The Inferior Colliculus

Abstract

The mammalian inferior colliculus sits as a protuberance on the dorsal surface of the midbrain (Fig. 7.1) and is composed of several subdivisions (see Oliver and Huerta 1992 for a discussion of various subdivisions). The largest division is the central nucleus of the inferior colliculus (ICc), which is the target of the ascending auditory projections of the lateral lemniscus (Adams 1979; Schweizer 1981; Zook and Casseday 1982; Aitkin 1986; Irvine 1986; Ross, Pollak, and Zook 1988; Ross and Pollak 1989; Frisina, O’Neill, and Zettel 1989; Vater and Feng 1990; Casseday and Covey 1992; Oliver and Huerta 1992). In most echolocating bats, the ICc is especially large while the other divisions of the inferior colliculus are considerably smaller than in other animals (e.g., Pollak and Casseday 1989). The ICc is truly a nexus in the mammalian auditory system where the ascending fibers from 10 or more of the lower auditory nuclei make an obligatory synaptic connection (Fig. 7.2). The ICc neurons, in turn, send a large projection to the medial geniculate (Olsen 1986; Frisina, O’Neill, and Zettel 1989; also see review by Winer 1992, and Wenstrup, Chapter 8) and lesser projections to other divisions of the inferior colliculus as well as to the superior colliculus (Covey, Hall, and Kobler 1987; Oliver and Huerta 1992).