High-field functional MRI (7 Tesla) mapping of Tonotopy in Human primary auditory cortex in patients with unilateral hearing loss and tinnitus.
Tinnitus, the chronic perception of ringing or other phantom sounds, is typically associated with hearing loss. Recent studies in mouse models have alleviated tinnitus symptom by reversing plastic reorganization observed in the tonotopic maps of primary auditory cortex (A1) (Engineer et al. Nature, 2011, Yang et al. PNAS 2011). Here, we use high-resolution functional MRI at 7 Tesla (1.5 mm isotropic voxels) to test whether human tinnitus sufferers also have similar disorganization within the auditory cortical maps. Our fMRI mapping paradigm (Da Costa et al. J Neurosci 2011) allows fine-scale imaging of two primary tonotopic maps in human cortex corresponding to regions A1 and R. Ten tinnitus patients with chronic unilateral hearing loss and tinnitus and age-matched normal controls (ages 26-49) were tested. (Patients had chronic subjective non-pulsatile tinnitus associated with moderate to severe unilateral sensorineural hearing loss in one ear only with at least PTA>40dB on three consecutive frequencies between 1 and 4 KHz; tinnitus duration < 6 months). The recruitment of patients with unilateral hearing loss allowed unimpaired sound delivery via the unimpaired ear, bypassing any abnormal responsiveness at the peripheral level. Our first finding is that orderly tonotopic maps were seen bilaterally in all patients similar to control subjects, suggesting that unilateral hearing loss with tinnitus does not result in gross distortions of cortical tonotopic organization. Second, we find a tendency for larger response amplitudes in response to low-to-middle frequency tones in patients compared to controls. This result is similar to that observed in the above-mentioned animal studies and was most apparent in those patients with the strongest tinnitus severity (Tinnitus Handicap Inventory). The results suggest that unilateral hearing loss with tinnitus is associated with neural hyper-excitability in human primary auditory cortex. Reduction of this hyper- excitability may be important to the development of treatments to cure tinnitus in humans.