Cochlear implant listeners today enjoy reasonably good speech understanding in quiet but generally experience severe deterioration in performance in the presence of background sounds. To provide usable speech information through the cochlear implant in the presence of competing sources (speech or nonspeech), basic principles underlying the perception of complex, multichannel stimuli by the electrically stimulated auditory system need to be investigated. This proposal aims to investigate mechanisms underlying complex pattern perception by cochlear implant listeners. The long-term goal of this research is to discover fundamental principles of complex stimulus perception by both electrically stimulated and normal hearing auditory systems. Two hypotheses will be investigated: a) that principles of perceptual organization can be used by cochlear implant listeners for separating out sounds from multiple sources and b) noise is important in signal processing by biological systems in general and by the auditory system in particular, and the right kind of noise introduced into cochlear implants will be beneficial. The proposed experiments are designed to measure the perceptual interactions between pulse-train carriers with similar / dissimilar temporal envelopes exciting adjacent / distant tonotopic locations. Methods will include detection and discrimination of complex stimuli, perceptual difference measures, and auditory streaming measures. Effects of noise on the detection of envelope cues in single- and multi-channel complex stimuli will be measured. It is expected that this research will yield fundamental insight into mechanisms of auditory perception with cochlear implants. Because of their fundamental nature, the findings are likely to also further our understanding of the normally functioning auditory system.