人腦視覺皮質功能性核磁共振影像信號與複雜自然視覺刺激物理特性關聯性分析

Abstract

受測者間相關性分析(Inter-subject correlation analysis, ISC)被廣泛應用於腦神經科學領域的自然刺激研究中。從過往研究1,2中可以得知多位受測者在接受相同視覺刺激下，受測者間時序上的血氧濃度訊號在視覺皮質存在顯著相關性，本研究欲探究的即是造成此相關性產生的原因。自然視覺刺激本身包含對比度、亮度，以及各種不同的閃爍頻率等已知會激發視覺皮質腦神經活化的刺激成份。因此在本研究中，我們使用功能性核磁共振影像，搭配針對人腦視覺皮質的線性迴歸分析(General linear model analysis, GLM)與ISC分析，來研究對比度、亮度以及特定的閃爍頻率在ISC分析於視覺皮質的關聯性。 首先，我們針對受測者視覺皮質的血氧濃度訊號與不同刺激物理特性分別做GLM分析，實驗結果說明對比度和閃爍頻率(0.5赫茲、1赫茲、4赫茲、5赫茲、8赫茲和10赫茲)的刺激是視覺皮質活化的原因，與過往研究3-10相符。接著，我們進一步利用GLM分析去除受測者血氧濃度訊號中與對比度和特定閃爍頻率(0.5赫茲、1赫茲、4赫茲、5赫茲、8赫茲和10赫茲)存在線性相依的部分。經統計檢定發現：去除對比度和特定閃爍頻率前後的ISC分析結果在視覺皮質呈現顯著(p<0.01)。我們認為這代表對比度和特定閃爍頻率為自然視覺刺激中造成受測者間血氧濃度訊號於視覺皮質存在高相關性的原因。此外，在受測者血氧濃度訊號與不同閃爍頻率分別進行GLM分析的結果中，我們發現人腦視覺皮質具有不同深度偏好特定閃爍頻率的特徵。首先，我們成功利用自然視覺刺激實驗重現過往區組設計(block design)研究11的發現：1赫茲閃爍頻率的刺激對應到視覺皮質的內側。相較之下，10赫茲閃爍頻率的刺激對應到視覺皮質的外側。有趣的是，我們發現以往研究未曾討論的閃爍頻率0.1赫茲的刺激則是對應到視覺皮質的外側。同時，在實驗中我們發現相較於低頻的閃爍頻率（0.1赫茲），位於內側的視丘對於高頻的閃爍頻率（10赫茲）較為敏感。綜合以上的實驗發現，本篇論文提供了證據說明在自然視覺刺激中，對比度和閃爍頻率即是造成ISC分析於視覺皮質關聯性的關鍵因素。

Inter-subject correlation (ISC) analysis was generally used in naturalistic stimuli studies. Hemodynamic activity in human visual cortex was shown to correlate across subjects during natural viewing a movie clip1,2. This study aimed to investigate the physical properties of naturalistic audiovisual stimuli what cause the correlation across subjects' hemodynamic activity in visual cortex. Naturalistic audiovisual stimuli include multiple physical properties, such as contrast, brightness and flickering frequencies in different frequency bands. To investigate what physical properties of naturalistic audiovisual stimuli cause the correlation across subjects' hemodynamic activity in visual cortex, we used functional magnetic resonance imaging (fMRI) signal in general linear model (GLM) analysis and ISC analysis. First, the results of GLM analysis examining the correlation between the hemodynamic activity of visual cortex and each physical properties of naturalistic audiovisual stimuli turned out: visual cortex was activated by contrast and specific flickering frequencies (0.5Hz, 1Hz, 4Hz, 5Hz, 8Hz and 10Hz), same as the finding of previous studies3-10. And then, we demonstrated significant (p<0.01) t-value in the difference between results of voxel-by-voxel inter-subject correlation before and after removed contrast and specific flickering frequencies in visual cortex. Therefore, we suggest contrast and flickering frequency cause the correlation of hemodynamic activity in visual cortex across subjects. Furthermore, we revealed the selectivity of layer-specific frequency-preference in visual cortex. Same as the finding of the previous study11, we found out that the medial side of occipital lobe was activated by 1Hz of flickering frequency. And the lateral side of occipital lobe was activated by 10Hz of flickering frequency. Interestingly, we demonstrated 0.1Hz of flickering frequency, which has not been studied before, activated the lateral side of occipital lobe. Moreover, we also revealed that thalamus is more sensitive to higher flickering frequency (10Hz). Taken together, our findings provide evidence for contrast and flickering frequency cause the correlation of hemodynamic activity in visual cortex across subjects.