Listening effort and difficult listening conditions: Lexical processing and neural entrainment with noise and vocoders
Listening effort, the increased attention associated with hearing under non-optimal conditions, has been measured in three ways: subjective measures, behavioural measures, and physiological measures. In this study, we examined all three with two aims in mind: first, to explore the relationship between attention, signal degradation types, and neural entrainment; and second, to relate different ways of measuring listening effort with one another. We created stimuli for a dichotic listening experiment with different types and levels of degradation. We prepared five different stimuli conditions: no signal degradation, three levels of SNR (+3 dB, -0.5dB, -4dB), and one vocoded condition (14 channels). Noise was speech-shaped noise matched to the spectrum of the target talker. SNR levels were determined in a pilot experiment and fixed to match different intelligibility levels. Participants were presented with a dichotic listening task and were instructed to focus on the female (target) speaker, while ignoring the male (distractor) speaker. Stimuli consisted of single sentences, and were either semantically predictable, unpredictable or semantically incoherent. In order to ensure participants’ attention to the task, and to gain a behavioural measure of listening effort, they were instructed to press a button whenever they heard a semantically incoherent sentence. Subjective measures of listening effort were collected through a questionnaire administered at the end of the experiment. Results of this study indicate that cortical entrainment to the target speaker is higher than the distractor, which is in line with previous research. Entrainment and lexical processing are robust with increasing noise, then fall when the stimulus becomes (almost) unintelligible (-4 dB SNR). Unexpectedly, results from the vocoded condition indicate no entrainment advantage for the target speaker, even at an intelligible level. In future studies, we intend to explore the issue with different stimuli and listener groups.