觀看自然刺激下之人腦血液動力信號同步性受低階視覺特性影響分析

Abstract

跨受試者間血氧動力學反應的同步性量測，已廣泛應用於檢測人類受自然和複雜刺激下時大腦活動的區域。使用此種分析方式，在較高階區域的同步性通常不如感覺及其關聯區域。考慮到感覺刺激的分層處理，處理低階或高階特徵的腦區皆會產生跨受試者間的同步性。然而，認知神經科學的研究主要關心後者，故期望能去除由前者所導致的同步大腦活動，從而獨立出負責高階反應活動的腦區。 本實驗中，我們藉由控制視覺刺激以找出大腦負責處理低階視覺刺激相關的腦區。同時，我們讓受試者透過影片中的角色代入產生高階的大腦意識活動(競爭與合作)。我們讓兩個受試者分別從兩個不同的視角的錄影觀察同一場遊戲，並找出兩個受試者之間功能性磁振造影訊號的相關性與錄影畫面相似度有關的腦區。我們藉由錄影畫面的重疊率來測量兩個不同視角之間畫面的相似性。我們假設這些大腦區域與低階視覺處理有關。之後我們通過聚類分析找到這些區域的特徵訊號，隨後以線性回歸方式去除這些特徵訊號後計算受試者間的大腦活動相關性。 在去除特徵訊號之後，受試者之間大腦活動的同步性在視覺和側向枕葉皮質上顯著減少，但在扣帶皮層，額葉皮質和島葉中顯著增加。實驗結果表明以畫面的重疊性來量化受試者在觀察自然刺激時相關特徵的可行性。

Measuring synchronized hemodynamic responses across subjects has been widely used to detect active human brain areas under naturalistic and complex stimuli. Using this analysis, higher order areas are typically less synchronized than sensory and association areas. Considering the hierarchical processing of sensory stimuli, synchronized brain activity can occur at brain areas dynamically processing either low-level or high-level features. To isolate the latter for which cognitive studies are mainly interested in, it is important to suppress the synchronized brain activity due to the former causes. Here we proposed an experiment by controlling the effect due to low-level visual stimulation. We prepared montage movie clips of the same session of a video game but from four different perspectives. The subjects were instructed to project themselves into the players in the movie, including imagining cooperative and competitive behaviors with others. We first identified brain areas where the correlated fMRI signals between two subjects during viewing the movie but from two distinct perspectives related to the similarity between two perspectives. The similarity between two distinct perspectives was measured by the degree of overlapped field-of-view (FOV) of video frames. These brain areas were hypothesized to be related to low-level processing. The characteristic time courses at these areas were then found by clustering analysis. Inter-subject correlation (ISC) was subsequently calculated after removing these characteristic time courses by regression. The synchronization of brain activity between subjects showed significantly reduced at visual and lateral occipital cortex, but significantly increased at cingulate cortex, frontal cortex and insula. The result suggested the feasibility of using the overlapped field-of-FOV to quantify low-level visual related features during naturalistic viewing between subjects.