- 學位論文
鼠腦前額葉皮質在風險選擇行為扮演的角色
The role of rat prefrontal cortex in risk-dependent choice
摘要
在決策相關的研究中，前額葉皮質一直被認為是負責處理不確定情境下決策的重要腦區。但是，前額葉的次級區域在風險選擇中的具體角色，卻仍難以捉摸。有鑑於此，本研究透過破壞大白鼠的旁側眼眶前額葉皮質，以及內側前額葉皮質，探討破壞腦區的效果對受試風險選擇行為的影響。本研究採用T迷津風險選擇作業。在此作業中，受試針對兩個選項進行選擇，分別是酬賞量小而確定會獲取的選項，以及相對的酬賞量大而不確定的選項。這項作業包含三種不同比率的酬賞，因特別設定兩個選項的期望值相等於1，其分別代表高中低三種不同程度的風險。本實驗採用受試者間實驗設計，在行為測試之前，大白鼠被隨機分派至不同的實驗組，分別是旁側眼眶前額葉皮質破壞組、內側前額葉皮質破壞組，以及控制組。上述各組大白鼠會再被隨機分派至三個不同程度的風險組別中進行行為測試。結果顯示不同風險程度的控制組大白鼠，習得不同的風險選擇模式。在低風險情境下，受試傾向風險選擇；在高風險情境下，受試傾向規避風險；而在中等風險情境下，受試表現出介於傾向風險和規避風險之間的選擇模式。破壞旁側眼眶前額葉皮質顯著的干擾了受試的風險選擇行為，使受試增加風險選擇。相反的，破壞內側前額葉皮質，並未改變受試在任何一種風險情境下的選擇行為。藉由其它三種行為測試證明，破壞旁側眼眶前額葉皮質造成的行為改變，並非因受試運動功能受損、出現類焦慮反應，或無法區辨酬賞數量大小所致。綜合以上結果，旁側眼眶前額葉皮質和內側前額葉皮質參與風險選擇具有異質性功能。本研究更進一步證實，在以酬賞大小和出現機率操弄的特定風險程度之下，旁側眼眶前額葉皮質對風險選擇行為的習得扮演重要角色。
並列摘要
Prefrontal cortex (PFC) is suggested to be important in decision making under uncertainty, but the specific contribution of PFC subregions to risk-based decision making remains elusive. This study thus evaluated the lesion effects of lateral orbitofrontal cortex (lOFC) and medial PFC (mPFC) on risk choice behavior. A rat T-maze risk choice task was used where the subject would choose enter either a small and certain reward arm of the maze or a large but uncertain reward arm of the maze. This task was specifically set up to run with the expected value being equal (i.e., EV = 1) between the binary choice options which tests were carried out with three reward ratios representing different levels of risk, correspondingly notated as high, medium, and low. The rats were randomly assigned by following between-subject design in receiving excitotoxic lesion of lOFC or mPFC and sham controls prior to behavioral tests given by three reward ratios. For the results, the sham control rats acquired dissociable patterns of risk choice as depended on different risk levels; that is, risk-prone and risk-averse patterns respectively appeared in low-risk and high-risk conditions, whereas an intermediate choice style was shown in the medium-risk condition. The lOFC lesion significantly interfered with the risk choice behavior by shifting the choice toward a risk-prone style. In contrast, the mPFC lesion did not alter choice behavior given in any of three risk levels. The behavioral alterations in rats with lOFC lesion were not the result of motor deficit, anxiety-like response, or incapability to discriminate reward magnitudes from a large to a small one. Together, these findings highlight differential involvements of lOFC and mPFC in risk decision making and further suggest that the lOFC contributes to the acquisition of risk-dependent choice behavior given at a particular level of risk based on reward magnitude and probability.