1 Introduction When a uniform connected region in an optical image is completely surrounded by another such region, its perceptual interpretation is ambiguous. It might arise from viewing a smaller object in front of a larger object or a single object with a hole through it. Figure 1 illustrates the problem. In figure 1a, the fully enclosed white region in the center can be perceived as a white object on top of a black surface, or as a hole through a black surface, revealing the white background behind it. Particular image conditions can affect the interpretation strongly in either direction. In figure 1b, for example, although the same two logical possibilities exist (ie a sphere in front of a black surface, or a hole through the surface revealing a shaded or curved background), there is a strong tendency to perceive it as an object in front of its surrounding back- ground. In figure 1c, however, there is a strong tendency to perceive it as a hole through a dark square. The perceptual outcome of viewing such an image, like many underdetermined problems in vision, may be the result of integrating a number of factors. The goal of this paper is to discover what sorts of image structure affect the perception of holes versus occluding objects. In previous work, Cavedon (1980) performed a series of experiments on the perception of screens, some of which had holes cut through them, and some of which had shapes pasted on the top. Subjects were asked simply to report on what they saw upon viewing the displays. A few of the findings from these experiments were (i) an actual hole was not perceived when there were no detectable depth cues, (ii) a convex surrounded region was more likely to be perceived as a hole, and (iii) a region was more likely to be perceived as a hole when it contained texture elements similar to those of an outer region. Further experiments were conducted by Masin (1995), who compared the lighting conditions of three concentric surfaces at different distances from observers. They perceived either a transparent floating disc or a circular hole through a surface, depending on the ratio of light on adjacent surfaces. Others have made predictions about what conditions will lead to a stronger perception of a hole or an object. Bozzi (1975) has suggested that nonaccidental relations can influence hole perception, while others (eg Casati and Varzi 1994; Hershenson 1999; Palmer 1999) have intimated that grouping processes and continuation behind surfaces play a role in hole perception. However, in most of these cases demonstrations of the Of holes and wholes: The perception of surrounded regions Perception, 2001, volume 30, pages 1213 ^ 1226 Rolf Nelson, Stephen E Palmer Department of Psychology, 3210 Tolman Hall #1650, University of California, Berkeley, CA 94720-1650, USA; e-mail: rnelson@socrates.berkeley.edu Received 24 October 2000, in revised form 18 June 2001 Abstract. Fully enclosed regions in a two-dimensional image can often be perceived either as an object in front of a surface or as a hole through a surface. Several experiments were conducted to determine what factors affect perception of holes versus objects. Three types of factors were tested and found to influence this outcome. First, depth factors directly indicate that the enclosed region lies behind its surrounding surface. Second, grouping factors relate the enclosed region to an outer region that is generally perceived as a continuation of the surface seen through the hole. Finally, figural factors indicate whether the enclosed region is to be perceived as figure or ground. Relations among these factors and their implications for perceptual organization are discussed. DOI:10.1068/p3148