雙語處理的腦神經理論：舊題新探

Neural Bilingualism: A New Look at an Old Problem

全球化及數位化的時代為人類的生活帶來急劇的變遷與多語言的環境，日常生活中使用雙語已經是一種常態現象。語言是人類演化下的產物，具有演化歷程中的複雜性與多樣性，而人類的歷史反映在不斷演化的大腦中，大腦本身就是一個因應演化需求發展而成的適應系統，透過統計學習的基本能力，人們能夠習得語言，包括母語及第二語言。同時，在大腦可塑性的基礎上，透過長期經驗或訓練學習產生神經迴路的有效性連結，強化了語言處理及一般執行監控的認知處理的神經迴路，所以雙語發展是大腦神經迴路的重組。透過非侵入性的腦造影設施，研究證據揭露了即使是過了所謂語言學習關鍵期的成年人也可以成功習得第二語言，可見得第二語言習得年齡並非唯一關鍵因素。語言的精熟程度，個體差異，訓練的方式等也都會影響第二語言學習的成效。腦中的雙語，在使用中建構了雙語的心理詞彙，非意識控制的跨語言激發導致雙語競爭，帶來抑制非目標語的認知控制增強的後果，產生跨越年齡層的大腦的結構上和功能上的改變，這樣的效果持續到在老年人身上則具有認知保留的優勢。在現今的數位時代，從日新月異的新媒體載具中擷取資訊的方式對於人類大腦的感知、認知、記憶、搜尋、提取、執行監控等能力都將產生影響。透過數位科技的輔助，可能改變我們使用雙語的方式及學習的機制，在一腦多文的複雜適應系統的運作之下，我們提出「何時停止左腦側化？」的新問題取代「左腦側化何時開始？」的舊議題，有助於揭露大腦演化的本質，預期數位時代將改寫未來人類智慧的定義。

Our brain is an inherited survival system, the result of a relentless struggle to live, evolving and adapting to countless primordial challenges. As such, brain evolution can be viewed as a mirror image of the history of the species: language has evolved in as many diverse and complex forms as there have been historical and sociocultural settings. And now, here we are, in this digital age, where multiple languages co-existing in the same brain is (more often than not) the norm. Digitalization and globalization, furthermore, are driving a paradigm shift that relentlessly challenges us to keep pace with an ever accelerating life. Recent research has shown that language acquisition is based upon an innate ability of statistical learning formed by evolution, an idea supported also by second language acquisition studies. Moreover, there is abundant evidence indicating that both long-term bilingual experience and second language learning results in effective structural as well as functional connectivity in the brain, due to neural plasticity. Evidence suggests that effective connectivity due to second language learning enhances the capacity for language processing and general executive control by reorganizing neural circuitries. By using non-invasive brain imaging technology, effective reorganization of structural and functional connection in the brains of successful second language learners has been shown in lots of recent studies, even when dealing with adult learners whose age was way past the so called critical period of language acquisition. It must be noted that age of acquisition is not the only critical factor responsible for successful second language performance; language proficiency, individual differences, and training paradigm also have to be considered. Research in the last several decades shows us that the bilingual learner integrates two separate language-specific lexica into an effective bilingual lexicon. In this regard, neural imaging evidence shows that cross-language code-switching enhances inhibition control of the non-target language and also the ability to carry out code-switch executive control. As a result, throughout a bilingual lifespan, brain circuitries are being constantly rewired, giving an enhanced cognitive control advantage to the bilingual elderly. What other effects can modern life have on the brain? Human interaction with emerging multisensory media and communication technology nowadays in the digital environment must affect the operations of our brains: our perception, cognition, memory, navigation, access, and executive control. The impact of digital-technological tools may well be changing the underlying mechanisms of bilingualism and learning. Based on what we now know about “one brain for all languages”, both spoken and written, the old question of “When does lateralization begin?” has to be replaced with “When does lateralization stop (if ever)?” This new perspective can help us understand linguistic aspects of brain evolution and may even redefine the meaning and measurement of human intelligence in the future.