Comparison and Contrast in Perceptual Categorization.

People categorized pairs of perceptual stimuli that varied in both category membership and pairwise similarity. Experiments 1 and 2 showed categorization of 1 color of a pair to be reliably contrasted from that of the other. This similarity-based contrast effect occurred only when the context stimulus was relevant for the categorization of the target (Experiment 3). The effect was not simply owing to perceptual color contrast (Experiment 4), and it extended to pictures from common semantic categories (Experiment 5). Results were consistent with a sign-and-magnitude version of N. Stewart and G. D. A. Brown's (2005) similarity-dissimilarity generalized context model, in which categorization is affected by both similarity to and difference from target categories. The data are also modeled with criterion setting theory (M. Treisman & T. C. Williams, 1984), in which the decision criterion is systematically shifted toward the mean of the current stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Discrimination of artificial categories structured by family resemblances: A comparative study in people (Homo sapiens) and pigeons (Columba livia).

Adult humans (Homo sapiens) and pigeons (Columba livia) were trained to discriminate artificial categories that the authors created by mimicking 2 properties of natural categories. One was a family resemblance relationship: The highly variable exemplars, including those that did not have features in common, were structured by a similarity network with the features correlating to one another in each category. The other was a polymorphous rule: No single feature was essential for distinguishing the categories, and all the features overlapped between the categories. Pigeons learned the categories with ease and then showed a prototype effect in accord with the degrees of family resemblance for novel stimuli. Some evidence was also observed for interactive effects of learning of individual exemplars and feature frequencies. Humans had difficulty in learning the categories. The participants who learned the categories generally responded to novel stimuli in an all-or-none fashion on the basis of their acquired classification decision rules. The processes that underlie the classification performances of the 2 species are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of background knowledge on object categorization and part detection.

Previous research has shown that background knowledge affects the ease of concept learning, but little research has examined its effects on speeded categorization of instances after the category is well learned. Subjects in 4 experiments first learned novel categories. At test, they categorized a new set of novel stimuli that were either consistent or inconsistent with background knowledge given about the categories. Background knowledge affected categorization responses in an untimed task, with usual reaction time instructions, with a response deadline, or when the stimuli were presented for 50 ms followed by a mask. Three other experiments using a part-detection task showed that subjects were more likely to notice missing parts that were critical than noncritical according to background knowledge. The mechanisms by which background knowledge affects categorization and part detection are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Comparing exemplar and prototype models of categorization.

Five experiments contrasted the prototype and exemplar theories of categorization. The stimuli for the experiments were simple perceptual figures that varied along two multi-valued dimensions. During the learning phase of each experiment, participants assigned each of a series of stimuli to one of two categories and received feedback concerning their assignments. During the generalization phase, participants made category judgments with respect to novel stimuli, including some crucial test stimuli that were (a) more similar to the prototype of one of the two categories, and at the same time, (b) more similar, on average, to the exemplars of the other category. Participants assigned the test stimuli to the latter category. This was taken as evidence favoring the exemplar over the prototype theory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Similarity, frequency, and category representations.

This article studies the joint roles of similarity and frequency in determining graded category structure. Perceptual classification learning experiments were conducted in which presentation frequencies of individual exemplars were manipulated. The exemplars had varying degrees of similarity to members of the target and contrast categories. Classification accuracy and typicality ratings increased for exemplars presented with high frequency and for members of the target category that were similar to the high-frequency exemplars. Typicality decreased for members of the contrast category that were similar to the high-frequency exemplars. A frequency-sensitive similarity-to-exemplars model provided a good quantitative account of the classification learning and typicality data. The interactive relations among similarity, frequency, and categorization are considered in the General Discussion. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sequence effects in categorization of simple perceptual stimuli.

Categorization research typically assumes that the cognitive system has access to a (more or less noisy) representation of the absolute magnitudes of the properties of stimuli and that this information is used in reaching a categorization decision. However, research on identification of simple perceptual stimuli suggests that people have very poor representations of absolute magnitude information and that judgments about absolute magnitude are strongly influenced by preceding material. The experiments presented here investigate such sequence effects in categorization tasks. Strong sequence effects were found. Classification of a borderline stimulus was more accurate when preceded by a distant member of the opposite category than by a distant member of the same category. It is argued that this category contrast effect cannot be accounted for by extant exemplar or decision-bound models of categorization. The effect suggests the use of relative magnitude information in categorization. A memory and contrast model illustrates how relative magnitude information may be used in categorization. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Blinded by the accent! The minor role of looks in ethnic categorization.

The categories that social targets belong to are often activated automatically. Most studies investigating social categorization have used visual stimuli or verbal labels, whereas ethnolinguistic identity theory posits that language is an essential dimension of ethnic identity. Language should therefore be used for social categorization. In 2 experiments, using the “Who Said What?” paradigm, the authors investigated social categorization by using accents (auditory stimuli) and looks (visual stimuli) to indicate ethnicity, either separately or in combination. Given either looks or accents only, the authors demonstrated that ethnic categorization can be based on accents, and the authors found a similar degree of ethnic categorization by accents and looks. When ethnic cues of looks and accents were combined by creating cross categories, there was a clear predominance of accents as meaningful cues for categorization, as shown in the respective parameters of a multinomial model. The present findings are discussed with regard to the generalizability of findings using one channel of presentation (e.g., visual) and the asymmetry found with different presentation channels for the category ethnicity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Learning correlations in categorization tasks using large, ill-defined categories.

The experiments revealed whether individual participants are sensitive to exemplar information in the form of within-category correlations between stimulus dimensions after training on large overlapping categories. Participants were trained in 1 of 2 categorization conditions. The sign of the correlation between dimensions differed across conditions, but the categorization rules that best separated the categories were identical. An unannounced attribute-prediction task followed categorization training. Several participants produced predictions consistent with the correct correlation between the dimensions. For other participants, the predictions reflected the correlation only within the region they had associated with the given category, even though the categories overlapped, suggesting that the decision boundary was explicitly represented in memory. Finally, for other participants, no correlational information appeared to be accessible for the prediction task. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Categorizing Others: The Dynamics of Person Construal.

A fundamental question in social cognition is whether people categorize others on the basis of the social groups to which they belong. Integrating ideas from related work on face processing, the current research explored the emergence and boundary conditions of person categorization. Using speeded responses to facial stimuli as a marker of category activation, the authors showed in 3 experiments that person categorization: (a) occurs only under active-encoding conditions and (b) does not extend to applicable but task-irrelevant categorical dimensions, but (c) is sensitive to overlap in the perceptual features that support multiple categorical construals. The authors consider the implications of these findings for models of social-cognitive functioning and the component processes that support person perception. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Size contrast as a function of conceptual similarity between test and inducers

In four experiments, the effect of the semantic relationship between test and inducing stimuli on the magnitude of size contrast in an Ebbinghaus-type illusion was explored. In Experiments 1 and 2, the greatest illusion was found when test and inducing stimuli were identical in shape and differed only in size. Decreased size contrast was found when inducing stimuli were drawn from the same category as the test stimulus, but were not visually identical. Even less size contrast was found when inducing stimuli were from a near conceptual category, with the least effect when they were drawn from a completely different category. In Experiment 3, it was demonstrated that even if test and inducing stimuli are drawn with identical geometric elements, the size contrast illusion is greatly reduced if they represent apparently different conceptual categories (through the manipulation of orientation and perceptual set). In Experiment 4, any geometric or spatial confounds were ruled out. These results suggest that size contrast is strongly influenced by the conceptual similarity between test and inducing stimuli.     

Decoupling object detection and categorization.

We investigated whether there exists a behavioral dependency between object detection and categorization. Previous work (Grill-Spector & Kanwisher, 2005) suggests that object detection and basic-level categorization may be the very same perceptual mechanism: As objects are parsed from the background they are categorized at the basic level. In the current study, we decouple object detection from categorization by manipulating the between-category contrast of the categorization decision. With a superordinate-level contrast with people as one of the target categories (e.g., cars vs. people), which replicates Grill-Spector and Kanwisher, we found that success at object detection depended on success at basic-level categorization and vice versa. But with a basic-level contrast (e.g., cars vs. boats) or superordinate-level contrast without people as a target category (e.g., dog vs. boat), success at object detection did not depend on success at basic-level categorization. Successful object detection could occur without successful basic-level categorization. Object detection and basic-level categorization do not seem to occur within the same early stage of visual processing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The Role of Bottom-Up Processing in Perceptual Categorization by 3- to 4-Month-Old Infants: Simulations and Data.

Disentangling bottom-up and top-down processing in adult category learning is notoriously difficult. Studying category learning in infancy provides a simple way of exploring category learning while minimizing the contribution of top-down information. Three- to 4-month-old infants presented with cat or dog images will form a perceptual category representation for cat that excludes dogs and for dog that includes cats. The authors argue that an inclusion relationship in the distribution of features in the images explains the asymmetry. Using computational modeling and behavioral testing, the authors show that the asymmetry can be reversed or removed by using stimulus images that reverse or remove the inclusion relationship. The findings suggest that categorization of nonhuman animal images by young infants is essentially a bottom-up process. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conjunctions of visually based categories: Overextension and compensation.

The degree to which overextension effects found with conjunctions of semantic categories of visual stimuli was tested in 4 experiments. Overextension occurs when participants categorize a stimulus in the conjunction of 2 categories but fail to categorize the same stimulus as belonging to 1 of the 2 constituent categories considered individually. Stimuli for the present experiments were ambiguous colored letter shapes and cartoon faces that could vary along dimensions of happiness and either apparent intelligence or apparent age. Overextension was found with both stimulus sets, thus showing that the phenomenon is not restricted to categorization in superordinate semantic categories. There was also evidence that typicality in 1 category could compensate for borderline membership of the other. More overextension was found for faces than for letters, and there was evidence for asymmetric compensation between category dimensions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Comparing Categorization Models.

Four experiments are presented that competitively test rule- and exemplar-based models of human categorization behavior. Participants classified stimuli that varied on a unidimensional axis into 2 categories. The stimuli did not consistently belong to a category; instead, they were probabilistically assigned. By manipulating these assignment probabilities, it was possible to produce stimuli for which exemplar- and rule-based explanations made qualitatively different predictions. F. G. Ashby and J. T. Townsend's (1986) rule-based general recognition theory provided a better account of the data than R. M. Nosofsky's (1986) exemplar-based generalized context model in conditions in which the to-be-classified stimuli were relatively confusable. However, generalized context model provided a better account when the stimuli were relatively few and distinct. These findings are consistent with multiple process accounts of categorization and demonstrate that stimulus confusion is a determining factor as 10 which process mediates categorization. (PsycINFO Database Record (c) 2011 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)     

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)     

Learning of ill-defined categories by monkeys.

Three rhesus monkeys were trained on a categorization task using a successive-discrimination paradigm. The category members were geometric stimuli differing in color, form, size, and orientation; and the component features of category exemplars were neither singly necessary nor jointly sufficient to determine category membership. Original learning was followed by transfer tests involving both training stimuli and new exemplars. After this initial learning and transfer phase, the assignment of categories to positional responses was reversed. After the reversal learning criterion was met, transfer tests were again given. This procedure was continued until 5 reversals had been completed. Ss were able to learn this task involving ill-defined categories and showed both appropriate transfer to new examples and improvement across successive reversals. Both the learning and transfer data are in accord with the idea that component features are encoded or combined in an interactive, rather than an independent, manner and show parallels with human classification data. (French abstract) (26 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Categorization creates functional features.

Many theories of object recognition and categorization claim that complex objects are represented in terms of characteristic features. The origin of these features has been neglected in theories of object categorization. Do they form a fixed and independent set that exists before experience with objects, or are features progressively extracted and developed as an organism categorizes its world? This article maintains that features can be learned flexibly as a consequence of categorizing and representing objects. All 3 experiments reported in this article used categories of unfamiliar, computer-synthesized 2-dimensional objects ("Martian cells"). The results showed that varying the order of category learning induced the creation of different features that changed the perceptual appearance and the featural representation of identical category exemplars. Network simulations supported a flexible rather than a fixed-feature interpretation of the data. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Recognition memory for exceptions to the category rule.

Experiments were conducted to demonstrate the utility of a rule-plus-exception model for extending current exemplar-based views of categorization and recognition memory. According to the model, exemplars that are exceptions to category rules have a special status in memory relative to other old items. In each of 4 experiments, participants first learned to categorize items organized into 2 ill-defined categories and then made old–new recognition judgments. Although the categories afforded no perfect rules, a variety of imperfect rules could be formed combined with memorization of exceptions to those rules. In each experiment, superior recognition of exceptions to imperfect logical rules was found. In addition, participants demonstrated better memory for old exemplars than new ones. A mixed model, which assumed a combination of rule-plus-exception processing and residual exemplar storage, provided good quantitative accounts of the data. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Memory-based judgments about multiple categories: A revision and extension of Tajfel's accentuation theory.

Accentuation theory states that the classification of stimuli produces encoding biases. Contrast effects enhance intercategory differences; assimilation effects enhance intracategory similarities. Do these biases affect the retrieval of stimuli distributed across many categories? The calendar superimposes arbitrary intermonth boundaries on day-to-day variations in temperature. In Exp 1, Ss estimated the average temperatures of 48 days. Differences between estimates for 2 days belonging to neighboring months were greater (contrast) and differences between estimates for 2 days belonging to the same month were smaller than actual differences (assimilation). Exp 2 showed that assimilation accounted for all categorization effects. When modified by assumptions from an exemplar model of category learning, accentuation theory accounts for the results. The relevance of these findings for social categorization is discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)