Biasing speech perception with transcranial alternating current stimulation

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.