By Julia Campbell, Ph.D., Au.D. 

While it is well known that hearing loss degrades speech perception, especially in noisy environments, less is understood as to why some individuals with typical hearing may also struggle with speech perception in noise (SPiN).  

Several factors appear to contribute to SPiN abilities in adults with typical hearing, including the top-down cognitive functions of attention, working memory, and inhibition.  Specifically, inhibition at the cognitive level may be considered as the ability to successfully suppress external auditory information (e.g., background noise) in order to correctly identify and understand the speech signal of interest, requiring the conscious participation of the listener. 

However, inhibition also exists at a bottom-up, sensory stage as an automatic process even when the listener is not actively involved. Inhibitory function at this level is considered to “gate” sensory information leading to cognitive centers. Thus, it is plausible that deficits in sensory inhibition may allow an excess of auditory noise to reach cognitive centers, overwhelming these resources and resulting in poor SPiN performance. 

Therefore, the goal of this study was to determine whether sensory inhibition is related to SPiN ability in listeners with typical hearing, as well as to identify the underlying cortical inhibitory networks in this population. Extended high-frequency thresholds at 10, 12.5, and 16 kilohertz were also assessed to determine if auditory sensitivities at these frequencies might interact with sensory inhibition and SPiN. 

Using high-density EEG (an electroencephalogram with 128 electrodes), we recorded cortical auditory evoked potentials (CAEPs) in response to a sensory gating paradigm in listeners with typical SPiN performance and mild SPiN deficits. In this paradigm, identical sound pairs (S1 and S2) are presented while the participant watches a silent movie and is instructed not to pay attention to the sound. Listeners with typical sensory gating or inhibitory function should show an amplitude suppression and latency decrease of the CAEP S2 response in comparison to the CAEP S1 response as the repetitive sound is deemed non-novel.  

We found this expected response in listeners with typical SPiN, but not in listeners with mild SPiN impairment, consistent with reduced sensory inhibition in this group. In addition, lack of amplitude suppression of the CAEP P2 component significantly correlated with worse SPiN performance, indicating that decreased sensory inhibition during this particular time frame is related to poorer SPiN. 

Elevated extended high-frequency threshold averages correlated with worse SPiN, but were not related to gating function, suggesting that mild loss in this frequency range was not related to decreases in sensory inhibition. 

Finally, using source modeling, we observed that listeners with mild SPiN deficits presented with atypical cortical inhibitory networks underlying the gating response, affecting prefrontal, frontal, and parietal cortical regions. 

Together, these findings suggest that individuals with typical hearing and mild SPiN impairment may present with decreased inhibition at the sensory level, which is reflected in incomplete and atypical activation of cortical inhibitory networks.  

This lack of sensory inhibition may allow extraneous noise to reach cognitive centers and interfere with speech perception. Our laboratory is currently conducting studies to investigate the factors that may affect sensory inhibition in the presence of typical hearing (such as noise exposure). We are also examining the relationship between sensory and cognitive inhibition in listeners with variable SPiN to examine how these levels of auditory processing interact in challenging environments.

Julia Campbell, Ph.D., Au.D., CCC-A, FAAA, is an assistant professor of communication sciences and disorders in the Central Sensory Processes Laboratory at the University of Texas at Austin. Campbell, who is a 2016 Emerging Research Grants scientist generously funded by the Les Paul Foundation, is recruiting participants for this research; email julia.campbell@austin.utexas.edu.