Directionality Derived From Pinna-Cue Spectral Notches in Cat Dorsal Cochlear Nucleus

Abstract

We tested two hypotheses to determine whether dorsal cochlear nucleus (DCN) neurons are specialized to derive directionality from spectral notches: DCN neurons exhibit greater spectral-dependent directionality than ventral cochlear nucleus (VCN) neurons, and spectral-dependent directionality depends on response minima (nulls) produced by coincidence of best frequency (BF) and spectral-notch center frequency. Single-unit responses to 50-ms noise and tone bursts were recorded in barbiturate-anesthetized cats (BFs: 4-37 kHz). Units were classified using BF tone poststimulus time histograms. Pauser, onset-G (type II interneurons), and some chopper units were recorded from the DCN. Primary-like, onset-CIL (onset other than onset-G), and most choppers in the sample were recorded from the VCN. Many pauser and onset-G units were highly directional to noise. Chopper, onset-CIL, and primary-like units (collectively referred to as C-O-P units) were not. The difference in directionality depends on a monaural mechanism as pausers were more directional to monaural noise than C-O-P units. Contralateral inhibition produced a small increase in pauser directionality to noise simulation but had no effect on directionality of C-O-P units. Pauser and C-O-P units exhibited similar low directionality to BF tone, showing that the difference in noise directionality between groups depends on spectral cues. These results show that spectral-dependent directionality is a DCN specialization. Azimuth functions of highly directional units exhibited response nulls, and there was a linear relationship between BFs in the range of 8-13 kHz and azimuthal locations of nulls. This relationship parallels the known spatial distribution of spectral-notch center frequencies on the horizontal plane. Furthermore spatial receptive fields of pausers show response nulls that follow the expected diagonal trajectory of the spectral notch in this frequency range. These results show that DCN spectral-dependent directionality depends on response nulls produced by coincidence of unit BF and spectral-notch center-frequency.

Auditory Processing of Spectral Cues for Sound Localization in the Inferior Colliculus

“…1D) predict that these stimuli will produce a spatial receptive field in which excitatory discharge rates parallel the unit's preferred notch frequency contour. A similar pattern of inhibitory activity has been previously described for DCN type IV units (Imig et al 2000).…”

“…Electrophysiological studies in cats have shown that the projection neurons of the dorsal cochlear nucleus (DCN) are especially sensitive to spectral notches Imig et al 2000). When tested with pure tones, DCN principal cells exhibit a type IV frequency response map that is characterized by inhibitory responses at most combinations of frequency and level.…”

Acoustic Basis of Directional Acuity in Laboratory Mice

“…Neurophysiological studies in other mammalian species have identified neural populations that perform wideband spectral integration in the dorsal cochlear nucleus (DCN;Young et al 1992;Imig et al 2000) and the central nucleus of the inferior colliculus (ICC; Davis et al 2003;Chase and Young 2005). Surgical lesions of this spectral processing pathway produce localization deficits that are most obvious for the identification of sound source elevation (Sutherland et al 1998;May 2000).…”

Contralateral Effects and Binaural Interactions in Dorsal Cochlear Nucleus

“…Type IV units also respond with inhibition when a spectral notch is placed at their BF (Spirou and Young 1991). By virtue of their unusual sensitivity to narrowband peaks and notches, such as those added to sounds by the directionally dependent filtering properties of the pinna (Musicant et al 1990;Rice et al 1992), DCN principal cells are thought to serve a role in the detection of the spatial location of sound sources Imig et al 2000). Consistent with this interpretation, lesioning the output pathway of the DCN disrupts the sound orientation behaviors of cats (Sutherland et al 1998;May 2000).…”

“…The latencies of these inhibitory responses are suggestive of mono-and disynaptic connections from the contralateral cochlear nucleus. Finally, single-unit studies have demonstrated that DCN neurons are sensitive to acoustic stimulation of the contralateral ear (Mast 1970(Mast , 1973Young and Brownell 1976;Evans and Zhao 1993;Joris and Smith 1998;Imig et al 2000). Some units are excited by monaural stimulation of the contralateral ear, but others are inhibited.…”