Cortical Alpha Oscillations Predict Speech Intelligibility

By Andrew Dimitrijevic, Ph.D.

Hearing Health Foundation Emerging Research Grants recipient Andrew Dimitrijevic, Ph.D., and colleagues recently published “Cortical Alpha Oscillations Predict Speech Intelligibility” in the journal Frontiers in Human Neuroscience.

The scientists measured brain activity that originates from the cortex, known as alpha rhythms. Previous research has linked these rhythms to sensory processes involving working memory and attention, two crucial tasks for listening to speech in noise. However, no previous research has studied alpha rhythms directly during a clinical speech in noise perception task. The purpose of this study was to measure alpha rhythms during attentive listening in a commonly used speech-in-noise task, known as digits-in-nose (DiN), to better understand the neural processes associated with speech hearing in noise.

Fourteen typical-hearing young adult subjects performed the DiN test while wearing electrode caps to measure alpha rhythms. All subjects completed the task in active and passive listening conditions. The active condition mimicked attentive listening and asked the subject to repeat the digits heard in varying levels of background noise. In the passive condition, the subjects were instructed to ignore the digits and watch a movie of their choice, with captions and no audio.

Two key findings emerged from this study in regards to the influence of attention, individual variability, and predictability of correct recall.

First, the authors concluded that the active condition produced alpha rhythms, while passive listening yielded no such activity. Selective auditory attention can therefore be indexed through this measurement. This result also illustrates that these alpha rhythms arise from neural processes associated with selective attention, rather than from the physical characteristics of sound. To the authors’ knowledge, these differences between passive and active conditions have not previously been reported.

Secondly, all participants showed similar brain activation that predicted when one was going to make a mistake on the DiN task. Specifically, a greater magnitude in one particular aspect of alpha rhythms was found to correlate with comprehension; a larger magnitude on correct trials was observed relative to incorrect trials. This finding was consistent throughout the study and has great potential for clinical use.

Dimitrijevic and his colleagues’ novel findings propel the field’s understanding of the neural activity related to speech-in-noise tasks. It informs the assessment of clinical populations with speech in noise deficits, such as those with auditory neuropathy spectrum disorder or central auditory processing disorder (CAPD).

Future research will attempt to use this alpha rhythms paradigm in typically developing children and those with CAPD. Ultimately, the scientists hope to develop a clinical tool to better assess listening in a more real-world situation, such as in the presence of background noise, to augment traditional audiological testing.

Andrew Dimitrijevic, Ph.D., is a 2015 Emerging Research Grantee and General Grand Chapter Royal Arch Masons International award recipient. Hearing Health Foundation would like to thank the Royal Arch Masons for their generous contributions to Emerging Research Grants scientists working in the area of central auditory processing disorders (CAPD). We appreciate their ongoing commitment to funding CAPD research.

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