What good is goodness? The effects of reference points on discrimination and categorization of shapes

Abstract

Earlier research reported a category boundary effect on perception: Differences between stimuli belonging to the same category are perceived as smaller than differences between stimuli belonging to different categories even when the physical dissimilarity between the stimuli in the pairs is the same. In this paper, we propose that the existence of reference points (i.e., exemplars that serve as a point of comparison) can explain the occurrence of the category boundary effect as well as the directional asymmetries in within-category pairs. We investigated how reference points influence categorization and discrimination performance, using three different tasks: categorization, successive discrimination, and similarity judgment. We used both recognizable and non-recognizable morph figures as stimuli, assuming that recognizable series have clearer reference points. We replicated the overall category boundary effect for both discrimination and similarity and show the effect’s dependence on the strength of the reference points involved. The general category boundary effect is not a proper category boundary effect, however: rather than the type of stimulus pair presented (i.e., within- or between-category) one needs to take into account the distance from the reference points for each of the individual stimuli in the pair to actually predict discrimination performance and similarity judgments. These results provide evidence that reference points on a dimension and their strength have tangible consequences for how we perceive, categorize, and react to stimuli on that dimension. Moreover, our findings remind us of the danger of averaging without looking at underlying data patterns, and of the gains that can be made by seriously exploring consistent variability in extensive data sets.