Abstract: Code-switching has attracted much attention from researchers with different perspectives. It has widely accepted that when bilinguals switch from one language to another, there is generally a transient cost to performance, namely the so-called switching cost. Switching cost is the main aspect when we explore the essence of the language switching mechanism. The recent study focused on where these costs arise from. The costs could be caused by different mechanisms. It may arise as a result of processes within the bilingual lexicon and/or because of processes outside the lexicon associated with task performance. Although the investigator have done a lot of researches with different experimental paradigms and materials, non-controversial conclusion has not been arrived.
Until now, most conclusions were driven from lexical level or sub-lexical level and most researches was involved by two alphabetic characters and their orthographic specificity. If we select some language units (phrases or sentences)as experimental materials which are involved with grammatical pattern and/or syntax, this will help us to ascertain the source of switching cost in language representation system more than in lexicon or mental dictionary. For phrases, we mean prepositional phrases and noun phrases which represent spatial orientation and quantity separately. This study focused on ideogram characters such as Mandarin characters and phrase level to explore the mechanism of language switching with proficient Chinese-English bilinguals in picture naming task.
29 undergraduates or post-graduates whose major are English participated in two experiments. They were asked to name the picture in either Chinese or English as fast and accurately as possible according to different background color in two experiments. The material of experiment 1 was prepositional phrases. The material of experiment 2 consisted two kinds of noun phrases, either having quantifier or not. All the participants were tested individually on computers. Reaction times for correct responses and error rates were analyzed by subject and item variance.
The results showed that for the proficient Chinese-English bilinguals, the reaction time was shorter and the error rates were lower in non-switch condition. Typical language switching cost was found in both experiments. Furthermore, proficient bilinguals showed shorter naming latencies with their weaker language (English) than with their dominant language (Chinese) in switch condition. However, the switching costs of phrase with quantifier were not significantly different from that of phrase without quantifier. This means that the grammatical pattern and/or syntax did affect the processing of language switching but did not affect the switching cost.
It is claimed that in phrase level, even the proficient bilinguals choose the mechanism of inhibition control to complete the process of word. Therefore, the asymmetry of switching costs appeared without surprise. As language information did not affect the language switching cost in phrase lexical level, we can conclude that the switching cost does not come from the word - recognition system itself but is caused by processes outside the lexicon associated with task performance.

Key words: Chinese-English bilinguals, phrase level, language switching, switching cost