Contrast enhancement improves the representation of /ɛ/-like vowels in the hearing-impaired auditory nerve

This study examines the neural representation of the vowel /ɛ/ in the auditory nerve of acoustically traumatized cats and asks whether spectral modifications of the vowel can restore a normal neural representation. Four variants of /ɛ/, which differed primarily in the frequency of the second formant (F2), were used as stimuli. Normally, the rate-place code provides a robust representation of F2 for these vowels, in the sense that rate changes encode changes in F2 frequency [Conley and Keilson, J. Acoust. Soc. Am. 98, 3223 (1995)]. This representation is lost after acoustic trauma [Miller et al., J. Acoust. Soc. Am. 105, 311 (1999)]. Here it is shown that an improved representation of the F2 frequency can be gained by a form of high-frequency emphasis that is determined by both the hearing-loss profile and the spectral envelope of the vowel. Essentially, the vowel was high-pass filtered so that the F2 and F3 peaks were amplified without amplifying frequencies in the trough between F1 and F2. This modification improved the quality of the rate and temporal tonotopic representations of the vowel and restored sensitivity to the F2 frequency. Although a completely normal representation was not restored, this method shows promise as an approach to hearing-aid signal processing.