Joint work with Nikolai G. Bibikov, Pierre Poirier, and Frank K. Samson.
Is the dorsal cochlear nucleus (DCN) specialized to derive directional sensitivity from spectral notches (SNs)? To answer this question we tested two hypotheses. 1. DCN neurons exhibit greater spectral-dependent directionality than ventral cochlear nucleus (VCN) neurons. 2. Directionality depends on response nulls produced by coincidence of best frequency (BF) and SN center-frequency. Single unit responses to short (50 ms) noise and tone-bursts were recorded in barbiturate anesthetized cats (BFs: 4 * 37 kHz). Units were classified using BF post-stimulus time histograms. Pauser and Burst (Type II interneurons) units were recorded from the DCN. Primary- like, Onset, and many Choppers were recorded in the VCN (collectively referred to as VCN units even though some Choppers were in the DCN). The Burst and Onset unit sample was small. Unit responsiveness (normalized threshold response, NTR) was calculated from level- response functions at seven azimuths (30° intervals along the 180° arc in front of the head and in the horizontal plane) by averaging over the range of 20 * 29 dB re: threshold. NTR azimuth function modulation [100%*(NTRmax-NTRmin)/ NTRmax] provided a measure of directionality. Many DCN units were highly directional to noise, but VCN units were not. As a group Pausers were more directional to binaural noise than the entire VCN group, and more directional than either the Primary-like or Chopper subgroups. There was no difference between Primary-like and Chopper subgroups. Binaural suppression produced a small increase in Pauser directionality to noise simulation, but had no effect on VCN units. Nevertheless, Pausers were more directional to monaural noise than VCN units showing that differential directionality depends upon a monaural mechanism. Pauser and VCN units exhibited similar, low directionality to BF-tone bursts showing that the difference in noise directionality between DCN and VCN depends upon spectral cues. Thus, DCN neurons are specialized in their capacity to derive dir NTR azimuth functions of highly directional units exhibited response nulls, and there was a linear relationship between their azimuthal locations and BFs in the range of 8-13 kHz. Nulls were located in the contralateral field for lower BFs and in the ipsilateral field for higher BFs which corresponds with the known spatial distribution of SNs on the horizontal plane. Furthermore, spatial receptive fields of DCN units show response nulls that follow the expected diagonal trajectory of the SN. These observations support the hypothesis that DCN neurons derive directional sensitivity from spectral notches.
(Supported by NIDCD DC00173).