Bias Effects in the Possible/impossible Object Decision Test with Matching Objects

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In the possible/impossible object decision test, priming has consistently been found for structurally possible, but not impossible, objects, leading Schacter, Cooper, and Delaney (1990) to suggest that priming relies on a system that represents the global 3-D structure of objects. Using a modified design with matching objects to control for the influence of episodic memory, Ratcliff and McKoon (1995) and Williams and Tarr (1997) found negative priming for impossible objects (i.e., lower performance for old than for new items). Both teams argued that priming derives from (1) episodic memory for object features and (2) bias to respond "possible" to encoded objects or their possible parts. The present study applied the matched-objects design to the original Schacter and Cooper stimuli-same possible objects and matching impossible figures-with minimal procedural variation. The data from Experiment 1 only partially supported the bias models and suggested that priming was mediated by both local and global structural descriptions. Experiment 2 showed that negative priming for impossible objects derived from the structural properties of these objects, not from the influence of episodic memory on task performance. Supplemental materials for this study may be downloaded from

Exposure to a stimulus or a related item can change how subjects respond to that stimulus on a subsequent exposure, even in the absence of conscious awareness of the previous encounter. This form of memory is referred to as priming and is thought to be distinct from explicit memory, or the ability to consciously recognize or recollect information (Schacter & Buckner, 1998; Tulving & Schacter, 1990). Priming is typically measured as a change in the speed, accuracy, or bias with which a stimulus is classified or identified on a repeated presentation.

Perceptual priming, in which the change in a subject's behavior is thought to result from a modification in the processing of the perceptual properties of a previously experienced stimulus, can be viewed not only as a form of memory, but also as a by-product of the normal operation of perceptual systems that learn (Bowers & Marsolek, 2003; Henson, 2003). For this reason, the phenomenon of perceptual priming has informed the study of the nature of object representation and the different stages of perceptual analysis, particularly in the visual modality (for examples, see Bentin & Golland, 2002; Biederman & Cooper, 1991; Burgund & Marsolek, 2000; Cooper & Schacter, 1992; Gauthier & Tarr, 1997; Itier & Taylor, 2004).

A well-characterized perceptual priming paradigm that has been used to study how structural information about visual objects is represented is the possible/impossible object decision test developed by Cooper, Schacter, and colleagues (e.g., Schacter, Cooper, & Delaney, 1990). The standard block-based implementation of this test consists of two phases: encoding and test. During the encoding phase, participants make a specific decision about unfamiliar line drawings; this decision is unrelated to the object decision performed at test. Half of the drawings depict structurally possible figures, which could exist as 3-D objects in the real world, and half represent structurally impossible figures, which cannot be rendered as coherent 3-D structures (see Figure 1). In the test phase, previously encoded figures are intermixed with an equivalent set of new possible and impossible items, and subjects are asked to classify them as either structurally possible or impossible. Priming has consistently been reported for possible objects in the form of greater classification accuracy or speed for old than for new test items. In contrast, impossible objects generally fail to elicit priming under standard test conditions (Liu & Cooper, 2001; Schacter & Cooper, 1993; Schacter et al., 1990; Schacter, Cooper, Delaney, Peterson, & Tharan, 1991; Schacter, Cooper, Tharan, & Rubens, 1991; Schacter, Cooper, & Valdiserri, 1992; Soldan, Mangels, & Cooper, 2008). …


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