磁共振帶氧血紅素反應與大腦訊息傳遞迴路

An Information System in the Brain: Evidence from fMRI BOLD Responses

本研究採活化再現性分析方法（Liou et al., 2006），分析三組功能性磁共振造影視覺實驗資料，認知作業包括偵測變化的視覺注意力（visual attention in change detection）、物件導向的空間注意力（visual attention in objectbased selection in space）、及語言轉換的數字認知（language switching in numerical cognition）等。分析以個別受試者為主，且未將受試者資料校準至國際標準腦，以減少影像扭曲造成的誤差。重要的分析結果包括︰（1）多數大腦皮質同時參予不同的視覺認知作業，且當受試者執行完作業並凝視螢幕中央「+」時的活化反應區，與執行空間注意力作業反應區相似。此結果顯示，無論複雜或簡單的視覺認知作業，大腦皮質皆同時參予執行。由於大腦為一完整的訊息系統，本研究建議功能性磁振造影實驗研究應重視大腦不同區域的功能性聯結（神經網路），而非特殊區域的活化反應（localization）。（2）大腦聯絡皮質（associative regions）在所有作業中僅出現正向的反應，視覺皮質在所有作業中同時出現正向及負向反應，且與作業難度無關。掌管注意力及記憶的非特殊皮質（non-specific regions）在被動觀看刺激時僅出現正向的反應，但在需集中注意力的作業中，會同時出現正向及負向反應。由於聯絡皮質中γ-胺基丁酸（GABA）的濃度較其他皮質區少，且主要負責訊息的整合及長距離訊息傳輸，研究假設磁共振實驗中帶氧血紅素反應下降和γ-胺基丁酸的活動有關。本研究最後提出三個重要的訊息傳遞迴路：注意力、記憶、及情緒，做為探討功能性聯結研究的參考。

In this study, we will show in three visual experiments that there are several brain regions in which induced blood oxygenation level-dependent (BOLD) responses are reproducible between visual tasks and between individual subjects. The reproducible BOLD responses indicate that the specific-sensorial structure (e.g., the lingual gyrus) and non-specific structure (e.g., the precuneus) both exhibit increased and decreased BOLD responses, while the associative structure (e.g., the intraparietal sulcus) shows only increased responses. The response patterns remain stable even during task switching from visual-spatial attention to central eye-fixation. Based on the response patterns in different cortical structures, this study will discuss two issues pertinent to applying BOLD contrast to research on high-level cognitive functions. First, there may exist a large-scale information system reproducible between visual tasks that only affect changes in the response sign in the system nodal regions (i.e., positive or negative BOLD responses). If the information system remains faithful to all visual tasks, localization of cognitive functions might not be a priority in functional magnetic resonance imaging (fMRI) studies. Second, the reproducible BOLD responses have partially reflected functional roles of neurotransmitter systems in different cortical structures. Recent studies using MR spectroscopy have suggested that γ-aminobutyrate (GABA) activity is negatively correlated with BOLD responses. Since GABA concentration is relatively low in the associative regions as compared with that in other regions, the spatial resolution of BOLD signal may be insufficient in recovering possible negative activity in the associative regions. This also calls special attention to a recent discovery on glutamate- and GABA-mediated signaling in regulating blood flows and BOLD signal changes. We conclude the study by hypothesizing three inter-related functional loops for future research on the information system using BOLD contrast.