3.36 - Encoding of Interaural Level Differences for Sound Localization

Abstract

One of the primary acoustical cues to the spatial location of a sound source in azimuth is the interaural level difference (ILD). The ILD cue results from the frequency- and direction-dependent modifications of the sound waves by the head and pinnae and is defined as the difference in sound pressure level of the signals arriving at the two ears as a function of frequency. The neurons comprising the lateral superior olive (LSO) are thought to be the most peripheral neurons in the ascending auditory pathway to encode the ILD cue. Although it has been known for decades that LSO neurons are sensitive to ILDs, the specific mechanisms for ILD selectivity have been elucidated only recently. Here we review the mechanisms of ILD processing by first discussing the physical bases of the ILD cue itself. Then we examine the anatomical, physiological, and biophysical specializations present in the neural circuitry through the neurons of the LSO that enable the encoding of ILDs. The specializations in the input neurons to the LSO permit the spectral and temporal attributes of the sound arriving at the two ears to be represented accurately while the specializations at the LSO allow ILDs to be encoded based on these representations on nearly a frequency-by-frequency and time-by-time basis. Comparative studies that have examined the functional role of the LSO through combined anatomical, physiological, and behavioral observations confirm that the LSO is necessary for sound source localization based on ILDs.