A role for low‐frequency sensitive neurons in the auditory brainstem in the encoding of source location distance‐dependent interaural level difference cues.

Abstract

The duplex theory posits that low‐ and high‐frequency sounds are localized using two different acoustical cues, interaural time delays (ITDs) and interaural level differences (ILDs), respectively. Psychophysical data have generally supported the theory for pure tones. Anatomical and physiological studies have revealed two parallel brainstem pathways that appear to encode ITDs and ILDs separately. ITDs are extracted by medial superior olive neurons. ILDs are extracted by lateral superior olive (LSO) neurons. ILD‐sensitive neurons are also found in the inferior colliculus (IC). ILDs are a complex function of both source location and frequency such that lower and higher frequencies exhibit smaller and larger ILDs, respectively. LSO and IC neurons encode ILDs for high‐frequency sounds where the cues are physically available, but there are discrepancies regarding low‐frequency neurons. Although acoustically, low‐frequency ILDs are small, humans are sensitive to them and physiological studies have found low‐frequency neurons in the LSO and IC that could encode them. Here the hypothesis that low‐frequency ILDs are useful when sound source distance is varied is explored. These data demonstrate that a population of IC neurons is sufficient to encode the range of acoustic ILDs that would be experienced as a joint function of source location and distance.