Page 24 - Volume 12, Issue 2 - Spring 2012

by the inherent limitation in the speech processors that are used in today’s CIs. The hardware and signal processing in the implantable devices are far from ideal as far as providing binaural cues with fidelity. Bilateral CI users are essentially fit with two separate monaural systems. Speech processing strategies in clinical processors utilize pulsatile, non-simulta- neous multi-channel stimulation, whereby a bank of band pass filters is used to filter the incoming signal into numer- ous frequency bands (ranging from 12 to 22), and to send specific frequency ranges to individual electrodes. The enve- lope of the signal is extracted from the output of each band and is used to set stimulation levels for each frequency band; thus, fine-structure is discarded. Although ITDs in the envelopes may be present, because the processors have inde- pendent switch-on times, the ITD can vary dynamically and unreliably (van Hoesel, 2004, 2011). In addition, the micro- phones are not placed in the ear in a manner that maximizes the capture of directional cues such as spectrum and level cues. Microphone characteristics, independent automatic gain control and compression settings distort the monaural and interaural level directional cues that would otherwise be present in the horizontal plane. In an ideal situation, speech processors would provide bilateral CI users with binaural cues, similar to those available to normal-hearing listeners.
In summary, this paper focuses on the ability of humans to understand speech in complex noisy environments, and to benefit from spatial separation of maskers from target speech. Both binaural and monaural cues play a role in pro- viding this benefit. The effect size can depend on the extent to which listeners “need” spatial cues for source segregation. That is, when other auditory cues are unavailable, such as when target/masker similarity is high, spatial cues are espe- cially relevant, thus SRM can be large. SRM also varies, depending on the population of listeners being tested, and the integrity of their binaural auditory system. While mon- aural cues are generally seen in all listeners, when head shad- ow is available, binaural cues are only useful when preserved and presented to the auditory system with fidelity.AT
References
Agrawal, S. (2008). “Spatial hearing abilities in adults with bilateral cochlear implants,” unpublished dissertation, University of Wisconsin-Madison, Memorial Library, 728 State Street, Madison, WI 53706.
Arbogast, T. L., Mason, C. R., and Kidd, G., Jr. (2002). “The effect of spatial separation on informational masking of speech in nor- mal-hearing and hearing-impaired listeners,” J. Acoust. Soc. Am. 117, 2169–2180.
Bernstein, L. R., and Trahiotis, C. (1992). “Discrimination of inter- aural envelope correlation and its relation to binaural unmask- ing at high frequencies,” J. Acoust. Soc. Am. 91, 306–316.
Best, V., Ozmeral, E. J., and Shinn-Cunningham, B. G. (2007). “Visually-guided attention enhances target identification in a complex auditory scene,” J. Assn. Res. Otolaryngol. 8, 294–304.
Blauert, J. (1997). Spatial Hearing: The Psychophysics of Human Sound Localization, revised edition (MIT Press, Cambridge, MA).
Bronkhorst, A. W. (2000). “The cocktail party phenomenon: A review of research on speech intelligibility in multiple-talker
conditions,” Acustica 86, 117–128.
Brungart, D. S., Simpson, B. D., Ericson, M. A., and Scott, K. R.
(2001). “Informational and energetic masking effects in the per-
ception of multiple talkers,” J. Acoust. Soc. Am. 110, 2527–2538. Cherry, E. C. (1953). “Some experiments on the recognition of speech, with one and with two ears,” J. Acoust. Soc. Am. 25,
975–979.
Culling, J. F., and Summerfield, Q. (1995). “Perceptual separation of
concurrent speech sounds: Absence of across-frequency group- ing by common interaural delay,” J. Acoust. Soc. Am. 98, 785–797.
Culling, J. F., Hawley, M. L., and Litovsky, R. Y. (2004). “The role of head-induced interaural time and level differences in the speech reception threshold for multiple interfering sound sources,” J. Acoust. Soc. Am. 116, 1057–1065.
Drullman, R., and Bronkhorst, A. W. (2002). “Multichannel speech intelligibility and talker recognition using monaural, binaural, and three-dimensional auditory presentation,” J. Acoust. Soc. Am. 107, 2224–2235.
Durlach, N. I., Mason, C. R., Kidd, G., Jr., Arbogast, T. L., Colburn, H. S., and Shinn-Cunningham, B. G. (2003). “Note on informa- tional masking,” J. Acoust. Soc. Am. 11, 2984–2987.
Freyman, R. L., Balakrishnan, U., and Helfer, K. S. (2001). “Spatial release from informational masking in speech recognition,” J. Acoust. Soc. Am. 109, 2112–2122.
Freyman, R. L., Helfer, K. S., McCall, D. D., and Clifton, R. K. (1999). “The role of perceived spatial separation in the unmask- ing of speech,” J. Acoust. Soc. Am. 106, 3578–3588.
Gabriel, K. J., and Colburn, H. S. (1981). “Interaural correlation dis- crimination: I. Bandwidth and level dependence,” J. Acoust. Soc.
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