FREQUENCY-SHAPED AMPLIFICATION CHANGES THE NEURAL REPRESENTATION OF SPEECH WITH NOISE-INDUCED HEARING-LOSS

Temporal response patterns of single auditory nerve fibers were used t o characterize the effects of a common hearing-aid processing scheme, frequency-shaped amplification, on the encoding of the vowel /epsilon/ in cats with a permanent noise-induced hearing loss. These responses were contrasted with responses to unmodified stimuli in control and im paired cats. Noise-induced hearing loss leads to a degraded representa tion of the formant frequencies, in which strong phase locking to the formants is not observed in fibers with best frequencies (BFs) near th e formants and there is a wide spread of formant phase locking to fibe rs with higher BFs (Miller et al., 1997a,b). Frequency shaping effecti vely limits the upward spread of locking to Fl, which improves the rep resentation of higher frequency components of the vowel. However, it a lso increases phase locking to harmonics in the trough between the for mants, which decreases the contrast between Fl and the trough in the n eural representation. Moreover, it does not prevent the spread to high er BFs of responses to the second and third formants. The results show a beneficial effect of frequency shaping, but also show that interact ions between particular gain functions and particular spectral shapes can result in unwanted distortions of the neural representation of the signal. (C) 1998 Published by Elsevier Science B.V.