Scientists have established how people succeed in perceiving words and have conversations in noisy and crowded environments. This ability, known as the 'cocktail party effect', might be made possible by rapid and dynamic changes in the auditory cortex, which clarifies unintelligible phrases.
In two papers published in Nature Communications, scientists have investigated the 'cocktail party effect' further to pinpoint how our brain allows us to focus on one particular stimulus – such as a conversation – while filtering out a range of other noisy stimuli.
They have focused on studying the way the auditory cortex in the temporal lobe of the brain functions when people are in a noisy environment like a street, a party or a restaurant.
The first study, led by Edward Chang from the University of California, San Francisco, showed that when parts of the words in a conversation you are trying to focus on are covered with other noises, the auditory cortex steps in to replace the missing sounds.
The second study, led by Christopher Holdgraf from the University of California, Berkley, identified rapid changes in the auditory cortex after people heard a phrase, which allowed them to understand if they hear it again in a noisy context.
Factor or faster?
Chang's team developed word stimuli that differed only in a phoneme – 'faster' and 'factor' or 'babies' and 'rabies' for example – which the five trial participants had to listen to. Below is an example of the type of recorded words that were played out to them.
Next, these participants heard the same stimuli again, but with the critical phoneme replaced with broadband noise, so as to mimic what would happen in a noisy environment. The brain appeared to compensate for the noise, replacing the missing phoneme and allowing people to hear one of two closely-related words.
The experience was repeated over the course of different trials, with participants reporting which words they had heard.
During these trials, the scientists also recorded direct cortical activity in the brain of participants. They showed that the superior temporal auditory cortex was activated in the same way whether participants heard the stimuli with or without noise interruption.
This suggested that the auditory cortex filled in the missing sound in real time. It may therefore be central to the 'cocktail party effect'.
Quick changes in auditory cortex
The second research into the 'cocktail party effect' focused on the case of seven patients who had been implanted with electrodes as an epilepsy treatment. The scientists made them listen to the same sentences twice, the difference being that on one occasion the sentence was clear and the other it was unintelligible because it had been recorded in a noisy environment.
Some participants started by listening to the the noisy sentences first and the brain activity picked up by the electrodes showed their brain only processed it as incomprehensible noise. However, another group started by listening to the clear sentence. When the noisy sentence was played, it appeared to be processed as speech by the auditory cortex – the participants were able to easily understand what was previously incomprehensible.
This suggested that changes rapidly occur in the auditory cortex in response to experience to enhance speech and make it more intelligible even when it is surrounded by noise.
What was somewhat surprising and really interesting about this study is that we could show this more subtle pattern of how your brain interacts with sounds in speech, and that this pattern changes so quickly and automatically. Most neuroscience studies tend to just ask 'did brain activity in a given region increase or decrease?' and this study was trying to answer a much more complex question about how your brain tracked features of the sound over time", lead author Chris Holdgraf said.