A Neural Network Model of Concept-Influenced Segmentation

A Neural Network Model of Concept-influenced Segmentation Robert L. Goldstone ( rgoldsto@indiana.edu ) Department of Psychology, Indiana University Bloomington, IN 47405 USA Abstract Several models of categorization assume that fixed perceptual representations are combined together to determine categorizations. This research explores the possibility that categorization experience alters, rather than simply uses, descriptions of objects. Based on results from human experiments, a model is presented in which a competitive learning network is first given categorization training, and then is given a subsequent segmentation task, using the same network weights. Category learning establishes detectors for stimulus parts that are diagnostic, and these detectors, once established, bias the interpretation of subsequent objects to be segmented. Concept Learning and Perception The current research explores the influence that learning a new concept has on the segmentation of objects into component parts. Recently a number of researchers have argued that in many situations, concept learning influences the featural descriptions used to describe a set of objects. Rather than viewing perceptual descriptions as fixed by low-level sensory processes, this view maintains that perceptual descriptions are dependent on the higher-level processes that use the descriptions (Goldstone, Steyvers Spencer-Smith, & Kersten, 2000; Schyns, Goldstone, & Thibaut, 1998). Evidence for this view comes from the study of expert/novice differences (Lesgold et al., 1988), influences of acquired concepts on the interpretation of stimuli (Wisniewski & Medin, 1994), and influences of category learning on psychophysical measurements of perceptual sensitivity (Goldstone, 1994). Experiential Influences on Object Segmentation One type of influence of concept learning on perceptual learning may be to alter how objects are segmented into parts. Objects often have more than one possible segmentation. The letter “X” can be viewed as comprised of two crossing diagonal lines, or as a “V” and an upside- down “V” that barely touch at their vertices. The segmentation of scenes into parts depends upon experience. Behrmann, Zemel, and Mozer (1998) found that judgments about whether two parts had the same number of humps were faster when the two parts belonged to the same object rather than different objects. Further work has found an influence of experience on subsequent part comparisons. Two stimulus components are interpreted as belonging to the same object if they have co-occurred many times (Zemel, Behrmann, Mozer, & Bavelier, 1999). Thus, experience with particular feature combinations determines whether or not features will be integrated into a single object. Pevtzow and Goldstone (1994; reported in Goldstone et al., 2000) explored the influence of category learning on segmentation with the materials shown in Figure 1. We pursued the idea that how psychologically natural a part is might depend on whether it has been useful for previous categorizations. Naturalness was measured by how quickly subjects could confirm that the part was contained within a whole object (Palmer, 1978). To test this conjecture, we gave participants a categorization task, followed by part/whole judgments. During categorization, participants were shown distortions of the objects A, B, C, and D shown in Figure 1. The objects were distorted by adding a random line segment that connected to the five segments already present. Subjects were given extended training with either a vertical or horizontal categorization rule. For participants who learned that A and C were in one category, and B and D were in another (a vertical categorization rule) the two component parts at the bottom of Figure 1 were diagnostic. For participants who learned that A and B belonged in one category, and C and D belonged to the other category (a horizontal rule), the components on the right were diagnostic. A B C D Component Parts of Objects Figure 1: Pevtzow and Goldstone (1994) stimuli During part/whole judgments, participants were shown a whole, and then a part, and were asked whether the part was contained in the whole. Participants were given both present and absent judgments, and examples of these judgments are shown in Figure 2. Note that the two parts shown in Figure 2 were both potentially diagnostic during