色彩對比增益控制機制在整體圖形知覺扮演的角色

Abstract

葛拉斯(Glass)圖形是由隨機散佈的「雙點 (dipole)」部件所組成，而這些雙點之間的排列遵照特定的幾何規則，不同的規則將決定該圖形的整體知覺。知覺一個葛拉斯圖形仰賴兩階段的處理機制，局部處理—將兩個點組合成雙點部件—以及整體處理—將這些局部的雙點部件整合成整體的圖形。有別於傳統的葛拉斯圖形，在我們的研究中我們使用局部由「三點 (tripole)」部件組成的葛拉斯圖形，以之研究色彩對比 (color contrast) 對於人類群聚視知覺 (visual grouping) 的影響。每個三點部件包含一個定錨點 (anchor dot) 與兩個周邊點 (context dot)，將定錨點與其中一個周邊點整合則葛拉斯圖形整體看起來會呈現為一個逆時針螺旋，與另一個周邊點整合則會看成順時針螺旋。在我們的色彩操弄中，每個三點葛拉斯圖形中的所有點色調 (hue) 一致，只有色彩對比有所不同。我們使用四種色調的刺激，對比根據+/-(L-M)以及+/-S主軸軸線變化。實驗參與者必須在看完每個三點葛拉斯圖形之後判斷該圖形是順時針還是逆時針的螺旋並按下相對應的反應按鍵。結果發現判斷圖形為順時針或逆時針螺旋的機率會隨著其中一個周邊點的色彩對比上升，直到超過特定的對比值後該機率又會下降；而整體的機率又會隨著另外一個周邊點的對比上升而下降。這樣的結果無法用過去提出的理論模型來解釋，而必須使用我們提出的「除法抑制模型(divisive inhibition model)」才能解釋資料變異。本研究得到的結果與之前操弄明暗對比(luminance contrast)類似，都有呈現倒U(inverted-U)的趨勢，相異之處在於色彩對比模型中的抑制部件較明暗對比來的弱。

A Glass pattern consists of randomly distributed dot pairs, or dipoles, whose orientation is determined by a geometric transform, which defines the global percept perceived by an observer. The perception of Glass patterns involves a local process that associates dot pairs into dipoles and a global process that groups the dipoles into a global structure. In the present study, we used a variant of Glass patterns, which was composed of randomly distributed tripoles instead of dipoles, to estimate the influence of color contrast on perceptual grouping. Each tripole contained an anchor dot and two context dots. Grouping the anchor dot with one of the context dot would result in a global percept of a clockwise (CW) spiral while grouping with the other dot, a counterclockwise (CCW) spiral. All dots in each pattern were modulated in the same color direction but different contrasts. Four types of patterns were involved, namely modulating in +/-(L-M), and +/-S cardinal directions. The observers were to determine whether the spiral in each trial was CW or CCW. The probability of a context dot being grouped with the anchoring dot increased along with its color contrast to certain level before the probability started to drop. Our result cannot be explained by the existing models for perceptual grouping but a divisive inhibition model. The isoluminance contrast result observed is similar to the inverted U-shaped function for luminance contrast result previously reported (by us); except that color contrast model comprises a weaker self-inhibition component.