FENS Abstr. 3, A180.16 , 2006.
R. Schaette¹, O. König², M. Gross² and R. Kempter¹ ³.
¹Institute for Theoretical Biology, Biology Department, Humboldt-Universität zu Berlin, 10115 Berlin, Germany
²Klinik für Audiologie und Phoniatrie,
Charité-Universitätsmedizin Berlin,
Campus Benjamin Franklin, 14195 Berlin, Germany
³Bernstein Center for Computational Neuroscience Berlin, 10115 Berlin, Germany.
Tinnitus is often associated with hearing loss, but how hearing loss could lead to the development of tinnitus has remained unclear. In animals, hearing loss through cochlear damage can lead to behavioral signs of tinnitus. These animals also displayed increased spontaneous firing rates or hyperactivity of neurons in the auditory brainstem. Mechanisms for the development of hyperactivity, however, have also not been identified.
We have developed a model that shows how hyperactivity could arise after hearing loss. Our main assumption is that the mean activity of central auditory neurons is stabilized through homeostatic plasticity. Decreased auditory nerve activity after hearing loss is counteracted by homeostatic plasticity through an increase of excitatory and a decrease of inhibitory synaptic strengths. This restores the mean rate, but it can lead to increased spontaneous firing rates depending on the type and degree of cochlear damage. In our model, the amount of hyperactivity along the tonotopic axis therefore depends on the shape of the audiogram.
Here we test the model's ability to predict tinnitus pitch from patients' audiograms. We therefore evaluate a sample of patients with both noise-induced hearing loss and tone-like tinnitus. Given the audiograms, the model is used to determine the spontaneous firing rate along the tonotopic axis. The resulting patterns of hyperactivity exhibit distinct peaks that are associated with steep drops in the audiograms. If such peaks are interpreted as the basis for a tone-like tinnitus sensation, the model predicts tinnitus frequencies that are close to the tinnitus pitch of the patients. The deviations between predicted and observed pitch are within the range of errors typically obtained by psychophysical tinnitus pitch matching. Our model thus constitutes a plausible hypothesis of how hearing loss could lead to tinnitus.