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higher layers of the auditory hierarchical stream. The change results in enhanced slow-wave activity reflecting altered corticothalamic and corticolimbic interplay. Such enhancement facilitates and sustains gamma activity as a neural code of phantom perception, in this case auditory.

VII Pathophysiology

Acoustic Trauma Induces Long-Term Temporal Correlations in DCN. (Abstract of ARO Meeting Denver, Colorado) Benjamin Letham, Wei-Li D. Ma, Shanqing Cai, Eric D. Young Center for Hearing and Balance, Department of Biomedical Engineering, Johns Hopkins University

Changes to the functional properties of the dorsal cochlear nucleus (DCN) that follow acoustic trauma are not well understood. Past studies (e.g. Kaltenbach et al.) have reported an increase in mean spon- taneous firing rates following acoustic trauma. However, recent work from our lab in DCN principal cells has found no change in mean spontaneous firing rate. This finding has led us to study temporal patterns in DCN spontaneous activity that may change independent of the mean rate. One pattern of specific inte- rest is long-range dependence (LRD). LRD is an effect of fractal rate fluctuations, where rate fluctuations have weak correlations on the scale of minutes. Cats were acoustically traumatized by exposure to 10kHz noise at 107dB SPL for four hours. Compound action potentials showed a >60dB threshold shift at and above 10kHz. Ten minutes of spontaneous acti- vity were recorded from isolated single DCN units in these deaf cats as well as a group of normal hearing cats. LRD can be quantified by computing the Fano factor for a range of counting times. The Fano factor has a power-law dependence on counting time, and the exponent of the power-law is called the fractal dimension. This dimension is a measure of the spike count variance and LRD. The average fractal dimension of the 10kHz-exposed DCN units is significantly higher than that of nor- mal DCN units. Surprisingly, there is no correlation between higher fractal dimension and the degree of threshold shift (or spontaneous activity). The increased fractal dimension implies that neurons in exposed cats have more long-term correlations and higher spike count variance. Increased spike count variance could contribute to tinnitus by providing a fluctuating rate signal that would be interpreted as resulting from a fluctuating sound. (Supported by NIH grant DC00109 and the Tinnitus Consortium).

Alterations In Spontaneous Discharge Rates Of Single Units In The Dorsal Cochlear Nuc- leus Induced By Intense Sound Exposure. (Abstract of ARO Meeting Denver, Colorado) Paul Finlayson, James Kaltenbach Wayne State University

Hyperactivity in the dorsal cochlear nucleus (DCN), characterized by increases in spontaneous activity, has been implicated as a possible factor contributing to tinnitus following noise exposure. Evidence for the phenomenon of noise induced hyperactivity in the DCN has been obtained mostly using methods that examine activity of neural clusters (multiunit recordings) or populations of neurons (c-fos, 2-deoxy- glucose), an exception being a study by Brozoski et al., 2002, demonstrating noise-induced increases in single unit spontaneous discharge rates in the chinchilla DCN. Evidence for noise-induced hyperactivity from our laboratory has been based almost entirely on multiunit recordings, raising the question of whe- ther these increases reflect increased discharge rates at the single unit level, or instead, might be due to some other change. Increased multiunit activity could result from increases in extracellular voltages caused by increases in single unit spike amplitude, increased synchronous discharges, increases

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