Ipsilateral sound-evoked inhibition in the MNTB
There now exist rigorous accounts of how processing arises in auditory brainstem. Two prominent examples are the coding of interaural time differences (ITDs) and interaural level differences (ILDs), the major cues to azimuthal sound location. Nevertheless, recent findings have led us to revise our concepts of how these circuits operate. In this thesis, I focus on one such finding: the occurrence of ipsilateral sound-evoked inhibition in the medial nucleus of the trapezoid body (MNTB), an important nucleus in the ILD coding brainstem circuit. Previously, theory and empirical work had suggested that the MNTB is a monaural nucleus, receiving only contralateral excitation. However, I demonstrate that inhibition is stronger in young (P16–P19) than in adult gerbils, that it operates at physiologically realistic ILDs, and that the locus of inhibition is presynaptic to the MNTB, probably at the globular bushy cells in the ventral cochlear nucleus (VCN) that innervate it. In support of these findings, I present acoustic data which defines the range of natural ILDs for young and adult gerbils. The physiological findings are discussed in the contexts of development, ILD coding, ITD coding, and non-azimuth related coding. The most likely sources of the inhibition are considered and experiments to further pinpoint the source and the consequences for the MNTB and its targets are suggested.