Subcallosal brain structure: correlation with hearing threshold at supra-clinical frequencies (>8 kHz), but not with tinnitus.

This study tested for differences in brain structure between tinnitus and control subjects, focusing on a subcallosal brain region where striking differences have been inconsistently found previously. Voxel-based morphometry (VBM) was used to compare structural MRIs of tinnitus subjects and non-tinnitus controls. Audiograms of all subjects were normal or near-normal at standard clinical frequencies (≤8 kHz). Mean threshold through 14 kHz, age, sex and handedness were matched between groups. There were no definitive differences between tinnitus and control groups in modulated or unmodulated maps of gray matter (GM) probability (i.e., GM volume and concentration, respectively). However, when the image data were tested for correlations with parameters that were either not measured or not matched between the tinnitus and control groups of previous studies, a notable correlation was found: Threshold at supra-clinical frequencies (above 8 kHz) was negatively correlated with modulated GM probability in ventral posterior cingulate cortex, dorsomedial prefrontal cortex, and a subcallosal region that included ventromedial prefrontal cortex and coincided with previously-reported differences between tinnitus and control subjects. The results suggest an explanation for the discrepant findings in subcallosal brain: threshold at supra-clinical frequencies may have differed systematically between tinnitus and control groups in some studies but not others. The observed correlation between (1) brain structure in regions engaged in cognitive and attentional processes and (2) hearing sensitivity at frequencies generally considered unnecessary for normal human auditory behavior is surprising and worthy of follow up.