Languages may differ in terms of the proximate units of phonological encoding in spoken word production. It is widely accepted that phonemes are the primary units used in Indo-European languages. However, it is controversial what the functional units (syllables or phonemes) are in Chinese spoken word production. In the framework of proximate unit principle hypothesis for Mandarin Chinese spoken word production, phonological content is activated in a syllable while its syllabic frame is retrieved. Both syllabic content and frame are linked sequentially and tone is also specified at this point. Crucially, syllables are retrieved at an early stage of word form encoding, and then the phonemes of the syllable are retrieved in parallel and linked to their position in the syllable frame. The present study investigated the temporal courses of syllables and phonemes in word production using an implicit priming paradigm, and electrophysiological signals were measured concurrently. Specifically, participants were young native Chinese speakers with low level of English ability for excluding the influence of the second language. In an implicit priming paradigm, four sets of word pairs served as experimental stimuli. Each set consisted of four pairs, and the first word of a pair was the cue word, while the second was the response word. In the homogeneous conditions, four response words in a set with common phonological properties. For instance, the four response words in a set were (/wei1ji1/, crisis), (/wei2qiang2/, bounding wall), (/wei3ba/, tail), and (/wei4xing1/, satellite), which shared the first syllable but not tone. The heterogeneous conditions used the same word pairs, but assigned them to sets in which they didn't share any properties. Two homogeneous conditions (syllable shared and initial phoneme shared) were used in the study. Participants were asked to memorize the sets of four pairs of associate words, after which they were presented with the cue words, and were required to produce the response words. Behavioral results didn't show any syllable or phoneme effects. EEG results revealed that syllable overlap condition elicited significant effects in the time window of 100 similar to 400 ms, and the effect firstly showed up in the left and right anterior regions, following in the middle anterior region, and finally in the left and right anterior regions, right and middle posterior regions. While the phoneme effects only appeared between 500 similar to 600 ms after the cue words were presented, and were distributed in the right anterior and posterior regions. Both the syllabically and phonemically homogeneous conditions were characterized by more positive ERP waveforms when compared to the heterogeneous conditions. We suggest that syllable overlap effect arises at an early stage of phonological encoding, whereas the phoneme overlap effect possibly arises at a late stage of phonological encoding, or phonetic encoding in Chinese spoken word production. Our findings provide support for the assumption of the proximate unit principle.
