Matching performance and the similarity structure of the stimulus set.

Abstract

To compare two general approaches toward understanding how objects are classified, this article explores performance in a series of studies that required matching visually presented letters. One general approach has stressed the analysis of details or elements of the input pattern. The other approach has focused on the relationships between the various elements and objects that the observer might expect to occur in the task. In empirical support of the first approach, many researchers have observed that the time it takes a subject to report that two visually presented letters have the same name is less when these letters are physically identical (e.g., A-A) than when they are physically different (e.g., A-a). This relative ease of matching physically identical letters has been attributed to a visual process that matches the letters on the basis of their physical characteristics. In support of the second approach, the studies in this article replicate this finding, but only for data averaged across letter parts. The data from individual letter pairs do not reflect this temporal hierarchy. Individual letter performance for an entire set of letters has not previously been reported, and the importance of analyzing fine structure in data is stressed. It is shown that physical identity matches can be reliably faster or slower than name identity matches. The similarity structure of the total stimulus set reliably predicts which result will occur. Within the limits of the variables studied, this conclusion is shown to be independent of the criterion for the type of match the subject is asked to make (physical identity or name identity) and of the temporal and physical separation of the letters. The readiness with which the presented stimuli can be discriminated from other members of the stimulus set controls performance. For example, as Experiment 1 shows, it is easy or difficult to report o-o as "same" depending on what other letters are used in the task. Lockhead's holistic-discriminability model provides a framework for interpreting these demonstrations that matching performance does not depend only on the stimuli physically present. Performance depends on the similarity or discriminability between the presented stimuli and other stimuli the observers know might be presented.

An exemplar-retrieval model of speeded same–different judgments.

“…This idea about the role of memory for exemplars in influencing same-different judgments was brought out in compelling fashion in a previous study reported by Crist (1981). The theme of Crist's (1981) experiments was to demonstrate that the similarity structure of the total set of stimulus possibilities in a given experimental context plays an important role in same-different judgment.…”

“…In other words, it is not simply the properties of the two objects that are judged on a given trial that determine performance but rather their relationship to the other exemplars in the stimulus set. In particular, Crist (1981) demonstrated that observers are faster to judge two identical objects as same if the object in the pair is located in an isolated region of similarity space than if it is located in a dense region of similarity space. This idea is illustrated schematically in Figure 1.…”

“…In this illustrative example, note that the letter f is located in an isolated region of the similarity space, whereas the letters e and especially o and a are located in a dense region. The type of phenomenon observed by Crist (1981) was that participants were faster to respond same to the pair (jS, 39, which is an (3 (I Figure 1. A subset of the stimuli used by Crist (1981).…”

“…In this example, there are four objects in the stimulus set, the lower case letters a, e, o, andf. (These letters formed a subset of the ones actually used by Crist, 1981, in some of his experiments.) In this illustrative example, note that the letter f is located in an isolated region of the similarity space, whereas the letters e and especially o and a are located in a dense region.…”

“…The theme of Crist's (1981) experiments was to demonstrate that the similarity structure of the total set of stimulus possibilities in a given experimental context plays an important role in same-different judgment. In other words, it is not simply the properties of the two objects that are judged on a given trial that determine performance but rather their relationship to the other exemplars in the stimulus set.…”