Changing similarity: Stable and flexible modulations of psychological dimensions

Successfully categorizing objects requires discriminating between relevant and irrelevant dimensions (e.g., shape, color). Categorization can lead to changes in the visual system that stretch psychological space, making relevant dimensions more distinct and irrelevant dimensions more similar. These changes are known as dimensional modulation (DM) and they can be both stable and flexible in nature. The current study examined the interaction between stable DM and flexible DM, as well as the time course of relative changes in similarity. Using a two-dimensional space of cars, participants learned to categorize the space and then completed a target identification task during EEG recording. We found that attention, operationally defined as the selection negativity, was sensitive to category-relevance and appeared to selectively enhance previously irrelevant differences in the service of a target detection task. In contrast, we found that late decisional stages, operationally defined as the P3 b, were less sensitive to relevance and instead more sensitive to the number of morphsteps that separated targets from non-targets. Thus, it appears that relative similarity between targets and non-targets dynamically changed over the time course of individual decisions. Similarity between exemplars was greater along the irrelevant than the relevant dimension early on in the time course but a compensatory allocation of attention led to similarity being optimized among all dimensions for later stages. This finding is important because it 1) provides a new source of converging evidence for stable DM and 2) links a neural measure of attentional modulation with facilitation of an unpracticed, but task-relevant perceptual dimension.