| Summary: | 習得年齡對語言處理有相當影響，但其成因及作用階段至今仍無定案，根據以知的行為研究成果及假說，結合事件相關電位技術，及雙重選擇按鍵與否典範，我們將嘗試找出習得年齡在言語產出中的作用階段。假定不欲按鍵則無側化動作準備電位，且側化準備電位起始時間取決於，何時取得用以決定反應手的訊息，我們請受試者根據圖片刺激的兩種語言屬性做出正確反應。
在實驗一中，語意屬性（人造或非人造）決定反應手，圖片名稱第一音節的語音屬性(一、三聲或二、四聲)決定按鍵與否。若習得年齡影響語意處理，我們預期在按鍵情形下，習得年齡早晚兩組的側化準備電位起始時間將有所不同；而不按鍵情形下的側化準備電位持續時間，將反映習得年齡對語音處理的影響。實驗二中，圖片經更嚴格挑選，以控制所有變項，且語言屬性與反應類別間的兩種對應都被測試。
實驗一和實驗二都發現不按鍵時，若反應手非由語意屬性決定，則側化準備電位不會出現。這個結果符合文獻記載，並暗示語意處理早於語音處理。實驗一中，早習得圖片的唸名反應時間、圖片分類反應時間、及側化準備電位起始時間都較短，支持習得年齡越早，語意提取速度越快。但實驗二中，習得年齡對側化準備電位起始時間沒有顯著影響，儘管前測顯示早習得圖片的唸名反應時間較短，其分類反應時間卻較長，。實驗三進一步肯定，習得年齡對唸名及圖片分類作業有相反效果，以及習得年齡對圖片分類作業的效果不因重複練習而喪失。
根據完整語音理論 (Brown & Watson, 1987)，我們認為當作業要求完整的語音訊息，則習得年齡越早，提取速度越快，唸名作業如是；當作業要求片段的語音訊息，則習得年齡越早，提取速度越慢，額外時間用於解構完整語音訊息，我們的圖片判斷作業如是。實驗二中因少數圖片重複出現，習得年齡對語意處理的正面作用減少，習得年齡對片段語音處理的負面作用卻維持，使得習得年齡組別間，側化準備電位沒有顯著差異，且早習得組圖片分類反應時間反而較長。
總而言之，本研究支持概念/詞目競爭理論的說法，亦即習得年齡越早，語意提取速度越快。我們的資料也間接反映，習得年齡越早，語音提取速度越慢，但此推論仍需未來研究進一步支持。
=== Age-of-acquisition (AoA) is one of the most robust variables for language processing. But its cause and locus during the processing are still under debate. Based on previous behavioral studies and established hypotheses, this study aims to find electrophysiological evidence for the locus of AoA effect during speech production. The continuous measure of speech production is approached by event-related potential (ERP) in conjunction with a manual, dual choice Go/Nogo paradigm. In this paradigm, participants need to choose the correct response according to two linguistic attributes of the target picture. The presumption of this paradigm is that the lateralized readiness potential (LRP), a motor preparation related component, would be absent once the Nogo decision has been made, and its onset marks the time point at which critical information for hand selection is available.
In Experiment 1, which hand to respond depends on the target picture’s semantic attribute (artifact or non-artifact), to respond or not relies on its phonological attribute (the tone value of its first syllable). It is expected that if AoA influences lexical/semantic processing, LRP onset latency would be different under early and late AoA condition. AoA effect on phonological stage would be reflected on the duration of Nogo LRP. In Experiment 2 the reverse mapping between linguistic attribute and response types was also tested and variables other than AoA were more strictly controlled.
Both experiments replicate previous finding in that Nogo LRP only occurs when response hand is contingent on semantic information, which indicates that semantic stage precedes phonological stage. In Experiment 1, naming latency, picture classification RT (reaction time), and LRP onset latency were shorter for the early AoA group, indicating accelerated lexical/semantic retrieval. However, in Experiment 2, AoA effect on LRP onset latency was not found. And for items with early AoA, picture classification RT is significantly longer, despite that their naming latency is shorter in the pretest. Experiment 3 further validates the coexistence of the positive AoA effect on naming and negative AoA effect on tone classification and the resistance of the negative AoA effect to repetition.
Following the phonological completeness hypothesis (Brown & Watson, 1987), we suggest that when the unitary phonological form is demanded, as in the naming task, the early learned words has advantage. When a fraction of the unitary form is demanded, as in the tone classification task, extra time is needed to decompose the retrieved unitary form. The positive AoA effect on semantic stage is larger in Experiment 1 due to material selection and number of repetition. Thus in Experiment 1, the positive AoA effect on semantic stage remains, but in Experiment 2, it is overpowered by the negative AoA effect on fractional phonological extraction, resulting in the absence of AoA effect on LRP pattern and longer picture classification RT under early AoA condition.
In sum, results of this study indicate that semantic feature can be retrieved faster under early AoA condition, which is in line with concept/lemma competition hypothesis. Our results also imply slower tone retrieval under early AoA condition, which need to be further testified by future studies.
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