Learned Visual Categorical Perception Effects Depend on Method of Assessment and Stimulus Discriminability

Learned Visual Categorical Perception Effects Depend on Method of Assessment and Stimulus Discriminability Joshua R. de Leeuw (jodeleeu@indiana.edu) Department of Psychological and Brain Sciences, Program in Cognitive Science Indiana University, Bloomington, IN Jan Andrews (andrewsj@vassar.edu) Ken Livingston (livingst@vassar.edu) Department of Cognitive Science Vassar College, Poughkeepsie, NY Abstract important step towards a thorough understanding of the mechanisms that cause CP. Several studies have demonstrated that whether CP is observed at all in certain scenarios depends on various experimental factors, such as the availability of verbal labels during perceptual testing (Kikutani, Roberson, & Hanley, 2008), the particular kind of perceptual assessment used to determine whether CP is present (Gerrits & Schouten, 2004), and how stimulus morphspaces are created (Folstein, Gauthier, & Palmieri, 2012). However, few studies have reported differences in the kind of CP effect observed based on experimental manipulations, although Goldstone, Lippa, & Shiffrin (2001) and Livingston & Andrews (2005), both reported two qualitatively different CP effects exhibited by the same subjects when they were tested in two different ways. Thus, one possible reason for the diversity of CP effects is that different tasks used to assess CP are sensitive to different aspects of CP or invoke different processes, only some of which exhibit particular CP effects. In addition to the task used to measure CP, it is possible that incidental differences in stimuli between experiments are responsible for the different kinds of CP effects observed. If subjects learn that stimuli with small differences belong in different categories, successful categorization necessitates the ability to differentiate the stimuli, which might naturally lead to expansion effects. However, if the differences between stimuli are obvious prior to training, then expansion might be less likely to occur. Pevtzow and Harnad (1997) found larger expansion effects for stimulus sets that were harder to differentiate, which is consistent with this hypothesis. In this experiment, we directly tested the influence of stimulus discriminability and assessment task on CP. We created an artificial stimulus set that varied on two dimensions, and selected two subsets of stimuli from this set, one with only half as much variation between neighboring pairs as the other. We expected that the stimulus set with less variation would be more likely to produce expansion effects, while the stimulus set with more variation would be more likely to produce compression effects. Three commonly used tasks were implemented to assess CP: a similarity rating task, a same/different task, and an XAB forced-choice task. We expected task to interact Learned categorical perception (CP) effects were assessed using three different measures and two sets of stimuli differing in discriminability, both of which varied on one category-relevant and one category-irrelevant dimension. Two different kinds of analysis produced patterns of results that depended on both of these variables and show that categorical perception effects are sensitive to variations in assessment task and stimulus discriminability. Only the similarity-rating task produced evidence of between-category expansion effects, suggesting that participants used different strategies for subjective and objective tasks. Generally, there was evidence that category training caused a decrease in the salience of category-irrelevant variation, but when the assessment task cued participants to category-irrelevant differences they were equally apt at identifying category- irrelevant variation as a control group. Keywords: compression; experiments. Categorization; categorical perception; expansion; learning; similarity; online Introduction Learning to categorize stimuli in a new way can change how those stimuli are perceived or judged. This phenomenon is called learned categorical perception (CP). There are numerous kinds of reported CP effects (for a recent review, see Goldstone & Hendrickson, 2009). Learning that two stimuli belong to the same category can increase their similarity or perceptual confusability, an effect known as compression (e.g., Livingston, Andrews, & Harnad, 1998), while learning that two stimuli belong to different categories can have the opposite effect, often called expansion (e.g., Goldstone 1994; Notman, Sowden, & Ozgen, 2005). Categorizing stimuli based on particular sets of features may increase the relative influence of those features at the expense of other stimulus features, regardless of whether or not the stimuli belong to the same or different categories (e.g., Goldstone, 1994). Although there are several possible consequences of learning to categorize stimuli, most studies of learned CP only report finding one of the possible effects, even though different kinds of CP effects are not logically mutually exclusive. Determining why categorization training leads to different CP effects in different experimental contexts is an