The Coding of Spatial Location by Single Units in the Lateral Superior Olive of the Cat. II. The Determinants of Spatial Receptive Fields in Azimuth

The lateral superior olive (LSO) is one of the most peripheral nuclei in the auditory pathway to receive inputs from both ears, and its cells are sensitive to interaural level disparities (ILDs) when stimulated by sounds presented over earphones. It has, accordingly, long been hypothesized that the functional role of the LSO is to encode a correlate of ILDs, one of the acoustical cues to the spatial location of sound. In the companion paper, we used the virtual space (VS) technique to present over earphones stimuli containing all the acoustical cues to the location of broadband stimuli and measured the spatial receptive fields (SRFs) in azimuth of single LSO cells. The shapes of the SRFs were generally consistent with the ILD sensitivity of the cells (Tollin and Yin, 2002), but because the only variable under our control was azimuth, and not ILD directly, the precise cues responsible for the SRFs could not be unambiguously determined. Here, we test more directly the hypothesis that ILDs are the primary determinants of the SRFs in azimuth of LSO cells by digitally manipulating the head-related transfer functions used to create the VS stimuli by independently varying (or holding constant) in azimuth each of the primary localization cues in isolation while holding constant (or varying) the others. Our results support the classical view of the LSO that the form of the SRFs of the cells in azimuth is determined primarily by the ILDs in a small band of frequencies around the characteristic frequencies of the cells.