Inferior colliculus stimulation and changes in 2f1-f2 distortion product otoacoustic emissions in the rat

The external, central and dorsal subnuclei of the inferior colliculus (ICX, ICC, ICD respectively) have different patterns of descending projections to the periolivary region. We found that electrical stimulation of these subnuclei in anesthetized rats causes suppression of 2f1-f2 distortion product otoacoustic emissions (DPOAE). The responses in DPOAEs to stimulation of ICX and ICC are characterized by a large (7-25 dB), initial suppression which lasted 25-30 s (early phase) followed by a sustained smaller (3-15 dB) suppression (late phase). ICX stimulation produces the largest suppressions of DPOAEs in both contralateral and ipsilateral ears equally. Stimulation of the ICC causes large suppressions in the contralateral ear DPOAEs but much smaller than the suppressions of DPOAEs caused by ICX stimulation. Stimulation of the ICC causes a much smaller suppression in the ipsilateral ear DPOAEs than in the contralateral ear DPOAEs. ICD stimulation, however, produces little or no suppression of DPOAEs in either ear when compared to other subnuclei. No frequency-specific changes in DPOAEs were seen with stimulation of any of the subnuclei of the inferior colliculus (IC). Sectioning of middle ear muscles did not negate the suppressive effects of IC stimulation on DPOAEs. These findings verify that the IC has a subnucleus-specific influence over cochlear micromechanics.