People are able to perceive words and sentences in noisy environments thanks to rapid and dynamic changes in the auditory cortex of the brain, show two papers published this week in Nature Communications. The ability to focus on one conversation in a noisy room is known as the cocktail party effect. One group finds that when certain parts of the words are replaced by noise, a region in the auditory cortex fills in the missing sound in real time. A second team finds that rapid changes in the auditory cortex due to prior experience of a phrase can allow people to understand words and sentences that are initially completely unintelligible because of the presence of noise.
Edward Chang and colleagues developed a set of words that differed in a single critical letter / sound (for example, ‘faster’ versus ‘factor’). When they replaced the critical sound with noise and play it to five people, the participants reported hearing one word or the other on a trial-to-trial basis. While the participants listened to these stimuli, the direct activity of cortical neurons was recorded. The authors found that in the superior temporal auditory cortex, neural responses to the noise interrupted stimulus closely tracked the neural responses to the perceived original words, indicating that the auditory cortex fills in the missing sound in real time. Interestingly, the team also found that neural activities in higher cognitive areas in the brain before the onset of the noise could predict the word that the participant would report hearing. This observation suggests that internal neural states greatly bias and influence subsequent speech perception.
Christopher Holdgraf and colleagues investigated how experience of language rapidly changes how speech signals are processed by the brain. They asked seven patients who had been implanted with electrodes (for epilepsy treatment) to listen to recorded sentences. Some of these sentences were recorded with noise, so were initially unintelligible, and some were clear. When patients heard the noisy sentences first, they could not understand them and the brain treated the signal like noise. However, when patients heard the clear sentences first, the auditory cortex changed its activity, to enhance the speech signal. This change, in turn, changed the way the noisy stimuli were processed, enhancing the speech signal to such an extent that the participants could easily understand what was previously incomprehensible. These new findings show that the human auditory cortex changes rapidly in response to experience, and explains how these dynamics perceptually enhance speech.