Manipulating speech perception with amplitude-modulated kilohertz magnetic perturbation (AM-kTMP).

Speech perception has been hypothesized to exploit neural activity that entrains to the rhythmic properties of the utterance. Perceptual sensitivity varies with the phase of oscillatory activity induced by speech, and with an external oscillatory perturbation induced by transcranial alternating current stimulation (tACS) over the temporal lobe. Here we used a new form of non-invasive brain stimulation (NIBS), kilohertz transcranial magnetic perturbation (kTMP), in which a low frequency perturbation is created by the amplitude modulation (AM) of a continuous high-frequency carrier signal (3.5 kHz). As a magnetic induction method, kTMP can produce much higher electrical fields at the cortical surface than tACS and reduces cutaneous co-stimulation. Human participants (n=40) listened to sequences of rhythmic speech while receiving kTMP with an AM component of 3.125 Hz. The phasic modulation of perceptual sensitivity was greater during AM-kTMP than during sham stimulation, suggesting that AM-kTMP entrains neural activity underlying speech perception. Participants' ratings of rhythmic sensations and other side effects did not differ between AM-kTMP and sham stimulation. Together with its reduced peripheral stimulation, our results highlight kTMP as a promising NIBS method for modulating frequency-specific brain dynamics in speech perception.