健康老齡化對違反預期的語言訊息理解之影響

Abstract

人類的語言富有創造力，在各種語言層面（例如語意、句法等）皆涵蓋了許多預期之外的用法。人類處理這些預期之外的用法的能力可能會受到健康老化的影響，這對於成年人的語言理解和有效溝通至關重要。本論文主要使用非侵入性且具時間敏感性的腦電圖（EEG），在三項不同的研究中，探討健康老化如何影響個體處理違反預期的語意和句法訊息的能力。 在研究一中，我們著重在違反預期的語意訊息處理，旨在檢驗兩項研究問題：（一）過去常誘發於違反強烈語境預期的頭殼前區正向波（the anterior positivity，AP）是否一定得仰賴語境中強烈的語意約束？（二）該頭殼前區正向波是否反映了重新排序弱勢語意的過程。我們記錄了 32 位年輕的台灣華語母語者在閱讀單義詞或名動歧義詞（如英文中的watch）時的腦電反應，並刻意讓出現在這些詞前面的單詞語境不具語意預測力，僅提供語法上的限制。結果顯示，相較於單義詞和歧義詞的主要語義，當單詞語境在語法上偏向歧義詞的次要語義時，受試者誘發了較大的 AP 反應。這表明 AP 的誘發並不一定得仰賴語句/篇章語境中強烈的語意約束，而是反映了重新排序弱勢語意的過程。 在研究二中，我們使用與研究一相同的材料和實驗典範，招募了 40 位年長的台灣華語母語者，進一步探討健康老化是否會阻礙重新排序弱勢語意的能力。結果顯示，年長受試者與年輕受試者一樣，相較於單義詞和歧義詞的主要語義，歧義詞的次要語義也誘發了較大的 AP 反應。這表明重新排序弱勢語意的過程並沒有受到健康老化的影響。 在研究三中，我們轉而著重在違反預期的語法訊息處理，旨在探討健康老化過程中右腦半球（RH）在語法處理時額外增強的反應如何受到胼胝體結構完整性的調節。我們在70位台灣華語母語者的左視野或右視野呈現了違反語法規則的短語，同時測量其事件相關電位（ERPs）。此外，我們也加入擴散張量影像技術（DTI）量測其中44位志願受試者的胼胝體結構完整性。腦電圖結果顯示，年輕和年長受試者的左腦半球（LH）對違反語法規則的短語都誘發了 P600 反應，但只有年長受試者額外引發了右腦半球的 P600 反應。擴散張量影像結果顯示，年長受試者的右腦半球 P600 反應與胼胝體壓部（splenium）結構的擴散不等向性（FA）值呈現正相關，但在胼胝體膝部（genu）結構並未觀察到類似的關聯性。這些結果突顯出除了英語之外，以台灣華語為母語的老年人其右腦半球也能誘發本質上類似左腦半球的語法反應（P600）。更重要的是，我們發現胼胝體壓部結構的完整性可以調節老年人在處理語法訊息時兩腦半球之間的協調。 總結來說，本論文透過整合具時間敏感性的腦電圖和擴散張量影像等神經影像技術，提供了新的證據，顯示某些認知功能在健康年長者中仍然得到良好的保存。此外，年長者的大腦結構完整性也會調節其運用額外的神經認知資源之能力。這些發現有助於理解健康老化如何影響我們處理違反預期的語言訊息之認知和神經機制。

Human languages are inherently creative and feature a wide range of unexpected usages across different linguistic aspects, such as semantics and syntax. Our ability to handle these unexpected usages can be influenced by healthy aging, which is crucial for successful language comprehension and effective communication across the adulthood. This dissertation aims to investigate how healthy aging affects individuals’ capacity to process semantic and syntactic unexpectedness using non-invasive, time-sensitive Electroencephalography (EEG) through three distinct studies. In Study 1, we focused on semantic unexpectedness and examined (1) whether an elicitation of anterior positivity (AP), an event-related potential (ERP) component often observed for disconfirmed semantic predictions, hinges on a strong semantic contextual constraint, and (2) whether the AP reflects a process of weaker meaning reprioritization. Thirty-two younger Taiwan Mandarin native speakers’ EEG responses were recorded as they read unambiguous words or biased noun-verb homographs (e.g., watch) following one-word contexts that were semantically non-predictive but syntactically well-defined. Our results showed that the participants elicited larger AP responses when the one-word contexts syntactically favored the less frequent (subordinate) meanings of the homographs as opposed to the more frequent (dominant) meanings of the homographs and the unambiguous words, which suggests that the AP reflects a process of weaker meaning reprioritization that is not limited to sentence/discourse predictive processing. In Study 2, 40 older Taiwan Mandarin native speakers were recruited to further investigate whether healthy aging hampers the process of weaker meaning reprioritization. With the same stimuli and experimental paradigm of Study 1, the older participants also elicited larger AP responses to the subordinate meanings of the homographs relative to the dominant meanings of the homographs and the unambiguous words, which suggests that the process of weaker meaning reprioritization is preserved in healthy aging. In Study 3, we focused on syntactic unexpectedness and investigated how increased right-hemisphere (RH) responses in healthy older adults during syntactic processing, a highly left-lateralized function in younger adults, are modulated by callosal integrity. Syntactic phrase violations in Taiwan Mandarin were presented to either the left or right visual field with 70 (35 younger and 35 older) native speakers’ ERPs measured. A subset of 44 (24 younger and 20 older) participants’ microstructural tissue integrity of the corpus callosum was measured with diffusion tensor imaging (DTI). Younger and older adults elicited left-hemisphere (LH) P600 responses to grammatical violations, but only older adults additionally elicited a RH P600 grammaticality effect. Larger RH P600 effects in older adults were associated with larger fractional anisotropy (FA) values of the splenium. This association was not observed for the genu, which evinced significantly lower FA in older relative to younger adults. These results add to English literature highlighting that LH-similar syntactic responses are recruited in older adult RH for Taiwan Mandarin. Critically, splenium integrity modulates how older individuals engage inter-hemispheric coordination for syntactic information processing. In sum, by integrating time-sensitive EEG with neuroimaging techniques such as DTI, this dissertation provides novel evidence that certain cognitive functions remain well-preserved in healthy older adults. Additionally, older adults’ ability to utilize extra neural-cognitive resources is influenced by the integrity of their brain’s microstructure. These findings advance our theoretical understanding of how healthy aging affects the cognitive and neural mechanisms involved in processing unexpectedness during language comprehension.