Gap detection in single- and multiple-channel stimuli by LAURA cochlear implantees

Gap-detection thresholds were determined for different complex patterns of electrical stimulation in four postlingually deafened LAURA cochlear implan tees, to examine the nature of within- and across-channel auditory processe s in more detail. Gap detectability was examined as a function of stimulus complexity (one, two, or three channels), channel distance within and acros s multichannel pre- and post-gap markers, stimulus asymmetry, and pulse rat e. All markers roved in duration from 200 to 500 ms to ensure that subjects were not using overall stimulus duration as a cue. Gap-detection threshold s for all subjects were short (<5 ms) when the pre- and post-gap markers st imulated the same single or multiple channels, even when the distance betwe en simultaneously stimulated channels was large (exp. 1). For some subjects , gap detectability was more difficult in the across-channel condition, whe n the pre- and post-gap markers each stimulated different channels, althoug h performance improved substantially in most subjects after extensive train ing (exp. 2). Additional tests with random maskers also suggest that neural interaction only affects performance mildly, and that the magnitude of the gap-detection threshold probably depends more on the subject's cognitive ( in)ability to attend to the temporal gap than on the temporal acuity of the ir auditory system. Other stimulus conditions showed a difference in perfor mance related to the order of the markers: gap thresholds were longer when the pre-gap marker stimulated one channel and the post-gap marker stimulate d two or more channels, than vice versa (exp. 3). In addition, gap threshol ds of three of the subjects increased with decreasing pulse rate from 1250 to 400 pps, a finding which may be related to the rate of the speech proces sing strategies used by each individual implantee (exp. 4). (C) 1999 Acoust ical Society of America. [S0001-4966(99)01010-3].