Academic journal article Perception and Psychophysics

Perceptual Saliency of Points along the Contour of Everyday Objects: A Large-Scale Study

Academic journal article Perception and Psychophysics

Perceptual Saliency of Points along the Contour of Everyday Objects: A Large-Scale Study

Article excerpt

The aim of this large-scale study was to find out which points along the contour of a shape are most salient and why. Many subjects (N= 161) were asked to mark salient points on contour stimuli, derived from a large set of line drawings of everyday objects (N = 260). The database of more than 200,000 marked points was analyzed extensively to test the hypothesis, first formulated by Attneave (1954), that curvature extreme are most salient. This hypothesis was confirmed by the data: Highly salient points are usually very close to strong curvature extrema (positive maxima and negative minima). However, perceptual saliency of points along the contour is determined by more factors than just local absolute curvature. This was confirmed by an extensive correlational analysis of perceptual saliency in relation to ten different stimulus factors. A point is more salient when the two line segments connecting it with its two neighboring salient points make a sharp turning angle and when the 2-D part defined by the triplet of salient points is less compact and sticks out more.

It has been a well-known fact since the early days of vision science (e.g., Alhazen, ca. A.D. 1030; see Sabra, 1989) that not all points along an object's boundary contour are equally informative about that object's shape. Attneave (1954) was probably the first to explicitly formulate the hypothesis that curvature extrema (i.e., points along the contour where curvature reaches a local maximum) are most informative about shape. He used two demonstrations to support this hypothesis. In one demonstration, he asked subjects to mark salient points along the contour of a random shape and he showed that the frequency plots were centered on the curvature extrema (see Figure IA). In a second demonstration that has become known as Attneave s sleeping cat he created a version of a line drawing of his sleeping cat by connecting the curvature extrema by straight lines and he showed that this straight-line version was still easy to recognize.

Kennedy and Domander (1985), however, questioned Attneave's (1954) hypothesis that points of maximum curvature are most informative for shape recognition. In three experiments, with a small number of fragmented contour stimuli depicting manmade objects, they showed that identification of these stimuli was better when fragments were placed midway between extrema, and best when fragments were placed midway between midpoints and extrema. Kennedy and Domander concluded that the shapes of objects are best represented by samples of the contour that are selected to be evenly distributed, even if this means eliminating all of the points where curvature changes direction maximally (i.e., curvature extrema).

In a more recent study, with 12 silhouette stimuli derived from the shadows cast by sweet potatoes, Norman, Phillips, and Ross (2001) attempted to replicate Attneave's (1954) first demonstration more closely. Their results supported Attneave's hypothesis. More specifically, 12 subjects were asked to "copy" the silhouettes by positioning 10 points until the dotted contour version resembled the original silhouette version as closely as possible. The frequency plots (see Figure IB) looked very similar to the one from Attneave's study (see Figure IA). In addition, by doing a curvature analysis, they were able to derive the strongest curvature extrema, positive maxima, as well as negative minima (indicated by closed and open circles, respectively; see Figure 1C). By superimposing the points selected by at least half of the observers (indicated by arrows; see also Figure 1C), they then showed that the most informative points were almost always very close to the curvature extrema.

Norman et al. (2001 ) attributed the discrepancy between their results and those of Kennedy and Domander (1985) to stimulus and task differences. Kennedy and Domander had used fragmented contour stimuli depicting manmade objects (a window, a box, a stove, an electric clothes dryer, etc. …

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