Academic journal article Perception and Psychophysics

Differences between Searching among Objects and Searching among Holes

Academic journal article Perception and Psychophysics

Differences between Searching among Objects and Searching among Holes

Article excerpt

We report results from six experiments in which participants had to search for a "C" among "O" distractors. The search items were either holes or objects, defined by motion, contrast, or both. Our main findings were (1) it was easier to search among objects than to search among holes, and (2) the difference between search among objects and search among holes was primarily caused by grouping with the background. The data support the hypothesis that the shape of a hole is only available indirectly. We further note that, in our experiments, search performance for both holes and objects depended on the surface medium used to define the search items.

How do we perceive holes? Nelson and Palmer (2001) discussed which factors determine whether a surrounded region is perceived as a hole, or whether it is perceived as an object. They used a direct reporting task, where participants had to indicate on a scale from -100 to 100 whether they saw a hole or an object. They found that participants tended to detect a hole as a function of three factors: (1) the relative depth of the inner part of the hole area (e.g., based on shading); (2) the figurai properties of the boundary surrounding the hole; and (3) the degree to which the inner part of the hole grouped with a "far" region, surrounding the local region, which itself immediately surrounds the hole. A hole was more likely to be seen if the inner part was further in depth than the local surround, if the boundary around the hole was concave and simple, and if the hole grouped with an outer, relative to an inner, surrounding region.

The question of whether and when a hole is perceived to have a shape of its own remains to be answered. Rock, Palmer, and Hume (cited in Palmer, 1999) found that recognition memory for holes was as good as for objects, even though the participants accurately perceived the holes as holes and the objects as objects on their initial presentation. According to Palmer, "the hole is ground for purposes of defining depth relations and what is material versus open space, but is figure for purposes of describing shapes" (p. 287). A similar suggestion is made by Peterson (2003).

A different proposal has been put forward by Bertamini and Croucher (2003), who examined the ease with which observers judged the relative heights of two vertices. Normally, such judgments are made more rapidly when the vertices are convex than when they are concave (Bertamini, 2001; Gibson, 1994). Bertamini and Croucher presented either a convex shape (an hourglass) or a concave shape (a barrel), either on a large background (the object condition) or with a local surround, with the color of the inside part then matching the wider background (the hole condition; see Figure 1). The participants' task was to judge which of the two vertices was lower. In the object condition, judgments were faster for the convex/barrel stimulus than for the concave/hourglass stimulus. However, this advantage was reversed when the local surround was present (now performance was better with the hourglass than with the barrel; in a follow-up study, Bertamini & Mosca [2004] confirmed this finding using stereograms). Bertamini and Croucher interpreted their result as indicating that the contours were assigned to the local surround and not to the inner hole. With the contours assigned to the local surround, the surrounding object had convex contours around the hourglass hole, whereas it had concave contours around the barrel hole. Bertamini and Croucher suggested that "a hole is defined by the contour of the enclosing object, rather than the hole itself possessing the contour" (p. 52).

In the present study, we used visual search procedures to assess whether, in addition to any differences in representation when single stimuli are presented, holes contrast with objects in their ability to attract attention, even when holes and objects share the same contours. We report data from six experiments where, in each study, participants had to search for a target "C" among "O" distractors. …

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