RESOLVING VISUAL AMBIGUITY

What happens when alternative probabilities are about equal? The outline (Necker) cube appears to change its orientation spontaneously. Sometimes the lower square face of the cube appears nearer, and sometimes the upper square face. This reflects the absence of depth information from shading, perspective or stereopsis (3-D vision based on differences in the visual information received by each eye) that would normally reveal the orientation of the cube. Faced with two equally good interpretations, the visual system oscillates between them. But why does our visual system fail to generate a single stable percept, which is veridical (i.e. matches the characteristics of the scene exactly), namely a flat drawing on a flat sheet of paper? The reason that the brain chooses to interpret the scene as ‘not flat’ seems to reflect the power or salience of the depth cues provided by the vertices within the figure. Further evidence for this comes from the fact that we can bias the appearance of the cube by changing our point of view. So if you fixate the vertex marked 1 in figure 8.15, the lower face will seem nearer. Fixate the vertex marked 2 and the upper face tends to appear nearer. But why does this happen? Again, the answer brings us back to probabilities. When we fixate a particular vertex, it is seen as protruding (i.e. convex) rather than receding (concave). This is probably because convex junctions are more likely in the real world. To be sure, you will see concave corners (for example, the inside corners of a room), but most concave corners are hidden at the back of an object and therefore outnumbered by convex corners at the front. You can easily test this out by simply counting how many of each type you can see from where you are sitting now.