Effects of bilateral olivocochlear lesions on vowel formant discrimination in cats

Operant conditioning procedures were used to measure the effects of bilateral olivocochlear lesions on the cat's discrimination thresholds for changes in the second formant frequency (ΔF₂) of the vowel /ε/. Three cats were tested with the formant discrimination task under quiet conditions and in the presence of continuous broadband noise at signal-to-noise ratios (S/Ns) of 23, 13, and 3 dB. In quiet, vowel levels of 50 and 70 dB produced average ΔF₂s of 42 and 47 Hz, respectively, and these thresholds did not change significantly in low levels of background noise (S/Ns = 23 and 13 dB). Average ΔF₂s increased to 94 and 97 Hz for vowel levels of 50 and 70 dB in the loudest level of background noise (S/N = 3 dB). Average ΔF₂ thresholds in quiet and in lower noise levels were only slightly affected when the olivocochlear bundle was lesioned by making bilateral cuts into the floor of the IVth ventricle. In contrast, post-lesion ΔF₂ thresholds in the highest noise level were significantly larger than pre-lesion values; the most severely affected subject showed post-lesion discrimination thresholds well over 200 Hz for both 50 and 70 dB vowels. These results suggest that olivocochlear feedback may enhance speech processing in high levels of ambient noise.