Gray matter in the brain: differences associated with tinnitus and hearing loss.

Tinnitus, usually associated with hearing loss, is characterized by the perception of sound without an external sound source. The pathophysiology of tinnitus is poorly understood. In the present study, voxel-based morphometry (VBM) was employed to identify gray matter differences related to hearing loss and tinnitus. VBM was applied to magnetic resonance images of normal-hearing control subjects (n = 24), hearing-impaired subjects without tinnitus (n = 16, HI group) and hearing-impaired subjects with tinnitus (n = 31, HI + T group). This design allowed us to disentangle the gray matter (GM) differences related to hearing loss and tinnitus, respectively. Voxel-based VBM analyses revealed that both HI and HI + T groups, relative to the controls, had GM increases in the superior and middle temporal gyri, and decreases in the superior frontal gyrus, occipital lobe and hypothalamus. We did not find significant GM differences between both patient groups. Subsequent region-of-interest (ROI) analyses of all Brodmann Areas, the cerebellum and the subcortical auditory nuclei showed a GM increase in the left primary auditory cortex of the tinnitus patients compared to the HI and control groups. Moreover, GM decreases were observed in frontal areas and mainly GM increases in limbic areas, both of which occurred for hearing loss irrespective of tinnitus, relative to the controls. These results suggest a specific role of the left primary auditory cortex and the additional involvement of various non-auditory brain structures in tinnitus. Understanding the causal relation between these GM changes and tinnitus will be an important next step in understanding tinnitus mechanisms.