利用時間特徵來辨別由聽視覺一致與聽視覺不一致之刺激所引發之功能性核磁共振影像信號

Abstract

過往研究發現，當我們經歷麥格克效應時，在我們 Superior Temporal Sulcus 腦 區會呈現出較大的血氧濃度相依對比訊號(Blood-Oxygen-Level Dependent)。然而， 受限於傳統功能性磁振造影(functional magnetic resonance imaging, fMRI)較差的時間 解析度，過往研究皆無法探討 BOLD 訊號的時間特徵，因此本研究致力於利用時間特 徵來辨別由聽視覺一致與聽視覺不一致之刺激所引發之功能性核磁共振影像信號。在 本研究中，我們利用取樣頻率 10 赫茲的超快速 fMRI 來研究和聽視覺整合相關的血氧 訊號，藉由使用超快速的磁振造影技術，BOLD 訊號因此可以被量化出不同的時間特 徵，包含 Onset、Time-to-half(TTH)、Time-to-peak(TTP)、Time-to-half-off (TTHoff)以及半高全寬。實驗結果發現，雖然聽視覺一致與聽視覺不一致的實驗刺 激所引起的 BOLD 訊號在訊號強度上沒有差別，但兩種不同實驗刺激所引起的 BOLD 訊號在時間特徵上確實有差異。具體而言，在聽視覺不一致的刺激狀況下，在聽覺區 的地方，BOLD 訊號會比較晚出現(TTH 時間指標長了 200 毫秒)。此外，在 STS 這 個腦區，也呈現了比較慢的 BOLD 訊號(Onset 時間指標長了 700 毫秒)。因此，本研 究證實了利用超快速 fMRI 技術來研究 BOLD 訊號時間特徵的重要性，尤其是研究大 腦聽視覺整合歷程為目標的實驗。

Previous studies have reported that people perceiving McGurk effect have stronger blood-oxygen-level dependent (BOLD) response at superior temporal sulcus (STS). However, under the limitation of the temporal resolution of conventional functional magnetic resonance imaging (fMRI) scan, how temporal features of BOLD waveforms differ in audiovisual congruent and incongruent conditions are less explored. In this thesis, we aimed at studying how BOLD waveforms differ between McGurk and audiovisual congruent conditions. We used fast fMRI sampled at 10-Hz to study hemodynamic responses elicited by audiovisual stimuli. BOLD waveforms were quantified by several temporal indices, including Onset, Time-to-half (TTH), Time-to-peak (TTP), and Time-to-half-off (TTHoff) and full-width-at-half-maximum (FWHM). Among McGurk illusion perceivers, we found that the primary auditory cortex had significant delayed (larger TTH; ~200ms) BOLD responses in McGurk than congruent condition. STS also shows delayed onset time (larger Onset; ~700ms) when perceiving McGurk illusions. Our results suggest the importance of using a fast fMRI scan to study the temporal characteristics of BOLD waveforms to better discern perceptions during auditory and visual stimuli integration.