How the Brain Filters Out Tinnitus Signals in Mild Cases
By Julia Campbell, Ph.D., Au.D.
In our previously published study that appeared in the Journal of Speech, Language, and Hearing Research in January 2023, our team found that atypical sensory gating was significantly correlated with increased tinnitus severity in typical-hearing adults with minimal degrees of tinnitus.
Sensory gating is an electrophysiologic response that reflects an early, automatic stage of central inhibition. It can be thought of as a filter that suppresses information that is not of interest to conscious perception. So it is possible that atypical gating function in listeners with tinnitus may indicate that this particular stage of inhibition is failing to suppress internal auditory signals that are then perceived as tinnitus.
However, that study did not examine the underlying cortical generators for the gating response, leaving it open to question whether these networks were the same or different in the participants with minimal tinnitus.
In our follow-up study published in the American Journal of Audiology (AJA) in January 2024, we performed source localization analyses for the gating components in three groups of adults with typical hearing: with no tinnitus; with tinnitus and a Tinnitus Handicap Index (THI) score < 6 (low tinnitus); and with tinnitus and a THI score > 6 (but < 15) (high tinnitus).
This is a visualization of sensory gating brain activity seen on MRI scans among low tinnitus group. Credit: Ralston et al./AJA
We observed that, in the adults without tinnitus, gating-related networks were mainly localized in temporal, frontal, and prefrontal regions, which is in agreement with previous studies. In contrast, the tinnitus group with low tinnitus presented with an absence of prefrontal activation, along with additional cortical networks in medial temporal and parietal regions.
While this is a noted difference in gating-related generators when compared with the control group, it appears that these networks provide sufficient or compensatory gating function, as this group also presents with an extremely minimal tinnitus severity and a similar sensory gating response to the non-tinnitus group.
Meanwhile, the tinnitus group with high tinnitus showed a migration of gating-related networks posteriorly to the parietal regions, which corresponds with decreased gating function in the early part of the sensory gating response. This finding suggests maladaptive instead of compensatory plasticity in the high tinnitus group.
On the other hand, we found increased activation of prefrontal areas in this group to underlie enhanced, or better, gating function in the later part of the sensory gating response for this group.
Taken together, these results reflect that, while there is gating-related cortical plasticity for those with low tinnitus, it appears to be compensatory and still supports gating processes. For those with high tinnitus, parietal gating-related generators are maladaptive and underlie decreased gating function in the early part of the gating response, which may trigger compensatory, enhanced gating in the later part of the response which is supported by additional prefrontal activation.
So although these individuals report increased tinnitus severity, it is still maintained at a subclinical level (minimal handicap). As a result, increased prefrontal activity may be helpful in improving gating function, a topic for future research.
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’s 2016 Emerging Research Grant was generously funded by the Les Paul Foundation. She presented on this topic in her “Auditory Gating in Tinnitus” research webinar in June 2023.