Volume 117, Issue 3, March 2005
Index of content:
- SPEECH PERCEPTION 
117(2005); http://dx.doi.org/10.1121/1.1861158View Description Hide Description
Ito et al. [J. Acoust. Soc. Am. 110, 1141–1149 (2001)] demonstrated that listeners can reliably identify vowel stimuli on the basis of relative formant amplitude in the absence of, or in spite of, peak frequency. In the present study, formant frequencies and global spectral tilt are manipulated independently in synthetic steady-state vowels. Listeners’ identification of these sounds demonstrate strong perceptual effects for both local (formant frequency) and global (spectral tilt) acoustic characteristics. Subsequent experiments reveal that effects of spectral tilt are attenuated in synthetic stimuli for which formant center frequencies change continuously. When formant peaks are kinematic, perceptual salience of the relative amplitudes of low- and high-frequency formants (as determined by spectral tilt) is mitigated. Because naturally produced English vowels are rarely spectrally static, one may conclude that gross spectral properties may play only a limited role in perception of fluently produced vowelsounds.
117(2005); http://dx.doi.org/10.1121/1.1856273View Description Hide Description
While new electrode designs allow deeper insertion and wider coverage in the cochlea, there is still considerable variation in the insertion depth of the electrode array among cochlear implant users. The present study measures speech recognition as a function of insertion depth, varying from a deep insertion of 10 electrodes at 28.8 mm to a shallow insertion of a single electrode at 7.2 mm, in four Med-El Combi users. Short insertion depths were simulated by inactivating apical electrodes.Speech recognition increased with deeper insertion, reaching an asymptotic level at 21.6 or 26.4 mm depending on the frequency-place map used. Başkent and Shannon [J. Acoust. Soc. Am. 116, 3130–3140 (2004)] showed that speech recognition by implant users was best when the acoustic input frequency was matched onto the cochlear location that normally processes that frequency range, minimizing the spectral distortions in the map. However, if an electrode array is not fully inserted into the cochlea, a matched map will result in the loss of considerable low-frequency information. The results show a strong interaction between the optimal frequency-place mapping and electrode insertion depth. Consistent with previous studies, frequency-place matching produced better speech recognition than compressing the full speech range onto the electrode array for full insertion ranges (20 to 25 mm from the round window). For shallower insertions (16.8 and 19.2 mm) a mild amount of frequency-place compression was better than truncating the frequency range to match the basal cochlear location. These results show that patients with shallow electrode insertions might benefit from a map that assigns a narrower frequency range than patients with full insertions.