Electrophysiological measures of listening effort and comprehension: Speech in noise, vocoders, and competing talkers

This study explored the relationship between signal degradation and electrophysiological measures of speech processing, in order to understand how these measures relate to intelligibility and listening effort. Subjects heard two talkers presented to different ears, with the target talker presented in quiet, three levels of speech-shaped noise ( + 3 dB, -0.5dB, -4dB SNR), or a 14-channel noise vocoder. The speech materials were simultaneous sentences that varied in final-word predictability; subjects were asked to monitor the sentences for catch trials (i.e., semantically anomalous final words). The results demonstrated that cortical entrainment to the amplitude envelope of the target was higher than for the distractor, as found in previous studies. Entrainment and lexical processing are robust with increasing noise, then fall when the stimulus becomes very poorly intelligible. Unexpectedly, results from the vocoded condition indicate no entrainment advantage for the target speaker, even when this condition is intelligible.