The analysis of interaural time differences in the chick brain stem

Abstract

The brain stem auditory system of the chick has proven to be a useful model system for analyzing how the brain encodes temporal information. This paper reviews some of the work on a circuit in the brain stem that compares the timing of information coming from the two ears to determine the location of a sound source. The contralateral projection from the cochlear nucleus, nucleus magnocellularis (NM), to nucleus laminaris (NL) forms a delay line as it proceeds from medial to lateral across NL. NL neurons function like coincidence detectors in that they respond maximally when input from the two ears arrive simultaneously. This arrangement may allow NL to code sound space by the relative level of activity across the nucleus. The head anatomy of the chick allows for enhancement of the functional interaural time differences. Comparing the functional interaural time differences to the length of the neural delay line suggests that each NL can encode approximately one hemifield of sound space. Finally it is suggested that inhibitory input into the NM-NL circuit may provide a means to dynamically adjust the gain of the circuit to allow accurate coding of sound location despite changes in overall sound intensity. KeywordsAuditory system; Sound localization; Nucleus magnocellularis; Nucleus laminaris; Coincidence detection; Interaural canal; GABA Temporal information is important in the auditory system both for identification of different sources of sound and for determining the location of sounds. Identification of different sound sources can be accomplished, to a large extent, using monaural cues. While localization of a sound source can also be accomplished, to some extent, with only one ear, significant localization cues are contained in the comparison of binaural information. Sounds located to one side will arrive at the near ear slightly before they arrive at the distant ear and, in general, these offset sounds will be slightly louder in the near ear than in the distant ear. By analyzing differences in the timing and intensity of information at the two ears, the brain can compute the location of a sound source.The circuitry for computing the location of a sound source begins in the brain stem. This report will review some of our work over the past several years on the circuit that begins the neural analysis of interaural time differences in the chick. Emphasis will be given to the anatomical, physiological and neurochemical adaptations in this model system to accomplish binaural sound localization. This manuscript is not meant to be an exhaustive review of the literature in the field, but rather, a story that exemplifies how adaptations at different levels of a system each contribute to enhancing the precision of an important task. Fig. 1A, provides a circuit for transforming time differences at the two ears into a place of maximal activation along an array of neurons. There are two main features of this model: 1) delay lines to an array of neurons and 2) neurons that function as coincidence detectors. As wi...

GABAergic and glycinergic inhibition modulate monaural auditory response properties in the avian superior olivary nucleus

“…Although we presented tones monaurally, the contralateral SON could have been driven in one of two ways. First, it has long been known that the middle ears of the chicken are acoustically coupled by the interaural canal, through which low frequency sound waves can readily pass and thereby influence the opposite ear drum (for review, see Hyson 2005). Second, ipsilateral NM cells project bilaterally to each NL (Parks and Rubel 1975;Rubel and Parks 1975), and the contralateral NL that projects in part to the contralateral SON (Conlee and Parks 1986) can be monaurally driven (Peña et al 1996).…”

Development of Glutamatergic Synaptic Transmission in Binaural Auditory Neurons

“…Functionally, mature NL neurons are highly specialized for encoding temporal information of sound (Kuba et al 2006). These neurons possess a variety of physiological specializations (Kuba et al 2005) that facilitate coincidence detection (Kuba et al 2002a(Kuba et al ,b, 2003 thought essential for binaural hearing (Hyson 2005;Young and Rubel 1983).…”

Beyond timing in the auditory brainstem: intensity coding in the avian cochlear nucleus angularis

“…These projection targets include the superior olivary nucleus (SON), two lemniscal nuclei, and a direct projection to the inferior colliculus ( Figure 1A). Three of these targets are associated with different roles; the superior olive mediates descending control of gain (Monsivais et al, 2000;Pena et al, 1996;Takahashi and Konishi, 2002; for review see Hyson, 2005), the lemniscal nuclei encode sensitivity to ILDs (Adolphs, 1993;Manley et al, 1988;Mogdans and Knudsen, 1994;Takahashi et al, 1995), and the inferior colliculus mediates the emergence of responses to biologically relevant stimuli (for review see Konishi, 2003).…”