of no-BB conditions were the least and secondleast in the 10 types of stimuli. The main effect of areas[F(2,16) � 6.01, P � 0.011] was significant. A post hoc testusing Bonferroni’s multiple comparison procedure revealedthat the RMS intensity in the center area was smaller than inboth the left (P � 0.050) and right (P � 0.015) areas. Therewas no significant difference between the left and right areas,and no significant stimuli-area interaction was found.

Furthermore, the RMS intensities in BB and no-BB condi-tions were compared separately in each of the three areas bytwo-way repeated-measure ANOVA adopting tone frequency(240 or 480 Hz) and interaural frequency difference (IFD �FL – FR � �6.66, �4, 0, �4, or �6.66) as within-subjectfactors. In each of the three areas, neither significant maineffect of tone frequency nor a tone frequency-IFD interactionwas found. In each of the three areas, the RMS intensity in IFDof 0 (namely, under no-BB condition) was least in the fivetypes of IFD; however, there was no significant main effect forIFD. These findings indicate a lack of systematic effect of tonefrequency or IFD on RMS intensity in the three areas.

Spectral analysis on each channel

In each subject, we explored demonstrative channels onwhich a distinct peak was recognized at the corresponding BBfrequency under BB condition and verified whether the peakrepresented specific synchronization to BB frequency. Figure2, H and I, demonstrates the strategy adopted in our study tosystematically explore the channels that show a spectral peakpotentially relevant to a specific BB frequency.

Figure 2H shows the FFT spectra on the same channel asthat of F. The top panels show the FFT of response to thecombination for 4-Hz BB, which consists of 240 Hz in the leftear and 244 Hz in the right ear (L240R244 in our nomencla-ture). The middle and lower panels show responses toL240R246.66 (6.66-Hz BB) and L240R240 (no-BB), respec-tively. On this channel, in L240R244, the amplitude at just 4

FIG. 3. Grand means � SE values (n � 9) of the temporal mean of rootmean square (RMS) intensity determined separately in the left and righttemporal channels and the other remaining 124 channels (center). The RMSmeasured without a subject (8 types of BB and 2 types of no-BB conditions)was smaller in each of the 3 areas. The intensity of magnetic fields in BBcondition was �2.5 times as great as the noise intensity demonstrated in theno-body condition, and the intensity of magnetic fields in the no-BB conditionwas about twice as great as the control.

FIG. 2. Steady-state responses by BBs and spectral analysis of each channel of a representative subject. A: averaged waveforms on 203 planar gradiometers,excluding 1 noisy channel. Responses evoked by BBs with 240-Hz tone in the left ear and 244-Hz tone in the right ear (4-Hz BB: black lines) and those evokedby a binaural presentation of 240-Hz tone (no-BB: gray lines) are arranged in parallel. Note the 4 peaks under 4-Hz BB condition in some channels, mainly inbilateral temporal areas. Demonstrative channels surrounded by rectangles are magnified in F and G. In the corresponding no-BB condition, there was no periodicfluctuation. The analysis time is equivalent to four cycles of 4-Hz BB � 1,000 ms. Temporal channels selected for evaluating amplitude in magnetic fields aresurrounded by polygons (40 channels in the right hemisphere; 39 channels in the left hemisphere because of the exclusion of 1 noisy channel). The selectedtemporal channels covered the perisylvian areas, including the auditory cortices. B: isofield contour map on the sensor array in the left hemisphere. Outflux (redlines) and influx (blue lines) are stepped by 2 fT. An arrow in F indicates the time point of this magnetic field, which was selected within 1 BB cycle at the centerof the analysis period to demonstrate a typical dipolar pattern over the temporal area in the left hemisphere. C: isofield contour map on the sensor array in theright hemisphere. An arrow in G indicates the time point of this magnetic field, which was selected within 1 BB cycle at the center of the analysis period todemonstrate a typical dipolar pattern over the temporal area in the right hemisphere. Note that the time points of B and C are not the same because they wereselected independently in each hemisphere. D: root mean square (RMS) of amplitude in magnetic fields was calculated to represent the time course of recordedfields in the left temporal channels. We adopted the temporal mean of the RMS in 1 BB cycle at the center of the analysis period to represent the mean intensityof the fields. E: RMS of amplitude in magnetic fields represented the time course of recorded fields in the right temporal channels. We adopted the temporal meanof the RMS in 1 BB cycle at the center of the analysis period to represent the mean intensity of the fields. F: magnification of the demonstrative channels inthe left temporal area surrounded by the rectangles in A. Top: magnetic field under 4-Hz BB condition (L240R244 in our nomenclature). Four peaks are clearlyrecognized in the analysis time of 4 BB cycles. Bottom: magnetic field under the corresponding no-BB condition (L240R240 as control). Note the lack of periodicfluctuation. G: magnification of the demonstrative channels in the right temporal area surrounded by the rectangles in A. Top: magnetic field under 4-Hz BBcondition (L240R244). Four peaks are clearly recognized in the analysis time of 4 BB cycles. Bottom: magnetic field under the corresponding no-BB condition(L240R240 as control). Note the lack of periodic fluctuation. H: fast Fourier transform (FFT) spectra on the same channel as that of F. Top: FFT spectra ofresponse to L240R244 (4-Hz BB). Right: enlargement of the left panel around the BB frequency of 4 Hz. Thin horizontal line: the mean amplitude in the widthof 1 Hz (from 3.5 to 4.5 Hz); dashed horizontal line: 95% confidence limit (1.64 times the SD) for response detection. The peak amplitude at just 4 Hz (arrows)is beyond the limit. Middle: FFT spectra of the response to L240R246.66 on the same channel are added (6.66-Hz BB as control). The amplitude at just 4 Hzis not beyond the 95% confidence limit. Furthermore, it was confirmed that the 4-Hz peak value in the top panels is also beyond the 95% confidence limit inthe middle panels. Bottom: FFT spectra of response to L240R240 (no-BB as control). The amplitude at just 4 Hz is not beyond the 95% confidence limit.Furthermore, it was confirmed that the 4-Hz peak value in the upper panels is beyond the 95% confidence limit in the lower panels. Because the doublerequirements described in the text were fulfilled, we considered that the 4-Hz peak on this channel was specific to 4-Hz BB. I: FFT spectra on the same channelas that of G. Top: FFT spectra of response to L240R244 (4-Hz BB). The peak amplitude at just 4 Hz (arrows) is beyond the 95% confidence limit in the widthof 1 Hz (from 3.5 to 4.5 Hz). Middle: FFT spectra of response to L240R246.66 on the same channel are added (6.66-Hz BB as control). The amplitude at just4 Hz is not beyond the 95% confidence limit. Furthermore, we confirmed that the 4-Hz peak value in the upper panels was beyond the 95% confidence limitin the middle panels. Bottom: FFT spectra of response to L240R240 (no-BB as control). The amplitude at just 4 Hz is not beyond the 95% confidence limit.Furthermore, we confirmed that the 4-Hz peak value in the upper panels is beyond the 95% confidence limit in the lower panels. Because the double requirementsdescribed in the text were fulfilled, we considered that the 4-Hz peak on this channel was specific to 4-Hz BB.



Hz was beyond the 95% CI of amplitudes in the width of 1 Hz(from 3.5 to 4.5 Hz). To evaluate the spectral peak on eachchannel, we used two control conditions, namely, another BB(6.66-Hz BB) and no-BB stimulations, and we used thesecontrols in two ways. The first application was to calculate the95% CI of amplitudes in the width of 1 Hz (from 3.5 to 4.5 Hz)similarly in both controls (L240R246.66 and L240R240) and torecognize that the 4-Hz peak in L240R244 was adequately highonly when the peak was also beyond both of the control 95%CIs on the same channel of the same subject. The resultsconfirmed that the 4-Hz peak in L240R244 was beyond all ofthe three kinds of the 95% CIs. The second application of thetwo controls was to verify the specificity of the peak at BBfrequency by confirming that a similar peak at 4 Hz was foundon the same channel of the same subject in neither of the twotypes of control conditions. In other words, when the tone inthe right ear was changed from 244 to 246.66 or 240 Hz, theamplitude at just 4 Hz did not exceed 95% CI in the width of1 Hz. When the double requirements (i.e., a peak at BBfrequency beyond each of the 3 kinds of 95% CI, no such peaksin any of the 2 control conditions) were fulfilled, we consideredthat the 4-Hz peak on this channel was specific to 4-Hz BB.Figure 2I shows the FFT spectra on the same channel as that ofG. Also on this channel in the right hemisphere, the amplitudeat just 4 Hz was beyond each of three kinds of 95% CI in thewidth of 1 Hz (in 4-Hz BB, 6.66-Hz BB, and no-BB), but 4-Hzpeak was detected in neither the corresponding 6.66-Hz BB northe no-BB condition. Therefore we considered that the 4-Hzpeak on this channel was specific to 4-Hz BB. Another notice-able finding was that in the three conditions, we found a broadpeak that might represent elicited alpha waves around 13 Hz.

With regard to 6.66-Hz BB, we selected channels with aspecific spectral peak using the same two requirements; namely1) the amplitude at just 6.66 Hz was beyond 95% CI in a widthof 1 Hz (from 6.16 to 7.16 Hz) in 6.66-Hz BB condition and intwo control conditions (4-Hz BB and no-BB) and 2) in thecorrespondent channel of the same subject in both 4-Hz BBand no-BB, the amplitude at just 6.66 Hz was within 95%CI.

Table 2 shows the median values of the nine subjects for thenumber of channels that satisfied the requirements mentionedin the preceding text. For example, in L240R244, on average,two, five, and two channels were synchronized to 4-Hz BB inthe left, center, and right zones, respectively. As shown in

Table 2, the number and spatial distribution of such “BB-synchronized” channels varied across subjects. For example, inthe left temporal area in L480R484, few subjects presented achannel that met the strict requirements, and the mediannumber of positive channels was zero. However, there was nodifference in the number of synchronized channels between 4-and 6.66-Hz BBs.

Figure 4 displays the spatial distribution of BB-synchronizedchannels in the arrangement of 204 sensors. On each channel inFig. 4A, the sums of numbers of BB-synchronized channels innine subjects and in four kinds of 4-Hz BB conditions areindicated by the diameter of the circle. Besides the temporalareas defined in Fig. 2, the remaining central area was dividedinto three zones: frontal, parietal, and occipital. Although theanatomical variations of individual brains were not taken intoaccount in this division, the results confirmed the presence inboth temporal zones bilaterally of comparatively many chan-nels showing frequent synchronization to 4-Hz BB. However,large circles were also found in the frontal, parietal, andoccipital zones, and small circles were scattered throughout thefive zones. As indicated in Table 2, these findings emphasizethat synchronization to 4-Hz BB was detected not only in thetemporal zones but also in the other zones. Figure 4B shows thesums of numbers of BB-synchronized channels in nine subjectsand in four kinds of 6.66-Hz BB conditions. Synchronization to6.66-Hz BB was frequently observed, especially in the frontaland parietal zones, in addition to the temporal zones.

MCE

The MCEs suggested several areas as possible generators ofBB ASSR. Figure 5 shows typical examples of MCE in one BBcycle in L244R240. In most subjects, dominant current sources�0.5 nAm were found in the superior temporal areas bilater-ally, which were considered to contain bilateral auditory cor-tices (Fig. 5, A and B). In some subjects, current sources werealso found in the posterior parietal cortex including the supe-rior or inferior parietal lobule (Fig. 5, C and D). However, wefound no specific IPD at which these current sources wereactivated within one BB cycle across conditions or subjects. Inconsideration of the great variability in IPD of the activations,we investigated within one cycle in each subject the presenceof activated sources over 0.5 nAm in the temporal and poste-

TABLE 2. Number of channels with a specific spectral peak that represented specific synchronization to BB frequency

FL, Hz FR, Hz

Left 39 Channels Center 124 Channels Right 40 Channels

Minimum Maximum Median Minimum Maximum Median Minimum Maximum Median

240.00 244.00 0 8 2 0 15 5 0 13 2244.00 240.00 0 5 2 0 28 3 0 8 1480.00 484.00 0 3 0 0 16 3 0 10 3484.00 480.00 0 7 1 0 16 3 0 10 1240.00 246.66 0 5 2 3 23 9 1 7 4246.66 240.00 0 5 1 2 14 6 0 12 4480.00 486.66 0 13 2 1 9 5 1 4 2486.66 480.00 0 11 3 3 10 7 0 8 2

To verify the specificity of the peak at BB frequency, we examined whether there was a similar peak on the same channel of the same subject in two typesof control conditions; another -BB (e.g., 6.66-Hz BB for 4-Hz BB evaluation) and no-BB stimulations. Double requirements ([1] a peak at BB frequency beyondall the 3 kinds of 95% confidence limits in BB, another -BB, and no-BB conditions; [2] no such peaks in any of the 2 control conditions) were required to confirmthat the peak on this channel was specific to BB frequency. We counted the number of channels that fulfilled the requirements, using data of the nine subjectsin eight types of BB conditions. The table shows minimum, maximum, and median across the nine subjects under each condition.



rior parietal areas. Table 3 shows the number of subjects whoshowed a current source in each brain area. Furthermore,weakly activated regions were found scattered mainly in thefrontal lobe, although no specific tendency was noted in thespatial distribution. These findings were consistent with thespatial distribution of BB-synchronized channels shown in Fig.4. Table 3 also shows that there was no difference between 4-and 6.66-Hz BBs in the tendency of the activation.

Phase analysis of magnetic fields on BB-synchronizedchannels

BB stimulation consists of regularly repeated IPD cycles. Toexplore the correlation between the IPD cycle and wave con-

figuration of the steady-state response to BB, we investigatedthe IPD at which waves formed peaks and troughs in BB-synchronized channels. In particular, we analyzed the phase ofextracted FFT components of BB frequency to identify a peakand a trough within one BB cycle.

The top panels in Fig. 6, A and B, display the samewaveforms on the demonstrative channels in L240R244 asshown in Fig. 2, F and G, respectively (the middle panels aretheir magnifications in 1 BB cycle). The sine waves shown ingray lines are FFT components of BB frequency of 4 Hz. Thebottom panels show dichotic sine waves to make possible acomparison of the changing IPD with responses in one BBcycle. From the standpoint of the channel in the left hemi-sphere (Fig. 6A), the IPD (� right phase – left phase) shiftedcontinuously from the ipsilateral (left) ear leading (�� IPD � 0) to the contralateral (right) ear leading (0 � IPD ��). In contrast, with regard to the channel in the right hemi-sphere (Fig. 6B), the IPD shifted continuously from the con-tralateral (left) ear leading (�� IPD � 0) to the ipsilateral(right) ear leading (0 � IPD � �). Arrows in the middle panelsindicate the IPD at which the sine component of BB frequencyreached a peak value within one BB cycle.

Each panel of Fig. 6C is a summed plotting of peak IPDs onBB-synchronized channels of all nine subjects in each BBstimulation. The ordinate represents the amplitude of the FFTcomponent of BB frequency. BB-synchronized channels arerepresented by five symbols according to the five zones used inFig. 4. The two arrows in the panel of L240R244 correspond tothe channels the IPDs with peak values of which are indicatedin Fig. 6, A and B. In this subject and with L240R244 stimu-lation, the IPD with peak value was located approximately at 0on each hemisphere. However, when the data of all the ninesubjects were plotted together, each panel of Fig. 6C showsthat the peak IPDs in the left temporal area (blue triangles) andin the right temporal area (red triangles) did not correlate withlaterality (positive vs. negative IPD). If the arrangement andangles of 203 employed sensors are considered, a delay of 180°should be considered an identical phase. However, in each ofthe eight types of BB condition and in each of the five brain

FIG. 4. Spatial distribution of the BB-synchronized channels in the arrange-ment of 204 channels. A: sums of numbers of BB-synchronized channels in 9subjects and in 4 kinds of 4-Hz BB conditions are indicated by a diameter ofa circle. Besides the temporal zones explained in Fig. 2, the remaining centralarea was divided into 3 zones: frontal, parietal, and occipital (gray lines).Although the anatomical variations of individual brains are not taken intoaccount in this schema, it is confirmed that in both temporal zones, bilaterallythere are many channels where synchronization to 4-Hz BB was frequentlyobserved. However, large circles are found also in frontal, parietal, andoccipital zones, and small circles are scattered over the 5 zones. B: sums ofnumbers of BB-synchronized channels in 9 subjects and in 4 kinds of 6.66-HzBB conditions. A synchronization to 6.66-Hz BB was frequently observed,especially in the frontal and parietal zones, besides the temporal zones.

Left hemisphere Right hemisphere

AA

C

B

D

FIG. 5. Examples of source distribution (color coded) by minimum-normcurrent estimates (MCEs) in the same subject shown in Fig. 2. In this subjectin L244R240, dominant current sources �0.5 nAm were recognized in the lefttemporal (A: at 0.81*� in 1 IPD cycle), right temporal (B: at �0.68*� in 1 IPDcycle), left posterior parietal (C: at �0.80*� in 1 IPD cycle), and rightposterior parietal (D: at 0.64*� in 1 IPD cycle) areas.



areas, there was no specific phase for channels synchronized to4- or 6.66-Hz BB. Furthermore, within a single subject therewas no consistent correlation between the left and right tem-poral areas, unlike the example shown in Fig. 6, A and B. It isalso noticeable that most BB-synchronized channels with ahigh amplitude of BB-frequency component belonged to theleft or right temporal areas. This finding suggests that theBB-synchronized channels in the temporal areas tend to have acomparatively high spectral peak of BB frequency, althoughTable 2 and Fig. 4 show that the number of BB-synchronizedchannels was not exceedingly large in the temporal areascompared with other areas.

D I S C U S S I O N

Synchronization to BB and possible sources of BB ASSR

In an analysis of magnetic fields on each channel, we usedtwo methods to search for channels that represented ASSR toBB in each subject. One method examined the configuration ofaveraged magnetic field in each channel (Fig. 2, A, F, and G),and the other involved searching the channels for FFT with aspecific peak at BB frequency (Fig. 2, H and I). The compar-ison of another BB and no-BB condition confirmed that themagnetic field itself under BB condition was ASSR to BB.Such spectral analysis of a magnetic field on a single channelhas been performed and validated in the study of ASSR to AMtones (Fujiki et al. 2002; Patel and Balaban 2000; Ross et al.2000). However, to identify more strictly the channels thatwere synchronized to a specific BB frequency, we adopted tworequirements, namely, a peak at BB frequency beyond the 95%confidence limits and no such peaks in any of the two controlconditions (another-BB and no-BB conditions). Especially, aspectral peak at BB frequency was considered significantlyevident only when it exceeded all of the three 95% CIs (BB,another-BB, and no-BB conditions). These strict requirementsusing two types of controls reduced the number of channelsthat were considered to represent synchronization to BB. It ispossible that some channels had false negative synchronizationto BB; however, the selected channels showed with adequatereliability that periodic responses with the same frequency asIFD are evoked in the cerebral cortex.

Consequently, the small numbers of selected channels mightcause a large variance in the spatial distribution of BB-syn-chronized channels. This might be partly because anatomical

variations of individual brains were not taken into account inthis calculation. Gender difference in the location of auditorycortex (Elberling et al. 1982; Nakasato et al. 1995; Ohtomo etal. 1998; Reite et al. 1995) might be another cause of the varieddistribution of BB-synchronized channels because we pooledthe data of six male and three female subjects. Furthermore, theEdinburgh handedness questionnaire confirmed that one of ournine subjects was ambidextrous. Because the only single neu-rophysiological study on BB perception in humans (Schwarzand Taylor 2005) did not discriminate the handedness ofsubjects and because auditory processing using IFD was con-sidered to be less influenced by handedness than by otherauditory mechanisms, such as the processing of languagesounds (Griffiths and Warren 2002; Jancke et al. 2002; Josse etal. 2003; Liegeois-Chauvel et al. 1999; Mazoyer et al. 1993;Zatorre et al. 2002), we did not exclude the data of theambidextrous subject. Actually, the values of this subject werenot different from those of others and did not affect the resultsof any analysis.

MCE requires no a priori information concerning the possi-ble source configuration or restriction of the MEG channelsincluded in the model. Therefore the MCE was consideredsuitable for making an approximate estimation of brain areasactivated by BB stimulation. As shown in Table 3, mostsubjects showed dominant activation in the left and righttemporal areas, which contained the auditory cortices. Thisfinding supports the notion that the main source for BB ASSRmight be within the temporal area similar to conventionalASSR, although MCE cannot demonstrate whether these ac-tivities contain BB specific spectral components.

In humans, the primary auditory cortex (PAC) is generallyidentified with Brodmann (1909) area 41, which is in the depthof the Sylvian fissure where it occupies a sizable part ofHeschl’s gyrus (HG). Studies using implanted electrodes insurgical patients confirm the localization of the PAC in humansto HG, particularly its middle part (Howard et al. 1996;Liegeois-Chauvel et al. 1991). Homologies of the belt andparabelt subdivisions defined in the macaque (Kaas and Hack-ett 2000) have never been precisely determined in the humanbrain, and thus areas surrounding the PAC are simply referredas nonprimary auditory cortex (Hall et al. 2003). Extensivestudies using MEG have been made of conventional ASSRevoked by stimulus rates near 40 Hz. In previous studies, thesource of ASSR was located on the PAC (Engelien et al. 2000;Forss et al. 1993; Pantev et al. 1993, 1996). Animal studies inwhich intracortical (Karmos et al. 1993) and subdural fieldpotentials (Franowicz and Barth 1995) were recorded sug-gested PAC as the source of the 40-Hz ASSR.

Previous neurophysiological studies provided evidence thatthe auditory cortex can represent BB frequency component.BBs demonstrate that the discharges of the auditory nervefibers preserve information on the phase of acoustic stimuli.Neural spikes tend to occur at a particular phase of thesinusoidal waveform (phase locking), and the central auditorysystem has the capacity to preserve temporal information(frequency coding). Animal studies confirmed that the centralauditory system preserves and utilizes information of continu-ously changing IPD on a presentation of BBs. Reale andBrugge (1990) examined the sensitivity to IPD in single neu-rons of PAC in anesthetized cats. They demonstrated thesensitivity of neurons to fixed IPD produced by a dichotic

TABLE 3. Number of subjects who showed a current source ineach brain area

FL, Hz FR, Hz

Temporal RegionPosterior Parietal

Region

Left Right Left Right

240.00 244.00 8 9 3 3244.00 240.00 9 9 4 4480.00 484.00 7 7 5 3484.00 480.00 9 8 4 5240.00 246.66 9 9 5 2246.66 240.00 9 8 3 1480.00 486.66 8 8 4 1486.66 480.00 8 9 3 2Mean 8.4 8.4 3.9 2.6



presentation of two tones that differed from each other only inthe starting phase. Approximately 26% of cells that weresensitive to static IPD were also sensitive to dynamicallychanging IPD created by BBs. They used tone combinations of600 and 600–635 Hz to elicit BB, and BB-synchronizedneuronal spikes were recorded in the range of IFD of 5–29 Hz.

No remarkable difference was found in spike numbers between5 and 9 Hz BB. The IPD sensitivity of some PAC neurons wasmarkedly similar to that of IPD-sensitive neurons in the medialsuperior olive and central nucleus of the inferior colliculus.Based on these results, Reale and Brugge (1990) concludedthat the sensitivity to IPD is transmitted from the lower brainstem to PAC, where it is preserved essentially undistorted. Intheir experiments with primates, Malone et al. (2002) reportedthe responses to changing IPD of BB in the auditory cortex ofawake macaques. Their results were recorded in the coreregion of the auditory cortex (Hackett et al. 1998).

In human EEG recordings of ASSR to BB (Schwarz andTaylor 2005), a systematic phase shift raised the possibility ofmore than one current source along the rostrocaudal axis. Inthis EEG study, a BB-specific spectral component was detectedin the frontal and parietal electrodes as well as in the temporalareas. In the current study, BB-specific spectral peaks werefound also on channels in the parietal, frontal, and occipitalareas. MCE demonstrated current sources in the posteriorparietal area and scattered weak activation in the frontal lobe,in addition to the temporal areas. These results supported thepossibility of multiple sources in the human EEG study.

A phase analysis of positive channels showed that BB-specific components with high amplitude tended to be fre-quently found in the temporal areas. The RMS intensity ofaveraged magnetic fields was larger in the temporal areasbilaterally than in other areas. These findings suggest that themain source for BB ASSR might be located in the auditorycortex, such as for conventional ASSR, but they do not confirmthat the auditory cortex is the exclusive source.

IPD for BB

Rose et al. (1966) and later Yin and Kuwada (1983b) foundthat for certain cells of the cat central nucleus of the inferiorcolliculus there existed an ITD that evoked the same relativedischarge rate regardless of stimulus frequency. Rose et al.(1966) referred to this ITD as the characteristic delay (CD) and

-375 -125 0 125 375-20

0

20

-10

0

10

IPD = - IPD = 0 IPD =

FL IpsilateralFR ContralateralFR Contralateral

FL Ipsilateral

-375 -125 0 125 375

IPD = - IPD = 0 IPD =

FR Ipsilateral FR Ipsilateral

A

Am

plitu

de (

fT/c

m)

(ms)(ms)

FL Contralateral FL Contralateral

0

-π -π/2 0 π/2 π -π -π/2 0 π/2 π

IPDIPD

0

1

2

3

4

5

6

0

1

2

3

4

5

6

0

1

2

3

4

5

6

0

1

2

3

4

5

6

L: 240 Hz R: 244 Hz L: 244 Hz R: 240 Hz

L: 480 Hz R: 484 Hz L: 484 Hz R: 480 Hz

L: 240 Hz R: 246.66 Hz L: 246.66 Hz R: 240 Hz

L: 480 Hz R: 486.66 Hz L: 486.66 Hz R: 480 Hz

Right TemporalLeft Temporal

Frontal Parietal Occipital

B

C

FIG. 6. A and B: phase of extracted FFT components of BB frequency wasanalyzed within 1 BB cycle. Top: waveforms on the demonstrative channels inL240R244 as shown in Fig. 2, F and G, respectively. The sine waves of theFFT component of BB frequency of 4 Hz are added (gray lines). Middle:magnifications of waves in the top. The abscissa is set in 1 BB cycle withinwhich the IPD changes continuously. Arrows indicate the IPD with a peakvalue of the BB frequency component. Bottom: schematic dichotic sine wavesto cause BB of 1 cycle. In this example, the frequency of the sinusoid deliveredto the left ear (FL: thin line) was lower than that delivered to the right ear (FR:thick line). For a schematic illustration, the amplitudes of the 2 sine waves aredifferent and the frequencies of the tones are much lower than those actuallyused under L240R244 condition. From the standpoint of the left hemisphere,the IPD (� right phase – left phase) shifted continuously from the ipsilateral(left) ear leading (�� IPD � 0) to the contralateral (right) ear leading (0 �IPD � �). In contrast, from the standpoint of the right hemisphere, the IPDshifted continuously from the contralateral (left) ear leading (�� IPD � 0)to the ipsilateral (right) ear leading (0 � IPD � �). C: each panel is a summedplot of peak IPDs on BB-synchronized channels of all the 9 subjects in eachBB stimulation. The ordinate expresses the amplitude of the FFT componentof BB frequency. BB-synchronized channels are represented by 5 symbolsaccording to the 5 zones applied in Fig. 4 (left temporal, right temporal, frontal,parietal, and occipital). Two arrows in the panel of L240R244 correspond tothe channels the IPDs with peak value of which are indicated in A and B. Notethat in each of the 8 types of BB conditions and in each of the 5 brain areas,there is no specific phase for channels synchronized to 4- or 6.66-Hz BB.Furthermore, within a single subject, there is no consistent correlation betweenthe left and right temporal areas, unlike the examples shown in A and B.



argued that it may be a property of some binaural neurons thatallows them to detect the location of a sound source in space.Benson and Teas (1976) were unable to find in their resultsfrom chinchilla auditory cortex a registration of peaks ortroughs in spike count versus IPD functions examined at threedifferent frequencies. However, in the PAC of anesthetizedcats, the discharges of neurons sensitive to dynamically chang-ing IPD were highly periodic and tightly synchronized to aparticular phase of the BB cycle (Reale and Brugge 1990). Inthe latter study, the ITD sensitivity of a single neuron wasanalyzed at a relatively large number of frequencies within theresponse area, and further consideration was given to thecortical presence of a CD. Their findings revealed that the IPDsensitivity in the cortex differs little from that recorded in theinferior colliculus or medial superior olive.

Because BB stimulation consists of regularly repeated IPDcycles, we had expected before our experiment to find acorrelation between the IPD cycle and the wave configurationof steady-state response to BB. Reale and Brugge (1990) founda marked tendency for each neuron to fire maximally when thephase of the tone to the ipsilateral ear lagged that to thecontralateral and for cell firing to reach its minimum when thereverse was true. Based on this finding, as illustrated in Fig. 1,we employed BB stimulation in which IPD shifted from oneear leading to another ear leading through one complete cycleof BB. For example, when the right ear lagged to the left ear(-� � right phase – left phase � 0), the response in the rightauditory cortex had been expected to show maximal amplitude.In contrast, when the left ear lagged to the right ear (0 � rightphase – left phase � �), the response in the left auditory cortexhad been expected to show maximal amplitude. However, asshown in Fig. 6, our results showed that the distribution in thephase of peak amplitude did not exhibit such laterality. TheIPD for peak showed wide interindividual variability withineach type of BB stimulation. Furthermore, also within eachsubject, the peaks of BB-synchronized waves in the left andright temporal areas did not always show symmetry or mirrorimage. These findings might mean that there is no uniformphase of neuronal activity for BB detection in synchronizedareas. Alternatively, the great variability in phase might implythat BB ASSR does not represent changing IPD per se butreflects a higher order of cognitive process corresponding tosubjective fluctuations of BB. Our results did not show asystemic phase shift that was reported in the EEG study(Schwarz and Taylor 2005). This discrepancy in phase mightreflect the essential difference between 40-Hz BB and slowBB, or the cognitive process for subjective fluctuations of slowBB. In summary, our results in source and phase analysessuggest the contribution of several cortical regions and implythe sequential recruitment of these regions for the high-ordercognitive process of BB.

Previous studies indicated that the great variability in phasecould be assumed within the auditory cortex. Reale and Brugge(1990) indicated that IPD-sensitive cells in the auditory cortexexhibited peaks and troughs in spike count at certain IPDwithin one BB cycle; however, the specific IPD of the peaksand troughs were different for each cell. This result impliedthat the auditory cortex contained a mixture of neurons thesuitable IPDs of which differed from one another, and thisvariance may contribute to realizing auditory function such assound localization. In fact, in experiments involving cats,

Furukawa et al. (2000) suggested a model in which sound-source location is coded by a population of cortical neuronsthat are distributed widely throughout the auditory cortex, eachof which can carry information of sound-source location by itsown spike pattern (Middlebrooks et al. 1994, 1998). Ourresults were not contradictory to the model of functionalensembles of cortical neurons.

Intensities of ASSR

The intensity of magnetic field under BB condition wassignificantly greater than those of no-BB and no-body condi-tions. The recorded fields showed relatively weak responses toBB; however, our findings proved that the fields were strongenough to be distinguished from noise, which might be causedby acoustic stimulations or the circumstances of our record-ings.

Although BB ASSR was greater than noise level and evi-dently contained a spectral component of BB frequency, itsweakness may be due to the small number of IPD-sensitiveneurons. Reale and Brugge (1990) demonstrated that approx-imately a fourth of the IPD-sensitive neurons isolated in of thecat’s PAC responded to continuously changing IPD. On theother hand, �90% of IPD-sensitive neurons in the inferiorcolliculus responded to both static and dynamic phase shifts(Yin and Kuwada 1983a).

In the study of ASSR to BB (Schwarz and Taylor 2005), BBASSR was more evident when the subjects were instructed toattend to one warbling tone than when they were not. Oursubjects watched video movies instead of attending to BB tomaintain a vigilance level based on precedent finding thatdrowsiness and sleep reduce the amplitude of response inconventional ASSR to AM tone with rates around 40 Hz(Galambos et al. 1981; Jerger et al. 1986; Linden et al. 1985).It is possible, however, that the neglect of BB might attenuateASSR. Although it was reported that selective attention to AMtone does not affect ASSR with rates around 40 Hz whenarousal is controlled (Linden et al. 1987), a recent MEG studyusing an AM discrimination task, which required focusedattention to the modulation rhythm, showed an enhancement inamplitude during auditory attention (Ross et al. 2004). Suchsignificant effects of attention on amplitude have been reportedalso in steady-state visual- (Morgen et al. 1996; Muller et al.1998) and somatosensory- (Giabbiconi et al. 2004) evokedpotentials. Alternatively, BB ASSR could reflect both a cog-nitive process, which is influenced by attention, and an uncon-scious transporting process of IPD information, which is notaffected by attention.

Effects of tone frequency and IFD

Previous behavioral studies (Licklider et al. 1950; Perrottand Musicant 1977; Perrott and Nelson 1969) reported thatboth 4- and 6.66-Hz BBs elicited similar sensations of abeating tone, which was also described as a “periodic fluctu-ation in loudness.” All the participants in our study alsoreported that both 4- and 6.66-Hz BBs containing 240- or480-Hz tones allowed them to perceive BB in the narrowsense. This equality in subjective evaluations by subjects wasreflected also in the properties of BB ASSR. The intensity ofmagnetic fields, spatial distribution of BB-synchronized chan-



nels, or MCEs: these properties of BB ASSR were not influ-enced by either tone frequency or IFD. Especially, an exchangeof frequencies between ears causes an inversion of IPD, andthe direction of rotating tone should be inverted if slow BB wasperceived as a movement of auditory image. However, theproperties were not influenced systematically by such ex-changes of stimuli. Furthermore, whether a higher tone waspresented to the ipsilateral or the contralateral ear made nospecific effect on responses in a temporal area. These findingsare consistent with results of previous animal studies. Althougha sensitivity to the direction of BBs was reported in a smallnumber of neurons in the inferior colliculus (Yin and Kuwada1983a), Reale and Brugge (1990) could find no such sensitivityin the PAC. Most cells sensitive to BB in the PAC respondedequally to BB, regardless of the direction of IPD change.

Originally, conventional ASSR are more difficult to recordat low modulation frequencies because the EEG noise levelsare higher. However, reliable responses can be recorded atlower frequencies, especially at 2–5 Hz for amplitude-modu-lated tones and at 3–7 Hz for frequency-modulated tones(Picton et al. 1987). These results confirm that ASSR could besynchronized even to fluctuation with a frequency �10 Hz. Toour knowledge, there are no studies in which synchronizedresponses to BB were recorded using lower frequency tones inhumans. In this regard, two neuroimaging studies employed1-Hz BB; however, neither the PET studies (Griffiths et al.1994) nor the functional MRI studies (Bremmer et al. 2001)detected a BB-specific component because of their limitations.Our results showed that BB ASSR can be elicited by suchlower IFD. However, one limitation of our study is the use ofonly two types of BB frequencies. Further studies using BBs�4 Hz and beyond 6.66 Hz are required because any technicalor physiological factors, such as intensity of BB stimulation(Karino et al. 2005), can influence the responses.

Conclusions

In our study, periodical steady-state responses with smallamplitudes were evoked by slow BB. A spectral analysis ofmagnetic field on each single channel revealed that responsesevoked by BB contained a specific spectral component of BBfrequency, and the evoked responses were the ASSR to BB.The spatial distribution of BB-synchronized channels andMCEs suggested multiple BB ASSR sources in the parietal andfrontal cortices in addition to the temporal areas, includingauditory cortices.

A C K N O W L E D G M E N T S

We thank two anonymous reviewers for helpful comments.

G R A N T S

This study was supported by a grant-in-aid for Scientific Research(15500312 and 16790990) from the Ministry of Education, Culture, Sports,Science and Technology, Japan.

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Neuromagnetic Responses to Binaural Beat in Human Cerebral Cortex

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