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An Exception to the 90° Corner Bias

Simple line drawings depicting objects are extremely ambiguous. Each depiction can, in fact, represent an infinite number of objects. It is generally believed that viewers rely on biases to reduce the ambiguity. For example, many researchers (eg Gregory 1970; Mackworth 1973; Perkins 1972; Shepard 1981) have argued that there is a bias to interpret three lines intersecting at a point as a 90° corner whenever such an interpretation is consistent with projective geometry. A 90° corner is a corner in which three surfaces, each containing a 90° angle, meet at 90° angles. All eight corners of a rectangular box are 90° corners (see figure 1). Several researchers have provided psychophysical evidence for the 90° corner bias (Butler 1984; Perkins 1982; Shepard 1981), but the research has not found the bias to be as strong as the above hypothesis suggests. Some research suggests that the strength of the 90° bias depends upon experience. Perkins (1972) and later Butler (1984) observed that naive subjects did not have as strong a 90° bias as did the researchers. Recently, Butler and Baker (1984) found that students of architecture and others with substantial drawing experience had stronger 90° corner biases than students without as much experience. The present paper provides an exception to the 90° corner bias that cannot be attributed to individual differences in experience. Figures 2 and 3 present two views of a three-dimensional object that is composed of 90° corners. The two views differ only by a rotation about the vertical axis. Many observers report that figure 3 depicts a threedimensional object, one that probably contains 90° angles. But I have not found anyone who reports a compelling three-dimensional interpretation of the intersections in figure 2. One of the important implications of figures 2 and 3 concerns local cues. Some observers (eg Hochberg 1982) argue that corners in line drawings act as local depth cues. Hochberg further argues that perception of the local features is not determined by the whole configuration. The example provided in figure 2 is inconsistent with this hypothesis. Figure 2 contains the same Y-shaped, three-line intersection as that found in figure 1. Yet, the Y-intersection is perceived as a 90° corner in figure 1, but not in figure 2. The lack of a 90° corner bias for figure 2 must result from either global cues or perceptual constraints among features that have precedence over local depth cues. Research by Butler (1982), Hemenway and Palmer (1978), and Hochberg and Brooks (1960) indicates that relatively simple drawings of objects are seen as two-dimensional,