MONAURAL BRAINSTEM PROCESSING IS CONTROLLED BY INHIBITION

Monaural auditory brainstem processing within the superior olivary complex (SOC) and cochlear nucleus, specifically the neuronal coding of temporal, spectral and intensity cues are a prerequisite for both sound localization and differentiation of acoustic objects. Neurons which are innervated strictly monaurally are located in the anteroventral cochlear nucleus (AVCN), the medial nucleus of the trapezoid body (MNTB) and in the superior paraolivary nucleus (SPN) (66% of the neurons). In all three nuclei, 58% of the investigated neurons receive a prominent inhibitory input. Different sets of acoustic stimuli were used to characterize the interaction between excitation and inhibition. In 19% of the AVCN neurons, inhibition causes an upper threshold in the response areas, while in a number of MNTB neurons thresholds are elevated due to inhibition. In another population of AVCN neurons (33%) the inhibitory influence increases the dynamic range. Many SPN neurons show a very broad excitatory or inhibitory frequency response. Cells which are dominated by inhibition are further characterized by a strong off-response. Both, excitatory and inhibitory SPN neurons show strong synchronized responses to AM-stimuli. This indicates that transients in sound amplitude are the main features processed by these neurons.