Anatomy and physiology of binaural hearing

Binaural hearing improves performance in most auditory tasks and is essential for some. This paper introduces the brain stem pathways and nuclei involved in binaural interaction and outlines some recent approaches to understanding binaural mechanisms. It also provides examples of basic science approaches to the effects of infant hearing loss on those pathways and mechanisms. Binaural interaction occurs primarily and almost simultaneously at three levels of the brain: the superior olivary complex (SOC), the nuclei of the lateral lemniscus (NLL) and the inferior colliculus (IC). The SOC derives its input from the anterior ventral cochlear nucleus (CN) through branching axons that innervate several SOC subdivisions on both sides of the brain. At least some of these anteroventral CN axons project on up to the contralateral NLL and IC. The IC and NLL also receive direct, major projections from the contralateral CN, via the dorsal and intermediate acoustic striae, and from the SOC bilaterally. The IC receives additional input from the NLL bilaterally, and is thus innervated by every nuclear group within the auditory brain stem. There is little evidence for strict, functional segregation in these binaural pathways, although subdivisions of the SOC appear to be predominantly involved in analysing either interaural time or level differences (ITD, ILD). ITD- and ILD-sensitive neurones are also found in abundance in the central IC. There is emerging evidence that binaural information is coupled with spectral cues derived from the outer ear in several auditory mid-brain regions [the NLL, the external IC and the superior colliculus (SC)] to produce topographic representations of auditory space. Throughout the higher auditory system the response of neurones to stimulation of each ear is either excitatory or inhibitory, and there is a spatial segregation of neurones receiving predominantly excitatory or inhibitory input from the ipsilateral ear in both the medial geniculate body of the thalamus and the auditory cortex. Neonatal, unilateral hearing loss leads to a rearrangement of binaural connections in the auditory brain stem, to changes in the physiology of IC neurones in response to stimulation of the normal ear and to compensatory alterations in the auditory space map in the SC. The same hearing losses in adulthood do not produce these changes. The evidence from this and other work suggests that binaural mechanisms are more sensitive to hearing loss, over a longer developmental period, than mechanisms subserving monaural processing.