Unintelligible low-frequency sound enhances simulated cochlear-implant speech recognition in noise

Speech can be recognized by multiple acoustic cues in both frequency and time domains. These acoustic cues are often thought to be redundant. One example is the low-frequency sound component below 300 Hz, which is not even transmitted by the majority of communication devices including telephones. Here, we showed that this low-frequency sound component, although unintelligible when presented alone, could improve the functional signal-to-noise ratio (SNR) by 10-15 dB for speech recognition in noise when presented in combination with a cochlear-implant simulation. A similar low-frequency enhancement effect could be obtained by presenting the low-frequency sound component to one ear and the cochlear-implant simulation to the other ear. However, a high-frequency sound could not produce a similar speech enhancement in noise. We argue that this low-frequency enhancement effect cannot be due to linear addition of intelligibility between low- and high-frequency components or an increase in the physical SNR. We suggest a brain-based mechanism that uses the voice pitch cue in the low-frequency sound to first segregate the target voice from the competing voice and then to group appropriate temporal envelope cues in the target voice for robust speech recognition under realistic listening situations.