Bias in Spatial Memory: Prototypes or Relational Categories?

Bias in Spatial Memory: Prototypes or Relational Categories? L. Elizabeth Crawford (lcrawfor@richmond.edu) Department of Psychology, University of Richmond, 28 Westhampton Way Richmond, VA 23173 USA David Landy (dlandy@indiana.edu) Department of Psychological & Brain Sciences, Indiana University, 1101 E. 10 th St., Bloomington IN 47405 USA Amanda N. Presson (Nicole.Presson@richmond.edu) Department of Psychology, University of Richmond, 28 Westhampton Way Richmond, VA 23173 USA Abstract Two experiments examined participants’ use of independent and relational category structures in a spatial memory task. When estimating the locations of dots presented on a rectangular display, many participants appeared to divide the space into left and right relational categories, biasing estimates away from the center of the screen and toward the outer edges, in contrast to prior work showing biases toward centrally located prototypes. More participants showed this relational pattern at short interstimulus intervals than at long ones. The results suggest that participants flexibly make use of different types of spatial structure under different task demands. Keywords: spatial memory; categorization. Introduction Categories profoundly affect our interpretation of situations. But how are categories constructed, and how do particular situations impact this construction? Recent studies distinguish between categories defined in isolation and those that are defined in relation to other categories (see Chin- Parker & Ross, 2004; Genter & Kurtz, 2005; Goldstone, 1996; Yamauchi & Markman, 1998). This distinction between relational and non-relational category structures comes out of the literature on categories of objects. To our knowledge, there has been little discussion of this issue in the spatial domain, despite the large literature on spatial category effects. Thus it is not known what factors might influence the kinds of category structure people use to organize space. To the extent that categories of objects and of space serve similar functions, the distinction between relational and independent categories may emerge in the spatial domain as well. Research on categories of objects has shown that when a category is construed as independent, the category center is generally treated as the prototype, leading to fast, accurate classification and high typicality ratings for central values (e.g., Goldstone, 1996; Posner & Keele, 1968; Rosch, 1975). However, when categories are established in a contrasting relation to one another, the values that show advantages in classification and higher typicality ratings tend to be more extreme than the central values (e.g., Atran, 1999; Barsalou, 1985; Goldstone, Steyvers, & Rogosky, 2003; Davis & Love, 2010). That is, the best exemplars of contrasting categories are idealized caricatures that exaggerate the differences between the categories, rather than the most typical values. These different category structures have also been shown to influence how individual category members are remembered. When items within a single category vary along a continuous dimension such as size or hue, estimates of individual items are biased toward the central value of the presented set (e.g., Huttenlocher, Hedges, & Vevea, 2000), but when that set is divided into two distinct categories, estimates may be biased away from the value corresponding to the category boundary (Goldstone, 1995, but see also Sailor & Antoine, 2005). Patterns of bias in estimates have also been used to investigate categorization in the spatial domain. A common finding is that when participants reproduce the location of a stimulus within a bounded spatial frame, their location estimates are systematically biased toward some locations and away from others. A prominent model explains these biases as category effects that arise during reconstructive memory (Huttenlocher, Hedges, & Duncan, 1991). In a process likened to Bayesian estimation, the model posits that people use the category information as a prior distribution in order to adjust for the inexactness of fine- grained memory, leading to systematic biases toward the center of the prior distribution. As predicted by this account, when fine-grained memory is worsened by adding a delay, bias becomes more extreme, suggesting that the category is given more weight during memory reconstruction. This model offers an account of the large literature on how categories influence memories of stimulus features and suggests that bias in estimates can reveal the category structure that people impose on space. The categories that are revealed by bias in spatial memory tend to be consistent across many variations in experiment procedures. In general, estimates are biased away from outer edges and internal axes of symmetry and toward centrally located prototypes. For example, within a circle, estimates are biased toward the centers of mass of the four quadrants bounded by the horizontal and vertical axes of symmetry (Huttenlocher, et al., 1991; Huttenlocher, Hedges, Corrigan, & Crawford, 2004). Stretching the circle along one dimension so that it becomes an ellipse moves the prototypes accordingly (Wedell, Fitting, & Allen, 2007). Stimuli within a rectangular frame, such as sandbox or a rectangle drawn on paper (Huttenlocher, Newcombe, &