Spectral edge sensitivity in neural circuits of the dorsal cochlear nucleus.

One possible function of the dorsal cochlear nucleus (DCN) is discrimination of head-related transfer functions (HRTFs), spectral cues used for vertical sound localization. Recent psychophysical and physiological studies suggest that steep, rising spectral edges may be the features used to identify HRTFs. Here we showed, using notch noise and noise band stimuli presented over a range of frequencies, that a subclass of DCN type IV neurons responded with a response peak when the rising spectral edge of a notch or band was aligned near best frequency (BF). This edge sensitivity was correlated with weak or inhibited responses to broadband noise and inhibition in receptive fields at frequencies below BF. Some aspects of the inhibition shaping the response peak, namely inhibition to rising edges below BF and to falling edges at BF, could be explained by the properties of type II interneurons with BFs below those of the type IV neurons. However, many type IV neurons also showed inhibitory responses with the rising spectral edge just above BF, and these responses could not be reproduced by current models of DCN circuitry. Therefore, a new component of the DCN circuit is needed to fully explain the responses to rising spectral edges. This shaping of edge sensitivity by inhibition to rising spectral edges both below and above BF suggests the specialization of DCN for spectral edge coding along the tonotopic gradient.