Theta and Gamma Transcranial Alternating Current Stimulation (tACS) Modulate Mandarin Consonant and Lexical Tone Perception.

A theta/gamma neural activity mechanism has been postulated to explain the auditory sampling of hierarchical syllable-phoneme structure with corresponding speech rates or linguistic hierarchies (theta for syllable and gamma for phoneme). Yet, whether such a mechanism is generalizable to Mandarin Chinese, a tonal language, where lexical tones are at syllable length but function as phonemic cues, is unclear. In this study, we applied transcranial alternating current stimulation (tACS) with theta or gamma currents over bilateral temporal areas in healthy Mandarin-speaking participants during a consonant or tone identification task in quiet or noisy environments. Results showed that theta tACS impaired both consonant and tone identification in quiet and prolonged reaction times across tasks and environments. Moreover, it reduced the psychometric slope for consonant identification in quiet, indicating decreased perceptual sensitivity and less distinct category boundaries. Gamma tACS, however, only delayed tone identification in quiet. Besides, both theta and gamma tACS modulated perceptual decision-making parameters in quiet, leading to increased boundary thresholds (cautiousness of decision) and altered response biases in the perceptual decision of both consonants and tones, as evidenced by the hierarchical drift-diffusion model. These findings indicate that the perception of Mandarin consonants and lexical tones is causally supported by syllable-length theta activity, presumably reflecting syllable-level holistic representations, regardless of environment. Gamma activity, however, is presumably engaged in supporting fast-changing and fine-grained acoustic features in a modulatory manner. Theoretical explanations from historical linguistics and implications for clinical or educational applications are also provided.