Single Auditory Units in the Superior Olivary Complex: II: Locations of Unit Categories and Tonotopic Organization

Abstract

The locations of various categories of units are described and correlations are found between unit properties and unit locations. Units on the edges of the medial superior olive (MSO) and the lateral superior olive (LSO) appear to have properties different from units inside these nuclei. Many units in the dorsomedial periolivary nucleus had wide tuning curves and some had very long latencies. Almost all units in the medial nucleus of the trapezoid body (MNTB) had irregular firing patterns, but most units in the LSO and the dorsolateral periolivary nucleus had regular firing patterns. Class 2 (off) units may correspond to ‘stellate’ cells of the MNTB.With few exceptions, units dorsolateral to the MSO were excited by ipsilateral sounds, and units ventromedial to the MSO, or in the ventral nucleus of the lateral lemniscus (VNLL), were excited by contralateral sounds.Best frequencies were arranged from high to low (1) from ventromedial to dorsolateral in the MSO, (2) from the medial limb to the lateral limb in the LSO, and (3) from ventromedial to dorsolateral for units with complex-waveshapes in the MNTB. Other units in the MNTB were not arranged in tonotopic sequences. Units in the VNLL did not appear to be arranged in a tonotopic sequence which extended throughout the entire nucleus. In the MSO there are more high-frequency neurons than low-frequency neurons, whereas in the LSO the opposite is true.