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in the number of active units, or breakdown in intercellular insulation (demyelination). The present study was undertaken to determine whether intense sound exposure causes increases in the discharge rates of single units in the DCN. We performed three separate experiments, each comparing spontaneous discharge rates of single units in the DCNs of tone-exposed and control hamsters. In each experiment, a different combination of electrode impedance, electrolyte solutions, exposure conditions and recovery times were used. Animals were exposed to an intense (115-127 dB SPL) 10 kHz tone for 4 hours, either while anesthetized or while awake and freely mobile. Post-exposure recovery times were varied from 5 to 30 days. Recordings were obtained from more than 200 units at varying depths below the DCN sur- face. The results from all three experiments revealed considerable overlap in the range of spontaneous rates recorded in the two animal groups. Despite this overlap, the mean single unit discharge rates from exposed animals were consistently higher than those from control animals and were statistically signifi- cant. These results make it likely that multiunit hyperactivity reflects, at least in part, increases in single unit discharge rate. The possibility that one or more other factors might also contribute to multiunit hyper- activity cannot be ruled out. (Supported by NIH grant DC003258).

Correlated neural activity as the driving force for functional changes in auditory cortex. Hear Res. 2007 Jan 16. Eggermont JJ Department of Physiology and Biophysics, Department of Psychology University of Calgary, 2500 Uni- versity Drive N.W., Calgary, Alberta, Canada T2N 1N4

The functional role of neural synchrony is reflected in cortical tonotopic map reorganization and in the emergence of pathological phenomena such as tinnitus. First of all experimenter-centered and subject- centered views of neural activity will be contrasted; this argues against the use of stimulus-correction procedures and favors the use of a correction procedure based on neural activity without reference to stimulus timing. Within a cortical column neurons fired synchronously with on average about 6% of their spikes in a 1ms bin and occasionally showing 30% or more of such coincident spikes. For electrode separations exceeding 200mum the average peak correlation strength only occasionally reached 3%. The experimental evidence for coincidence of neural activity, neural correlation and neural synchrony shows that horizontal fibers activity can induce strong neural correlations. Cortico-cortical connections for a large part connect cell groups with characteristic frequencies differing by more than one octave. Such neurons have generally non-overlapping receptive fields but still can have sizeable peak cross-cor- relations. Correlated neural activity and heterotopic neural interconnections are presented as the subst- rates for cortical reorganization; increased neural synchrony and tonotopic map reorganization go hand in hand. This links cortical reorganization with hypersynchrony that can be considered as an important driving force underlying tinnitus.

Cortical and subcortical fMRI of unilateral tinnitus. (Abstract of ARO Meeting Denver, Colorado) Cris Lanting1,2, Emile de Kleine1,2, Hilke Bartels1, Dave Langers1,2, Pim van Dijk1,2 1Department of Otorhinolaryngology / Head and Neck Surgery, University Medical Center Groningen, 2Fac. of Medical Sciences, School of Behavioral and Cognitive Neurosciences, University of Groningen

The current understanding of possible mechanisms of tinnitus generation is still poor. Our goal is to find a possible neural correlate of tinnitus, using fMRI. In this work we used 10 patients (5 male) with unilateral tinnitus (5 left sided, 5 right sided) and 9 healthy subjects (4 male). Subjects had no or minor hearing deficits in both ears (max. 30 dB HL). Experiments were performed on a 3T Philips Intera scanner. 41 coronal slices (2 mm) were acquired using a matrix of 128x128 voxels (1.75 x 1.75 mm2) using sparse sampling (TR=10 s). Stimuli consisted of right and left

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