Biasing the perception of spoken words with tACS

Abstract

Recent neuroimaging evidence suggests that the frequency of entrained oscillations in auditory cortices influences the perceived duration of speech segments, impacting word perception (Kösem et al. 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 Hz 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 frequency of the tACS current would influence neural entrainment and therewith how speech is perceptually sampled, leading to a perceptual over-or underestimation of the vowel duration. Experiment 1 revealed no significant result. In contrast, results from experiment 2 showed a significant effect of tACS frequency on target word perception. Faster tACS lead to more long-vowel word percepts, in line with previous findings suggesting that neural oscillations are instrumental in the temporal processing of speech. The different results from the two experiments suggest 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 a continuous stream of 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.

No evidence for modulation of sound rise-time perception by 4-Hz brain oscillations

“…However, whether these measures sufficed to prevent toneinduced phase resets remains unclear and would have required directly measuring brain oscillations. The strengths of our tACS and experimental manipulation were likely sufficient to modulate brain oscillations as they have proven effective in some, although not all, speech-perception studies with similar statistical power [7,10]. A perhaps more exciting interpretation is that slow cortical oscillations contribute less to the perception of ART, but more to its linguistic interpretation.…”

“…The short tone stimuli in our experiment provided a continuum of basic acoustic differences without any linguistic information. When linguistic stimuli are used, subtle changes in ART or cortical phase have been observed to affect the categorical perception of phonemes (e.g.,/d/vs/t/, short/a/vs long/ a:/) [6,10]. Thus slow cortical oscillations may adapt the boundaries of linguistic categories rather than the temporal representation of individual ARTs [7].…”