雖然情節記憶的功能性神經影像研究穩定發現左側後頂葉腦區(Left posterior parietal cortex, LPPC)在執行記憶提取時有活化的現象，但是關於探討左側後頂葉腦區與記憶提取兩者之間的因果關係仍缺乏神經心理學或跨顱刺激的直接研究證據。本研究選用非侵入性的跨顱電刺激與事件相關腦電位，檢驗左側後頂葉腦區是否涉及記憶提取的功能；同時更進一步探究當左側後頂葉腦區神經元活動受跨顱電刺激調節後，是否會影響重拾記憶(recollection)或記憶熟悉感(familiarity)的運作歷程。具體而言，正極(anodal)與陰極(cathodal)直流電刺激會分別增加與減少神經的興奮性，若左側後頂葉腦區涉及記憶提取的功能，施打正極或陰極的跨顱電刺激勢必會加強或降低記憶表現。實驗一探究左側後頂葉腦區與記憶提取間的因果關係。實驗參與者被隨機分成兩組，依照分派的組別，每位實驗參與者都必須在兩次不同天的實驗過程中分別經歷兩種不同情況的跨顱電刺激：其中一組接受正極刺激與假性(sham)跨顱電刺激；另一組則接受陰極刺激與假性跨顱電刺激，之後進行來源再認記憶測驗。跨顱電刺激的施打位置為P3（依照國際10-20 系統的定位）；施打的時間點介於學習與測驗兩階段之間；共持續施打十分鐘。實驗結果發現：正極刺激增會進再認記憶表現；而陰極刺激則削弱再認記憶表現。然而並無直接證據顯示跨顱電刺激；另一組則接受陰極刺激與假性跨顱電刺激，之後進行來源再認記憶測驗。跨顱電刺激的施打位置為P3（依照國際10-20 系統的定位）；施打的時間點介於學習與測驗兩階段之間；共持續施打十分鐘。實驗結果發現：正極刺激增會進再認記憶表現；而陰極刺激則削弱再認記憶表現。然而並無直接證據顯示跨顱電刺激足以影響來源記憶表現。實驗一的結果支持左側後頂葉腦區與記憶提取之間存在因果關係之論點；但並無直接證據支持左側後頂葉腦區僅選擇性參與重拾記憶的運作歷程。實驗二與實驗一採用同樣的實驗設計，並進一步紀錄實驗參與者接受跨顱電刺激之後執行再認記憶測驗時的腦電波變化。藉由左側頂葉新舊腦電波效果(left-parietal old/new effect)與中額葉區新舊腦電波效果(mid-frontal old/new effect)探討重拾記憶歷程與記憶熟悉感歷程在本實驗的的運作情形。結果發現，接受陰極跨顱電刺激之後，左側頂葉新舊腦電波效果相較於接受假性跨顱電刺激，有降低的現象，但並無發現奇對中額葉腦電波效果的影響。此電生理變化結果顯示：重拾記憶運作歷程與左側後頂葉腦區可能存在因果關連性。實驗三採用受試者內設計，每位實驗參與者在三次不同天的實驗過程中分別經歷三種不同情況的跨顱電刺激：正極刺激、陰極刺激與假性刺激，檢驗施打跨顱電刺激於左側後頂葉腦區的效果是否受到實驗參與者間不同記憶能力的影響。實驗三針對個別受試者計算接受跨顱電刺激後的記憶提昇效果量，該指標計算方式為：以施打正極或陰極跨顱電刺激後所得的再認記憶分數減去施打假性跨顱電刺激後的再認記憶分數。實驗結果發現正極與陰極跨顱電刺激的效果量與施打假性電刺激時的再認記憶表現之間存在顯著著負相關。此結果顯示執行再認記憶作業時，不同受試者的跨顱電刺激效果量存有個體差異，若實驗參與者的基本記憶能力表現較差，無論施打正極或陰極跨顱電刺激皆會增進其再認記憶表現。根據以上實驗發現，本研究認為：左側後頂葉腦區與記憶提取間確實存在因果關係；重拾記憶的運作歷程會因刺激左側後頂葉腦區而受到干擾；然而跨顱電刺激會因實驗參與者的記憶能力不同而展現不同程度的效果量。Although functional neuroimaging studies of episodic memory retrieval consistently found activations over the left posterior parietal cortex (LPPC), very few neuropsychological or brain-stimulation studies provide evidence for the causal relationship between the LPPC and memory retrieval. This thesis thus employed the non-invasive transcranial direct current stimulation (tDCS) and event related brain potentials (ERPs) to examine the issues of whether LPPC is necessarily involved in memory retrieval and whether recollection- or familiarity-based recognition would be affected when the neuronal activity in the LPPC is modulated. Specifically, anodal and cathodal direct current stimulations are known to increase and decrease neuronal excitability, respectively. If LPPC is necessarily involved in memory retrieval, applying anodal or cathodal stimulations over this region should accordingly increase or decrease memory performance.The issue of whether LPPC is causally related to memory retrieval was investigated in experiment 1, in which participants were randomly assigned to two groups, with one group receiving sham and anodal stimulations and the other groups receiving sham and cathodal stimulations in source memory tests on their two visits to the lab. The stimulation was conducted over the P3 site of the 10-20 system for an interval of 10 minutes between study and test. The results showed that the old/new recognition performance was increased after anodal stimulation and decreased after cathodal stimulation over the LPPC. There was however no evidence showing source memory performance being affected by stimulation over the LPPC. The findings of Experiment 1 therefore support the claim that the LPPC is causally related to memory retrieval yet did not provide evidence for the argument that recollection is selectively supported by the LPPC. The issue was further addressed in Experiment 2 in which ERPs were recorded during the test phase after tDCS stimulation. The left posterior parietal and mid-frotnal ERP old/new effects, known to be reflecting recollection-based and familiarity-based recognition, was used to index how recollection and familiarity processes are affected by the LPPC tDCS stimulations. The results showed that only parietal ERP old/new effect was diminished after cathodal stimulations in comparison to the sham stimulation condition, suggesting that the LPPC might be causally related to recollection processes in recognition memory. Experiment 3 adopted a within-subjects design to examine whether the effect of stimulating the LPPC on recognition memory is contingent on participants’ memory capacity. Each participant received sham, anodal, and cathodal stimulations on their three visits to the lab. The effect of tDCS on the LPPC was derived by subtracting recognition scores after sham stimulation from the scores after anodal or cathodal stimulation. Negative correlations between tDCS effect and recognition scores in the sham condition were found whether the stimulation was anodal or cathodal. This finding revealed individual difference of the LPPC tDCS effect onmemory retrieval and suggests that memory is improved after LPPC stimulation only when the participants did relatively poor in the sham stimulation condition. To conclude, the experiments reported in this thesis provide solid evidence that the LPPC is indeed causally related to memory retrieval. Recollection-based recognition was found to be modulated by stimulating the LPPC. And the tDCS effect might be interacting with the individual mnemonic ability.
