Perception of Rhythmic Speech Is Modulated by Focal Bilateral Transcranial Alternating Current Stimulation

Abstract

Several recent studies have used transcranial alternating current stimulation (tACS) to demonstrate a causal role of neural oscillatory activity in speech processing. In particular, it has been shown that the ability to understand speech in a multi-speaker scenario or background noise depends on the timing of speech presentation relative to simultaneously applied tACS. However, it is possible that tACS did not change actual speech perception but rather auditory stream segregation. In this study, we tested whether the phase relation between tACS and the rhythm of degraded words, presented in silence, modulates word report accuracy. We found strong evidence for a tACS-induced modulation of speech perception, but only if the stimulation was applied bilaterally using ring electrodes (not for unilateral left hemisphere stimulation with square electrodes). These results were only obtained when data were analyzed using a statistical approach that was identified as optimal in a previous simulation study. The effect was driven by a phasic disruption of word report scores. Our results suggest a causal role of neural entrainment for speech perception and emphasize the importance of optimizing stimulation protocols and statistical approaches for brain stimulation research.

Sustained neural rhythms reveal endogenous oscillations supporting speech perception

“…We hypothesized that entrained neural activity due to tACS (irrespective of whether it involves endogenous oscillations; Fig. 3B) will produce a phasic modulation of speech perception in the ongoing tACS condition, as reported previously (8)(9)(10). However, in the pretarget tACS condition, such a phasic modulation can only be explained by sustained neural oscillations which lead to rhythmic changes in perception ( Fig.…”

“…This finding has important implications for future studies: Given the previous reports of tACS-induced changes in speech processing (7)(8)(9)(10)(11), tACS may be a promising tool to treat conditions associated with deficits in speech comprehension. However, individual differences in have so far hampered this goalexisting data suggest that different tACS phases will lead to optimal perception for each individual participant and extensive testing might therefore be needed to determine this optimal phase before further interventions.…”

“…Previous studies have reported that "preferred" tACS phase (leading to highest accuracy) varies across participants (7)(8)(9)(10). Indeed, in neither of the two conditions did we find evidence for a non-uniform distribution of φ To statistically evaluate the hypothesized phasic modulation of word report accuracy, we compared the observed cosine amplitudes ( Fig.…”

“…Several studies have reported modulation of speech perception outcomes by tACS, and conclude that changes in neural entrainment, produced by varying the phase relation between tACS and speech rhythm, are responsible (8)(9)(10)(11). However, thus far these effects could reflect the rhythmic nature of the applied current, which might interfere with processing of speech presented with the same rhythm without any involvement of neural oscillations (15).…”

“…Indeed, brain responses seem to align more reliably to intelligible than to unintelligible speech (5,6). Similarly, rhythmic electrical stimulation applied to the scalp (tACS) is assumed to "entrain" brain oscillations and has been shown to modulate speech processing and perception (7)(8)(9)(10)(11). Despite the prominence of entrainment theories in speech research and elsewhere (1, [12][13][14], it has been surprisingly difficult to demonstrate that stimulus-aligned brain responses indeed involve endogenous neural oscillations.…”