Feedback interference and dissociations of classification: Evidence against the multiple-learning-systems hypothesis

Researchers have argued that different categorization problems are learned by separate and distinct cognitive systems. They propose that an explicit system is responsible for learning rule-based categories and that a separate implicit system learns information-integration categories. One source of supporting evidence involves experiments in which observers perform a concurrent memory-scanning task that interferes with the processing of feedback. Researchers have reported a dissociation in which this manipulation impairs learning of a rule-based category but not an information-integration category. In the present research, we test the hypothesis that the dissociation was the result of lowered perceptual discriminability in the rule-based structure in comparison with the information-integration one. We demonstrate an example of an alternate rule-based category with easy-to-discriminate stimuli in which performance is unaffected by the interfering memory-scanning task. Furthermore, we demonstrate that learning of an information-integration category with low perceptual discriminability is impaired by the memory-scanning task. These demonstrations of the reverse dissociation challenge the interpretation that rule-based and information-integration category structures are learned by separate cognitive systems.     

Further tests of an exemplar-similarity approach to relating identification and categorization

Further tests were provided of an exemplar-similarity model for relating the identification and categorization of separable-dimension stimuli (Nosofsky, 1986). On the basis of confusion errors in an identification paradigm, a multidimensional scaling (MDS) solution was derived for a set of 16 separable-dimension stimuli. This MDS solution was then used in conjunction with the exemplar-similarity model to accurately predict performance in four separate categorization paradigms with the same stimuli. A key to achieving the accurate quantitative fits was the assumption that a selective attention process systematically modifies similarities among exemplars across different category structures. The tests reported go well beyond earlier ones (Nosofsky, 1986) in demonstrating the generalizability and utility of the theoretical approach. Implications of the results for alternative quantitative models of classification performance, including Ashby and Perrin's (1988) general recognition theory, were also considered.     

Incremental Bayesian Category Learning From Natural Language

Models of category learning have been extensively studied in cognitive science and primarily tested on perceptual abstractions or artificial stimuli. In this paper, we focus on categories acquired from natural language stimuli, that is, words (e.g., chair is a member of the furniture category). We present a Bayesian model that, unlike previous work, learns both categories and their features in a single process. We model category induction as two interrelated subproblems: (a) the acquisition of features that discriminate among categories, and (b) the grouping of concepts into categories based on those features. Our model learns categories incrementally using particle filters, a sequential Monte Carlo method commonly used for approximate probabilistic inference that sequentially integrates newly observed data and can be viewed as a plausible mechanism for human learning. Experimental results show that our incremental learner obtains meaningful categories which yield a closer fit to behavioral data compared to related models while at the same time acquiring features which characterize the learned categories. (An earlier version of this work was published in Frermann and Lapata 2014 .)     

Classification response times in probabilistic rule-based category structures: Contrasting exemplar-retrieval and decision-boundary models

Experiments were conducted to contrast the predictions from exemplar models and rule-based decisionboundary models of perceptual classification. Observers classified multidimensional stimuli into categories that could be described in terms of easily verbalized logical rules. The critical manipulation was that some pairs of stimuli received probabilistic feedback, whereas other control pairs received deterministic feedback. Despite the probabilistic feedback, the probabilistic pairs and the deterministic pairs were the same distance from idealobserver, rule-based decision boundaries. Across two experiments with varying category structures, observers classified the probabilistic pairs with slower response times (RTs) and lower accuracies than the comparison deterministic pairs. The effects were relatively long term, extending into test blocks in which all feedback was withheld. The results were as predicted by exemplar models, but challenged models that posit that RT is a function solely of the distance of a stimulus from rule-based boundaries. The studies add considerable generality to previous ones and suggest that, even in domains involving rule-based category structures, exemplar-retrieval processes play a significant role. Supplemental materials related to this article may be downloaded from http:// mc.psychonomic-journals.org/content/supplemental.     

Free sorting with stimuli clustered in a multidlmensional attribute space

An experiment was conducted to determine whether Ss leam, without knowledge of results or prior familiarization with the prototypes, to sort histoform stimuli generated by three probablistic rules into categories consistent with these mies. Seven Ss were asked to sort 30 stimuli (10 from each schema population) on each trial. No constraints were placed on the number of categories to be used. A statistic was developed to measure consistency between schema-defined categories and subjeet-defined categories. Four of the seven Ss met the leaming criterion, and demonstrated increasing consistency with the schema-defined categories across trials. In general, more categories than necessary were used to classify the stimuli correctly. The results raise several questions about classification strategies used by Ss in perceptual learning tasks with stimuli generated by probabilistic rules, and indicate the need for further research conceming the variables that influence perceptual category formation.     

Inferior Frontal Regions Underlie the Perception of Phonetic Category Invariance

ABSTRACT— The problem of mapping differing sensory stimuli onto a common category is fundamental to human cognition. Listeners perceive stable phonetic categories despite many sources of acoustic variability. What are the neural mechanisms that underlie this perceptual stability? In this functional magnetic resonance imaging study, a short-interval habituation paradigm was used to investigate neural sensitivity to acoustic changes within and between phonetic categories. A region in the left inferior frontal sulcus showed a pattern of activation consistent with phonetic invariance: insensitivity to acoustic changes within a phonetic category and sensitivity to changes between phonetic categories. Left superior temporal regions, in contrast, showed graded sensitivity to both within- and between-category changes. These results suggest that perceptual insensitivity to changes within a phonetic category may arise from decision-related mechanisms in the left prefrontal cortex and add to a growing body of literature suggesting that the inferior prefrontal cortex plays a domain-general role in computing category representations.     

Human adults and human infants show a "perceptual magnet effect" for the prototypes of speech categories, monkeys do not

Many perceptual categories exhibit internal structure in which category prototypes play an important role. In the four experiments reported here, the internal structure of phonetic categories was explored in studies involving adults, infants, and monkeys. In Experiment 1, adults rated the category goodness of 64 variants of the vowel i parallel on a scale from 1 to 7. The results showed that there was a certain location in vowel space where listeners rated the i parallel vowels as best instances, or prototypes. The perceived goodness of i parallel vowels declined systematically as stimuli were further removed from the prototypic i parallel vowel. Experiment 2 went beyond this initial demonstration and examined the effect of speech prototypes on perception. Either the prototypic or a nonprototypic i parallel vowels was used as the referent stimulus and adults' generalization to other members of the category was examined. Results showed that the typicality of the speech stimulus strongly affected perception. When the prototype of the category served as the referent vowel, there was significantly greater generalization to other i parallel vowels, relative to the situation in which the nonprototype served as the referent. The notion of a perceptual magnet was introduced. The prototype of the category functioned like a perceptual magnet for other category members; it assimilated neighboring stimuli, effectively pulling them toward the prototype. In Experiment 3, the ontogenetic origins of the perceptual magnet effect were explored by testing 6-month-old infants. The results showed that infants' perception of vowels was also strongly affected by speech prototypes. Infants showed significantly greater generalization when the prototype of the vowel category served as the referent; moreover, their responses were highly correlated with those of adults. In Experiment 4, Rhesus monkeys were tested to examine whether or not the prototype's magnet effect was unique to humans. The animals did not provide any evidence of speech prototypes; they did not exhibit the magnet effect. It is suggested that the internal organization of phonetic categories around prototypic members is an ontogenetically early, species-specific, aspect of the speech code.     

Perceptual differentiation as a source of category effects in object processing: Evidence from naming and object decision

The locus of category effects in picture recognition and naming was examined in two experiments with normal subjects. Subjects carried out object decision (deciding whether the stimulus is a “real” object or not) and naming tasks with pictures of clothing, furniture, fruit, and vegetables. These categories are distinguished by containing either relatively many exemplars with similar perceptual structures (fruit and vegetables;structurally similar categories), or relatively few exemplars with similar perceptual structures (clothing and furniture;structurally dissimilar categories). In Experiment 1, responses to the stimuli from the structurally similar categories were slower than responses to stimuli from the structurally dissimilar categories, and this effect was larger in the naming than in the object decision task. Further, prior object decisions to stimuli from structurally similar categories facilitated their subsequent naming. In Experiment 2, we orthogonally manipulated object decision and naming as prime and target tasks, again with stimuli from the four categories. Category effects, with responses slower to objects from structurally similar categories, were again larger in naming than in object decision, and these category effects in naming were reduced by priming with both naming and object decision. We interpret the data to indicate that category effects in object naming can reflect visually based competition which is reduced by the preactivation of stored structural knowledge for objects.     

Within-category discrimination of musical chords: Perceptual magnet or anchor?

Recent speech research has begun to evaluate the internal structure of categories. In one such study, Kuhl (1991) found that discrimination was poorer for vowel stimuli that were more representative of the category (prototype, or P, set) than it was for less representative stimuli (nonprototype, or NP, set). This finding was interpreted as indicating that a category prototype may function as a “perceptual magnet,” effectively decreasing perceptual distance, and thus discriminability, between stimuli. The present study examines the function of prototypes in a musical category—another natural, but nonspeech category. Paralleling the Kuhl study, representative (P) and less representative (NP) sets of major triad stimuli were constructed, based on equal temperament. Musically experienced subjects rated the stimuli in each set for goodness as a major triad, with the highest rated stimulus serving as a prototype standard for a subsequent discrimination task. Results from the discrimination task demonstrated better performance in the P context than in the NP context. The current nonspeech results indicate that a prototype functions as an anchor rather than a magnet. In addition to providing a natural, nonspeech standard for comparison with speech findings, the results provide some important insights into the nature of musical categories.     

Procedural learning in perceptual categorization

In two experiments, observers learned two types of category structures: those in which perfect accuracy could be achieved via some explicit rule-based strategy and those in which perfect accuracy required integrating information from separate perceptual dimensions at some predecisional stage. At the end of training, some observers were required to switch their hands on the response keys, whereas the assignment of categories to response keys was switched for other observers. With the rule-based category structures, neither change in response instructions interfered with categorization accuracy. However, with the information-integration structures, switching response key assignments interfered with categorization performance, but switching hands did not. These results are consistent with the hypothesis that abstract category labels are learned in rule-based categorization, whereas response positions are learned in information-integration categorization. The association to response positions also supports the hypothesis of a procedural-learning-based component to information integration categorization.     

Memory storage and retrieval processes in category learning

The detailed course of learning is studied for categorization tasks defined by independent or contingent probability distributions over the features of category exemplars. College-age subjects 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. The 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.     

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.     

Identifying stimuli of different perceptual categories in pure and mixed blocks of trials: evidence for stimulus-driven switch costs.

Responding to stimuli of different perceptual categories is usually faster when the categories are presented isolated from each other, in pure blocks, than when they are presented randomly intermixed, in mixed blocks; a difference denoted as perceptual mixing costs. The present study examined the contribution of strategic and stimulus-driven factors to these costs. The first two experiments showed that perceptual mixing costs were not reduced when participants were informed at the start of each trial in mixed blocks about the impending category. Furthermore, Experiments 1 and 3 showed that mixing costs were concentrated on those trials of mixed blocks where the perceptual category was different from that of the preceding trial. These results support the view that perceptual mixing costs derive from stimulus-driven trial-by-trial adjustments in processing. Some general implications for processing models are discussed.     

Discrimination and categorization of photographs of natural objects by chimpanzees (Pan troglodytes)

Two experiments assessed the ability of four adult female chimpanzees to categorize natural objects. Chimpanzees were initially trained to match different color photographs of familiar objects from four possible categories. In training, all the comparison stimuli were from the same category in one condition, and from different categories in another condition. For all subjects, training performance was consistently better for the "different category" than for the "same category" trials. Probe trials were shown after training. In probe trials, the sample and positive comparison stimuli were different items from the same category, and the foils were selected from among the three other test categories. Individual performance was above chance in probe trials, suggesting that categorization by chimpanzees may transcend perceptual resemblance. These results were later replicated with novel stimulus items from the same four categories (experiment 2). Altogether, this research demonstrates that chimpanzees grouped perceptually different exemplars within the same category, and further suggests that these animals formed conceptual representations of the categories. Accepted after revision: 10 August 2001 Electronic Publication     

Attention, similarity, and the identification-categorization relationship

A unified quantitative approach to modeling subjects' identification and categorization of multidimensional perceptual stimuli is proposed and tested. Two subjects identified and categorized the same set of perceptually confusable stimuli varying on separable dimensions. The identification data were modeled using Shepard's (1957) multidimensional scaling-choice framework. This framework was then extended to model the subjects' categorization performance. The categorization model, which generalizes the context theory of classification developed by Medin and Schaffer (1978), assumes that subjects store category exemplars in memory. Classification decisions are based on the similarity of stimuli to the stored exemplars. It is assumed that the same multidimensional perceptual representation underlies performance in both the identification and categorization paradigms. However, because of the influence of selective attention, similarity relationships change systematically across the two paradigms. Some support was gained for the hypothesis that subjects distribute attention among component dimensions so as to optimize categorization performance. Evidence was also obtained that subjects may have augmented their category representations with inferred exemplars. Implications of the results for theories of multidimensional scaling and categorization are discussed.     

Human adults and human infants show a “perceptual magnet effect” for the prototypes of speech categories,monkeys do not

Many perceptual categories exhibit internal structure in which category prototypes play an important role. In the four experiments reported here, the internal structure of phonetic categories was explored in studies involving adults, infants, and monkeys. In Experiment 1, adults rated the category goodness of 64 variants of the vowel /i/ on a scale from 1 to 7. The results showed that there was a certain location in vowel space where listeners rated the /i/ vowels as best instances, or prototypes. The perceived goodness of Iii vowels declined systematically as stimuli were further removed from the prototypic Iii vowel. Experiment 2 went beyond this initial demonstration and examined the effect of speech prototypes on perception. Either the prototypic or a nonprototypic IM vowel was used as the referent stimulus and adults’ generalization to other members of the category was examined. Results showed that the typicality of the speech stimulus strongly affected perception. When the prototype of the category served as the referent vowel, there was significantly greater generalization to other /i/ vowels, relative to the situation in which the nonprototype served as the referent. The notion of aperceptual magnet was introduced. The prototype of the category functioned like a perceptual magnet for other category members; it assimilated neighboring stimuli, effectively pulling them toward the prototype. In Experiment 3, the ontogenetic origins of the perceptual magnet effect were explored by testing 6-month-old infants. The results showed that infants’ perception of vowels was also strongly affected by speech prototypes. Infants showed significantly greater generalization when the prototype of the vowel category served as the referent; moreover, their responses were highly correlated with those of adults. In Experiment 4, Rhesus monkeys were tested to examine whether or not the prototype’s magnet effect was unique to humans. The animals did not provide any evidence of speech prototypes; they did not exhibit the magnet effect. It is suggested that the internal organization of phonetic categories around prototypic members is an ontogenetically early, species-specific, aspect of the speech code     

An investigation of categorical speech discrimination by rhesus monkeys

The categorical discrimination of synthetic human speech sounds by rhesus macaques was examined using the cardiac component of the orienting response. A within-category change which consisted of stimuli differing acoustically in the onset of F2 and F3 transitions, but which are identified by humans as belonging to thesame phonetic category, were responded to differently from a no-change control condition. Stimuli which differed by the same amount in the onset of F2 and F3 transitions, but which human observers identify as belonging toseparate phonetic categories, were differentiated to an even greater degree than the within-category stimuli. The results provide ambiguous data for an articulatory model of human speech perception and are interpreted instead in terms of a feature-detector model of auditory perception.     

The ABCs of categorical perception

Studies of speech perception first revealed a surprising discontinuity in the way in which stimulus values on a physical continuum are perceived. Data which demonstrate the effect in nonspeech modes have challenged the contention that categorical perception is a hallmark of the speech mode, but the psychophysical models that have been proposed have not resolved the issues raised by empirical findings. This study provides data from judgments of four sensory continua, two visual and two tactual-kinesthetic, which show that the adaptation level for a set of stimuli serves as a category boundary whether stimuli on the continuum differ by linear or logarithmic increments. For all sensory continua studied, discrimination of stimuli belonging to different perceptual categories was more accurate than discrimination of stimuli belonging to the same perceptual category. Moreover, shifts in the adaptation level produced shifts in the location of the category boundary. The concept of Adaptation-level Based Categorization (ABC) provides a unified account of judgmental processes in categorical perception without recourse to post hoc constructs such as implicit anchors or external referents.     

Neural correlates of acquired color category effects

Category training can induce category effects, whereby color discrimination of stimuli spanning a newly learned category boundary is enhanced relative to equivalently spaced stimuli from within the newly learned category (e.g., categorical perception). However, the underlying mechanisms of these acquired category effects are not fully understood. In the current study, Event-Related Potentials (ERPs) were recorded during a visual oddball task where standard and deviant colored stimuli from the same or different novel categories were presented. ERPs were recorded for a test group who were trained on these novel categories, and for an untrained control group. Category effects were only found for the test group on the trained region of color space, and only occurred during post-perceptual stages of processing. These findings provide new evidence for the involvement of cognitive mechanisms in acquired category effects and suggest that category effects of this kind can exist independent of early perceptual processes.     

Category-structure effects in picture comparisons

Subjects indicated for a series of trials whether or not two pictures of common objects were from the same category. Reaction time (RT) was recorded. The pool of stimuli used in the series of trials consisted of two pictures representing each of several categories. Half were S categories, members of which were similar; the other half were D categories, which had dissimilar members (this variable is termed category structure). In Experiments 1 and 3, the similarity manipulation defining category structure was based on the concept of basic level categories (Rosch, Mervis, Gray, Johnson, & Boyes-Braem, 1976). In Experiment 2, category labels were homonyms, and the two members of S or D categories represented one or two senses of the labels, respectively. The principal finding was an effect of category structure (faster RT on trials using stimuli from S categories rather than D categories). These results are interpreted in terms of a model that postulates the use of semantic codes in the task, even on trials where the two stimuli are physically identical.