Spatial release from masking with noise-vocoded speech.

This study investigated how confusability between target and masking utterances affects the masking release achieved through spatial separation. Important distinguishing characteristics between competing voices were removed by processing speech with six-channel envelope vocoding, which simulates some aspects of listening with a cochlear implant. In the first experiment, vocoded target nonsense sentences were presented against two-talker vocoded maskers in conditions that provide different spatial impressions but not reliable cues that lead to traditional release from masking. Surprisingly, no benefit of spatial separation was found. The absence of spatial release was hypothesized to be the result of the highly positive target-to-masker ratios necessary to understand vocoded speech, which may have been sufficient to reduce confusability. In experiment 2, words excised from the vocoded nonsense sentences were presented against the same vocoded two-talker masker in a four-alternative forced-choice detection paradigm where threshold performance was achieved at negative target-to-masker ratios. Here, the spatial release from masking was more than 20 dB. The results suggest the importance of signal-to-noise ratio in the observation of "informational" masking and indicate that careful attention should be paid to this type of masking as implant processing improves and listeners begin to achieve success in poorer listening environments.