Abstract
The interaural level difference (ILD) cue to sound location is first encoded in the lateral superior olive (LSO). ILD sensitivity results because the LSO receives excitatory input from the ipsilateral cochlear nucleus and inhibitory input indirectly from the contralateral cochlear nucleus via glycinergic neurons of the ipsilateral medial nucleus of the trapezoid body (MNTB). It is hypothesized that in order for LSO neurons to encode ILDs, the sound spectra at both ears must be accurately encoded via spike rate by their afferents. This spectral-coding hypothesis has not been directly tested in MNTB, likely because MNTB neurons have been mostly described and studied recently in regards to their abilities to encode temporal aspects of sounds, not spectral. Here, we test the hypothesis that MNTB neurons and their inputs from the cochlear nucleus and auditory nerve code sound spectra via discharge rate. The Random Spectral Shape (RSS) method was used to estimate how the levels of 100-ms duration spectrally stationary stimuli were weighted, both linearly and non-linearly, across a wide band of frequencies. In general, MNTB neurons, and their globular bushy cell inputs, were found to be well-modeled by a linear weighting of spectra demonstrating that the pathways through the MNTB can accurately encode sound spectra including those resulting from the acoustical cues to sound location provided by head-related directional transfer functions (DTFs). Together with the anatomical and biophysical specializations for timing in the MNTB-LSO complex, these mechanisms may allow ILDs to be computed for complex stimuli with rapid spectrotemporally-modulated envelopes such as speech and animal vocalizations and moving sound sources.
Highlights
The interaural level difference (ILD) cue to sound location requires neural encoding of the shapes and magnitudes of sound spectra
Neurons were classified as medial nucleus of the trapezoid body (MNTB) based on three properties: (1) responses only to stimuli presented to the contralateral ear (Guinan et al, 1972a,b; Smith et al, 1998); (2) the presence of a prepotential in the action potential waveform (Guinan and Li, 1990); and (3) a primary-like (PL) or a primary-like with notch (PLN) PST histograms (PSTHs) to short tone burst stimuli (Smith et al, 1998)
It is assumed that the spectrotemporal receptive field (STRF) for RESULTS Results are based on recordings of 103 MNTB and 51 globular bushy cells (GBCs) neurons collected from 22 subjects
Summary
The interaural level difference (ILD) cue to sound location requires neural encoding of the shapes and magnitudes of sound spectra. ILDs result from frequency- and direction-dependent modifications of sound by the head and pinnae and are defined as the difference in spectra of the signals at the two ears (Tollin and Koka, 2009a,b). The superior olivary complex contains a circuit comprising the ipsilateral medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO) that is essential for ILD encoding (Tollin, 2003). SBCs and GBCs receive excitatory inputs from the auditory nerve. These inputs confer upon single LSO neurons the ability to compute a neural correlate of ILDs (Boudreau and Tsuchitani, 1968)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
