Recognition of spectrally degraded and frequency-shifted vowels in acoustic and electric hearing

The present study measured the recognition of spectrally degraded and frequ ency-shifted vowels in both acoustic and electric hearing. Vowel stimuli we re passed through 4, 8, or 16 bandpass filters and the temporal envelopes f rom each filter band were extracted by half-wave rectification and low-pass filtering. The temporal envelopes were used to modulate noise bands which were shifted in frequency relative to the corresponding analysis filters. T his manipulation not only degraded the spectral information by discarding w ithin-band spectral detail, but also shifted the tonotopic representation o f spectral envelope information. Results from five normal-hearing subjects showed that vowel recognition was sensitive to both spectral resolution and frequency shifting. The effect of a frequency shift did not interact with spectral resolution, suggesting that spectral resolution and spectral shift ing are orthogonal in terms of intelligibility. High vowel recognition scor es were observed for as few as four bands. Regardless of the number of band s, no significant performance drop was observed for tonotopic shifts equiva lent to 3 mm along the basilar membrane, that is, for frequency shifts of 4 0%-60%. Similar results were obtained from five cochlear implant listeners, when electrode locations were fixed and the spectral location of the analy sis filters was shifted. Changes in recognition performance in electrical a nd acoustic hearing were similar in terms of the relative location of elect rodes rather than the absolute location of electrodes, indicating that coch lear implant users may at least partly accommodate to the new patterns of s peech sounds after long-time exposure to their normal speech processor. (C) 1999 Acoustical Society of America. [S0001-4966(99)00603-7].