Role of auditory cortex in noise- and drug-induced tinnitus.
Eggermont JJ.

Department of Psychology, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada. eggermon@ucalgary.ca

PURPOSE: To elucidate the role of auditory cortex in tinnitus.

METHOD: Neurophysiological findings in cat auditory cortex following noise trauma or the application of salicylate and quinine, all expected to induce tinnitus, were reviewed. Those findings were interpreted in the context of what is expected from studies in humans, specifically in the brains of people with tinnitus.

RESULTS: Tinnitus is an auditory percept to which several central structures in the auditory system may contribute. Because the central auditory system has both feed-forward connections and feedback connections, it can be described as a set of nested loops. Once these loops become activated in a pathological fashion, as they may be in tinnitus, it becomes hard to assign importance to each contributing structure. Strongly interconnected networks, that is, neural assemblies, may be determining the quality of the tinnitus percept.

CONCLUSION: It is unlikely that tinnitus is the expression of a set of independently firing neurons, and more likely that it is the result of a pathologically increased synchrony between sets of neurons. There is clear evidence for this from both evoked potentials and from neuron-pair synchrony measures.