The Effects of Alignability on Memory

According to structure-mapping theory. the process of comparison is one of alignment and mapping between representational structures. This process induces a focus on commonalities and alignable differences (i.e., those related to the commonalities). Nonalignable differences (i.e,, those not related to the commonalities) are held to be neglected. The theory thus predicts increased focus on the Corresponding information, whether these are commonalities or dqferences. In this article, we explore the implications of this claim for memon: Specifically, we test the prediction that alignable differences are more likely to be processed and stored than nonalignable differences. We present a study in which people made similarity comparisons betflpeen pairs of pictures and then were probed for recall. The recall probes were figures taken from the pictures and were either alignable or nonalignable differences between the pairs. The alignable differences were better memory probes than the nonalignable differences, tuggesting that people were more likely to encode and store the corresponding information than the noncorresponding information. Daily experience bombards a person with a wide array of information, only some of which is worthy of further attention. In order to make sense of the world, i t is necessary to filter out some aspects of the incoming information and focus on others. How does one determine which aspects to attend to and store? One important determinant of what will be processed and remembered is consistency with prior schemas or mental models (Bransford & Johnson, 1972, 1973; Brewer & Dupree, 1983; Rumelhart, 1980). For example, Bransford and Johnson (1973) gave subjects titled paragraphs to read. Sentences that were consistent with the schema suggested by the title of the paragraph were better remembered than were sentences that were inconsistent with that schema. Anderson and Pichert (1978) asked subjects to read a description either from the perspective of a home buyer or from the perspective of a burglar and then recall the description. Each group recalled details consistent with their perspective. Bower, Black, and Turner (1979) demonstrated effects of the restaurant script (Schank & Abelson, 1977) on subjects' recall of restaurant descriptions. Thus, there is abundant evidence that the likelihood that information in a complex situation will be remembered is related to its consistency with stored knowledge. But what happens when no clear schema applies, or (perhaps more commonly) when many schemas could apply? For example, in the scene shown in Figure la, there are a number of objects participating in a number of events: There is a barn, a pigsty, fences, grass, a pig spraying mud on a tractor, a farmer shouting angrily, a helicopter causing a breeze, a hayloft blowing over, and so on. When a person is presented with such a situation, how is it determined which information will be processed deeply enough to be recalled later? Address correspondence to Arthur B. Markman, Department of Psychology, Columbia University, 406 Schermerhom Hall, New York, NY 10027; e-mail: [email protected]. In this article, we suggest that one determinant of the information people attend to is the comparisons they make. We first review the structure-mapping theory of comparison; then, we examine the impli:ations of this view for memory and present an experimental test of the predictions. STRUCTURAL ALIGNMENT AND MAPPING IN SIMILARITY COMPARISON Comparison is a core cognitive process. It has typically been studied in the context of determining the similarity between two things (Gentner & Markman, 1995; Markman & Gentner, 1993b; Medin, Goldstone, & Gentner, 1993; Tversky, 1977), or in order to examine the impact of similarity on cognitive processes such as categorization (Goldstone, 1994a; Hampton, 1995), learning (Gentner, 1989: Kotovsky & Gentner, in press), and decision making (Medin. Goldstone, & Markman, 1995). However, comparison is equally important in determining which differences people find psychologically salient. The comparison of two scenes tells you what information to pay attention to: the aligned structure and its associated alignable differences. By the aligned structure, we mean the system of matching predicates and matching objects; objects can be placed in correspondence either on the basis of having shared attributes or by virtue of playing similar roles in the common relational structure. By alignable diferences. we mean nonidentical items that have been placed in correspondence (Gentner & Markman, 1994; Markman & Gentner, 1993a. 1996). Alignable differences contrast with nonalignable differences, which are differences that are not in correspondence: that is, elements in one scenario that have no correspondence in the other. For example, the pig in Figure l a and the baby in Figure 1 b are an alignable difference: They can be placed in correspondence because both are making a mess and are the object of another individual's anger. In contrast, the helicopter in Figure l a has no correspondence in Figure I b, and hence it is a nonalignable difference. These distinctions emerge within the framework of structuremapping theory. Structure mapping casts similarity as a process of alignment and mapping of structured representations (Falkenhainer. Forbus, & Gentner, 1989; Gentner, 1983, 1989; Gentner & Markman. 1995, 1997; Gentner & Toupin, 1986; see also Holyoak & Thagard. 1989; Keane, Ledgeway, & Duff, 1994). On this account, similarity is processed much as analogy is (Gentner & Markman, 1995, 1997; Markman & Gentner, 1993a. 1993b; Medin et al., 1993). This approach presupposes structured mental representations that contain explicit relations between their elements: for example, in Figure la, CAUSE [MESS (pig, tractor), ANGRY (man, pig)]. To compare representations of this type, one carries out a process of structural alignment to find the maximal structurally consistent match. A match is structurally consistent when it observes both parallel connectivity and one-to-one mapping. Parallel connectivity states that the arguments to corresponding representational elements must also be placed in correspondence. For example, the pig in Figure la corresponds to the baby in Figure lb , because both are making a mess. Likewise, the VOL. 8, NO. 5, SEPTEMBER 1997 Copyright O 1997 American Pqychological Society 363 PSYCHOLOGICAL SCIENCE Alignability and Memory Fig. 1. Sample triad of stimuli used in the experiment. The bast picture (a) could be compared with one of two other pictures (b or c) Each comparison changes which objects are alignable differences anc which are nonalignable differences. tractor in Figure 1 a corresponds to the wall in Figure 1 b, because bot1 are the receivers of this mess making. One-to-one mapping require that each element i n one representation match with at most one ele ment in the other. For example, if the pig in Figure l a is placed ii correspondence with the baby in Figure Ib, he cannot also be place1 in correspondence with the mother.' The distinction between alignable and nonalignable differences i a result of this comparison process. Alignable differences arise whei nonidentical elements are placed in correspondence (by virtue of play ing the same role in a matching relational structure). In contras1 nonalignable differences are those that either are not related to thi commonalities or are related in different ways. Nonalignable differ ences may be different elements that occupy different roles or tha lack assignable roles (because they are not connected to the cornmoi 1. More detailed descriptions of this process and of a computational mode that can implement i t can be found in Falkenhainer et al. (1989) and Markmai and Gentner (1993b). Other computational models with the same general char acteristics have also been developed (Goldstone, 1994b; Holyoak & Thagard 1989; Keane et al., 1994). structure); or (in a frequent operationalization) they may be elements in one scenario that have no correspondence in the other. Alignable differences are connected to the common system; therefore, whether a difference is considered to be alignable or nonalignable depends on which two scenarios are aligned and on how they are aligned. Thus, what is considered to be an alignable or a nonalignable difference will vary across comparisons. In the comparison of Figures la and 1 b, the pig in Figure l a and the baby in Figure Ib are an alignable difference, whereas the helicopter in Figure l a is a nonalignable difference. In contrast, Figures l a and I C share a different set of relations. In this case, the pig has no correspondence at all in Figure I C , and hence is a nonalignable difference. The helicopter in Figure la and the fan in Figure I C are placed in correspondence because they are both blowing something over, and so they form an alignable differepce. A central assumption of the structural alignment approach is that comparisons focus attention on the common system. This assumption predicts both that commonalities should be more focal than differences (Markman & Gentner, 1993a; Tversky, 1977) and that alignable differences should be more focal than nonalignable differences (by virtue of their connection to the common system). To test this second claim, we (Gentner & Markman, 1994) gave subjects word pairs and asked them to list one difference each for as many pairs as possible under time pressure. Subjects produced many more alignable differences than nonalignable differences, suggesting that the comparison process made the alignable differences salient. We (Markman & Gentner, 1996) have also tested the prediction that alignable differences should have stronger effects on the perception of overall similarity than nonalignable differences. Using picture pairs, we found that a change in an item that played the role of an alignable difference in a comparison had a greater impact on rated similarity than did the same change when the item played the role of a nonalignable difference. Thus, there is evidence that comparison of two items draws attention to the commonalities and the alignable differences of the pair. Returning to the issue of memory storage, we now draw a further implication. The structural alignment view predicts that when comparisons are available during encoding, the greater degree of attention paid to commonalities and alignable differences during the comparison process should manifest itself as greater memorability for these features of the pair than for nonalignable differences. We tested this prediction in a straightforward experiment. Participants were asked to rate the similarity of 10 pairs of pictures. After a 30-min delay, participants were shown an item taken from one of the pictures. The item was either an alignable difference (e.g., the pig from Fig. l a given the pair l a and Ib) or a nonalignable difference (e.g., the helicopter from Fig. l a given the pair la and Ib). The participant was then asked to remember as much as possible about the scene from which the cue came. We predicted that if the cue was an alignable difference of the scenes, subjects would be able to remember more about the scene than if the cue was a nonalignable difference. However, if comparisons do not focus on commonalities and their associated alignable differences, then there would not be a systematic advantage for alignable-difference cues over nonalignable-difference cues.