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

This-study examines the neural representation of the vowel-/epsilon/ 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 /epsilon/, which differed primarily in the frequency of the se cond formant (F2), were used as stimuli. Normally, the rate-place code prov ides a robust representation of F2 for these vowels, in the sense that rate changes encode changes in F2 frequency [Conley and Keilson, J. Acoust. Sec . Am. 98, 3223 (1995)]. This representationis lost after acoustic trauma [M iller et al., J. Acoust. Sec. Am. 105, 311 (1999)]. Here it is shown that a n improved representation of the F2 frequency can be gained by a form of hi gh-frequency emphasis that is determined by both the hearing-loss profile a nd the spectral envelope of the vowel. Essentially, the vowel was high-pass filtered so that the F2 and F3 peaks were amplified without amplifying fre quencies in the trough between F1 and F2. This modification improved the qu ality of the rate and temporal tonotopic representations of the vowel and r estored sensitivity to the F2 frequency. Although a completely normal repre sentation was not restored, this method shows promise as an approach to hea ring-aid signal processing. (C) 1999 Acoustical Society of America. [S0001- 4966(99)04011-4].