三度空間中平面結構對對稱偵測的影響

Abstract

要偵測一個對稱圖形，觀察者需要根據圖像中找出對稱軸左右兩個圖形的元素間的對應關係，這意味著在這些元素進行對應時，可能存在著知覺機制相互對應的一種關係。另外，人類在感知三維空間結構中的隨機點立體圖（RDS）時會有兩個知覺機制，這機制分別會抑制不同視差距(disparities)的隨機點和興奮同樣差距的隨機點。因此，我們將探討把對稱元素放到不同深度的結構時，是否會影響對稱性的偵測。實驗中的對稱圖形是根據垂直對稱軸所形成的，並由 0.19o × 0.19o 大小和0.5%密度的點所組成。我們利用2AFC的實驗方法，來測量受試者的一致性閾值(coherence threshold)，亦即在目標圖形中，需要有多少的訊號點，才能使受試者在 偵測目標圖形的整體形狀時達到百分之七十五的正確率。我們所操弄的3D圖像是由左右圖形的視差距(disparity，最大0.228o)所產生的。結果顯示，在對稱圖形左右分為兩半的條件下，受試者的表現受到會抑制，但在斜面的條件下，受試者的表現卻與平面時沒有差別。而當對稱圖形是在傾斜，但在不同平面的條件下時，抑制情況又再次出現。另一方面，當對稱圖形與3D 空間結構一致時，受試者的表現則有促進的效果。我們的研究結果指出，在偵測對稱圖形時，「共面」及2D-3D空間對稱結構的「一致性」都是重要的決定因素。

To perceive a symmetric pattern, an observer needs to find correspondence between two image elements across the symmetric axis. It implies a correspondence relationship between perceptual mechanisms responding to these elements. To perceive a 3D structure in random dot stereogram (RDS), it is shown that perceptual mechanisms responding to different disparities would inhibit each other; and would excite the neighborhoods where the disparity varies smoothly except for a small fraction at the object boundaries. We thus investigated whether putting corresponding elements of a symmetric pattern at different depths would affect symmetry detection. The symmetry patterns consisted of dots (0.19o × 0.19o) occupying 0.5% density of the display. We measured the coherence threshold at 75% accuracy with 2AFC paradigm and manipulated the symmetric pattern rendered on different 3D configuration structures that were produced by the disparity (max 0.228o) between the left and the right eye images. The results show that the two-half plane had inhibited observer’s performance. The coherence threshold for symmetric pattern on a slant surface was similar to that on a frontoparallel plane, even though the depth of the two sides of the symmetric axis was different. The threshold increased dramatically when one side of the axis inclined toward the observer while the other side inclined away even though the depth difference between the two sides was the same as that in the slant condition. The threshold reduced on a hinge configuration whose joint coincide with the symmetry axis. Our result suggests that the co–planarity and the consistency of 2D symmetry pattern and 3D structure are important factors for symmetry detection.