Volume 127, Issue 2, February 2010
Index of content:
- PSYCHOLOGICAL ACOUSTICS 
Effects of simulated spectral holes on speech intelligibility and spatial release from masking under binaural and monaural listening127(2010); http://dx.doi.org/10.1121/1.3273897View Description Hide Description
The possibility that “dead regions” or “spectral holes” can account for some differences in performance between bilateral cochlear implant (CI) users and normal-hearing listeners was explored. Using a 20-band noise-excited vocoder to simulate CI processing, this study examined effects of spectral holes on speech reception thresholds (SRTs) and spatial release from masking (SRM) in difficult listening conditions. Prior to processing, stimuli were convolved through head-related transfer-functions to provide listeners with free-field directional cues. Processed stimuli were presented over headphones under binaural or monaural (right ear) conditions. Using Greenwood’s [(Year: 1990). J. Acoust. Soc. Am.87, 2592–2605] frequency-position function and assuming a cochlear length of , spectral holes were created for variable sizes (6 and ) and locations (base, middle, and apex). Results show that middle-frequency spectral holes were the most disruptive to SRTs, whereas high-frequency spectral holes were the most disruptive to SRM. Spectral holes generally reduced binaural advantages in difficult listening conditions. These results suggest the importance of measuring dead regions in CI users. It is possible that customized programming for bilateral CI processors based on knowledge about dead regions can enhance performance in adverse listening situations.
Median-plane sound localization as a function of the number of spectral channels using a channel vocoder127(2010); http://dx.doi.org/10.1121/1.3283014View Description Hide Description
Using a vocoder, median-plane sound localization performance was measured in eight normal-hearing listeners as a function of the number of spectral channels. The channels were contiguous and logarithmically spaced in the range from . Acutely testing vocoded stimuli showed significantly worse localization compared to noises and click trains, both of which were tested after feedback training. However, localization for the vocoded stimuli was better than chance. A second experiment was performed using two different 12-channel spacings for the vocoded stimuli, now including feedback training. One spacing was from experiment 1. The second spacing (called the speech-localization spacing) assigned more channels to the frequency range associated with speech. There was no significant difference in localization between the two spacings. However, even with training, localizing 12-channel vocoded stimuli remained worse than localizing virtual wideband noises by 4.8° in local root-mean-square error and 5.2% in quadrant error rate. Speech understanding for the speech-localization spacing was not significantly different from that for a typical spacing used by cochlear-implant users. These experiments suggest that current cochlear implants have a sufficient number of spectral channels for some vertical-plane sound localization capabilities, albeit worse than normal-hearing listeners, without loss of speech understanding.