Recognizing Speech in a Noisy Environment

Post by Amanda McFarlan

What's the science?

Recognizing speech in noisy environments is a challenging task that most humans are able to master. However, exactly how the brain is able to do this is not known. Previous studies have shown that speech recognition in the absence of background noise is associated with modulation in the activity of the left ventral medial geniculate body (vMGB) of the auditory thalamus. This modulation of the vMGB was shown to be greater for tasks that required participants to discern speech rather than other stimuli (e.g. the speaker’s voice or sound intensity), suggesting that it is task-dependent. This week in the Journal of Neuroscience, Mihai and colleagues investigated how background noise affects the task-dependent modulation of the left vMGB for speech.

How did they do it?

The authors used ultra-high field functional magnetic resonance imaging (fMRI) to measure brain activity while participants performed two auditory tasks. The first auditory task was the speech task where participants listened to a series of auditory syllables (made up of a vowel-consonant-vowel) and were asked to identify whether each new syllable was different from the previous one. The second auditory task was the speaker task. For this task, participants were asked to determine whether the voice of the person reading the syllables changed from one syllable to the next. The auditory syllables for the speech and speaker tasks were either presented with background noise (noise condition) or without background noise (clear condition).     

What did they find?

The authors found that activity in the left vMGB was increased during the speech task compared to the speaker task in the noise condition. Conversely, activity in the left vMGB was not significantly different for the speech and speaker tasks in the clear condition. The authors also explored whether differences in activity could be detected in other areas of the brain including the right vMGB, the cerebral cortex, and the central nuclei of the inferior colliculi. Their analyses revealed that, unlike in the left vMGB, the noise condition had no detectable influence on activity for the speech vs. speaker tasks in these brain regions.   

What’s the impact?

This study showed that the activation of the left vMGB is stronger when decoding speech compared to identifying a speaker if the listening environment is noisy, suggesting that the auditory thalamus may be particularly important for recognizing speech in the presence of background noise. Together, these findings provide insight into how the brain decodes speech in noisy conditions. Further research in this field may have clinical relevance for the treatment of individuals with difficulties in understanding speech-in-noise, including individuals with autism spectrum disorder, developmental dyslexia, and auditory processing disorders.

Mihai et al. Modulation of the primary auditory thalamus when recognizing speech with background noise. Journal of Neuroscience (2021). Access the original scientific publication here.