Increases in Spontaneous Activity in the Dorsal Cochlear Nucleus Following Exposure to High Intensity Sound: A Possible Neural Correlate of Tinnitus.

The purpose of this study was to test the effects of intense tone exposure on the spontaneous activity of multiunit clusters in the mammalian dorsal cochlear nucleus (DCN). Adult hamsters (60-101 days of age) were exposed to a 10 kHz tone at levels between 125 and 130 dB SPL for a period of 4 hours. The effects of tone exposure were studied following a recovery period of 30-58 days and were quantified by measuring the spontaneous rates, response thresholds and frequency tuning properties of neural clusters at the surface of the DCN. Measures were performed at each of 10-15 sites along the tonotopic axis of the DCN. The effects of the tone exposure were examined by comparison with identical measures obtained from normal unexposed animals. Results indicate that tone exposure induced major chronic increases in the spontaneous activity of the DCN. Such increases were broadly distributed across the tonotopic range of the DCN and were generally found in tonotopic map areas characterized by tone-induced elevations of neural thresholds. Mean spontaneous rate reached its maximum value at or close to the tonotopic locus which normally represents the frequency of the exposure tone. The increased activity induced by tone exposure resembled the heightened activity in normal animals during presentation of a moderate level continuous tone. These changes in spontaneous activity indicate that central auditory neurons are in a state of elevated activity for extended periods following intense sound exposure and suggest that the affected neurons may signal the presence of acoustic stimulation even though such stimulation is not present. Possible mechanisms of these changes and their relation to the clinical problem of tinnitus are discussed.