A Study of Object-Location Memory Hongbin Wang (Hongbin.Wang@uth.tmc.edu) Todd R. Johnson (Todd.R.Johnson@uth.tmc.edu) Jiajie Zhang (Jiajie.Zhang@uth.tmc.edu) Yue Wang (Yue.Wang@uth.tmc.edu) School of Health Information Sciences, University of Texas Health Science Center at Houston 7000 Fannin, Suite 600, Houston, TX 77030 USA Abstract This paper aims to study the representational nature of human object-location memory. Two experiments are reported, including both performance data and eye movement data. The results show that multiple allocentric frames of reference are used to encode spatial relationships among objects and late computation in object-location memory retrieval in object- cued conditions is often inevitable. The implications on developing a general model of human spatial cognition are discussed. Introduction One important aspect of human spatial memory has to do with remembering the location of objects relative to each other. For example, you might recall that the book you read last night is on your office desk between your computer and the desk lamp. This type of memory for spatial relationships is an essential component of a more general type of memory for spatial layout and is obviously critical for many spatial tasks including locating and navigation (see Tversky, 2000, for a review). It is not clear, however, how spatial relationships among objects are encoded in memory. While it seems apparent that allocentric frames of reference (i.e., locations defined relative to external objects) rather than egocentric frames of reference (i.e., locations relative to the observer self) are often used to describe these spatial relationships, the representational and computational nature of this description is controversial (see Hunt & Waller, 1999; Klatzky, 1998). For example, are object-based spatial relationships encoded and stored directly (early computation)? Or do they have to be inferred much later at the retrieval stage (late computation)? What factors determine which representational scheme is used? In this paper we report two experiments we conducted to directly address these issues. The results show that multiple allocentric frames of reference are used to encode spatial relationships among objects and late computation in object- location memory retrieval in object-cued conditions is often inevitable. This paper consists of three major parts. In the first section, the experimental paradigm is briefly introduced. In the second section, the experiments are reported, including both performance data and eye movement data. In the final section, we briefly discuss our ongoing work of developing a computational model of the object-location memory and its implications on modeling human spatial cognition in general. The Experimental Paradigm We adopted an experimental paradigm developed by Milner and colleagues in the 1990s, which we call the Milner paradigm (e.g., Johnsrude, Owen, Crane, Milner, & Evans, 1999; Milner, Johnsrude, & Crane, 1997; Owen, Milner, Petrides, & Evans, 1996). Though their focus was on neuroimaging studies of the brain foundations of object- location memory, the Milner paradigm offers an elegant experimental design that allows a systematic evaluation of multiple schema for representing spatial relationships. In addition, the availability of neuroimaging data provides invaluable constraints on both understanding behavioral results and developing computational models (e.g., Wang, Johnson, & Zhang, 2001). There are two phases in the Milner paradigm. In the encoding phase (Figure 1A), eight drawings (objects) are individually presented on a computer screen to subjects, with each drawing accompanied by two landmarks (solid squares). Subjects are asked to remember the locations of drawings, relative to the landmarks. In the retrieval phase, subjects are presented some cues plus two identical drawings. One of the two identical drawings (target) is presented in its original location, and the other one (noise) is presented in a different location (or more accurately, it occupies the original location of another object). Subjects are required to perform a forced-choice recognition of the target, relative to the cues. Milner and colleagues originally used four retrieval conditions: 1. In the fixed-landmark condition (Figure 1C), the two landmarks were presented as cues, along with the target-noise pair. The absolute location of landmarks and objects on the screen was unchanged from their original encoding positions. 2. In the shifted-landmark condition (Figure 1D), the two landmarks were presented as cues, along with the target-noise pair. Though the spatial relationships among the landmarks/drawings remained unchanged, the absolute locations of the landmarks/drawings on the screen were shifted. 3. In the fixed-object condition (Figure 1E), two encoded drawings instead the two landmarks were
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