Although it is generally accepted that humans cannot perceive sounds in the
frequency range above 20 kHz, the question of whether the existence of suc
h "inaudible" high-frequency components may affect the acoustic perception
of audible sounds remains unanswered. In this study, we used noninvasive ph
ysiological measurements of brain responses to provide evidence that sounds
containing high-frequency components (HFCs) above the audible range signif
icantly affect the brain activity of listeners. We used the gamelan music o
f Bali, which is extremely rich in HFCs with a nonstationary structure, as
a natural sound source, dividing it into two components: an audible low-fre
quency component (LFC) below 22 kHz and an HFC above 22 kHz. Brain electric
al activity and regional cerebral blood flow (rCBF) were measured as marker
s of neuronal activity while subjects were exposed to sounds with various c
ombinations of LFCs and HFCs. None of the subjects recognized the HFC as so
und when it was presented alone. Nevertheless, the power spectra of the alp
ha frequency range of the spontaneous electroencephalogram (alpha-EEG) reco
rded from the occipital region increased with statistical significance when
the subjects were exposed to sound containing both an HFC and an LFC, comp
ared with an otherwise identical sound from which the HFC was removed (i.e.
, LFC alone). In contrast, compared with the baseline, no enhancement of al
pha-EEG was evident when either an HFC or an LFC was presented separately.
Positron emission tomography measurements revealed that, when an HFC and an
LFC were presented together, the rCBF in the brain stem and the left thala
mus 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 significantly with the rC
BF in the left thalamus. Psychological evaluation indicated that the subjec
ts 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 unreco
gnized response to complex sound containing particular types of high freque
ncies above the audible range. We term this phenomenon the "hypersonic effe
ct."