腦中的自由意志？對來自認知神經科學之實徵證據的擷選回顧   全文下載

Free Will in the Brain? A Selective Review of the Empirical Evidence From Cognitive Neuroscience

普羅大眾相信人類有自由意志，但透過測量大腦活動來了解人類功能和機制的認知神經科學研究顯示，在受試者任意進行抬手、自由決定以哪隻手按鍵或進行加法或減法運算時，先觀察到和後續行為相關之生理訊號，接著受試者才意識到其動作意圖，最後觀察到受試者之外顯行為；此種結果似乎強烈挑戰自由意志的存在。本文介紹相關研究之數個經典實驗，指出其實驗假設的內部矛盾、其實驗派典的潛在問題和侷限，並澄清其結果的意涵，再進一步根據人類記憶等其他認知範疇中的行為表現，提出「自由意志的運作並非單一歷程，包含難以意識到的內隱意志和可清楚意識到的外顯意志」。本文根據對科學實徵證據的擷選回顧，闡述即使我們相信人類所有認知功能（包括自由意志）都奠基於神經元活動，目前的腦科學研究仍尚未提供自由意志如何運作的直接證據，也未推翻自由意志的存在。

It is generally believed that people have free will, according to which we can freely choose and control our own behaviors. However, such belief is strongly challenged by empirical evidence from cognitive neuroscience, which investigates human cognitive functions and their neuronal mechanisms through measuring brain activities. Specifically, research has shown that when people spontaneously move their hands, freely choose to move their right or left hand, or freely decide to perform addition or subtraction, the neuronal signals that are relevant to the following behaviors are first observed, followed by participants’ subjective awareness of the intention to act, and then the explicit behaviors are observed. Such findings seem to be directly against the existence of free will. To better understand functional and neural mechanisms of free will in the brain, several classic studies addressing this issue via cognitive neuroscience methods are reviewed in the present article. The theoretical assumptions, experimental procedures, and behavioral and neuroimaging results of these experiments are introduced. The results and their implications are described and discussed. Critically, the inconsistency of the underlying assumptions, the problems of the paradigms, and the limitations of the findings of these experiments are highlighted and elaborated. It is further proposed that, similar to behaviors in other cognitive domains such as memory, the functioning of free will may not be a unitary process but encompassing the implicit phase that is unaware of and the explicit phase that is clearly aware of by an agent. Based on the selective review of the studies in cognitive neuroscience, it is concluded that despite the belief that all human behaviors are supported by neural activities in the brain, the present evidence has not elucidated the operations of free will in the brain, nor has it overthrown the existence of free will of human beings.