Cutaneous Alternating Current Stimulation Can Cause a Phasic Modulation of Speech Perception

Segregating important stimuli from distractors is crucial for successful speech perception. Neural activity synchronized to speech, also termed "neural speech tracking," is thought to be instrumental for this purpose. However, whether neural tracking of targets and distractors both play a similarly important role for speech perception in a setting with multiple competing speakers is rarely examined. In 61 human participants (30 males, 31 females), we used transcranial alternating current stimulation (tACS) to presumably manipulate neural tracking of two simultaneously presented sequences of rhythmic speech while participants attended to one of them. A random temporal relationship between speech streams allowed us to disentangle effects of tACS on target and distractor processing and to examine their combined effect on a behavioral measure of speech perception. We found that the phase relation between tACS and both target and distracting speech modulated word report accuracy to a similar degree. This effect was observed during bilateral tACS over auditory regions and the inferior frontal gyrus and, importantly, in a control "shunt" group that received near-identical cutaneous stimulation but ∼50% reduced brain stimulation. These results imply that, although tACS phase modulates the perception of both target and distracting speech, the cutaneous stimulation accompanying tACS can induce phase effects in speech perception that resemble those observed with conventional tACS. Our finding illustrates the urgent need to control for cutaneous stimulation in tACS studies.