Does airborne ultrasound lead to activation of the auditory cortex?

Abstract

As airborne ultrasound can be found in many technical applications and everyday situations, the question as to whether sounds at these frequencies can be heard by human beings or whether they present a risk to their hearing system is of great practical relevance. To objectively study these issues, the monaural hearing threshold in the frequency range from 14 to 24 kHz was determined for 26 test subjects between 19 and 33 years of age using pure tone audiometry. The hearing threshold values increased strongly with increasing frequency up to around 21 kHz, followed by a range with a smaller slope toward 24 kHz. The number of subjects who could respond positively to the threshold measurements decreased dramatically above 21 kHz. Brain activation was then measured by means of magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) and with acoustic stimuli at the same frequencies, with sound pressure levels (SPLs) above and below the individual threshold. No auditory cortex activation was found for levels below the threshold. Although test subjects reported audible sounds above the threshold, no brain activity was identified in the above-threshold case under current experimental conditions except at the highest sensation level, which was presented at the lowest test frequency.

A longitudinal, randomized experimental pilot study to investigate the effects of airborne ultrasound on human mental health, cognition, and brain structure

“…In general, however, the investigation of brain effects of ultrasound is in its infancy. When it comes to ‘pure’ ultrasound, an fMRI study found no evidence of auditory cortex activation for airborne ultrasound below the hearing threshold 3 , as already documented in an earlier MEG (magnetencephalography) that did not find auditory evoked magnetic field changes in response to frequencies at or above 20 kHz 9 .…”

“…As a rule of thumb, the term ultrasound comprises frequencies above the upper human hearing threshold (∼20 kHz)—however depending upon the sound pressure level (SPL), even higher frequencies are audible (or rather otherwise ‘perceivable’). Hearing thresholds could be determined at frequencies above 20 kHz 1 – 3 demonstrating an auditory perception at least in a subset of test persons. The experiments showed the highly individual properties of this perception process.…”

Effects of a Synthetic Clarinet Bore Liner on Spectral Centroid and Fundamental Frequency Error

“…Given the high interindividual variability of thresholds for sounds in the ultrasonic frequency range even for young normal hearing listeners [16,17,54], it is nonetheless possible that our sample did not include enough participants with sufficient sensitivity to observe major negative reactions to US exposure. Indeed only one of our participants was able to detect the presentation of the US tone, but this participant did not show any negative subjective reactions.…”

Exposure to High-Frequency Sound and Ultrasound in Public Places: Examples from Zurich, Switzerland

“…If an exposure to VHFS/US signals is able to trigger activation of the auditory cortex in humans was investigated by Kühler et al [20]. Although at first reading this study might be taken as suggesting that an activation of the auditory cortex could be also not detected in a study investigating neural effects correlated to a VHFS/US signals exposure, the study did not compare like with like: to quote the authors "stimuli at frequencies of 16.9 kHz, 19.1 kHz, 20.7 kHz and 24.2 kHz, being identical to the stimuli used in the hearing threshold experiments, were presented at two different SPLs: namely, at 2 dB below the individual hearing threshold of the subject (−2 dB SL) and at 5 dB above this hearing threshold (5 dB SL).…”

“…Although at first reading this study might be taken as suggesting that an activation of the auditory cortex could be also not detected in a study investigating neural effects correlated to a VHFS/US signals exposure, the study did not compare like with like: to quote the authors "stimuli at frequencies of 16.9 kHz, 19.1 kHz, 20.7 kHz and 24.2 kHz, being identical to the stimuli used in the hearing threshold experiments, were presented at two different SPLs: namely, at 2 dB below the individual hearing threshold of the subject (−2 dB SL) and at 5 dB above this hearing threshold (5 dB SL). A reference stimulus with a frequency of 14 kHz at 20 dB above the individual hearing threshold (20 dB SL) was used to compare the brain responses evoked by the ultrasound with a well-known brain response in the audible frequency range" [20]. Therefore, the 14 kHz control signal was had a considerably higher sensation level than the VHFS/US test signal, which might explain why the response from a VHFS/US did not elicit a comparable response in the auditory cortex.…”