Effect of unilateral noise exposure on the tonotopic distribution of spontaneous activity in the cochlear nucleus and inferior colliculus in the cortically intact and decorticate rat.

Effects of unilateral noise exposure on spontaneous activity (SA) in the anteroventral and dorsal cochlear nuclei (AVCN and DCN) and the central nucleus of the inferior colliculus (ICc) were studied in cortically intact and decorticate rats. SA was measured 1 week following exposure using uptake of 14C-labeled 2-deoxyglucose (2DG) in quiet. Optical density (OD) measurements were obtained in low- and high-frequency (LF and HF) areas of each nucleus. We refer to the ipsilateral AVCN and DCN (side of the noise-exposed ear) and the contralateral ICc as direct nuclei and to their opposite side counterparts as indirect nuclei. Noise exposure altered the tonotopic profile of SA in the direct pathway by causing a decrease in the ratio of HF OD to LF OD (HF/LF ratio). In intact animals, the decreased HF/LF ratio was due to decreased HF OD. In decorticate animals, it was due to decreased HF OD and increased LF OD, the latter occurring mainly in the DCN and ICc. Decorticate-intact differences may reflect corticofugal feedback inhibition. Lesion of the dorsal acoustic stria caused a substantial decrement of SA in the contralateral ICc. Furthermore, strong positive correlations between HF/LF ratios in the DCN, AVCN, and contralateral ICc suggest that the cochlear nucleus is a major contributor to SA in the ICc. Noise exposure had opposite and weaker effects on 2DG uptake in the indirect pathway that were attributed to crossed inhibition. Noise-induced changes in the tonotopic profile of SA may represent a neural correlate of tinnitus.

Spectral shape sensitivity contributes to the azimuth tuning of neurons in the cat's inferior colliculus.

We recorded high-best-frequency single-unit responses to free-field noise bursts that varied in intensity and azimuth to determine whether inferior colliculus (IC) neurons derive directionality from monaural spectral-shape. Sixty-nine percent of the sample was directional (much more responsive at some azimuths than others). One hundred twenty-nine directional units were recorded under monaural conditions (unilateral ear plugging). Binaural directional (BD) cells showed weak monaural directionality. Monaural directional (MD) cells showed strong monaural directionality, i.e., were much more responsive at some directions than others. Some MD cells were sensitive to both monaural and binaural directional cues. MD cells were monaurally nondirectional in response to tone bursts that lack direction-dependent variation in spectral shape. MD cells were unresponsive to noise bursts at certain azimuths even at high intensities showing that particular spectral shapes inhibit their responses. Two-tone inhibition was stronger where MD cells were unresponsive to noise stimulation than at directions where they were responsive. According to the side-band inhibition model, MD cells derive monaural directionality by comparing energy in excitatory and inhibitory frequency domains and thus should have stronger inhibitory side-bands than BD cells. MD and BD cells showed differences in breadth of excitatory frequency domains, strength of nonmonotonic level tuning, and responsiveness to tones and noise that were consistent with this prediction. Comparison of these data with previous findings shows that strength of spectral inhibition increases greatly between the level of the cochlear nucleus and the IC, and there is relatively little change in strength of spectral inhibition among the IC, auditory thalamus, and cortex.