Gray matter volumes in tinnitus and hyperacusis
Gray matter volumes in tinnitus and hyperacusis
The function of the brain is closely related to its structure. Understanding structural brain differences, therefore, provides an important window into the neural mechanisms underlying neurological and sensory disorders. This PhD thesisof Punit Makani investigates structural characteristics of the brain, with particular attention to gray matter differences, in individuals with and without tinnitus and hyperacusis. Across the neuroimaging studies included in this thesis, consistent gray matter differences were identified in regions implicated in auditory processing, attentional control, and internally directed cognition. These findings suggest that tinnitus and hyperacusis emerge from interactions between higher-order sensory and cognitive systems and demonstrate that these disorders are best understood as distributed brain conditions involving structural differences across multiple brain networks, rather than as disorders confined to the peripheral auditory system.
In individuals with tinnitus but clinically normal hearing (or clinically normal audiograms), structural alterations were relatively subtle and primarily characterized by lower grey matter in frontal and temporal cortical regions, including the ventromedial prefrontal cortex and auditory cortex. This suggests a combination of altered auditory processing and disrupted top-down cognitive regulation. In particular, lower ventromedial prefrontal cortex volumes may reflect diminished inhibitory or modulatory control over auditory percepts, potentially contributing to the persistence of tinnitus-related percepts despite normal peripheral hearing.
By contrast, tinnitus accompanied by hearing loss showed a different structural profile. Alongside auditory cortex alterations, higher gray matter was observed in posterior cortical regions such as the precuneus and lingual gyrus. This suggests broader cross-modal reorganization, potentially reflecting enhanced reliance on internally directed attention, self-referential processing, and multimodal integration in response to reduced auditory input.
Tinnitus accompanied by hyperacusis (increased sensitivity to everyday sounds) exhibited yet another distinct pattern, with functional differences in the auditory cortex combined with lower gray matter in the supplementary motor area. This implies involvement of both auditory gain mechanisms and sensorimotor systems responsible for preparing and regulating behavioral responses to sound, which may contribute to heightened sound sensitivity and reduced sensory filtering.
Overall, the findings demonstrate that tinnitus and hyperacusis involve overlapping but distinct patterns of brain network reorganization. While both conditions implicate the auditory network, tinnitus varies by hearing status in its broader network involvement, and hyperacusis uniquely engages sensorimotor regions. Together, these findings support the view that neither condition arises from a single neural mechanism, but rather from condition-specific alterations across interacting brain networks.