Training can improve the ability to understand speech in background noise
23 May 2008
The brain is as important as the ear for hearing. In addition to receiving and interpreting sound signals that have been converted by the inner ear into electrical impulses, the brain also sends signals back to earlier stages in the hearing pathway. One such stage is the brainstem – between the inner ear and the mid-brain – is thought to control the amplification of sounds in the inner ear.
Researchers Jessica de Boer and Roger Thornton at the MRC Institute of Hearing Research in Southampton have been studying the role this pathway plays in learning to pick out sounds from background noise.
A group of normal hearing adults was trained over a period of five days to discriminate speech sounds embedded in noise played to the right ear. The aim was to find out what happened to the signals sent from the brain to the ear over the period of training. This can be measured by recording evoked otoacoustic emissions (EOAEs), which are small sounds emitted by healthy ears in response to sound stimulation. Their size is controlled by signals from the brain; when a sound is played to one ear, the EOAE in the opposite ear is suppressed.
EOAEs were recorded from the right ears of both the trained group and another group who received no training, while a noise was played in their left ears. Results showed a correlation between improvement at the discrimination task and a change in the level of EOAE suppression: those who improved most had the greatest increase in suppression, whereas there was no change for those who showed no improvement. This is an important finding as it points to a mechanism in the brainstem that may, in some people, be responsible for trouble understanding speech in background noise and suggests that, if so, they may benefit from auditory training. This could be particularly relevant for children with language-based learning problems, who are thought to have brainstem-related difficulty in understanding speech in noise.
