Theta and gamma transcranial alternating current stimulation modulate Mandarin consonant and lexical tone perception

A theta/gamma oscillatory neural 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 language that both has suprasegmental phonemes in the form of lexical tones and contains more monosyllabic words, which may lead to higher reliance on either phonemic-level or syllabic-level perceptual representations, is unclear. In this study, we applied transcranial electric stimulation with either theta or gamma alternating currents (tACS) to bilateral auditory cortices in healthy Mandarin-speaking participants during a consonant or tone identification task in quiet or noisy environments. Results showed that theta tACS impaired consonant identification in quiet, specifically, and generally prolonged reaction times across tasks and environments. Gamma tACS, however, only delayed tone identification in quiet. Besides, both theta and gamma tACS modulated perceptual decision-making parameters, 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 (HDDM). These findings indicate that theta oscillations causally support the perception of Mandarin syllables, with consonants and lexical tones presumably embedded in syllable-level representations, regardless of task difficulty. Gamma activity, however, is presumably engaged in supporting fast-changing and fine-grained acoustic features in a modulatory manner. How the brain samples speech features for perception is a key question in language neuroscience. Previous research has shown that theta and gamma brain activity support the perception of suprasegmental syllables and segmental phonemes, corresponding to their length or linguistic hierarchy. However, for Mandarin, the existence of suprasegmental phonemes (lexical tones) and the large number of monosyllabic words can either lead to higher reliance on gamma oscillations for phoneme perception, or higher dependence on theta oscillations for holistic syllable integration. With the combination of brain stimulation to selectively modulate theta/gamma auditory activity and phoneme identification tasks, this study reveals a causal role of theta activity in supporting holistic syllable-based Mandarin phoneme perception, yet gamma engagement is modulatory.