THE JOURNAL OF COMPARATIVE NEUROLOGY 201:25-49 (1981) The Connections of the Inferior Colliculus and the Organization of the Brainstem Auditory System in the Greater Horseshoe Bat (Rhinolophus ferrumequinum)

The connections of the inferior colliculus, the mammalian midbrain auditory center, were determined in the greater horseshoe bat (Rhinolophus ferrumequinum), using the horseradish peroxidase method. In order to localize the auditory centers of this bat, brains were investigated with the aid of cell and fiber-stained material. The results show that most auditory centers are highly developed in this echolocating bat. However, the organization of the central auditory system does not generally differ from the mammalian scheme. This holds also for the organization of the superior olivary complex where a well-developed medial superior olivary nucleus was found. In addition to the ventral and dorsal nuclei of the lateral lemniscus a third well-developed nucleus has been defined which projects ipsilaterally to the inferior colliculus and which was called the intermediate nucleus of the lateral leminiscus. All nuclei of the central auditory pathway project ipsi-, contra-, or bilaterally to the central nucleus of the inferior colliculus with the exception of the medial nucleus of the trapezoid body and the medial geniculate body. The tonotopic organization of these projections and their possible functions are discussed in context with neurophysiological investigations. Insectivorous bats are mammals that hunt at night with the help of an active sonar system. They emit ultrasonic calls and extract from the returning echoes information about their environment and their prey with the aid of a highly developed auditory system. Since the discovery of this specialized orientation system in bats many neurophysiological experiments have been performed on acoustic processing in the central auditory system of these animals. The neurophysiological data show that the auditory system of echolocating bats is highly specialized and adapted to their specific orientation behavior (e.g., Neuweiler, '80; Pollack, '80; Suga and O'Neill, '80). Surprisingly, only a few anatomical data exist for the central auditory system in bats (for review see Henson, '70). Most anatomical investigations show that some of the auditory centers are highly developed. But, two main questions arise from earlier investigations: How is the auditory pathway organized up to the inferior colliculus and how well-developed are the higher auditory centers, the medial geniculate body, and the auditory cortex? The most detailed anatomical study of the auditory system in bats is the early work of Poljak ('26a,b) in Rhinolophus ferrumequinum and Nyctulus noctulu. Poljak concluded that the organization of the central auditory pathway in these bats does not differ significantly from that of other mammals. However, his studies of fiber connections were done only with myelin-stained material. In contrast to Poljak's findings, Harrison and Irving ('661, Irving and Harrison ('671, and Masterton and Diamond ('67) emphasized some differences in the central auditory system of bats, especially concerning the organization of the superior olivary complex. They advocated that bats do not have a medial superior olivary nucleus. Therefore it was generally agreed that the organization of the auditory system of echolocating bats differed from that of other mammals, and not until the auditory pathways of bats were recently investigated 0021-996'718112011-0025$07.00 @ 1981 ALAN R. LISS, INC.