Cutaneous alternating current stimulation can cause aphasic 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, the relative contribution of neural tracking of targets and distractors for speech perception in a setting with multiple competing speakers remained unclear. In 61 human participants, we used transcranial alternating current stimulation (tACS) to 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 behavioural 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, the inferior frontal gyrus (IFG) and, importantly, in a control group that received near-identical cutaneous stimulation but [~]50% reduced brain stimulation. These results imply that, although neural tracking of both target- and distracting speech causally modulates speech perception, the cutaneous stimulation that goes along with tACS can cause phase effects in speech perception that resemble those for conventional tACS. Our finding illustrates the urgent need to control for cutaneous stimulation in tACS studies. Significance StatementNeural activity synchronised to speech ("neural speech tracking") plays an important role in speech perception, yet its exact role in multi-speaker scenarios remains underexplored. We here use tACS to manipulate neural tracking in such scenarios. We find that neural tracking of both target and distracting speech causally modulates speech perception, irrespective of whether tACS is applied to target auditory cortex, inferior frontal gyrus, or in a control condition designed to produce similar cutaneous stimulation, but [~]50% reduced direct brain stimulation. Although our results demonstrate a causal role of neural tracking for speech perception in multi-speaker scenarios, they also illustrate the urgent need to control for effects of cutaneous stimulation in future tACS work on speech perception and beyond.