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Inaudible High-Frequency Sounds Affect Brain Activity:
Hypersonic Effect
TSUTOMU OOHASHI,
1,2
EMI NISHINA,
3
MANABU HONDA,
4,5
YOSHIHARU YONEKURA,
4,6
YOSHITAKA FUWAMOTO,
7
NORIE KAWAI,
8,9
TADAO MAEKAWA,
10
SATOSHI NAKAMURA,
6
HIDENAO FUKUYAMA,
4
AND HIROSHI SHIBASAKI
4
1
Department of KANSEI Brain Science, ATR Human Information Processing Research Laboratories, Kyoto 619-0288;
2
Department of Network Science, Chiba Institute of Technology, Narashino 275-0016;
3
Human Interface Research and
Development Section, National Institute of Multimedia Education, Chiba 261-0014;
4
Department of Brain Pathophysiology,
Kyoto University School of Medicine, Kyoto 606-8507;
5
Laboratory of Cerebral Integration, National Institute for
Physiological Sciences, Okazaki 444-8585;
6
Biomedical Imaging Research Center, Fukui Medical University, Fukui 910-
1193;
7
Department of Environmental and Information Sciences, Yokkaichi University, Yokkaichi 512-8512;
8
Institute of
Community Medicine, University of Tsukuba, Tsukuba 305-8577;
9
Foundation for Advancement of International Science,
Tsukuba 305-0005; and
10
Art and Technology Project, ATR Media Integration & Communications Research Laboratories,
Kyoto 619-0288, Japan
Oohashi, Tsutomu, Emi Nishina, Manabu Honda, Yoshiharu
Yonekura, Yoshitaka Fuwamoto, Norie Kawai, Tadao Maekawa,
Satoshi Nakamura, Hidenao Fukuyama, and Hiroshi Shibasaki. In-
audible high-frequency sounds affect brain activity: hypersonic effect.
J Neurophysiol 83: 3548 –3558, 2000. Although it is generally ac-
cepted that humans cannot perceive sounds in the frequency range
above 20 kHz, the question of whether the existence of such “inau-
dible” high-frequency components may affect the acoustic perception
of audible sounds remains unanswered. In this study, we used nonin-
vasive physiological measurements of brain responses to provide
evidence that sounds containing high-frequency components (HFCs)
above the audible range significantly affect the brain activity of
listeners. We used the gamelan music of Bali, which is extremely rich
in HFCs with a nonstationary structure, as a natural sound source,
dividing it into two components: an audible low-frequency component
(LFC) below 22 kHz and an HFC above 22 kHz. Brain electrical
activity and regional cerebral blood flow (rCBF) were measured as
markers of neuronal activity while subjects were exposed to sounds
with various combinations of LFCs and HFCs. None of the subjects
recognized the HFC as sound when it was presented alone. Never-
theless, the power spectra of the alpha frequency range of the spon-
taneous electroencephalogram (alpha-EEG) recorded from the occip-
ital region increased with statistical significance when the subjects
were exposed to sound containing both an HFC and an LFC, com-
pared with an otherwise identical sound from which the HFC was
removed (i.e., LFC alone). In contrast, compared with the baseline, no
enhancement of alpha-EEG was evident when either an HFC or an
LFC was presented separately. Positron emission tomography mea-
surements revealed that, when an HFC and an LFC were presented
together, the rCBF in the brain stem and the left thalamus increased
significantly compared with a sound lacking the HFC above 22 kHz
but that was otherwise identical. Simultaneous EEG measurements
showed that the power of occipital alpha-EEGs correlated signifi-
cantly with the rCBF in the left thalamus. Psychological evaluation
indicated that the subjects felt the sound containing an HFC to be
more pleasant than the same sound lacking an HFC. These results
suggest the existence of a previously unrecognized response to com-
plex sound containing particular types of high frequencies above the
audible range. We term this phenomenon the “hypersonic effect.”
INTRODUCTION
It is generally accepted that audio frequencies above 20 kHz
do not affect human sensory perception since they are beyond
the audible range (Durrant and Lovrinc 1977; Snow 1931;
Wegel 1922). Thus for example, most of the conventional
commercial digital audio formats [e.g., compact disks (CDs),
digital audio tapes (DATs), and digital audio broadcasting]
have been standardized to a frequency range that does not
allow such high-frequency components (HFCs) of sounds to be
included. As a premise for determining these formats, several
psychological experiments were performed to evaluate sound
quality subjectively by means of questionnaires, according to
the recommendation of the Comite´ Consultatif International
Radiophonique (CCIR 1978) or its modified versions. Studies
by Muraoka et al. (1978) and Plenge et al. (1979), as well as
other studies, concluded that listeners did not consciously
recognize the inclusion of sounds with a frequency range above
15 kHz as making a difference in sound quality. Nevertheless,
and interestingly enough, artists and engineers working to
produce acoustically perfect music for commercial purposes
are convinced that the intentional manipulation of HFC above
the audible range can positively affect the perception of sound
quality (Neve 1992). Indeed, the Advanced Audio Conference
organized by the Japan Audio Society (1999) proposed two
next-generation advanced digital audio formats: super audio
compact disk (SACD) and digital versatile disk audio (DVD-
audio). These formats have a frequency response of up to 100
kHz and 96kHz, respectively. However, the proposal was not
based on scientific data about the biological effects of the
HFCs that would become available with these advanced for-
mats. Although recently there have been several attempts to
explore the psychological effect of inaudible HFCs on sound
perception using a digital audio format with a higher sampling
The costs of publication of this article were defrayed in part by the payment
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3548 0022-3077/00 $5.00 Copyright © 2000 The American Physiological Society
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rate of 96 kHz (Theiss and Hawksford 1997; Yamamoto 1996;
Yoshikawa et al. 1995, 1997), none of these studies has con-
vincingly explained the biological mechanism of the phenom-
enon. This may reflect in part the limitations of the conven-
tional audio engineering approach for determining sound
quality, which is solely based on a subjective evaluation ob-
tained via questionnaires.
There are two factors that may have some bearing on this
issue. First, it has been suggested that infrasonic exposure may
possibly have an adverse effect on human health (Danielsson
and Landstrom 1985), suggesting that the biological sensitivity
of human beings may not be parallel with the “conscious”
audibility of air vibration. Second, the natural environment,
such as tropical rain forests, usually contains sounds that are
extremely rich in HFCs over 100 kHz. From an anthropoge-
netic point of view, the sensory system of human beings
exposed to a natural environment would stand a good chance of
developing some physiological sensitivity to HFCs. It is pre-
mature to conclude that consciously inaudible high-frequency
sounds have no effect on the physiological state of listeners.
In the present study, therefore, we addressed this issue by
using quantifiable and reproducible measurements of brain
activity. To measure human physiological responses to HFCs,
we selected two noninvasive techniques: analysis of electroen-
cephalogram (EEG) and positron emission tomography (PET)
measurements of the regional cerebral blood flow (rCBF).
These methods have complementary characteristics. EEG has
excellent time resolution, is sensitive to the state of human
brain functioning, and places fewer physical and mental con-
straints on subjects than do other techniques such as functional
magnetic resonance imaging (fMRI). This is of special impor-
tance because some responses might be distorted by a stressful
measurement environment itself. On the other hand, PET pro-
vides us with detailed spatial information on the neuroanatomi-
cal substrates of brain activity. Combining these two tech-
niques with psychological assessments, we provide evidence
herein that inaudible high-frequency sounds have a significant
effect on humans.
METHODS
Subjects
Twenty-eight Japanese volunteers (15 males and 13 females, 19 – 43
years old) participated in the EEG experiments; 12 Japanese volunteers (8
males and 4 females, 19 –34 years old) participated in the PET experi-
ment; and 26 Japanese volunteers (15 males and 11 females, 18 –31 years
old) participated in the psychological experiment. None of the subjects
had any history of neurological or psychiatric disorders. Written informed
consent was obtained from all subjects before the experiments. The PET
and EEG experiments were performed in accordance with the approval of
the Committee of Medical Ethics, Graduate School of Medicine, Kyoto
University. All subjects were familiar with the actual sounds of the
instruments used as a sound source.
Sound materials and presentation systems
Traditional gamelan music of Bali Island, Indonesia, a natural
sound source containing the richest amount of high frequencies
with a conspicuously fluctuating structure, was chosen as the sound
source for all experiments. A traditional gamelan composition,
“Gambang Kuta,” played by “Gunung Jati,” an internationally
recognized gamelan ensemble from Bali, was recorded using a
B&K 4135 microphone, a B&K 2633 microphone preamplifier,
and a B&K 2804 power supplier, all manufactured by Bru¨el and
Kjær (Nærum, Denmark). The signals were digitally coded by Y.
Yamasaki’s high-speed one-bit coding signal processor (United
States Patent No. 5351048) (Yamasaki 1991) with an A/D sam-
pling frequency of 1.92 MHz and stored in a DRU-8 digital data
recorder (Yamaha, Hamamatsu, Japan). This system has a gener-
ally flat frequency response of over 100 kHz.
Most of the conventional audio systems that have been used to
present sound for determining sound quality were found to be unsuit-
able for this particular study. In the conventional systems, sounds
containing HFCs are presented as unfiltered source signals through an
all-pass circuit and sounds without HFCs are produced by passing the
source signals through a low-pass filter (Muraoka et al. 1978; Plenge
et al. 1979). Thus the audible low-frequency components (LFCs) are
presented through different pathways that may have different trans-
mission characteristics, including frequency response and group de-
lay. In addition, inter-modulation distortion may differentially affect
LFCs. Therefore it is difficult to exclude the possibility that any
observed differences between the two different sounds, those with and
those without HFCs, may result from differences in the audible LFCs
rather than from the existence of HFCs. To overcome this problem,
we developed a bi-channel sound presentation system that enabled us
to present the audible LFCs and the nonaudible HFCs either sepa-
rately or simultaneously. First, the source signals from the D/A
converter of Y. Yamasaki’s high-speed, one-bit coding signal proces-
sor were divided in two. Then, LFCs and HFCs were produced by
passing these signals through programmable low-pass and high-pass
filters (FV-661, NF Electronic Instruments, Tokyo, Japan), respec-
tively, with a crossover frequency of 26 or 22 kHz and a cutoff
attenuation of 170 or 80 dB/octave, depending on the type of test.
Then, LFCs and HFCs were separately amplified with P-800 and
P-300L power amplifiers (Accuphase, Yokohama, Japan), respec-
tively, and presented through a speaker system consisting of twin
cone-type woofers and a horn-type tweeter for the LFCs and a
dome-type super tweeter with a diamond diaphragm for the HFCs.
The speaker system was designed by one of the authors (T. Oohashi)
and manufactured by Pioneer Co., Ltd. (Tokyo, Japan). This sound
reproduction system had a flat frequency response of over 100 kHz.
The level of the presented sound pressure was individually adjusted so
that each subject felt comfortable; thus the maximum level was
approximately 80 –90 dB sound pressure level (SPL) at the listening
position.
Using the bi-channel sound presentation system, four different
sound combinations were prepared as follows: 1) full-range sound
(FRS) 5HFC 1LFC; 2) high-cut sound (HCS) 5LFC only; 3)
low-cut sound (LCS) 5HFC only; and, 4) baseline 5no sound
except for ambient noise. All experiments were performed in an
acoustically shielded room. In the PET experiment, there was a
very low-level fan noise from the PET scanner, which did not
annoy the subjects. Figure 1Ashows the averaged power spectrum
of the source signal obtained from the music with a CF-5220 fast
Fourier transform (FFT) analyzer (Ono Sokki, Tokyo, Japan) over
an analysis period of 200 s. It contained a significant amount of
HFCs above the audible range, often exceeding 50 kHz and, at
certain times, 100 kHz. Figure 1Bshows the averaged power
spectra of the actual sounds reproduced with a 22 kHz cutoff
frequency for the filter and recorded at the subject’s head position.
The spectrum of FRS was essentially the same as that of the source
and contained both LFCs below and HFCs above 22 kHz. None of
the blindfolded subjects could distinguish LCS (i.e., HFC only)
from silence when it was presented alone. Therefore we concluded
that the HFC employed in the present experimental setting was, at
least, a consciously unrecognizable air vibration.
3549BIOLOGICAL EFFECT OF INAUDIBLE HIGH-FREQUENCY SOUNDS
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EEG recordings and analysis
The EEG experiments were performed in the EEG laboratory of the
National Institute of Multimedia Education. Subjects were asked to sit
on a chair in a relaxed position. The distance from the speakers to the
subjects’ ears was approximately 2.5 m. Special attention was paid to
the subjects’ immediate environment to avoid discomfort. For exam-
ple, the room was decorated with plants, lacquered masks, and land-
scape paintings. The equipment for the EEG recordings was hidden
from the subjects’ view and all cables for the experimental equipment
were in a pit below the floor. The subjects were instructed to enjoy the
music without any cognitive tasks during the sound presentation. The
subjects were able to see outdoors through a wide, double-glass
window that acoustically shielded the experimental room from outside
sounds. Two different EEG experiments were performed. In the first
experiment, to explore the physiological effect of sounds with a
nonaudible frequency range, we employed a strictly controlled exper-
imental setting of sound presentation combined with conventional
EEG measurements. In the second experiment, the same effect was
examined under more ordinary listening conditions.
EXPERIMENT 1. To examine the physiological effect of sounds with
an inaudible frequency range, 11 subjects were presented with the
FRS, HCS, and baseline conditions. In this experiment, a cutoff
frequency of 26 kHz with a steeper cutoff attenuation of 170 dB/
octave was employed to separate HFCs from LFCs. This relatively
high cutoff frequency was chosen because when a cutoff frequency
lower than 26 kHz is used the skirts of the power spectrum of the
filtered HFCs extend below 20 kHz and generate sounds containing
components below 20 kHz. It is widely known that the upper limit of
the audible range of humans varies considerably. It usually corre-
sponds to around 15 or 16 kHz in young adults and sometimes below
13 kHz in the elderly, and some people can recognize air vibrations of
20 kHz as sound. When a cutoff frequency of 26 kHz is employed
with the steeper cutoff attenuation, the power spectrum of the filtered
HFCs under 20 kHz falls below the system noise level. Therefore we
selected a cutoff frequency of 26 kHz, which is sufficiently high to
completely exclude contamination by audible sound components in all
of the subjects. In accordance with conventional recordings of back-
ground EEG activity, subjects were asked to keep their eyes naturally
closed during the experiment to eliminate any effects of visual input.
The presentation of the sounds in both FRS and HCS conditions lasted
200 s, which included the entire piece of music. The baseline condi-
tion also lasted 200 s without sound presentation. The inter-session
intervals were 10 s. Two recording sessions were repeated for each
condition in the following order: baseline–FRS–HCS–FRS–HCS–
baseline.
EXPERIMENT 2. The validity of the digital audio format internation-
ally employed for CDs was evaluated under more ordinary listening
conditions. Seventeen subjects were presented with sounds using a
cutoff frequency of 22 kHz, which corresponds to the upper range of
sounds recorded by a CD. Subjects were then asked to keep their eyes
naturally open as they usually do when they listen to music. The
open-eye condition was also appropriate to control the subjects’
vigilance. Each subject was presented with four types of conditions:
FRS, HCS, and baseline, as in Experiment 1, plus LCS to elucidate the
effect of an HFC when it is presented alone. As in Experiment 1, each
condition lasted 200 s. Before the actual recording sessions, HCS was
presented once to familiarize the subjects with the experimental
environment. To avoid any influence by the order of presentation, the
four different conditions were performed in random order across the
subjects. After a 10-min rest, the same four conditions were repeated
in reverse order. Neither the subjects nor the experimenters knew
which conditions were being performed.
The EEGs, recorded using the WEE-6112 telemetric system (Ni-
hon-Koden, Tokyo, Japan) to minimize constraint on the subjects,
were stored on magnetic tape for off-line analysis. The EEGs were
recorded continuously, including the intervals between the sessions.
Data were recorded from 12 scalp sites (Fp1, Fp2, F7, Fz, F8, C3, C4,
T5, Pz, T6, O1, and O2 according to the International 10-20 System)
using linked earlobe electrodes as the reference with a filter setting of
1– 60 Hz (23 dB). The impedance of all electrodes was kept below
5kV. The EEGs obtained were subjected to power spectra analysis.
The power spectrum of the EEG at each electrode was calculated by
fast Fourier transform (FFT) analysis for every 2-s epoch, with an
overlap of 1 s, at a frequency resolution of 0.5 Hz with a sampling
frequency of 256 Hz. Then the averaged power spectrum within a 10-s
time window was calculated. Each analysis window was designated
by the time at its middle point measured from the beginning of the
sound presentation. For example, the time window labeled as 100-s
contains data from 95 to 105 s from the beginning. Then the square
root of the averaged power level in a frequency range of 8.0 –13.0 Hz
FIG. 1. Power spectra of the sound used in this study. A: the averaged
power spectrum calculated from the entire 200-s period of the recorded sound
source signal using a CF-5220 fast Fourier transform (FFT) analyzer (Ono
Sokki, Tokyo, Japan). It contains a significant amount of high-frequency
components above the audible range. B: the averaged power spectra of the
sounds reproduced by the bi-channel sound presentation system (see text) in
different conditions. The power was calculated from the signal actually re-
corded at the subject’s head position using a B&K 4135 microphone (Bru¨ el
and Kjær, Nærum, Denmark). The top, middle, and bottom panels represent
full-range sound (FRS), high-cut sound (HCS), and low-cut sound (LCS),
respectively. The power spectrum of FRS is essentially identical to the spec-
trum of the source and contains both a low-frequency component (LFC) (i.e.,
the one used in the HCS condition) and a high-frequency component (HFC) (in
the LCS condition).
3550 OOHASHI ET AL.
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at each electrode position was calculated as the equivalent potential of
EEGs in an alpha band (alpha-EEG). To eliminate a possible effect of
inter-subject variability, the alpha-EEG at each electrode position was
normalized with respect to the mean value across all time epochs,
conditions, and electrode positions for each subject. To obtain an
overview of the data, to check for contamination by artifacts, and to
characterize the spatial distribution of the alpha-EEG, we constructed
colored contour line maps using 2,565 scalp grid points with linear
interpolation and extrapolation. This type of map is called a brain
electrical activity map (BEAM) (Duffy et al. 1979). To avoid con-
tamination by artifacts arising from eye movement, we calculated
occipital alpha-EEGs by averaging the alpha-EEGs at the electrodes
on the posterior one-third of the scalp. The BEAMs and occipital
alpha-EEGs were averaged over multiple time epochs and subjected
to a statistical evaluation of condition effects. Since the time course of
the alpha-EEG change revealed a considerable time lag with respect to
the sound presentation (see RESULTS and Fig. 2C), we made a statistical
evaluation of the data obtained from all time epochs as well as of the
data from only the latter half of the session (from the 100-s to 200-s
class marks). We used analysis of variance (ANOVA) followed by
Fishers’ protected least significant difference (PLSD) post hoc test to
assess statistical significance for the different conditions.
PET measurement and analysis
The sound presentation equipment was installed and calibrated in
the PET laboratory of Kyoto University Hospital. Subjects lay supine,
with their eyes naturally open, on the PET scanner bed in a quiet,
dimly lit room. Their heads were fixed in individually molded helmet-
shaped rests that were contoured to leave their ears undisturbed. The
distance from the speakers to the subjects’ ears was approximately
1.5 m. As in the EEG study, special attention was paid to the
immediate environment to minimize the subjects’ discomfort. Six of
the subjects were studied using FRS, HCS, and baseline conditions,
and the other six were studied using FRS, LCS, and baseline condi-
tions. The order of the conditions was randomized across the subjects
and a total of six scans was performed on each subject with intervals
of 7 min. For each of the FRS, HCS, and LCS presentations, 30 mCi
of
15
O-labeled water was injected into the right cubital vein 80 s after
the beginning of each session. The same procedure was carried out for
the baseline condition after a minimum 1-min rest without any pre-
sentation other than the ambient background noise of the PET scanner
room. Following the injection, the head was scanned for radioactivity
with a multi-slice PET scanner (PCT3600W, Hitachi Medical Co.,
Tokyo, Japan) for 120 s. The scanner acquired 15 slices with a
center-to-center distance of 7 mm and an axial resolution of 6.5 mm
full-width at half-maximum (FWHM) at the center (Endo et al. 1991).
The in-plane spatial resolution with stationary mode acquisition used
in this protocol was 6.7 mm of FWHM, which was blurred to ;10
mm in the reconstructed PET images. The field of view and pixel size
were 256 mm and 2 32 mm, respectively. Prior to the emission
measurements, transmission data were obtained using a
68
Ge/
68
Ga
standard plate source for attenuation correction. Reconstructed images
were obtained by summing up the activity throughout the 120-s
period. No arterial blood sampling was performed; therefore the
images collected were of tissue activity. Tissue activity recorded by
this method is linearly related to rCBF (Fox et al. 1984; Fox and
Mintun 1989).
The PET data were analyzed with statistical parametric mapping
(SPM96 software, Wellcome Department of Cognitive Neurology,
London, UK) implemented in MATLAB (Mathworks, Inc., Sherborn,
MA). Statistical parametric maps are spatially extended statistical
processes that are used to characterize regionally specific effects in
imaging data (Friston et al. 1991, 1994, 1995b; Worsley et al. 1992).
The scans from each subject were realigned using the first image as
the reference (Friston et al. 1995a). After realignment, the images
were transformed into a standard anatomical space (Friston et al.
1995a; Talairach and Tournoux 1988). As a result, each scan was
resampled into voxels that were 2 3234 mm each in the x
(right-left), y(anterior-posterior), and z(superior-inferior) directions.
FIG. 2. Normalized potentials from the alpha frequency range of the spontaneous electroencephalogram (alpha-EEG) under
each experimental condition (FRS, HCS, and baseline) and time course in the successive FRS and HCS conditions in EEG
Experiment 1. A: brain electrical activity maps (BEAMs) averaged across the 11 subjects over the entire time epoch of sound
presentation. Darker red indicates higher alpha-EEG potential. Note that the alpha-EEG is enhanced in the parieto-occipital region
exclusively in the FRS condition. B: mean and standard error of the occipital alpha-EEG for all 11 subjects. FRS significantly
enhanced the occipital alpha-EEG relative to HCS. C: time course of grand average BEAMs across all 11 subjects. Two sessions
for each condition were averaged in this figure. The occipital alpha-EEG shows a gradual increase during the FRS presentation and
a gradual decrease while HCS was successively presented.
3551BIOLOGICAL EFFECT OF INAUDIBLE HIGH-FREQUENCY SOUNDS
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Each image was smoothed with an isotropic Gaussian kernel
(FWHM 515 mm) to account for the variation in normal gyral
anatomy and to increase signal-to-noise ratio. The effect of global
differences in rCBF between scans was removed by scaling the
activity in each pixel proportional to the global activity so as to adjust
the mean global activity of each scan to 50 ml/100g/min. To explore
regions showing significant differences in rCBF among different
conditions, the general linear model with contrasts was employed at
each voxel (Friston et al. 1995b). Since the different conditions were
run in different subjects, the contrasts of FRS versus HCS and HCS
versus baseline were examined for six subjects, and those of FRS
versus LCS and LCS versus baseline were examined for the other six
subjects. The contrast of FRS versus baseline was examined for all 12
subjects, inclusive. The resulting set of voxel values for each contrast
constituted a statistical parametric map of the tstatistic. The tvalues
were transformed into the unit normal distribution (Zscore), which
was independent of the degree of freedom of error, and were thresh-
olded at 3.09. To account for multiple non-independent comparisons,
the significance of the activation in each brain region detected was
estimated by the use of distributional approximations from the theory
of Gaussian fields in terms of spatial extent and/or peak height
(Friston et al. 1994). An estimated Pvalue of 0.05 was used as a final
threshold for significance. The resulting set of Zscores for the
significant brain regions was mapped onto a standard spatial grid
(Talairach and Tournoux 1988).
In all of the subjects, EEGs were simultaneously recorded through-
out the PET measurement, which lasted approximately 60 min, from
12 electrodes as in the EEG experiment. The EEGs obtained during
the total 200-s sound presentation were subjected to power spectra
analysis and, in particular, those during each 120-s PET scan were
used for correlation analysis with the rCBF. The data of one subject
were excluded because of an excessive amount of electrical noise in
the EEG. We used ANOVA followed by Fisher’s PLSD post hoc test
to assess the statistical significance of the different conditions. In
addition, we used SPM software to calculate a correlation map be-
tween rCBF and the occipital alpha-EEG, to examine the relationship
between them. An estimated Pvalue of 0.05 with correction for
multiple comparisons was used as the final threshold for significance.
Psychological evaluation of sound quality
We also evaluated the subjective perception of sound quality. Since
the subjective impression of sounds is closely related to the subjects’
psychological condition, this evaluation was performed separately
from the EEG and PET experiments. We used the same piece of
gamelan music as was used for the EEG and PET experiments. First,
a pair of FRS and HCS, each lasting 200 s, was presented. The order
of the conditions was randomized across the subjects. After an inter-
mission of 3 min, another pair of FRS and HCS was presented in
reverse order. Therefore the stimuli were presented in an A-B-B-A
fashion, in which FRS and HCS were assigned to A and B or B and
A, respectively, in a randomly counterbalanced way across the sub-
jects. Neither the subjects nor the experimenter knew what the sound
conditions were, although they did know that the presentation was in
an A-B-B-A fashion. The subjects filled out a questionnaire to rate the
sound quality in terms of 10 elements, each expressed in a pair of
contrasting Japanese words (e.g., soft vs. hard). Each element of each
condition was graded on a scale of 5 to 1. The scores were statistically
evaluated by the paired comparison method described by Scheffe´
(1952). Note that the method used in the present study differs from
that recommended by the CCIR (1978) and its modified version,
which were widely used to determine the digital format of CDs around
1980 (e.g., Muraoka et al. 1978; Plenge et al. 1979). In the previous
studies, sound materials were never longer than 20 s and the interval
between two successive sound materials was 2–3 s or less. Therefore
if neuronal response to sound stimuli is characterized by delay and
persistence for longer than 20 s, it is difficult to exclude the possibility
that those studies might have introduced a subjective evaluation that
might not precisely correspond to each sound condition.
RESULTS
EEG Experiment 1
Figure 2, Aand B, shows the grand average BEAMs and
occipital alpha-EEGs, respectively, for the 11 subjects, calcu-
lated over the entire period of the sound presentation. The
alpha-EEGs were enhanced during FRS compared with those
during the other conditions. This enhancement was especially
predominant in the occipital and parietal regions (Fig. 2A).
ANOVA on the occipital alpha-EEG revealed a significant
main effect of condition [F(2,63) 53.74, P,0.05]. The post
hoc tests showed that the occipital alpha-EEG during FRS was
significantly greater than that during HCS (P,0.05) (Fig. 2B).
There was a similar tendency when FRS was compared with
the baseline (P50.10). Figure 2Cshows the averaged time
course of the BEAMs calculated for each 30 s of the FRS and
HCS conditions for all subjects, inclusive. The alpha-EEG
showed a gradual increase during the first several tens of
seconds of FRS; there was a gradual decrease at the beginning
of the following HCS. Taking into account the delay and
persistence of the enhancement of the alpha-EEG, statistical
evaluation was also made of the data from the latter half of the
recording session (from the 100-s to 200-s class mark). In this
analysis, compared with the data obtained by analyzing the
entire period of the sound presentation, ANOVA followed by
post hoc tests revealed a more significant main effect of con-
dition [F(2,63) 54.43, P,0.05] and a greater difference
between FRS and HCS (P,0.01).
EEG Experiment 2
The grand average BEAMs and occipital alpha-EEGs across
all 17 subjects over the latter half of the session (from the 100-s
to 200-s class mark) are shown in Fig. 3. The amount of eye
movement did not differ for different conditions. The alpha-
EEG showed significant enhancement in FRS compared with
the other conditions (Fig. 3A). This enhancement was predom-
inant in the occipital and parietal regions. ANOVA on the
occipital alpha-EEG revealed a significant main effect of con-
dition [F(3,131) 53.74, P,0.05]. The post hoc tests showed
that the occipital alpha-EEG in FRS was significantly greater
than that in the other three conditions (Fig. 3B). There was no
significant difference among HCS, LCS, and baseline (P.0.8
for all comparisons). A similar but weaker tendency was rec-
ognized when the data from the entire period of the sound
presentation were subjected to the analysis (main effect of
condition, P50.26; FRS vs. baseline, P50.05). This is
reasonable because the time course of the grand average oc-
cipital alpha-EEG in this experiment showed, as in Experiment
1, a gradual increase over the first several tens of seconds of
FRS (data not shown).
PET experiment
When the conditions with audible sounds (i.e., FRS or HCS)
were compared with those without audible sounds (i.e., LCS or
baseline), the bilateral temporal cortex, presumably the pri-
mary and secondary auditory cortex, always showed signifi-
3552 OOHASHI ET AL.
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cantly increased rCBF as expected (Table 1; see also Fig. 5C).
More importantly, when FRS was compared with HCS, deep-
lying structures in the brain were significantly more activated
during the presentation of FRS than during that of HCS (Fig. 4
and Table 1). The activated areas corresponded to the brain
stem (Fig. 4B) and the lateral part of the left thalamus (Fig.
4C). The same areas also showed an increased rCBF when FRS
was compared with either the baseline or LCS (Fig. 5, Aand
B). This tendency was also recognizable in the comparison of
FRS versus baseline with a lower threshold (Z.1.64 with
correction for multiple comparisons) (Fig. 5Cand Table 1).
Conversely, when HCS was presented, these areas in fact
showed a decreased rCBF compared with the baseline (Fig. 5,
Aand B). When LCS was compared with the baseline, no
significant differential activation was observed anywhere in the
brain and neither the left thalamus nor the brain stem showed
changes in rCBF.
EEG–rCBF correlation
The EEGs measured simultaneously with PET showed that
FRS significantly increased alpha-EEG activity compared with
HCS (P,0.05) (Fig. 6A), which is in complete agreement
with the findings of the EEG experiments performed indepen-
dently of the PET experiment. In contrast, when HCS was
compared with the baseline, alpha-EEG activity decreased
slightly in parallel with the changes we observed in the rCBF.
The normalized EEG potentials showed a significant correla-
tion with the rCBF equivalent value in the lateral part of the
thalamus (r50.539, P,0.0001). The maximum correlation
in the brain was observed at x5216, y5216, and z50(Z
score 54.30) in the stereotaxic space, which corresponds to
the pixel immediately adjacent to the maximally significant
point in the left thalamus as determined by the rCBF experi-
ments (Fig. 6, Band C, and Table 1).
Psychological evaluation of sound quality
Table 2 shows the subjective evaluation of sound quality
examined by Scheffe´’s paired comparison method (Scheffe´
1952). A significant difference was evident between FRS and
HCS in some elements of sound quality. Subjects felt that FRS
was softer, more reverberant, with a better balance of instru-
ments, more comfortable to the ears, and richer in nuance than
HCS.
DISCUSSION
Physiological effects of inaudible high-frequency sounds
Despite the fact that nonstationary HFCs were not perceived
as sounds by themselves, we demonstrated that the presenta-
tion of sounds that contained a considerable amount of non-
FIG. 3. Normalized alpha-EEG potentials in each experimental condition
(FRS, HCS, LCS, and baseline) during the latter half of the sound presentation
in EEG Experiment 2. A: BEAMs averaged across all 17 subjects over the time
period from the 100- to 200-s class marks. B: mean and standard error of the
occipital alpha-EEG for all 17 subjects. FRS significantly enhanced the occip-
ital alpha-EEG relative to the other conditions.
TABLE 1. Location and significance of activation in each area explored by SPM software
Analysis Location
Talairach Coordinate (mm) Magnitude of
Peak Activation
(Zscore)
Size of
Activation
(voxel)
Significance of
Activation
(corrected P)xyz
Subtraction analysis
FRS .baseline GTT, GTs (lt) 244 216 8 6.42 790 ,0.001
GTT, GTs (rt) 42 218 4 5.76 753 ,0.001
(brain stem) (4) (226) (28) (3.39) — —
[thalamus(lt)] (216) (218) (0) (3.32) — —
FRS .HCS brain stem 4 226 28 4.67 117 0.022
thalamus(lt) 216 218 0 4.50 60 0.039
HCS .baseline GTT, GTs (lt) 254 220 0 4.88 462 ,0.001
GTT, GTs (rt) 36 220 8 4.08 245 0.004
FRS .LCS GTT, GTs (lt) 246 220 8 3.99 179 0.026
GTT, GTs (rt) 48 28 4 5.40 476 0.001
LCS .baseline n.s.
Correlation analysis
rCBF vs. alpha-EEG thalamus (lt) 216 216 0 4.30 149 0.027
Zscores for the brain stem and thalamus in the comparison of FRS vs. baseline are reported in the parentheses to show a tendency of increased rCBF. x,y,
and z, stereotactic coordinates in the three orthogonal dimensions of the atlas by Talairach and Tournoux (1988). GTT, transverse temporal gyrus; GTs, superior
temporal gyrus; lt, left; rt, right.
3553BIOLOGICAL EFFECT OF INAUDIBLE HIGH-FREQUENCY SOUNDS
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FIG. 4. Projected images of statistical parametric maps (SPMs) contrasting
FRS with HCS with a standard threshold (Z.3.09 and P,0.05 with correction
for multiple comparisons). A:projectionimages.MapsofZscores for the regions
where activity was significantly increased during FRS as compared with HCS are
shown in a standard anatomical space (Talairach and Tournoux 1988) viewed from
the back (coronal view), the right side (sagittal view), and the top (transverse view)
of the brain. Maps are illustrated by a color scale, with the lower Zscore
represented in darker red and the higher Zscore in brighter yellow. Band C:
activated foci in the brain stem (B)andleftthalamus(C)aresuperimposedontothe
spatially normalized magnetic resonance image (MRI) as shown in neurological
convention. Maps are illustrated by a color scale, with the lower Zscore repre-
sented in darker red and the higher Zscore in brighter yellow.
FIG.5. Averaged regionalcerebralblood flow(rCBF)at theactivatedfoci in
acomparisonofFRSandHCSacrossthe12subjectsundereachcondition(see
Table 1). A:brainstem;B:leftthalamus.EachPvalue indicates the significance
calculated by Fisher’s protected least significant difference (PLSD) following
analysis of variance (ANOVA) without correction for multiple pixel-based com-
parisons. Both the brain stem and the left thalamus showed an increase in rCBF
during the FRS presentation compared with all other conditions (baseline, LCS,
and HCS). rCBF decreased during HCS compared with the baseline. On the other
hand, presentation of LCS did not lead to any change in rCBF compared with the
baseline values. C:projectionimagesofSPMcontrastingFRSwiththebaseline(in
the same format as in Fig. 4A)withalowerthreshold(Z.1.64 and P,0.05 with
correction for multiple comparisons).
3554 OOHASHI ET AL.
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stationary HFCs (i.e., FRS) significantly enhanced the power
of the spontaneous EEG activity of alpha range when com-
pared with the same sound lacking HFCs (i.e., HCS). In par-
allel experiments employing exactly the same stimulus and
methods, PET rCBF measurement revealed that FRS activated
the deep-lying brain structures, including the brain stem and
thalamus, compared with HCS. In addition, subjective evalu-
ation by questionnaire revealed that FRS intensified the sub-
jects’ pleasure to a significantly greater extent than HCS did.
We conclude, therefore, that inaudible high-frequency sounds
with a nonstationary structure may cause non-negligible effects
on the human brain when coexisting with audible low-fre-
quency sounds. We term this phenomenon the “hypersonic
effect” and the sounds introducing this effect the “hypersonic
sound.” We do not think that the hypersonic effect is specific
to the sound material used in the present study because we
previously confirmed, by EEG analysis, that the same effect
can be introduced by different sound sources containing a
significant amount of nonstationary HFCs (e.g., Oohashi et al.
1994).
In contrast to the fact that the primary auditory cortex in the
bilateral temporal lobes was similarly activated by FRS and
HCS, it is noteworthy that the brain stem and thlamic foci
activated by the presentation of FRS showed a decrease in
rCBF when HCS was presented, as shown in Fig. 5. This
finding suggests that these areas may not belong to the con-
ventional auditory perception system. Moreover, it is the com-
bined presentation of HFCs and LFCs, not HFCs alone, that
specifically induces the enhancement of alpha-EEG and acti-
vation in the deep-lying structures. We interpret these findings
FIG. 6. Normalized alpha-EEG potentials under each experimental condi-
tion and their correlation with adjusted rCBF equivalent values as measured by
positron emission tomography (PET) scanning. A: grand average normalized
alpha-EEG potentials with standard error for 11 subjects. The data of one
subject were excluded because of excessive electrical noise in the EEG.
ANOVA followed by Fisher’s PLSD post hoc test showed a significant main
effect of condition (P,0.05) and a significant increase during the presentation
of FRS compared with HCS (P,0.05). B: the brain areas in which the rCBF
equivalent values were significantly correlated with the alpha-EEG potentials
are shown in a standard format (the same as in Fig. 4A). Maps are illustrated
in a color scale, with the higher Zscore represented in brighter green. C: the
same area as shown in B(green) and the activated area in the comparisons of
FRS and HCS as shown in Fig. 4 (yellow) are superimposed. The same area in
the left thalamus that was activated by FRS was most significantly correlated
with the alpha-EEG potentials. The maximum correlation in the brain was
observed at [x,y,z]5[216, 216, 0] (Zscore 54.30) in the stereotaxic space,
which is the pixel immediately adjacent to the maximally significant local
point observed in the rCBF experiment (FRS vs. HCS).
TABLE 2. Subjective evaluation of sound quality under FRS and
HCS conditions
Element for Evaluation*
Significance
Level (P)† q‡
Soft vs. hard ,0.01 5.33
Reverberant type vs. percussive type ,0.01 5.01
Instruments in balance vs. instruments in imbalance ,0.01 4.57
Comfortable to ears vs. uncomfortable to ears ,0.01 4.44
Rich in nuance vs. lacking in nuance ,0.05 3.63
Lower tone dominant vs. higher tone dominant — 2.25
Thick vs. thin — 1.70
Light vs. heavy — 1.13
Like vs. dislike — 1.12
Finely textured vs. roughly textured — 0.14
* Approximate English equivalents for pairs of Japanese words used for
evaluation of sound quality. The subjects rated sound quality on a scale of 5
(the former) to 1 (the latter). † Pindicates the significance level by which FRS
showed a higher score (more favorable) than HCS for each element. ‡ q
indicates each Student’s interval.
3555BIOLOGICAL EFFECT OF INAUDIBLE HIGH-FREQUENCY SOUNDS
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to mean that the hypersonic effect does not simply result from
a neurophysiological response to isolated frequencies above an
audible range, but from a more complex interaction to which
HFCs and LFCs both contribute.
The alpha rhythm of EEG is considered to occur in relaxed
yet alert subjects and to be sensitive to the subjects’ emotional
as well as arousal state (Drennen and O’reilly 1986; Iwaki et al.
1997). Although there is considerable inter-subject variability
in the amount of alpha rhythm, normal alpha rhythm can be
treated as an intra-individually stable trait in terms of its
test–retest reliability (Fernandez et al. 1993; Gasser et al. 1985;
Kohrman et al. 1989). Although the mechanisms underlying
generation of the alpha rhythm have yet to be fully clarified, an
animal model suggests the involvement of at least the thalamo-
cortical and intracortical networks (Steriade et al. 1990). Our
finding of a significant positive correlation between the rCBF
in the thalamus and the occipital alpha-EEG suggests that the
occipital alpha-EEG may reflect an aspect of activity in deep-
lying structures, including the thalamus. This finding does not
contradict our earlier report (Sadato et al. 1998), which did not
address the physiological effect of inaudible high-frequency
sounds.
Explanation of the discrepancy between the present and
previous studies
The fact that we used an entire piece of natural music
lasting 200 s as sound stimuli instead of short fragments of
sounds might explain the discrepancy between our findings
and those of previous studies carried out around 1980 to
determine the format for digital audio CDs (e.g., Muraoka et
al. 1978; Plenge et al. 1979), which concluded that the
presence of sounds containing a frequency range above 15
kHz was not recognized as making a difference in sound
quality. The CCIR (1978), and the current International
Telecommunication Union–Radio communication sector
(ITU-R 1997), have recommended that sound samples used
for the comparison of sound quality should not last longer
than 15–20 s (CCIR 1978; ITU-R 1997), and that intervals
between sound samples should be about 0.5–1 s (CCIR
1978) because of short-term human memory limitations.
Most of the previous psychological experiments, including
the studies by Muraoka et al. (1978) and Plenge et al.
(1979), were carried out using, essentially, the sound pre-
sentation method recommended by the CCIR. We also ex-
amined the psychological evaluation using the same mate-
rial and sound presentation system as was used for the
present study, but followed the presentation method recom-
mended by the CCIR, and confirmed that the results were in
agreement with the studies by Muraoka et al. (1978) and
Plenge et al. (1979).
In our EEG and PET experiments, we focused on physio-
logical brain responses and objectively evaluated the effect of
the combination of audible sounds and inaudible HFCs on
brain activity, independent of a subjective evaluation of sound
quality. According to the EEG measurements, the occipital
alpha-EEG gradually increased over several tens of seconds
after the exposure to FRS began, and this increase persisted for
several tens of seconds after FRS ended. These findings sug-
gest that the phenomenon that we call the hypersonic effect
may involve some neuronal mechanisms that can be charac-
terized by delay and persistence for as long as several tens of
seconds. It seems, therefore, that an exposure to FRS shorter
than 20 s, as recommended by the CCIR and ITU-R, may be
insufficient to introduce a physiological effect. By the same
token, a short exposure to HCS following FRS with a short
interval of 0.5–1 s may not be enough to withdraw physiolog-
ical effects, if any, induced by the preceding FRS. Based on
this physiological consideration, we performed our psycholog-
ical experiment with sound materials of longer duration. The
results showed a significant difference between FRS and HCS
in some elements of sound quality. That difference was evident
despite the fact that a long presentation time should make it
more difficult to detect subtle differences between two mate-
rials due to the limitation of short-term auditory memory. Our
findings suggest the possibility that the results of the previous
psychological studies may not be valid in a situation where
humans are continuously exposed to auditory stimuli such as
music or environmental sounds.
Hypothetical explanation of neuronal mechanisms of the
hypersonic effect
From an authentic view of human auditory physiology, it
is not straightforward to explain the neuronal basis of the
hypersonic effect characterized by the fact that HFCs
showed significant physiological and psychological effects
on listeners only when presented with audible sounds. Al-
though how inaudible HFCs produce a physiological effect
on brain activity is still unknown, we need to consider at
least two possible explanations. The first is that HFCs might
change the response characteristics of the tympanic mem-
brane in the ears and produce more realistic acoustic per-
ception, which might increase pleasantness. However, this
hypothesis is unlikely to explain the fact that the subjects
who showed significant hypersonic effect were not neces-
sarily aware of the difference of sounds in a conscious
manner. An alternative explanation is that HFCs might be
conveyed through pathways distinct from the usual air-
conducting auditory pathway and therefore might affect the
CNS, including the deep-lying brain structure. It was re-
ported that the vibratory stimulus of ultrasound modulated
by the human voice activated the primary auditory cortex
(Hosoi et al. 1998) and was successfully recognized by
people with normal hearing as well as those whose hearing
is totally impaired (Lenhardt et al. 1991). Recently evidence
has accumulated that stimuli outside the frequency and
amplitude boundaries of an auditory neuron’s receptive field
can influence responses to stimuli inside the classical recep-
tive field determined with pure tone stimuli (e.g., Schulze
and Langner 1999). This modulatory interaction between
inside and outside the classical auditory receptive range is
noteworthy. However, we cannot conclude that the neural
mechanisms incorporating ultrasound hearing, including the
bone-conducting auditory pathway, are the system respon-
sible for the hypersonic effect, which involves the brain
stem and thalamus. These regions showed decreased activity
compared with the baseline when HCS was presented and
thus may not belong to the conventional auditory perception
system. Therefore participation of nonauditory sensory sys-
tems such as somatosensory perception also needs to be
considered in further investigations.
3556 OOHASHI ET AL.
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We pay special attention to the fact that FRS is accompanied
by an intensification of the pleasure with which the sound is
perceived, and envisage the participation of the neuronal path-
ways in connection with reward-generating systems (Cooper
1991; Olds and Milner 1954; Wise 1980), which effectively
control various aspects of human behavior. The present PET
result does not seem to be contradictory to this view. The brain
stem contains distinct neuronal groups that are the major
source of monoaminergic projections to various parts of the
brain (Nieuwenhuys et al. 1988; Role and Kelly 1991). These
monoaminergic systems are thought to be the primary sites for
the action of many stimulants and antipsychotic drugs (Kandel
1991). The rCBF in this area was reported to increase after oral
amphetamine challenge (Devous et al. 1995). These fibers lie
in the medial forebrain bundle, which is considered to be
intimately connected with registering pleasurable sensations
(Thompson 1988). The monoaminergic neurons or the opioid-
peptidergic neurons in the deep-lying brain structures are char-
acterized by long neurotransmitter residence times at synaptic
junctions and the participation of an intracellular messenger in
the postsynaptic neurons (Hartzell 1981; Kehoe and Marty
1980; Schwartz and Kandel 1991). These characteristics seem
to support the delay and persistence of the hypersonic effect
observed in the present EEG experiments. The activation of the
thalamus may reflect its function as part of the limbic system,
which also plays an important role in the control of emotions
(LeDoux 1993; Vogt and Gabriel 1993). It might also reflect
the role of the thalamus in gating sensory input to the cortex
(Andreasen et al. 1994). We speculate that changes of activity
in the deep-lying structure may introduce some modulatory
effects on the perception of audible sounds and thus control
some aspects of human behavior. We have incorporated these
features in the two-dimensional sound perception model: sound
frequencies in the audible range function as a message carrier
and frequencies above the audible range, together with those in
the audible range, function as a modulator of sound perception
through the brain systems, including the reward-generating
system. Further investigations are clearly required to examine
this hypothetical model.
In conclusion, our findings that showed an increase in alpha-
EEG potentials, activation of deep-seated brain structures, a
correlation between alpha-EEG and rCBF in the thalamus, and
a subjective preference toward FRS, give strong evidence
supporting the existence of a previously unrecognized response
to high-frequency sound beyond the audible range that might
be distinct from more usual auditory phenomena. Additional
support for this hypothesis could come from future noninvasive
measurements of the biochemical markers in the brain such as
monoamines or opioid peptides.
We thank the staff of the Kyoto University PET Center for valuable
contributions to this work; Dr. Yoshio Yamasaki, Waseda University, for the
use of his recently developed signal processing system; the Yamashiro Institute
of Science and Culture for recording the sound sources; Dr. Norihiro Sadato,
National Institute for Physiological Sciences, for valuable comments on an
early version of the manuscript; and Dr. Masako Morimoto, Japan Society for
the Promotion of Science, for valuable technical support.
This work was supported in part by the Japan Ministry of Education, Science
and Culture, through the Grants-in-Aid for Scientific Research (A) (09490031)
to T. Oohashi, on Priority Areas to H. Shibasaki, and for International Scien-
tific Research Program (10041144) to T. Oohashi, and by the Japan Society for
the Promotion of Science through the Research for the Future Program JSPS-
RFTF 97L00201 to H. Shibasaki.
Address for reprint requests: T. Oohashi, Dept. of KANSEI Brain Science,
ATR Human Information Processing Laboratories, 2-2 Hikaridai, Seika-cho,
Soraku-gun, Kyoto 619-0288, Japan.
Received 15 November 1999; accepted in final form 6 March 2000.
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