The function of the medial superior olive in small mammals: temporal receptive fields in auditory analysis

Traditionally, the medial superior olive, a mammalian auditory brainstem st ructure, is considered to encode interaural time differences, the main cue for localizing low-frequency sounds. Detection of binaural excitatory and i nhibitory inputs are considered as an underlying mechanism. Most small mamm als, however, hear high frequencies well beyond 50 kHz and have small inter aural distances. Therefore, they can not use interaural time differences fo r sound localization and yet possess a medial superior olive. Physiological studies in bats revealed that medial superior olive cells show similar int eraural time difference coding as in larger mammals tuned to low-frequency hearing. Their interaural time difference sensitivity, however, is far too coarse to serve in sound localization. Thus, interaural time difference sen sitivity in medial superior olive of small mammals is an epiphenomenon. We propose that the original function of the medial superior olive is a binaur al cooperation causing facilitation due to binaural excitation. Lagging inh ibitory inputs, however, suppress reverberations and echoes from the acoust ic background. Thereby, generation of antagonistically organized temporal f ields is the basic and original function of the mammalian medial superior o live. Only later in evolution with the advent of larger mammals did interau ral distances, and hence interaural time differences, became large enough t o be used as cues for sound localization of low-frequency stimuli.