Scientists at University College London and Imperial College London have shown how the brain makes sense of speech in a noisy environment, such as a pub or in a crowd. The research suggests that various regions of the brain work together to make sense of what it hears, but that when the speech is completely incomprehensible, the brain appears to give up trying.
The study was intended to simulate the everyday experience of people who rely on cochlear implants, a surgically-implanted electronic device that can help provide a sense of sound to a person who is profoundly deaf or who has severe hearing problems.
Using MRI scans of the brain, the researchers, funded by the Wellcome Trust and the Medical Research Council, identified the importance of one particular region, the angular gyrus, in decoding distorted sentences. The findings are published in the Journal of Neuroscience.
In an ordinary setting, where background noise is minimal and a person's speech is clear, it is mainly the left and right temporal lobes that are involved in interpreting speech. However, the researchers have found that when hearing is impaired by background noise, other regions of the brain are engaged, such as the angular gyrus, the area of the brain also responsible for verbal working memory - but only when the sentence is predictable.
"In a noisy environment, when we hear speech that appears to be predictable, it seems that more regions of the brain are engaged," explains Dr Jonas Obleser, who did the research whilst based at the Institute of Cognitive Neuroscience (ICN), UCL. "We believe this is because the brain stores the sentence in short-term memory. Here it juggles the different interpretations of what it has heard until the result fits in with the context of the conversation."
The researchers hope that by understanding how the brain interprets distorted speech, they will be able to improve the experience of people with cochlear implants, which can distort speech and have a high level of background noise.
"The idea behind the study was to simulate the experience of having a cochlear implant, where speech can sound like a very distorted, harsh whisper," says Professor Sophie Scott, a Wellcome Trust Senior Research Fellow at the ICN. "Further down the line, we hope to study variation in the hearing of people with implants - why is it that some people do better at understanding speech than others. We hope that this will help inform speech and hearing therapy in the future."