Auditory supplements to speechreading: combining amplitude envelope cues from different spectral regions of speech

Many listeners with severe-to-profound hearing losses perceive only a narrow range of low-frequency sounds and must rely on speechreading to supplement the impoverished auditory signal in speech recognition. Previous research with normal-hearing subjects [Grant et al., J. Exp. Psychol. 43A, 621-645 (1991)] demonstrated that speechreading was significantly improved when supplemented by amplitude-envelope cues that were extracted from various spectral regions of speech and presented as amplitude modulations of carriers with frequencies at or below the speech band from which the envelope was derived. This experiment assessed the benefit to speechreading provided by pairs of such envelope cues presented simultaneously. In general, greater improvements in speechreading scores were observed for pairs than for single envelopes when the carrier signals were chosen appropriately. However, when pairs of envelope signals were transposed to low frequencies, the benefit to speechreading was no better than the most effective single-band envelope signal tested, or for a low-pass-filtered speech signal with the same overall bandwidth. Suggestions for improving the efficacy of frequency-lowered envelope cues for hearing-impaired listeners are discussed.