INCREASED NEURAL RESPONSE TO AUDITORY STIMULATION
IN UNILATERAL TINNITUS, AN FMRI STUDY OF THE INFERIOR COLLICULUS AND THE AUDITORY CORTEX

Pim van Dijk1, Cris Lanting1, Hilke Bartels1, Dave Langers2, Emile de Kleine1
1 Dept. of Otorhinolaryngology, University Medical Center Groningen, The
Netherlands
2 Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston, USA

Background:

Tinnitus is believed to be related to abnormal patterns of spontaneous neural activity in the central auditory system. These abnormal patterns may result from a decreased effectiveness of neural inhibitory mechanisms. Functional MRI provides a measure of neural activity. Here we investigated whether tinnitus is related to abnormal neural responses to auditory stimulation.

Study design:

Functional MRI was performed in 10 normal hearing controls and 8 patients with unilateral tinnitus (4 left-sided, 4 right-sided). The stimuli were 6-octave wide dynamically-rippled Gaussian noise, presented to the right and left ear, respectively.

Stimulus levels were 40 and 70 dB SPL. Together, this provides 4 stimulus conditions. A sparse sampling MRI protocol was used to acquire functional images (TR=10 s, scan duration 2 s). A region-of-interest analysis was performed on the left and right inferior colliculus and the left and right auditory cortex.

Results:
In normal hearing controls, the average relative response amplitude in the inferior colliculi ranged from 0.21% for sound at 40 dB in the ipsilateral ear, to 0.60% for a stimulus at 70 dB in the contralateral ear. In the cortex, these numbers were 0.34% (ipsilateral ear, 40 dB) and 1.40% (contralateral ear, 70 dB). In tinnitus patients, the average responses to the 4 stimulus conditions were larger, ranging over 0.49-0.79% in the colliculi and 0.50-1.55% in the cortex.

Conclusions:
The neural response to sound is enlarged in the inferior colliculi and the auditory cortex of patients with unilateral tinnitus. This finding is consistent with reduced effectiveness of neural inhibition in tinnitus patients.