Trends in Neurosciences
Dissecting the circuitry of the auditory system
Section snippets
IC neurons and lower auditory nuclei often have similar response properties
Similar response properties in IC and lower nuclei are well illustrated by neurons that are excited by stimulation of one ear and inhibited by stimulation of the other ear (EI neurons). These neurons encode interaural intensity disparities (IIDs), the principal cues animals use to localize high frequency sounds [21]. EI properties are revealed by simply presenting a sound of fixed intensity to the excitatory ear and simultaneously presenting sounds of progressively increasing intensity to the
EI neurons are formed in multiple ways in the IC
Subjecting IC neurons to more challenging tests, however, reveals that the EI properties of many IC cells are not simply a reflection of LSO projections. The first question these tests were designed to answer is simple: does the actual inhibition evoked by stimulation of the inhibitory ear occur in the IC or does it occur in a lower nucleus, presumably the LSO? If the inhibition occurs in the LSO, then blocking inhibition at the IC cell should have no effect on ipsilaterally evoked inhibition;
Properties of DNLL neurons predict emergent properties of some EI neurons in the IC
The DNLL, like the IC, receives a large complement of inputs from lower nuclei 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45 and the neurons within it that are tuned to high frequencies are predominately, if not exclusively, EI 24, 26, 27, 46. Stimulation of the contralateral (excitatory) ear typically evokes a strong, sustained discharge train 13, 47, 48 (Fig. 4). The duration of the discharge train corresponds to the duration of the stimulus, and is never longer. By contrast,
Functional relevance of emergent properties resulting from DNLL innervation
The demonstration that an initial signal can change the responsiveness of IC cells to a trailing signal suggests that the DNLL circuitry could contribute to a precedence-like effect 12, 51. The precedence effect, or law of the first wavefront, was discovered in human psychophysical studies and is due to a mechanism that suppresses the directional information carried by echoes. It explains how, in a reverberant room, a listener can localize only the first sound and not the sequence of echoes
Concluding comments
The circuitry linking the LSO, DNLL and IC can serve as a model for a more general understanding of how the integration of incoming information creates complex, but biologically meaningful, properties within the IC. Although the variety of response transformations that occur in the IC is far larger than described here for EI cells 12, 20, 60, 61, 62, 63, 64, 65, we now have a better understanding of roles played by innervation from the DNLL for binaural processing. The significance of this is
Acknowledgements
We thank Eric Bauer for his helpful comments. Our work is supported by NIH grant DC 00268.
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2015, Current BiologyCitation Excerpt :The complex circuit revolving around the precedence effect has been studied in vivo, in vitro, and on a psychophysical level. It has also been modeled [8–11], and has even been recreated in the engineering of household robots [14], allowing them to localize accurately in indoor environments. However, one major gap in our understanding has concerned the cellular or subcellular mechanisms by which DNLL neurons are suppressed for these long time periods.