Single-trial neuromagnetic analysis of auditory steady state responses and its application for the studies of chronic tinnitus

Tinnitus is an auditory phantom sensation that is often associated with alterations of the tonotopic map in the Primary Auditory Cortex. However recent research points also to changed activations in other regions of the cortex. The steady state paradigm is an elegant tool that allows to investigate brain responses at the level of single trials. Thus I tried to apply the neuromagnetic auditory steady responses to the research of chronic tinnitus. Nine Tinnitus sufferers and ten healthy control subjects were stimulated with auditory steady states at three different carrier frequencies. In the tinnitus patients, one tone was placed at the lesion edge frequency while the other two served as a control tone 1.1 and 2.2 octaves below. Frequencies for the control group were similar to these frequencies. Three different methodological approaches were used to analyze the data and test current hypotheses on tinnitus. First the amplitude of the steady state response at the contralateral temporal source location was compared for all carrier frequencies. There was no difference between the tinnitus and the control group, neither for the amplitude in the lesion edge condition nor for the control conditions. Second the locations of the dipoles were fitted in the temporal plane. All dipoles of the tinnitus patients shifted to a more anterior position compared to the control group. This effect was more dominant for the Right Hemisphere. A high correlation was found between the euclidean distance of the control frequencies and the tinnitus distress. Third the phase coherence across different regions of the brain was calculated to investigate differences in the activated network between tinnitus and control subjects. A changed architecture of the activated network was found in the tinnitus group. Some of the phase coherences that revealed differences between the groups also correlated highly with the measures of the Tinnitus Questionnaire. A decoupling of parietal and prefrontal regions was associated with the tinnitus as well as a coupling of parietal-temporal and parietal-frontocentral connections. The right parietal source location played a major role in this network