Input from the medial nucleus of trapezoid body to an interaural level detector

The medial nucleus of the trapezoid body (MNTB) contains components of a neural network that functions as an interaural level difference (ILD) detector. In the cat, lateral superior olivary (LSO) neurons compare the contralateral inhibitory input from the MNTB with an excitatory input from the ipsilateral anteroventral cochlear nucleus to extract information about binaural stimuli. To better specify the inhibitory inputs to the LSO and gain a better understanding of the inhibitory component of the LSO network, the response characteristics of MNTB neurons were examined in cats under stimulus conditions similar to those used to study LSO inhibitory responses. The inhibitory tuning curves of LSO units were wider than the tuning curves of MNTB units. Hence, MNTB neurons with similar, but not identical, characteristic frequencies converge to provide inhibitory input to single LSO neurons. Variations in the number of converging MNTB inputs produced a range of LSO excitatory-inhibitory threshold differences, thus creating a coding mechanism for representing the ILD. Convergence of MNTB inputs also increased the dynamic range over which contralateral stimulus level effects LSO binaural responses beyond the dynamic ranges of individual MNTB units, thus expanding the ILD range encoded by the LSO network. The differences between the first-spike latencies of MNTB and LSO tone burst responses were small and the precision of the LSO first-spike discharges was significantly greater than that of MNTB units. As tone bursts delivered simultaneously to the two ears can consistently inhibit LSO first-spike discharges, the inhibitory input must match the LSO precision by converging a number of the more variably timed MNTB discharges. Because of their precision LSO first-spike discharges may be used to encode interaural time-of-arrival differences of mid- to high-frequency transients. These findings add to the foundation for a comprehensive network model that describes the inputs to the LSO as point processes, delimits the biophysical mechanisms underlying excitatory and inhibitory interactions at the single neuron level, and reveals how these inputs determine the response to different binaural stimulus conditions.