Our ears still hear sounds after the sounds have stopped. These are the findings of researchers from Oregon Hearing Research Center in the US, through a cooperation with their colleagues from the Swedish Karolinska Institute in Stockholm. The researchers have reached these conclusions through a study of guinea pigs.
Before, it was assumed that the hair cells in the inner ear only moved when they were directly affected by sounds. The study has, however, shown that there are after-vibrations in the inner ear. How long the small hairs move after the sound depends on the strength of the sound and its frequency.
The goal of these after-vibrations, according to the research team, could be to function as a form of mechanical archive or short-term memory in the inner ear.
Our ability to understand
The researchers believe that the after-vibrations can have an effect on our ability to perceive sounds and language.
â€The ability to detect brief gaps in an ongoing stimulus is critical for speech recognition: gaps need to be longer than a minimal interval to be perceived. To the extent that after-vibrations excite the auditory nerve fibres, they may explain part of the difficulty in detecting such gaps,â€ explains Dr. Alfred L. Nuttall from the Oregon Hearing Research Center.
A minor hearing loss can lead to a substantial reduction in after-vibrations. That means, that it is more difficult to perceive the small gaps in an ongoing stimulus, which in turn have a bearing on speech recognition.
The researchers conclude, that there will need to be further study before the significance and purpose of the hair cells and after-vibrations can be established.
A complicated hearing process
The inner ear contains a structure called the cochlea that serves as the organ of hearing. The cochlea is a coiled, fluid filled structure that contains a basilar membrane and associated hair cells. Essentially, sound-evoked vibrations of the basilar membrane are sensed by the hair cells which in turn convey auditory information to the nervous system. Some hair cells respond to basilar membrane vibrations by producing forces that increase hearing sensitivity and frequency selectivity through mechanisms that are not completely understood.
Source: Hörakustik nr. 6 2011;