While listening to meaningful speech, auditory input is processed more rapidly near the end (vs. beginning) of sentences. Although several studies have shown such word-to-word changes in auditory input processing, it is still unclear from which processing level these word-to-word dynamics originate. We investigated whether predictions derived from sentential context can result in auditory word-processing dynamics during sentence tracking. We presented healthy human participants with auditory stimuli consisting of word sequences, arranged into either predictable (coherent sentences) or less predictable (unstructured, random word sequences) 42-Hz amplitude-modulated speech, and a continuous 25-Hz amplitude-modulated distractor tone. We recorded RTs and frequency-tagged neuroelectric responses (auditory steady-state responses) to individual words at multiple temporal positions within the sentences, and quantified sentential context effects at each position while controlling for individual word characteristics (i.e., phonetics, frequency, and familiarity). We found that sentential context increasingly facilitates auditory word processing as evidenced by accelerated RTs and increased auditory steady-state responses to later-occurring words within sentences. These purely top–down contextually driven auditory word-processing dynamics occurred only when listeners focused their attention on the speech and did not transfer to the auditory processing of the concurrent distractor tone. These findings indicate that auditory word-processing dynamics during sentence tracking can originate from sentential predictions. The predictions depend on the listeners' attention to the speech, and affect only the processing of the parsed speech, not that of concurrently presented auditory streams.

Recent neuroimaging evidence suggests that the frequency of entrained oscillations in auditory cortices influences the perceived duration of speech segments, impacting word perception [Kösem, A., Bosker, H. R., Takashima, A., Meyer, A., Jensen, O., & Hagoort, P. Neural entrainment determines the words we hear. Current Biology , 28 , 2867–2875, 2018]. We further tested the causal influence of neural entrainment frequency during speech processing, by manipulating entrainment with continuous transcranial alternating current stimulation (tACS) at distinct oscillatory frequencies (3 and 5.5 Hz) above the auditory cortices. Dutch participants listened to speech and were asked to report their percept of a target Dutch word, which contained a vowel with an ambiguous duration. Target words were presented either in isolation (first experiment) or at the end of spoken sentences (second experiment). We predicted that the tACS frequency would influence neural entrainment and therewith how speech is perceptually sampled, leading to a perceptual overestimation or underestimation of the vowel's duration. Whereas results from Experiment 1 did not confirm this prediction, results from Experiment 2 suggested a small effect of tACS frequency on target word perception: Faster tACS leads to more long-vowel word percepts, in line with the previous neuroimaging findings. Importantly, the difference in word perception induced by the different tACS frequencies was significantly larger in Experiment 1 versus Experiment 2, suggesting that the impact of tACS is dependent on the sensory context. tACS may have a stronger effect on spoken word perception when the words are presented in continuous speech as compared to when they are isolated, potentially because prior (stimulus-induced) entrainment of brain oscillations might be a prerequisite for tACS to be effective.