FUNCTIONAL-CHANGES IN THE VENTRAL COCHLEAR NUCLEUS FOLLOWING ACUTE ACOUSTIC OVERSTIMULATION

The effects of acute acoustic overstimulation on the discharge patterns of neurons in the ventral cochlear nucleus (VCN) were evaluated in anesthetized chinchillas. Response measures were obtained from the same neuron before and after presenting a 3- to 5-min intense tone (90-105 dB SPL) located one-half oct above the unit's characteristic frequency (CF). If a unit had an inhibitory response area at frequencies above CF and if the traumatizing tone reduced the magnitude of the inhibitory response, then the neuron's discharge rate to suprathreshold tones at CF increased (''enhancement'') by as much as 25%. However, if a unit lacked an inhibitory response area at frequencies above CF, then the traumatizing tone typically caused either no change or a decrease in the unit's discharge rate at CF. The traumatizing tone did not alter the shape of the post-stimulus time histograms. Moreover, the width of the excitatory response area was not altered by the exposure even when traumatizing stimulus reduced the magnitude of the inhibitory response above CF. The enhanced firing rate at CF following the exposure could conceivably contribute to the enhanced evoked-potential amplitudes observed in the auditory brain stem following acoustic trauma.