Abstract
Recent neuroimaging evidence suggests that the frequencyof entrained oscillations in auditory cortices influences the perceivedduration of speech segments, impacting word perception[Kösem, A., Bosker, H. R., Takashima, A., Meyer, A.,Jensen, O., & Hagoort, P. Neural entrainment determines thewords we hear. Current Biology, 28, 2867–2875, 2018]. We furthertested the causal influence of neural entrainment frequencyduring speech processing, by manipulating entrainment withcontinuous transcranial alternating current stimulation (tACS)at distinct oscillatory frequencies (3 and 5.5 Hz) above the auditorycortices. Dutch participants listened to speech and wereasked to report their percept of a target Dutch word, whichcontained a vowel with an ambiguous duration. Target wordswere presented either in isolation (first experiment) or at theend of spoken sentences (second experiment). We predictedthat the tACS frequency would influence neural entrainmentand therewith how speech is perceptually sampled, leadingto a perceptual overestimation or underestimation of thevowel’s duration. Whereas results from Experiment 1 did notconfirm this prediction, results from Experiment 2 suggested asmall effect of tACS frequency on target word perception:Faster tACS leads to more long-vowel word percepts, in linewith the previous neuroimaging findings. Importantly, the differencein word perception induced by the different tACS frequencieswas significantly larger in Experiment 1 versusExperiment 2, suggesting that the impact of tACS is dependenton the sensory context. tACS may have a stronger effect onspoken word perception when the words are presented incontinuous speech as compared to when they are isolated,potentially because prior (stimulus-induced) entrainment ofbrain oscillations might be a prerequisite for tACS to beeffective.
