Research paperBrief bursts of infrasound may improve cognitive function – An fMRI study
Introduction
The term ‘infrasound’ (IS) applies to sounds in the very low-frequency range (1 Hz < frequency < 20 Hz), which are ubiquitous in our environment. While IS can be a by-product of natural events (such as the flow of rivers or avalanches), it is emanating in abundance from man-made sources such as jet engines, road traffic and wind turbines. Exposure to IS is frequently reported to be annoying, irritating or disturbing (Kaczmarska and Łuczak, 2007) and over the past decades, a number of adverse health effects have been attributed to IS, ranging from psychological and behavioural alterations to respiratory and cardiovascular disorders (Huang et al., 2003, Ferreira et al., 2006, Pei et al., 2007). More recently, the debate on whether IS should be considered hazardous to human health reached a broader audience, as increasing numbers of people living in close proximity to wind parks reported several potentially IS-related symptoms, including cognitive impairments such as difficulties in concentration and memory loss (Punch and James, 2014). These reports have even motivated the description of a novel clinical condition, the so called “Wind Turbine Syndrome” (WTS) (Pierpont, 2009), in which symptoms are explained as the consequence of an overexposure to low- and very low-frequency noise produced by the turbines. There's an ongoing debate, whether such symptoms can be explained as the result of a communicated nocebo-effect (i.e. triggered by high expectations and shared believes about the harmfulness of low-frequency sounds), or emerge in a bottom-up fashion, in the sense that IS precipitates more global changes of nervous function due to its action via auditory (or even somatosensory) pathways. The cases presented above clearly illustrate that the identification of potentially hazardous health effects of IS is a relevant public health issue and further research is required in order to evaluate, whether protection standards against low frequency noise need to be established (Duck, 2007).
Traditionally, IS frequencies are considered to be too low for the sound to be audible by humans (humans are said to have a hearing range of about 20 Hz–20.000 Hz). Therefore, it has been a widely held view that such frequencies are unable to influence inner ear function. Consequently, neural processes upstream in the central nervous system that could lead to the emergence of adverse psychological effects should also remain unaffected. However, it has been demonstrated in a number of studies that this view fails to recognize the complex physiology that underlies the ears' response to IS (Salt and Hullar, 2010). For example, it has been shown repeatedly that given a high enough sound pressure level, IS can very well be perceived (Robinson and Dadson, 1956, Corso, 1958, Whittle et al., 1972, Yeowart and Evans, 1974, Landstroem et al., 1983, Verzini et al., 1999, Schust, 2004, Møller and Pedersen, 2004). In addition, 2 studies revealed IS-induced changes of the distortion product otoacoustic emissions (DPOAEs) in animals (Marquardt et al., 2007), as well as in normally hearing human participants (Hensel et al., 2007). Since the DPOAE response is generally used as an objective indicator to examine cochlear amplification mediated by the outer hair cells, these findings clearly speak against the traditional view that IS has no influence on inner ear function. A functional magnetic resonance imaging (fMRI) study also showed that monaural acoustical stimulation with tone bursts of 12 Hz leads to a bilateral activation of the primary auditory cortex (Brodmann area 41/42) for sound pressure levels of 110 and 120 dB, while no significant activation during stimulation with 90 dB was observed (Dommes et al., 2009).
Since IS has been associated with cognitive impairments and a growing number of studies linked IS exposure to changes of inner ear and brain function, we set out to address the question, whether IS also affects cognitive processing. We hypothesized that IS would exert a negative influence on performance in an n-back working memory task and that this impairment would be reflected in changes of cortical brain activity, measured via fMRI. We also expected participants, scoring higher on scales in which depression, anxiety or neuroticism was assessed, to be more prone to experience detrimental performance or brain activation effects.
Section snippets
Participants
Thirteen healthy participants (7 female) with a mean age of 23.7 years (SD = 2.9) took part in the study on the basis of informed consent. The study was conducted according to the Declaration of Helsinki, with approval of the local ethics committee. All participants had normal or corrected-to-normal vision and were otologically normal (as assessed by means of the ISO 389-9:2009 questionnaire filled out by all participants). No participant had a history of neurological, major medical, or
Behavioural data
Comparison of performance in the n-back working memory task revealed no significant difference between tone and no-tone blocks. Surprisingly, we observed a trend towards a better performance, namely more correct trials in tone blocks (89.2), compared to no-tone blocks (87.5) (t(12) = 1.86, p = 0.087).
Participant reports
All 13 participants reported that IS stimulation had been perceived and could be clearly distinguished from scanner noise. 3 out of 13 participants reported that IS exposure had a negative effect
Discussion
The main findings of this study can be summed up in the following way: 1. According to our imaging data, brief bursts of IS led an activation of the bilateral primary auditory cortex (Brodmann areas 41, 42). 2. A more thorough analysis revealed that IS exposure did not have a detrimental effect on cortical brain activity during n-back task performance. 3. In contrast to our expectations, we found that IS exposure was associated with a considerable, yet non-significant trend towards an
Conclusion
In spite of the recent progress in research targeting the physiological mechanisms of IS perception, proof for negative implications of IS is hard to come by. In this study, neither our imaging-, nor our behavioural analysis supports the assumption that IS has a negative effect on cognitive processing. At the same time, the trend towards a performance-enhancing effect due to short-term IS exposure adds an entirely new aspect to the discussion about IS-related phenomena. Consequentially, if
Acknowledgements
This study was conducted as part of the EMRP Joint Research Project Metrology for a universal ear simulator and the perception of non-audible sound (EMRP Call 2011 Health HLT01), briefly called the Ears Project. The study has received funding from the European Union on the basis of Decision No 912/2009/EC.
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