Neural substrates for the encoding of the contextual tonal alternation: An fNIRS study of Mandarin third-tone sandhi in word production

Phonological alternations are common in speech, but the neurocognitive mechanisms for their encoding during word production remain unclear. Mandarin Tone 3 sandhi is an example of phonological alternation, whereby the Tone 3 (T3), a low-dipping tone, changes to a Tone 2 (T2)-like rising tone when followed by another T3. Previous research indicates that both the underlying tonal category and the surface tonal variant are activated during T3 sandhi word production, but the neural substrates of these sub-processes remain unclear. Using Mandarin T3 sandhi as a case study, we employed functional near-infrared spectroscopy (fNIRS) to better understand the neural bases of phonological alternations. Participants completed a phonologically-primed picture naming task, with different tonal relationships between monosyllabic primes and T3 sandhi words manipulated. Behaviorally, we replicated the facilitatory effects of T3 and T2 primes on the naming latencies of T3 sandhi words, confirming the activation of both underlying and surface tonal information. Compared to control primes, the fNIRS data revealed reduced activation in left temporal and bilateral frontal regions during T3 sandhi word production following T3 primes, indicating facilitation in retrieving the underlying tonal category and/or the wordform of T3 sandhi words, which may proceed to the downstream articulatory planning and execution of the context-specific tonal contour. Conversely, increased activation in left temporal regions but decreased activation in frontal regions was found during T3 sandhi word production following T2 primes, implying higher lexical-phonological competition in the wordform retrieval but facilitation in articulatory planning. Our findings offer implications for understanding the neural encoding of phonological alternations.