| 摘要: | 生活中的各種認知行為都受到視覺工作記憶的協助，其保存短暫性的資訊並整合過去經驗再予以執行任務，擁有較高的工作記憶能力可以有更佳的學習效果或工作表現。在學齡童身上，理解、運算及邏輯推理等學習技能皆與工作記憶有關，較差的工作記憶表現會使其比同儕的學習力差。在失智症患者身上也觀察到其工作記憶容量逐漸減少。工作記憶的容量並不是無限大的，而透過行為學以及腦電波的研究後證明了此假設。研究中利用視覺改變偵測任務來測量其容量，在對側延遲活動(contralateral delay activity, CDA)的事件相關電位(event-related potential, ERP)發現其振幅會隨著記憶負載(memory load)增加而增加，但是增加的幅度直至四個物件為止。此實驗說明了記憶容量並非一個固定的數字，但是也不是無限制。維持或是增進工作記憶的容量可以幫助人們擁有較佳的資訊運作處理能力，許多研究表明非侵入性的腦刺激 (non-invasive brain stimulation, NIBS)可以增進認知功能。這是一種利用電或是磁場來刺激腦神經活動的技術。在電刺激的領域有兩種主要的方法，過去研究表明正極的經顱直流電刺激(transcranial direct current stimulation, tDCS)可以透過降低枕葉的alpha波能量使得低表現族群工作記憶表現增強；另一種方法使用自身的theta頻率藉由交流電刺激(transcranial alternating current stimulation, tACS) 增加相位-振福耦合強度(phase-amplitude coupling, PAC)，而此PAC的增強與有助於老年人工作記憶表現增加至年輕人的工作記憶表現。
具震盪波的經顱直流電刺激(oscillatory transcranial direct current stimulation, otDCS)是近年來一個新的電刺激方法，其結合直流電刺激與交流電刺激的優勢，在正極的直流電波形上增加頻率的參數，不僅可增強神經的興奮性也可以透過給予特定的頻率調節腦波的活動。與傳統正極tDCS相比，在聯想式記憶提升的幅度並沒有顯著差異但是使用otDCS後增強表現的人數比較多。此研究揭示，使用otDCS進行記憶表現的研究可能更有效。然而，此電刺激方法如何影響內在的神經運作機制並不明確，與工作記憶的關係並未受到廣泛的討論，因此我們需要更多的研究來闡明otDCS是否能利用調節神經震盪來增進工作記憶的容量。
先前的研究表明了受試者接受正極tDCS刺激後，額葉的theta頻率對beta頻率的振幅調控與工作記憶的維持呈正相關。因此，本實驗採用了theta對beta振幅調控的波形於經顱直流電刺激，並利用經典的改變偵測測驗(change-detection task)來探討工作記憶容量。並且使用完全適應性雜訊經驗模態分解法(complete ensemble empirical mode decomposition with adaptive noise, CEEMDAN)對腦波訊號進行希爾伯特頻譜分析(Holo-Hilbert spectrum analysis, HHSA)及希爾伯特跨頻率相位群聚分析(Holo-Hilbert cross-frequency phase clustering, HHCFPC)以了解其神經機制。另外，如同個體差異一般，工作記憶的容量也有表現低以及表現高的群體差別，先前研究顯示電刺激的效果在低表現族群上較有效。為了觀察個體差異的神經機制，我們採用先前相似研究結果中的群體平均Pashler’s K(Kp) 值來做為區分組別的客觀依據。本實驗使用受試者內設計(within-subject design)來探討同一群受試者接受正極電刺激以及假刺激間的差異。
本研究旨在探討工作記憶的表現是否受到具震盪波的正極經顱直流電刺激影響，與其神經機制以及個體的差異。我們假設視覺工作記憶的表現與額葉的theta頻率對beta頻率的振幅調控有關，並且在低表現族群上會顯得更有效。
本研究的結果顯示接受被theta所調製的beta頻率的正極直流電刺激對低表現族群的工作記憶容量有顯著的增加，整體而言也有提升表現的趨勢。與沒有接受電刺激的實驗數值相比，theta-beta的能量在枕葉顯著的下降並且與改變偵測的敏感度(D prime)呈顯著正相關。此結果與高表現族群的結果相近，可能是因為高表現族群的人數為低表現族群的一倍。相比於高表現族群，低表現族群的右枕葉能量增加且與行為結果呈現負相關。透過希爾伯特跨頻率相位群聚分析展示頻率在各腦區間的相位-振幅耦合強度。低表現族群在接受電刺激後，額葉與左側頂葉以及右後側區域顯示theta與beta的相位振幅耦合減少，並與工作記憶容量指數Pashler’s K 呈顯著正相關。相比之下，高表現族群僅在額葉的theta-beta耦合減少並與行為表現呈現負相關。
綜合上述結果，受theta調控的beta頻率正極直流電刺激是透過減少枕葉的能量強度來增加工作記憶的表現。而低表現族群的記憶表現增加是源自於減少右後腦區以及左頂葉對額葉的theta-beta調控。我們認為此電刺激方式在認知作業進行同時施打較有效益，且施打後的腦神經活動仍然受到特定頻率的調控並能在低表現族群身上提升其工作記憶的表現。
;Background: Working memory is closely related to our daily life. It stores temporal information and organizes it with experience to execute the next action. Having a higher working memory performance may have higher efficiency at work and learning. The previous studies have revealed that learning ability is related to the working memory performance of the child. There are some disorders related to deteriorated working memory capacity such as Alzheimer’s disease. The poor working memory performance is one of signatures of individual difference in the present study. The nature of the limited working memory capacity is explored through the behavioral and EEG experiments. Therefore, maintaining or improving the working memory capacity is a great progress for processing outcome information. Non-invasive brain stimulation (NIBS) is a technique in which it can excite or entrain the brain oscillation by transcranial direct current stimulation (tDCS) or transcranial alternating current stimulation (tACS). The working memory capacity can be improved by decreasing the alpha power at the occipital lobe by anodal tDCS. A previous study used individual theta tACS aimed to enhance the phase-amplitude coupling and then increase the working memory performance in old adults. Oscillatory transcranial direct current stimulation (otDCS) is a novel technique that combines the advantage of tDCS and tACS. It can apply a specific frequency on the anodal or cathode tDCS waveform. The method is frequently used in sleeping research. For instance, the slow-wave tDCS during (rapid-eye-movement，REM) is frequently applied in memory consolidation studies. Another research reflects that the number of participants who improve the associative memory performance in otDCS condition more than conventional tDCS condition (Vulic et al.,2021). Nevertheless, the neural mechanism of the otDCS and its effect on the working memory is unclear. We aim to clarify whether the otDCS can improve the working memory capacity by entraining the specific brain oscillation.
This study aims to explore the neural mechanisms and the individual difference effect of otDCS. We hypothesize that the visual working memory capacity is correlated with the theta-beta modulation and the otDCS may more efficient on the low performers.
Method: A previous study proposed that the power of theta-beta frequency at mid-frontal positivity correlated with working memory capacity after anodal tDCS. Hence, we explored the working memory performance by using the beta frequency with the theta amplitude modulation anodal tDCS in the change detection task. To understand the neural mechanism, we used the complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) to decompose the EEG signal, then project it on the Holo-Hilbert spectrum (HHS) and calculate Holo-Hilbert cross-frequency phase clustering (HHCFPC). To observe the individual difference,, we categorized the low and high performers according to their Pashler’s K values with the grand average as the cut-off point. A within-subject design was used in this study to explore the difference between the anodal and sham in the same participant.
Results: Our results show that working memory capacity increased after otDCS on the low performers group. The online block is more efficient than the offline. For the EEG activity, we observed the power is significant correlation with Pashler’s K value on low performers. The correlation frequency across 1 to 32 Hz and scattered around whole brain region. The correlation of Pashler’s K is positive during fixation and study array. It negative correlate with brain activity on maintenance and test array. The phenomenon doesn’t show on the group level and high performers.
Conclusion: To sum up, theta-beta otDCS over the frontal area improves the working memory capacity on low performers. The EEG evidence show the similar result with behavior. Though, the specific frequency is not particularly prominent. The stimulation does improvement working memory performance with affect whole brain area. Though the online block is more efficient, there is a limit for the online EEG analysis for remove the tES artifact. We will going to study how to analyze the online EEG data. |
