FunctiOnaL anatOmy OF the superiOr OLivary cOmpLex

fibers from the cochlea. Since both cochlear nuclei impinge upon cells in the SOC, it represents the initial site of binaural interaction. The SOC is so named because anatomically it is in a superior position relative to the inferior olivary complex. The term olivary is based on the olive-shaped protuberance on the ventral lateral surface of the medulla created by the underlying inferior olivary complex. The SOC extends from the rostral medulla to the caudal pons and is called a complex because it represents many nuclear groups. The SOC can be functionally divided into a feedback system to the cochlea (olivocochlear system), an inhibitory periolivary system and a sound-localization system. Anatomically, there is considerable overlap between these systems. The SOC varies in size and composition among species (Figure 7–1). The main nuclei, the lateral (LSO) and medial (MSO, Figure 7–1, green filled) superior olivary nuclei can usually be identified in most species, but the nuclei that surround the LSO and MSO, viz., the periolivary nuclei (Figure 7–1, gray filled), take many forms. The tuning of neurons in LSO is biased towards high frequencies, whereas those in the MSO are biased towards low frequencies (Guinan et al., 1972). Thus, the relative size of the LSO and MSO reflect the animal’s frequency range of hearing (Glendenning & Masterton, 1998). So, animals with a hearing bias towards high frequency sounds (e.g., mice and rats) have a large LSO and a small MSO. Animals with good low and high frequency hearing (e.g., cats) have a prominent LSO and MSO. Humans have a hearing bias towards low frequency sounds so the most prominent nucleus is the MSO. Learning Objectives