Infrasound sensation is mediated by intracochlear electrical potentials

Sound below 16 Hz, commonly referred to as infrasound, has no tonality and is often regarded as inaudible despite being clearly perceivable at sufficient intensity. Perception mechanisms have remained elusive. One factor decreasing hearing sensitivity towards the lowest sound frequencies is the velocity-coupled sound input to inner hair cells (IHCs), which convert cochlear mechanical vibrations into neural signals. We show that in the infrasound range, the velocity of the mechanical stimulus is reduced to such an extent that displacement-coupled outer hair cells (OHCs) start to be involved in the neural excitation, without mechanical activation of IHCs. Combining non-invasive methods in humans with a model of electrical cochlear excitation, we show further that OHC-generated electrical potentials act on the IHC cell membrane and so cause synaptic release that leads to auditory sensation. This mechanism explains perceptual characteristics specific to infrasound, such as the shallow slope of sensation threshold curves below 16 Hz and the abnormal growth of loudness with only small increases in sound pressure. This new physiological insight might help to understand globally spread complaints about very low-frequency environmental sounds.