過去研究發現大腦神經振盪不僅與視覺意識相關，甚至能因果性地影響其產生。然而目前大多數關於神經振盪的研究都是基於線性頻率分析方法，非線性振盪特性在視覺意識中的作用卻顯少被討論。因此，探討振幅調變在視覺感知中的功能性角色為本研究宗旨。本研究試著透過兩種不同的經顱電刺激實驗，誘發光幻視來探討振幅調變之振盪如何影響視覺感知。 實驗一旨在探討振幅調變經顱交流電刺激如何影響視覺感知。本實驗招募12位健康受試者，並使用交流電刺激來誘發光幻視。光幻視指的是在無亮光刺激視網膜的情況下仍會產生視覺感知的一種現象。研究結果顯示，振幅調變經顱交流電刺激需要較高的閾值強度才能誘發光幻視。此外，振幅調變頻率與載波頻率並無交互作用，且受試者在Beta波的感知較為敏感。此外，當兩種頻率結合時，振幅調變頻率似乎會覆蓋載波頻率對於視覺感知的影響，導致閃爍速度不受載波頻率影響。與單純的正弦波刺激相比，受試者普遍認為感受到的閃爍速度較慢，無論其載波頻率為何。 實驗二進一步探討電刺激誘發的光幻視是如何產生，透過振盪性經顱直流電刺激，分別給予正極、負極及交流電刺激等並操弄兩種刺激強度。我們的結果發現振幅調變效應在不同極性條件下，無論是閾值或閃爍速度皆並無顯著差異。從結果推論，視覺電刺激誘發的光幻視主要取決於電場的相對變化，而非極性或刺激強度。 本研究結果顯示，神經的相位與振幅調變的頻率保持一致時，可能是大腦如何將時間資訊整合為感知事件的關鍵機制之一。因此，本研究認為調幅頻率在視覺感知形成中扮演著關鍵作用，這為未來探討視覺意識的神經機制提供了新的方向。;Brain oscillations are known not only to correlate but also to causally affect visual perception. While most of the oscillatory studies are based on linear frequency methods, how the nonlinear oscillatory property plays a part in visual awareness is rarely discussed. This thesis includes two transcranial electric stimulation experiments to examine the functional role of amplitude-modulated (AM) oscillation in phosphene perception.
In Experiment 1, we used transcranial alternating current stimulation (tACS) on twelve healthy participants to study how AM tACS affects visual perception by inducing phosphenes. Our results showed that AM tACS can induce phosphenes with a higher intensity. Participants were more sensitive to frequencies around the beta band. The perceived flicker rate of AM tACS phosphenes was primarily determined by the AM frequency, which overrode the effect of the carrier frequency.
In Experiment 2, we utilized oscillatory transcranial direct current stimulation (otDCS) with a fixed polarity compared to tACS. This is based on previous research that shows tACS can elicit phosphenes. However, the specific mechanisms underlying the effects of rhythmic electric field fluctuations and the alternation of polarity remain unclear. To address this issue, twenty-five participants received otDCS and tACS over the occipital lobe under anodal, cathodal, and polarity-switching conditions. AM effects were consistent across all polarities, suggesting that relative changes rather than the absolute electrical field induce phosphenes perception. Additionally, otDCS conditions revealed polarity-dependent differences, with anodal stimulation producing brighter phosphenes. tACS led to faster response times than both cathodal otDCS. AM frequencies were associated with higher thresholds and slower response times than sinusoidal stimulation. These findings underscore the crucial role of AM in visual perception, likely via neural phase-locking to the modulation envelope. This thesis demonstrates that AM frequency, stimulation polarity, and oscillation properties are key factors influencing visual perception. These insights provide direction for future investigations into the neural mechanisms underlying visual perception.
