Academic journal article Psychonomic Bulletin & Review

Are There Capacity Limitations in Symmetry Perception?

Academic journal article Psychonomic Bulletin & Review

Are There Capacity Limitations in Symmetry Perception?

Article excerpt

Previous researchers have proposed that there are two types of symmetry detection: one based on crude preattentive symmetry judgments and another based on detailed scrutiny of individual parts (Barlow & Reeves, 1979; S. E. Palmer & Hemenway, 1978; Royer, 1981). Four experiments were conducted to examine capacity limits in different symmetry judgments. Observers were required to discriminate between random patterns and approximate symmetry (Experiments 1 and 3) or between perfect and approximate symmetry (Experiments 2 and 4). The patterns were divided into two sets of dots, presented either simultaneously or successively. A comparison of accuracy under these two presentation conditions suggested that symmetry detection involves an analysis that is spatially parallel but coarse, regardless of either task difficulty or task type (detecting symmetry vs. detecting asymmetry).

Mirror symmetry is often highly salient to human observers (Barlow & Reeves, 1979), and the detection of symmetry may play an important role in mammalian vision in general. Some writers view symmetry as one of the most important aspects of early visual analysis (Wagemans, 1995). The present article examines questions at the interface between symmetry perception and visual attention that inquire into the existence and nature of capacity limitations in the detection of symmetry.

A number of writers have proposed a two-stage model of symmetry detection (S. E. Palmer & Hemenway, 1978; Royer, 1981; for recent models, see Dakin & Hess, 1997; Gurnsey, Herbert, & Kenemy, 1998; Rainville & Kingdom, 2000; van der Helm & Leeuwenberg, 1996). According to these models, when observers are required to discriminate between a symmetrical pattern and a random pattern, a preattentive symmetry analysis takes place. For finer judgments (e.g., discriminating between random displays and random displays with partial symmetry, or discriminating between perfect symmetry and slightly perturbed symmetry), an attention-demanding pointwise matching process is undertaken. Various results have been interpreted to argue that symmetry detection is preattentive for crude and easy symmetry detection (Barlow & Reeves, 1979; Locher & Wagemans, 1993) but not for finer and more difficult judgments (Barlow & Reeves, 1979; Foster, 1991; Royer, 1981; Wenderoth, 1997; for a review of the two-stage models, see Wagemans, 1995).

The concept of preattentive processing has several different aspects that can sometimes be dissociated from each other. Preattentive processing is often assumed to be fast, automatic, and unlimited in capacity, as opposed to slower, more flexible, limited-capacity controlled processing. The paradigmatic case of preattentive processing is usually assumed to be pop-out in visual search, where a target differs on a salient featural dimension from homogeneous distractors. Thus, the notion of preattentive processing actually involves three potentially separate questions (Hochstein & Ahissar, 2002; Pashler, 1998).

Several different methods have been proposed and used to identify whether a process is preattentive. Julesz offered an operational definition of preattentive processing, according to which any property that can be detected in a very brief display is said to be detected preattentively (Julesz, 1981, p. 28). In the literature on symmetry detection, evidence has been presented to show that determination of crude symmetry can be performed very quickly (Barlow & Reeves, 1979; Locher & Wagemans, 1993; Royer, 1981 ; Wenderoth, 1997), whereas detecting more subtle symmetry or asymmetry requires longer exposure. These data appear consistent with the two-stage model (S. E. Palmer & Hemenway, 1978; Royer, 1981).

Another question is whether symmetry perception is automatic and hence immune to the effects of mental set. If the observers are aware of some property of the stimuli, can they voluntarily adjust their symmetry perception to exploit this property? …

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