The role of timing in the brain stem auditory nuclei of vertebrates

Vertebrate animals gain biologically important information from environment al sounds. Localization of sound sources enables animals to detect and resp ond appropriately to danger, and it allows predators to detect and localize prey. In many species, rapidly fluctuating sounds are also the basis of co mmunication between conspecifics. This information is not provided directly by the output of the ear but requires processing of the temporal pattern o f firing in the tonotopic array of auditory nerve fibers. The auditory nerv e feeds information through several parallel ascending pathways. Anatomical and electrophysiological specializations for conveying precise timing, inc luding calyceal synaptic terminals and matching axonal conduction times, ar e evident in several of the major ascending auditory pathways through the v entral cochlear nucleus and its nonmammalian homologues. One pathway that i s shared by all higher vertebrates makes an ongoing comparison of interaura l phase for the localization of sound in the azimuth. Another pathway is sp ecifically associated with higher frequency hearing in mammals and is thoug ht to make use of interaural intensity differences for localizing high-freq uency sounds. Balancing excitation from one ear with inhibition from the ot her in rapidly fluctuating signals requires that the timing of these synapt ic inputs be matched and constant for widely varying sound stimuli in this pathway. The monaural nuclei of the lateral lemniscus, whose roles are not understood (although they are ubiquitous in higher vertebrates), receive in put from multiple pathways that encode timing with precision, some through calyceal endings.