Currently the emission of infrasound at work places and in daily life is becoming more pronounced because of an increasing number of sources as wind parks, pumps of renewable energy systems or the permanently increasing road traffic. The evaluation of also increasing complaints is accompanied with a still limited understanding about the perception mechanisms at these frequencies hindering the development of efficient assessment methods or the definition of exposure limits. New insight into mechanisms of perception can be expected from neuroimaging techniques providing objective information about noise processing in the brain.
In the framework of a project funded by the European Union within the European Metrology Research Programme (EMRP) the brain response following the stimulation with sounds at infrasound frequencies was investigated. Using functional magnetic resonance imaging (fMRI) a significant response was detected which was localised within the auditory cortex and which was present down to the lowest frequency presented (8 Hz). First evidence indicates that the signal strength of the blood-oxygen-level dependent (BOLD) signal shows a minimum at 20 Hz and a further investigation of BOLD-signal in dependence on the loudness was carried out.
Since fMRI offers high spatial but low temporal resolution magnetoencephalography was employed as an alternative method. Significant brain responses could be detected down to a frequency of 20 Hz. By the aid of a theoretical model the dipoles were localised also within the auditory cortex. At lower frequencies sources could be identified but no clear assignment to physiological regions was possible. Within the test subject ensemble one person, however, showed a significant response also at 8 Hz.