From local inputs to global percept of surfaces

In classical illusions such as the Kanizsa figure (Fig. 75.1A) (Kanizsa, 1979) and the Varin configuration for neon color spreading (Fig. 75.1B) (Varin, 1971), we can observe completed surfaces even though the physical information is spatially limited. In the Kanizsa figure, in addition to illusory contours, the pacman-shaped stimuli create an illusory rectangular surface where the entire area appears slightly darker than the background. In the Varin figure, the color in the wedge-shaped portions appears to fill the square region. The term filling-in refers to the situation where a property such as brightness and color propagates beyond the region of physical stimulation to form a clear percept of a delineated surface. Filling-in phenomena are important because they provide a psychophysical paradigm to explore, as well as give insights into, the mechanism that integrates local inputs to form a global surface representation.

Figure 75.1..  

A, Kaniza square. In addition to illusory contours forming a square, the pacman-shaped stimuli give rise to the percept of an illusory surface in which the entire area appears slightly darker than the background, and amodally completed white discs. B, Varin figure for neon color filling-in. By adding colored wedge portions to A, a transparent, colored surface and white discs seen through it are induced. (See color plate 49.)

What is noteworthy is that filling-in effects often involve an impression of one surface in front of another (or others). An example may be seen in a white disc that appears to be partly occluded by the illusory rectangular surface in the Kanizsa figure (Fig. 75.1A). This is sometimes called amodal completion because the occluded surface is perceived but is not locally visible in the literal sense. In the same Kanizsa figure, the rectangular illusory surface can be considered modally completed because it is perceived as an entirely visible, occluding surface. Another example of modal completion is the illusory colored surface in the Varin figure (Fig. 75.1B). In this case, the colored portions are seen as parts of a single semitransparent colored surface through which the back surface is also visible, thus allowing visibility of two surfaces along the same line of sight. Thus, in short, filling-in and multiple surface layouts characterize visual surface perception from spatially sparse inputs.

Furthermore, it has been shown that minor changes in local visual inputs often lead to a drastic global change in surface perception. Figure 75.2 is an example demonstrating the effect of local disparity (Nakayama et al., 1989). The left-middle pair and the middle-right pair, which can fuse, contain opposite signs of local disparity, while the retinally stimulated areas are largely identical. Three discs through one window or one disc through three windows can be perceived, depending on the disparity, that is, on which pair is fused. Thus, the relationship between modal (occludding) and amodal (occluded) can be reversed by a local change in disparity. Nakayama et al. (1989) also showed that global surface layouts defined by local disparity influences recognition of a face occluded by stripes. The performance was better when the face was seen behind the stripes than in front, indicating that amodal completion of the face facilitated recognition.

Figure 75.2..  

Effect of local disparity information on global surface layouts. The observer is expected to see either three discs through one window or one disc and three windows, depending on the direction of disparity. The converging fuser should fuse the left and middle images to see three discs through one window and the middle and right images to see one disc through three windows. The diverging fuser should fuse the opposite pairs. (From Nakayama et al., 1989.)

Nakayama et al. (1990) studied the effect of local disparity on color filling-in using the Varin and other configurations. When the colored portions were defined as front (by a crossed disparity), color filling-in, subjective contours, and transparency were all enhanced. By contrast, when they were defined as behind, all these became amodal, thus suppressed.

Figure 75.3 presents examples of color filling-in derived from very limited colored areas (the original was developed by Ken Nakayama; an unpublished observation). Here, colored areas, as well as disparity and collinearity cues, are presented very locally and sparsely. Note also that one of the colored areas is even unpaired between the left- and right-eye images (Nakayama and Shimojo, 1990). Yet, a microscopic difference in edge orientation alone gives rise to a global difference in the completed surface (compare the top and the bottom stereograms when fused). Whereas collinearity itself may be considered a global property, the information that defines this property is edge location and orientation, which are given only very locally.

Figure 75.3..  

Effect of local edges on global surface completion. The converging fuser should fuse the left and middle images; the diverging fuser should fuse the middle and right images. The difference in the small colored regions leads to remarkably different filled-in surfaces: a diamond in A and a cross in B. Note that one of the colored portions is given only for one eye (the colored region on the left of the middle image is missing). Global filled-in surfaces can be formed even in the absence of binocularly matched inputs. (Courtesy of Ken Namayama.) (See color plate 50.)

Local changes in luminance-related cues, such as contrast at edges (Nakayama et al., 1990) and background luminance (Anderson, 1999), are also known to be critical in determining global surface properties. It should be noted that the local factors determining global surface properties, such as edge orientation, contrast at edges, disparity, and so on, are typical features that are detected in the early visual cortex (such as areas V1 and V2). Hence, the global aspects of surface perception do not exclude the critical role of local feature detection in the early visual cortex. As we will discuss later, one of the possible mechanisms underlying global surface representation is a propagation-like process starting from local features, which in effect can fill in a big gap in space to establish a surface representation. In such a mechanism, a local feature detector is local in the sense that it is activated by an isolated stimulus presented in a limited area (classical receptive field) but also global in the sense that the activity can be modulated by the global context outside the receptive field (Gilbert et al., 1990). Likewise, a local feature is local in that it is given in a limited retinal area, but it can also be global in that it can modulate the activity of distant local feature detectors.