Sequential similarity and comparison effects in category learning

Sequential similarity and comparison effects in category learning Paulo F. Carvalho (pcarvalh@indiana.edu) Department of Psychological and Brain Sciences, Indiana University 1101 East Tenth Street Bloomington, IN 47405 USA Robert L. Goldstone (rgoldsto@indiana.edu) Department of Psychological and Brain Sciences, Indiana University 1101 East Tenth Street Bloomington, IN 47405 USA Abstract Order effects in category learning have been previously demonstrated. Specifically, alternation between exemplars of two categories has been shown to improve category learning and discrimination, compared to presenting exemplars of each category in separate blocks. However, the mechanisms under- lying order effects are still not completely known. Remaining issues pertain to the relevance of within and between category similarities, and the role of comparing sequentially presented objects. We present two experiments: in Experiment 1 within- and between-category similarity are manipulated simultane- ously with presentation schedule. In Experiment 2, alternation between categories is compared to two blocked conditions: one in which very similar stimuli are presented successively, and another in which they are dissimilar. Our results show a clear overall advantage of low similarity in categorization performance, but no effect of presentation schedule. Also, al- ternation between categories is shown to result in better per- formance than the blocked condition with more dissimilar stimuli. Keywords: category learning; order effects; spacing; perceptual learning. Introduction The ability to discriminate between two similar stimuli has been shown to be influenced by the way stimuli are present- ed. Take, for example, two categories A and B. Exemplars from these two categories can be presented interleaved, i.e., A B A B A B. Another way to present the same exemplars is to show all exemplars of category A before starting presentation of the exemplars from category B, i.e., A A A B B B. This exemplar ordering manipulation can have a great impact on the ability to discriminate between A and B. Indeed, results show that performance improves considera- bly with interleaved exposure to the stimuli (e.g., Dwyer, Hodder, & Honey, 2004; Lavis & Mitchell, 2006). This effect has been replicated several times, using differ- ent tasks, stimuli, and sensory modalities both in humans and other species. Improved discrimination for interleaving presentations also seems to be highly transferable to new situations that make use of the same kind of knowledge (Mitchell, Kadib, Nash, Lavis, & Hall, 2008) and to result in better deductive extraction of the relevant categories, such as painter identities (Kornell & Bjork, 2008). In light of this evidence, alternation of exemplars has been described as highly beneficial for discrimination learning, and can be potentially implemented in several informal and formal learning contexts (Rohrer & Pashler, 2010; Taylor & Rohrer, 2010). However, a full description of how the interleaving ad- vantage occurs has not yet been provided. Some authors have argued for the long-known mnemonic powers of spaced rather than massed practice (Kornell, 2009; Kornell & Bjork, 2008; Vlach, Sandhofer, & Kornell, 2008) and have made a connection between interleaving and spacing. When different presentations of A are interleaved with B presentations, then the A presentations will be more spaced. However, evidence inconsistent with this proposal has been presented. Mitchell, Nash, and Hall (2008), for example, presented three highly similar stimuli, A, B and C, to partic- ipants. In one condition A and B were interleaved and C was presented spaced. In the spaced condition, two instanc- es of C were separated by an interval during which no in- formation was presented. The duration of this interval was equivalent to the presentation of another stimulus. This ma- nipulation still resulted in better discrimination for stimuli presented interleaved (for a similar demonstration using paitings see Kang & Pashler, in press). In a follow-up exper- iment, the authors added a new stimulus, D. This stimulus was very different from A, B and C. Interleaving A and B resulted in better performance than interleaving C and D. Mitchell and collaborators (Mitchell, Kadib, et al., 2008; Mitchell, Nash, et al., 2008) suggested that the advantage of interleaving might still result from different memory encod- ings promoted by each presentation schedule. Their pro- posal is that when highly similar stimuli are alternated, then the features that they share will be presented several times in a row, and thus attention shifts towards the differences, enriching the memory trace for the differences between the two categories. When stimuli are blocked, however, all of the features of each category, both those unique to each cat- egory and those that are shared across categories, are pre- sented several times in a row, resulting in a relative decrease in attention to unique features and a poorer memory trace for the purposes of discrimination. Also central to Mitchell and collaborators’ proposal (Mitchell, Kadib, et al., 2008; Mitchell, Nash, et al., 2008) is the process of comparison that results from shifting atten- tion to discriminating features. The importance of compari- son in the sequence of object presentations was also pro- posed by Eleanor Gibson (1969) in her influential theory of perceptual learning. Gibson proposed that the opportunity to compare the stimuli was crucial in facilitating a process of differentiation that would result in better discrimination. Unfortunately, the mechanism through which this differen-