Does airborne ultrasound lead to activation of the auditory cortex?

Validity of the 1984 Interim Guidelines on Airborne Ultrasound and Gaps in the Current Knowledge

ABSTRACT

Airborne ultrasound is used for various purposes both in industrial and public settings, as well as being produced as a by-product by a range of sources. The International Radiation Protection Association (IRPA) published interim guidelines on limiting human exposure to airborne ultrasound in 1984, based on the limited scientific evidence that was available at that time. In order to investigate whether research since 1984 requires the development of revised exposure guidelines we considered (a) within the context of ultrasound exposure the relevance to health of the biological endpoints/mechanisms listed in the IRPA guidelines, (b) the validity of the exposure limits, and (c) whether there are biological endpoints/mechanisms not covered in the guidelines. The analysis of the available evidence showed that the biological endpoints that form the basis of the guidelines are relevant to health and the guidelines provide limits of exposure based on the evidence that was available at the time. However, the IRPA limits and their associated dosimetry were based on limited evidence, which may not be considered as scientifically substantiated. Further, there is no substantiated evidence of biological endpoints/mechanisms not covered by the IRPA guidelines. These two observations could mean that IRPA's limits are too low or too high. Research since the IRPA guidelines has made some improvements in the knowledge base, but there are still significant data gaps that need to be resolved before a formal revision of the guidelines can be made by ICNIRP, including research needs related to health outcomes and improved dosimetry. This statement makes a number of recommendations for future research on airborne ultrasound.

Applying appropriate frequency criteria to advance acoustic behavioural guidance systems for fish

Deterrents that use acoustics to guide fish away from dangerous areas depend on the elicitation of avoidance in the target species. Acoustic deterrents select the optimum frequency based on an assumption that highest avoidance is likely to occur at the greatest sensitivity. However, such an assumption may be unfounded. Using goldfish (Carassius auratus) as a suitable experimental model, this study tested this as a null hypothesis. Under laboratory conditions, the deterrence thresholds of individual goldfish exposed to 120 ms tones at six frequencies (250-2000 Hz) and four Sound Pressure Levels (SPL 115-145 dB) were quantified. The deterrence threshold defined as the SPL at which 25% of the tested population startled was calculated and compared to the hearing threshold obtained using Auditory Evoked Potential and particle acceleration threshold data. The optimum frequency to elicit a startle response was 250 Hz; different from the published hearing and particle acceleration sensitivities based on audiograms. The difference between the deterrence threshold and published hearing threshold data varied from 47.1 dB at 250 Hz to 76 dB at 600 Hz. This study demonstrates that information obtained from audiograms may poorly predict the most suitable frequencies at which avoidance behaviours are elicited in fish.

Air-conducted ultrasound below the hearing threshold elicits functional changes in the cognitive control network

Air-conducted ultrasound (> 17.8 kHz; US) is produced by an increasing number of technical devices in our daily environment. While several studies indicate that exposure to US in public spaces can lead to subjective symptoms such as 'annoyance' or 'difficulties in concentration', the effects of US on brain activity are poorly understood. In the present study, individual hearing thresholds (HT) for sounds in the US frequency spectrum were assessed in 21 normal-hearing participants. The effects of US were then investigated by means of functional magnetic resonance imaging (fMRI). 15 of these participants underwent three resting-state acquisitions, two with a 21.5 kHz tone presented monaurally at 5 dB above (ATC) and 10 dB below (BTC) the HT and one without auditory stimulation (NTC), as well as three runs of an n-back working memory task involving similar stimulus conditions (n-ATC, n-BTC, n-NTC). Comparing data gathered during n-NTC vs. fixation, we found that task performance was associated with the recruitment of regions within the cognitive control network, including prefrontal and parietal areas as well as the cerebellum. Direct contrasts of the two stimulus conditions (n-ATC & n-BTC) vs. n-NTC showed no significant differences in brain activity, irrespective of whether a whole-brain or a region of interest approach with primary auditory cortex as the seed was used. Likewise, no differences were found when the resting-state runs were compared. However, contrast analysis (n-BTC vs. n-ATC) revealed a strong activation in bilateral inferior frontal gyrus (IFG, triangular part) only when US was presented below the HT (p < 0.001, cluster > 30). In addition, IFG activation was also associated with faster reaction times during n-BTC (p = 0.033) as well as with verbal reports obtained after resting-state, i.e., the more unpleasant sound was perceived during BTC vs. ATC, the higher activation in bilateral IFG was and vice versa (p = 0.003). While this study provides no evidence for activation of primary auditory cortex in response to audible US (even though participants heard the sounds), it indicates that US can lead to changes in the cognitive control network and affect cognitive performance only when presented below the HT. Activation of bilateral IFG could reflect an increase in cognitive demand when focusing on task performance in the presence of slightly unpleasant and/or distracting US that may not be fully controllable by attentional mechanisms.

Auditory brainstem responses to high-resolution audio sounds: Effects of anti-alias filters

Despite the superior technical specifications of high-resolution audio in comparison to conventional compact discs (CDs) and digital versatile discs (DVDs), whether and how high-resolution audio leads to an improved subjective listening experience remains unclear. The sound quality of conventional digital formats is deteriorated by temporal blur, which is caused by a high-cut filter applied to avoid aliasing of a digital sound source, resulting in spurious artificial sound energy before and after the actual sound onset. Using sound materials of high-resolution audio grade, this study investigated the effects of filter-induced temporal blur on the auditory neural pathway from the cochlea through the rostral brainstem by measuring the auditory brainstem responses (ABRs) of participants with normal hearing. The participants (N = 24) listened to 0.1-ms clicks, with and without a high-cut filter of 22 kHz or 11 kHz applied to them. The amplitudes and latencies of five ABR waves (<10 ms) revealed no significant effects caused by filtering. The participants could not distinguish between the original and the 22-kHz filtered click sounds, while they could distinguish between the original and the 11-kHz filtered sounds. These results indicate that a CD-level anti-alias filter does not affect initial auditory processing and the sound difference is not discernible, at least for artificial sounds with a simple acoustic structure.

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

Ultrasound-(US) emitting sources are highly present in modern human environments (e.g., movement sensors, electric transformers). US affecting humans or even posing a health hazard remains understudied. Hence, ultrasonic (22.4 kHz) vs. sham devices were installed in participants' bedrooms, and active for 28 nights. Somatic and psychiatric symptoms, sound-sensitivity, sleep quality, executive function, and structural MRI were assessed pre-post. Somatization (possible nocebo) and phasic alertness increased significantly in sham, accuracy in a flexibility task decreased significantly in the verum condition (indicating hastier responses). Effects were not sustained after p-level adjustment. Exploratory voxel-based morphometry (VBM) revealed regional grey matter (rGMV) but no regional white matter volume changes in verum (relative to placebo). rGMV increased in bilateral cerebellum VIIb/Crus II and anterior cingulate (BA24). There were rGMV decreases in two bilateral frontal clusters: in the middle frontal gyri/opercular part of inferior frontal gyrus (BA46, 44), and the superior frontal gyri (BA4 ,6, 8). No brain-behavior-links were identified. Given the overall pattern of results, it is suggested that ultrasound may particularly induce regional gray matter decline in frontal areas, however with yet unclear behavioral consequences. Given the localization of clusters, candidate behavioral variables for follow-up investigation are complex motor control/coordination, stress regulation, speech processing, and inhibition tasks.Trial registration: The trial was registered at NIH www.clinicaltrials.gov , trial identifier: NCT03459183, trial name: SonicBrain01, full trial protocol available here: https://clinicaltrials.gov/ct2/show/NCT03459183 .

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

The public is unknowingly exposed to very high-frequency sound (VHFS; 11.2–17.8 kHz) and ultrasound (US; >17.8 kHz) signals in air in public places, as evidenced by previously published reports. The present report provides evidence for the presence of VHFS/US signals in the air at public places in Zurich, Switzerland. The analysis of the signals measured revealed that they: (i) contain one, two or multiple frequencies; (ii) comprise frequencies ranged from 15.5 kHz to 36.0 kHz; (iii) were either quasi constant in their amplitude or exhibit a clear amplitude modulation; and (iv) were in their characteristics (frequencies, modulation, intensity) specific for each place. Based on the signal characteristic it is likely that the signals are generated by public-address voice-alarm (PAVA) systems. The work presented: (i) documents the presence of VHFS/US signals at public places in Zurich, possibly caused by PAVA systems; and should (ii) show that is easily possibly to measure the signals with an affordable measurement equipment as a “citizen scientist”, and stimulate others also to measure and analyse VHFS/US signals with this citizen scientist approach in other cities worldwide. Due to the possible negative health-related effects of a human exposure to VHFS/US signals, further research is needed to document VHFS/US signals at public places and to evaluate biological effects of this exposure with laboratory studies.