Inaudible high-frequency sounds affect brain activity: Hypersonic effect

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."