“The sound of speech has two components in the inner ear. One component consists of high frequency variations in the sound. The other component, known as the ‘envelope’, describes the outermost shape of the sound of speech”, says Anders Fridberger, professor of neuroscience at Linköping University, and one of two principal authors of the article.
Anders Fridberger Photo credit Thor BalkhedThe envelope, which varies relatively slowly, can be considered as the overall structure of the speech signal. Researchers have previously shown that the envelope is most important for understanding what someone is saying.
“But until now we have not known how the inner ear decodes the envelope of the speech signals. This is the new result we are presenting”, says Anders Fridberger.
The inner ear produces tiny electrical currents whenever sound enters – it converts sound to electrical impulses. These are led through the auditory nerve to the brain. By placing small electrodes into the ear canal of experimental subjects, and placing electrodes close to the cochlea of patients who have undergone surgery, the researchers have been able to record the way in which the inner ear codes speech-like sound.
Characteristic electrical signalsLiU researchers in collaboration with an international research team are investigating how the inner ear functions. Photo credit Thor BalkhedThe results have led the researchers to the understanding that speech gives rise to a particular form of electrical signals sent to the brain. These signals differ from those that arise in response to other forms of sound.
“This is both remarkable and surprising”, says Anders Fridberger.
Understanding inner ear function
The discovery is an important addition to our understanding of how the inner ear functions. The inner ear is embedded in a large bony structure, which makes it difficult to access and study it. Thus, diagnosing the exact location of damage to the inner ear is currently difficult.
“We believe that our results will improve diagnostic procedures for various hearing impairments, something that is sorely needed. Much remains to be done, however”, cautions Anders Fridberger.
Translation: George Farrants