整體形狀視覺的周邊調節效應

Abstract

我們運用Glass patterns所構成的中央－周邊刺激圖形，來呈現整體形狀知覺的周邊調節效應。Glass patterns是由許多成對的兩個點所組成，每一對點的方向由特定的幾何轉換來決定。藉由整合不同對點，受試者可以在影像中看到特定的整體形狀結構。我們利用2AFC的實驗方法，來測量受試者的一致性閾值 (coherence threshold)，亦即在目標圖形之中，需要有多少訊號點，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。我們測量受試者對於中央目標的Glass patterns的一致性閾值，在中央目標單獨呈現的情境下，或者在多種形狀的Glass patterns亦同時呈現於周邊的情境下。我們發現，相對於中央目標單獨呈現的情境，圓形 (concentric) 和螺旋狀 (spiral) 的周邊會提高受試者偵測圓形目標的一致性閾值，而放射狀 (radial) 的周邊則對於圓形的偵測沒有影響。偵測放射狀圖形的一致性閾值會因為螺旋狀的周邊而提高。螺旋狀以及平移狀 (translational) 的Glass patterns不會受到任何周邊的影響。當我們在中央與周邊加入一個空白的間隔後，這樣的效果仍然可以維持，而且當中央與周邊的距離為最佳時，此效應的強度最大。結果顯示，整體形狀的知覺會受到周邊訊息的調節。此周邊調節決定於中央目標與周邊情境兩者的形狀。

We demonstrate a novel surround modulation of global form perception by using Glass patterns in a center-surround configuration. Glass patterns contain randomly distributed dot pairs, or dipoles, whose orientations are determined by a geometric transform. By integrating across dipoles, an observer can perceive a specific global structure in the image. We measured the coherence threshold, the minimum proportion of signal dots needed in the target for an observer to detect the global form, at 75% accuracy with a 2AFC paradigm. The coherence thresholds of the central target Glass patterns were either measured alone or measured with the presence of various Glass pattern surrounds. Concentric and spiral surrounds increased the coherence threshold for the concentric target compared with that measured for the target alone, while a radial surround had no effect. The coherence threshold for the radial pattern, however, was elevated only by the spiral surround while the spiral and translational Glass patterns were not affected by any surrounds. The effect persisted when the center and the surround were segregated by a blank gap, and the amplitude of the modulation peaked at an optimal distance between the center and the surround. Our results show that global form perception can be modulated by a surround, and this modulation depends on the shapes of the central target and the surround context.