Recognition of spectrally asynchronous speech by normal-hearing listeners and Nucleus-22 cochlear implant users

This experiment examined the effects of spectral resolution and fine spectr al structure on recognition of spectrally asynchronous sentences by normal- hearing and cochlear implant listeners. Sentence recognition was measured i n six normal-hearing subjects listening to either full-spectrum or noise-ba nd processors and five Nucleus-22 cochlear implant listeners fitted with 4- channel continuous interleaved sampling (CIS) processors. For the full-spec trum processor, the speech signals were divided into either 4 or 16 channel s. For the noise-band processor, after band-pass filtering into 4 or 16 cha nnels, the envelope of each channel was extracted and used to modulate nois e of the same bandwidth as the analysis band, thus eliminating the fine spe ctral structure: available in the full-spectrum processor. For the 4-channe l CTS processor, the amplitude envelopes extracted from four bands were tra nsformed to electric currents by a power function and the resulting electri c currents were used to modulate pulse trains delivered to four electrode p airs. For all processors, the output of each channel was time-shifted relat ive to other channels, varying the channel delay across channels from 0 to 240 ms tin 40-ms steps). Within each delay condition, all channels were des ynchronized such that the cross-channel delays between adjacent channels we re maximized, thereby avoiding local pockets of channel synchrony. Results show no significant difference between the 4- and 16-channel full-spectrum speech processor for normal-hearing listeners. Recognition scores dropped s ignificantly only when the maximum delay reached 200 ms for the 4-channel p rocessor and 240 ms for the 16-channel processor. When fine spectral struct ures were removed in the noise-band processor, sentence recognition dropped significantly when the maximum delay was 160 ms for the 16-channel noise-b and processor and 40 ms for the 4-channel noise-band processor. There was n o significant difference between implant listeners using the 4-channel CIS processor and normal-hearing listeners using the 4-channel noise-band proce ssor. The results imply that when fine spectral structures are not availabl e, as in the implant listener's case, increased spectral resolution is impo rtant for overcoming cross-channel asynchrony in speech signals. (C) 2001 A coustical Society of America.