Abstract
Interaural time differences (ITDs) are a main cue for sound localization and sound segregation. A dominant model to study ITD detection is the sound localization circuitry in the avian auditory brainstem. Neurons in nucleus laminaris (NL) receive auditory information from both ears via the avian cochlear nucleus magnocellularis (NM) and compare the relative timing of these inputs. Timing of these inputs is crucial, as ITDs in the microsecond range must be discriminated and encoded. We modeled ITD sensitivity of single NL neurons based on previously published data and determined the minimum resolvable ITD for neurons in NL. The minimum resolvable ITD is too large to allow for discrimination by single NL neurons of naturally occurring ITDs for very low frequencies. For high frequency NL neurons (>1 kHz) our calculated ITD resolutions fall well within the natural range of ITDs and approach values of below 10 μs. We show that different parts of the ITD tuning function offer different resolution in ITD coding, suggesting that information derived from both parts may be used for downstream processing. A place code may be used for sound location at frequencies above 500 Hz, but our data suggest the slope of the ITD tuning curve ought to be used for ITD discrimination by single NL neurons at the lowest frequencies. Our results provide an important measure of the necessary temporal window of binaural inputs for future studies on the mechanisms and development of neuronal computation of temporally precise information in this important system. In particular, our data establish the temporal precision needed for conduction time regulation along NM axons.
Highlights
Unlike the visual or somatosensory system, the auditory system cannot rely on a spatial representation of signals on its receptor surface
The code used for sound location in chickens remains in question, because it is unknown whether nucleus laminaris (NL) neurons can discriminate between Interaural time differences (ITDs) within the natural range at all sound frequencies
In this study we determined the minimum resolvable ITD of single neurons in the avian NL derived with a computational model that was based on previously published ITD tuning curves (Christianson and Peña, 2006; Köppl and Carr, 2008)
Summary
Unlike the visual or somatosensory system, the auditory system cannot rely on a spatial representation of signals on its receptor surface. In particular the sound localization circuits of chickens and barn owls have received a lot of attention (e.g., Young and Rubel, 1983; Carr and Konishi, 1990; Overholt et al, 1992; Kuba et al, 2006, 2010; Sorensen and Rubel, 2006; Seidl et al, 2010; Wang and Rubel, 2012) These circuits are used to address the open questions of the mechanisms involved in the development of neural circuits for processing temporally precise information (Seidl et al, 2010; Yamada et al, 2013) and the neural code used for sound localization (Harper and McAlpine, 2004; Salomon et al, 2012)
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