雙眼競爭與雙重解釋圖形在知覺上的神經機制

Abstract

觀看一個刺激卻產生兩種不同的知覺，彼此交替出現的現象稱為「雙重知覺」（bistable perception），本研究將探討雙眼競爭（binocular rivalry）與雙重解釋圖形（ambiguous figure）是否循相同的機制以產生雙重知覺，該問題在文獻中仍未有明確的解答。雖然已知認知調控與基本刺激屬性皆會影響雙重知覺，但現有證據仍不足以斷定何者為雙重知覺的關鍵歷程。我們過去的研究指出額上回（superior frontal gyrus）內側和雙眼競爭與雙重解釋圖形知覺皆有關，本研究除再次檢驗額上回內側和雙重知覺的關連性外，並進一步探討該處神經活動在雙重知覺上可能扮演的功能。實驗一的功能性大腦磁振造影（functional magnetic resonance imaging）資料顯示，雙眼競爭刺激與雙重解釋圖形所引發的神經活動型態十分相似，唯有當受試者產生知覺交替時，才伴隨有額上回內側的神經活化，且其活化在知覺交替開始但尚未完成的期間特別明顯。實驗二比較受試者對知覺交替的自主控制程度，結果發現其在雙眼競爭與雙重解釋圖形知覺的影響力相當，支持二者有相同機制的說法。我們也觀察到，當受試者意圖控制知覺交替時，額上回內側的神經活化增加，暗示了影響雙重知覺的認知調控必須透過額上回的神經活動的可能性。實驗三藉由操弄刺激屬性的強弱以破壞知覺交替的平衡，其效果在雙眼競爭與雙重解釋圖形知覺上顯著不同，但額上回內側的神經活化並未隨之改變，顯示其在雙重知覺上所擔負的功能不受基本視覺訊息處理的干擾。綜合三個實驗結果，我們推論雙眼競爭與雙重解釋圖形在基本視覺訊息處理層次上由不同機制負責，一旦刺激的神經表徵建立，雙眼競爭與雙重解釋圖形應透過相同機制以產生知覺交替，而額上回內側在其間扮演重要的角色。

The function of human visual system is to analyze the input image and to extract information about objects in the environment. It puzzles investigators for years how human visual system is capable of reconstructing a stable and coherent three-dimensional world through two-dimensional retinal information. A collection of incident lights into our eyes can be interpreted as numerous possible objects, but most of the time, people are consciously aware of a single percept. It results from a series of processes and distributed neural networks are implicated to process incoming visual information. Hence, any impairment among the processes may prevent daily vision. For instance, macular degeneration causes loss of central vision, which harms one’s capability to see fine details of an object (Cacho, Dickinson, Reeves, & Harper, 2007). Damage to unilateral primary visual cortex leads to blindness of the contralateral visual field (Trevethan, Sahraie, & Weiskrantz, 2007). However, some patients seemed to exhibit residual visual ability unconsciously, which is called “blindsight”. Although patients insisted on their visual deficits, some of them performed better than chance when they were required to guess whether a stimulus was presented in their blind field (Trevethan et al., 2007). It has raised a lot of discussion of visual awareness and its neural correlates (Crick & Koch, 1998). To a normal brain, a stable percept is the best guess given the visual input (Crick & Koch, 2003). Occasionally, the stable percept may break down. Certain categories of visual stimuli lead to two or more percepts, rather than one, and those percepts alternate without any physical changes occurring in the stimulus. This phenomenon is called “bistable perception” or “multistable perception”, depending on how many percepts the stimulus evokes (Blake & Logothetis, 2002; Leopold & 9 Logothetis, 1999). Perceptual changes which lack corresponding stimulus changes offer an opportunity to dissociate perceptual representation from sensory representation of a stimulus (Moutoussis, Keliris, Kourtzi, & Logothetis, 2005), and address to the issue of visual awareness (Crick & Koch, 2003). Ambiguous figure is one type of stimuli which is capable of generating bistable perception. For example, the famous Necker cube, as Figure 1a shows, can be perceived as two cubes with different depth features (see Figure 1b & 1c). Continuously viewing it for a while results in a dynamic sequence of the two cube percepts. Binocular rivalry is another type of stimuli which also induces bistable perception. When two discrepant monocular images presented to two eyes separately, the two images rival for perceptual dominance and only one monocular image is perceived at a time while the other is suppressed. Figure 2 illustrates an example of binocular rivalry, in which the two gratings moving in the opposite directions would lead to perceptual alternations between one and the other. Although both binocular rivalry and ambiguous figure cause two percepts switching, they differ in how alternative percepts arise. In the above examples, the Necker cube needs reorganization of edges, so as to coherently construct it into the other depth, but different values of a feature, such as motion directions, for two eyes are sufficient to cause alternative percepts without further grouping.