Prototypes in category learning: The effects of category size, category structure, and stimulus complexity.

Although research in categorization has sometimes been motivated by prototype theory, recent studies have favored exemplar theory. However, some of these studies focused on small, poorly differentiated categories composed of simple, 4-dimensional stimuli. Some analyzed the aggregate data of entire groups. Some compared powerful multiplicative exemplar models to less powerful additive prototype models. Here, comparable prototype and exemplar models were fit to individual-participant data in 4 experiments that sampled category sets varying in size, level of category structure, and stimulus complexity (dimensionality). The prototype model always fit the observed data better than the exemplar model did. Prototype-based processes seemed especially relevant when participants learned categories that were larger or contained more complex stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Prototypes in the mist: The early epochs of category learning.

[Correction Notice: An erratum for this article was reported in Vol 25(1) of Journal of Experimental Psychology: Learning, Memory, and Cognition (see record 2008-09597-001). As a result of errors made in production, two equations in the article were printed incorrectly. The corrected equations are included in the erratum.] Recent ideas about category learning have favored exemplar processes over prototype processes. However, research has focused on small, poorly differentiated categories and on task-final performances—both may highlight exemplar strategies. Thus, we evaluated participants' categorization strategies and standard categorization models at successive stages in the learning of smaller, less differentiated categories and larger, more differentiated categories. In the former case, the exemplar model dominated even early in learning. In the latter case, the prototype model had a strong early advantage that gave way slowly. Alternative models, and even the behavior of individual parameters within models, suggest a psychological transition from prototype-based to exemplar-based processing during category learning and show that different category structures produce different trajectories of learning through the larger space of strategies. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Categorization of speech by infants: Support for speech-sound prototypes.

This study tested 6-month-old infants' categorization of speech stimuli to investigate whether infants organize speech categories around "prototypes." In Experiment 1, infants first discriminated single "good" exemplars from two different vowel categories. They were then tested with 64 novel exemplars, 32 from each vowel category. The test stimuli varied in the degree to which they conformed to adult-defined prototypes of the two categories. The results showed that infants correctly sorted the novel stimuli over 90% of the time. In Experiment 2, we trained two groups of infants, one with a good (prototypical) exemplar from a vowel category and the other with a poor (nonprototypical) exemplar. Then we tested both groups with 16 novel exemplars from that same vowel category. Generalization to novel members of the category was significantly greater following exposure to the prototypical exemplar. Results are consistent with a model of speech perception that holds that young human infants organize vowel categories around prototypes. This may contribute to their seemingly efficient processing of speech information, even in the first half year of life. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Exemplar models and prototype effects in similarity-based categorization.

I. P. L. McLaren, C. H. Bennett, T. Guttman-Nahir, K. Kim, and N. J. MacKintosh (1995) have argued that exemplar models have problems explaining their finding that people often respond more accurately to the prototypes from certain categories than to other exemplars. The author reviews McLaren et al's arguments and shows that two exemplar theories, the generalized context model with Euclidean distances and the weighted ratio model, can account for the prototype-superiority effects in their experiments. The author concludes that McLaren et al's results support, rather than challenge, exemplar models of categorization. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Alternative strategies of categorization

Psychological studies of categorization often assume that all concepts are of the same general kind, and are operated on by the same kind of categorization process. In this paper, we argue against this unitary view, and for the existence of qualitatively different categorization processes. In particular, we focus on the distinction between categorizing an item by: (a) applying a category-defining rule to the item vs. (b) determining the similarity of that item to remembered exemplars of a category. We begin by characterizing rule application and similarity computations as strategies of categorization. Next, we review experimental studies that have used artificial categories and shown that differences in instructions or time pressure can lead to either rule-based categorization or similarity-based categorization. Then we consider studies that have used natural concepts and again demonstrated that categorization can be done by either rule application or similarity calculations. Lastly, we take up evidence from cognitive neuroscience relevant to the rule vs. similarity issue. There is some indirect evidence from brain-damaged patients for neurological differences between categorization based on rules vs. that based on similarity (with the former involving frontal regions, and the latter relying more on posterior areas). For more direct evidence, we present the results of a recent neuroimaging experiment, which indicates that different neural circuits are involved when people categorize items on the basis of a rule as compared with when they categorize the same items on the basis of similarity.     

Journey to the center of the category: The dissociation in amnesia between categorization and recognition.

The authors' theoretical analysis of the dissociation in amnesia between categorization and recognition suggests these conclusions: (a) Comparing to-be-categorized items to a category center or prototype produces strong prototype advantages and steep typicality gradients, whereas comparing to-be, categorized items to the training exemplars that surround the prototype produces weak prototype advantages and flat typicality gradients; (b) participants often show the former pattern, suggesting their use of prototypes; (c) exemplar models account poorly for these categorization data, but prototype models account well for them; and (d) the recognition data suggest that controls use a single-comparison exemplar-memorization process more powerfully than amnesics. By pairing categorization based in prototypes with recognition based in exemplar memorization, the authors support and extend other recent accounts of cognitive performance that intermix prototypes and exemplars, and the authors reinforce traditional interpretations of the categorization-recognition dissociation in amnesia. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

"Prototypes in the mist: The early epochs of category learning": Correction to Smith and Minda (1998).

Reports an error in "Prototypes in the mist: The early epochs of category learning" by J. David Smith and John Paul Minda (Journal of Experimental Psychology: Learning, Memory, and Cognition, 1998[Nov], Vol 24[6], 1411-1436). As a result of errors made in production, two equations in the article were printed incorrectly. The corrected equations are included in the erratum. (The following abstract of the original article appeared in record 1998-12790-005.) Recent ideas about category learning have favored exemplar processes over prototype processes. However, research has focused on small, poorly differentiated categories and on task-final performances--both may highlight exemplar strategies. Thus, we evaluated participants' categorization strategies and standard categorization models at successive stages in the learning of smaller, less differentiated categories and larger, more differentiated categories. In the former case, the exemplar model dominated even early in learning. In the latter case, the prototype model had a strong early advantage that gave way slowly. Alternative models, and even the behavior of individual parameters within models, suggest a psychological transition from prototype-based to exemplar-based processing during category learning and show that different category structures produce different trajectories of learning through the larger space of strategies. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Memory storage and retrieval processes in category learning.

Studied the detailed course of learning for categorization tasks defined by independent or contingent probability distributions over the features of category exemplars in 2 experiments with 72 college-age Ss. Ss viewed sequences of bar charts that simulated symptom patterns and responded to each chart with a recognition and a categorization judgment. Fuzzy, probabilistically defined categories were learned relatively rapidly when individual features were correlated with category assignment, more slowly when only patterns carried category information. Limits of performance were suboptimal, evidently because of capacity limitations on judgmental processes as well as limitations on memory. Categorization proved systematically related to feature and exemplar probabilities, under different circumstances, and to similarity among exemplars of categories. Unique retrieval cues for exemplar patterns facilitated recognition but entered into categorization only at retention intervals within the range of short-term memory. Findings are interpreted within the framework of a general array model that yields both exemplar-similarity and feature-frequency models as special cases and provides quantitative accounts of the course of learning in each of the categorization tasks studied. (46 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of memory decay on predictions from changing categories.

In contrast to the static categories assumed in most categorization experiments, many real-world categories undergo gradual and systematic change in their definitions over time. Four experiments were carried out to study such category change. In these studies, participants successfully adjusted as category change occurred, but also showed a lingering and cumulative effect of past observations. The participants' performance was closely modeled by incorporating memory decay for past observations into J. R. Anderson's (1990, 1991) rational categorization algorithm and into a version of R. M. Nosofsky's (1986) exemplar categorization model. The resulting models suggest that the decay function is closer to a power law than to an exponential and that decay occurs both by item and by time, with the item decay being stronger than the time decay. The finding of power law decay gives additional support to claims that exemplar memories are used in categorization. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Prototypes and particulars: Geometric and experience-dependent spatial categories.

People use geometric cues to form spatial categories. This study investigated whether people also use the spatial distribution of exemplars. Adults pointed to remembered locations on a tabletop. In Experiment 1, a target was placed in each geometric category, and the location of targets was varied. Adults' responses were biased away from a midline category boundary toward geometric prototypes located at the centers of left and right categories. Experiment 2 showed that prototype effects were not influenced by cross-category interactions. In Experiment 3, subsets of targets were positioned at different locations within each category. When prototype effects were removed, there was a bias toward the center of the exemplar distribution, suggesting that common categorization processes operate across spatial and object domains. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of similarity in categorization: providing a groundwork

The relation between similarity and categorization has recently come under scrutiny from several sectors. The issue provides an important inroad to questions about the contributions of high-level thought and lower-level perception in the development of people's concepts. Many psychological models base categorization on similarity, assuming that things belong in the same category because of their similarity. Empirical and in-principle arguments have recently raised objections to this connection, on the grounds that similarity is too unconstrained to provide an explanation of categorization, and similarity is not sufficiently sophisticated to ground most categories. Although these objections have merit, a reassessment of evidence indicates that similarity can be sufficiently constrained and sophisticated to provide at least a partial account of many categories. Principles are discussed for incorporating similarity into theories of category formation.     

Multidimensional Rule, Unidimensional Rule, and Similarity Strategies in Categorization: Event-Related Brain Potential Correlates.

Forty participants assigned artificial creatures to categories after explicit rule instruction or feedback alone. Stimuli were typical and atypical exemplars of 2 categories with independent prototypes, conflicting exemplars sharing features of both categories, and "Others" with only 1 or 2 features of the well-defined categories. Ten feedback-only participants spontaneously adopted a unidimensional rule; 10 used a multidimensional similarity strategy. Event-related potentials (ERPs) recorded during the transfer phase showed a commonality between multidimensional rule and similarity strategies in late frontal brain activity that differentiated both from unidimensional rule use. Multidimensional rule users alone showed an earlier prefrontal ERP effect that may reflect inhibition of responses based on similarity. The authors also discuss the role of declarative memory for features and exemplars. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Memory bias toward normative and novel trait prototypes.

The "prototype" of a set of exemplars is the most typical exemplar. The present recognition memory experiment with 22 undergraduates demonstrated memory bias toward both normative (previously learned) and novel prototypes. Lists of traits were presented during the acquisition and recognition phases. The lists varied in degree of similarity to a prototype list that was composed of either 6 positive traits (positive condition), 6 negative traits (negative condition), or 3 positive and 3 negative traits (novel condition). In each condition, the most typical exemplar of the set of acquisition lists was the prototype list, which was not presented during acquisition. As predicted, in all 3 conditions recognition confidence was a positive, linear function of similarity to the prototype list and was highest for the prototype list. Contrary to prediction, the slope of this linear "bias-toward-prototype" effect was not steeper in the 2 normative conditions (the positive and the negative conditions) than in the novel condition. Results suggest that prototype abstraction occurred and that a comparison-to-prototype process was an important determinant of recognition confidence. (21 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Investigations of exemplar and decision bound models in large, ill-defined category structures.

Experiments involving large-size, ill-defined categories were conducted to distinguish between the predictions of an exemplar model and linear and quadratic decision bound models. In conditions in which the optimal classification boundary was of a more complex form than the quadratic model, the exemplar model provided significantly better accounts of study participants' data than did the decision bound models, even in situations in which a linear bound would have yielded nearly optimal performance. The results suggest that participants are not predisposed or constrained to use linear or quadratic decision bounds for classifying multidimensional perceptual stimuli and that exemplar models may provide a parsimonious process-level account of the complex types of decision bounds used by experiment participants. The results also suggest some limitations on the complexity of the decision bounds that can be learned, in contrast to the predictions of the exemplar model. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Multiple congruity effects in judgments of magnitude.

Four experiments investigated a novel finding in the area of symbolic magnitude comparisons: Congruity effects may occur with subsets of objects. Such multiple congruity effects appear to signal the creation of size-ordered categories. Exp 1 observed separate congruity effects for large and small pairs despite the intermingling of pairs within a session. Exp 2 determined whether this result was an artifact of the items used. Exps 3 and 4 examined whether linear separability on a dimension of size or on some other correlated dimension was a prerequisite for multiple size-ordered categorization. The results of these experiments suggest that congruity effects are properly regarded as indicating the presence of an organized structure or category. Thus, to the extent that congruity effects typify magnitude comparisons, the processing of relational information appears to implicate categorization. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

The role of similarity in artificial grammar learning.

The authors examine the role of similarity in artificial grammar learning (AGL; A. S. Reber, 1989). A standard finite-state language was used to create stimuli that were arrangements of embedded geometric shapes (Experiment 1), connected lines (Experiment 2), and sequences of shapes (Experiment 3). Main effects for well-known predictors from the literature (grammaticality, associative global and anchor chunk strength, novel global and anchor chunk strength, length of items, and edit distance) were observed, thus replicating previous work. However, the authors extend previous research by using a widely known similarity-based exemplar model of categorization (the generalized context model; R. M. Nosofsky, 1989) to fit grammaticality judgments, by nested regression analyses. The results suggest that any explanation of AGL that is based on the existing theories is incomplete without a similarity process as well. Also, the results provide a foundation for further interpreting AGL in the wider context of categorization research. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effect of category variability in perceptual categorization.

Exemplar and distributional accounts of categorization make differing predictions for the classification of a critical exemplar precisely halfway between the nearest exemplars of 2 categories differing in variability. Under standard conditions of sequential presentation, the critical exemplar was classified into the most similar, least variable category, consistent with an exemplar account. However, if the difference in variability is made more salient, then the same exemplar is classified into the more variable, most likely category, consistent with a distributional account. This suggests that participants may be strategic in their use of either strategy. However, when the relative variability of 2 categories was manipulated, participants showed changes in the classification of intermediate exemplars that neither approach could account for. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The Origin of Exemplar Effects in Rule-Driven Categorization.

S. W. Allen and L. R. Brooks (1991) have shown that exemplar memory can affect categorization even when participants are provided with a classification rule. G. Regehr and L. R. Brooks (1993) argued that stimuli must be individuated for such effects to occur. In this study, the authors further analyze the conditions that yield exemplar effects in this rule application paradigm. The results of Experiments 1-3 show that interchangeable attributes, which are not part of the rule, influence categorization only when attention is explicitly drawn on them. Experiment 4 shows that exemplar effects can occur in an incidental learning condition, whether stimulus individuation is preserved or not. The authors conclude that the influence of exemplar learning in rule-driven categorization stems from the attributes specified in the rule or in the instructions, not from the stimulus gestalts. (PsycINFO Database Record (c) 2010 APA, all rights reserved)