Figure 1.

Quantitative models for response times in the same-different task. (A) To elucidate how same-different responses are related to the underlying perceptual space, consider a hypothetical perceptual space consisting of many hierarchical stimuli. In this space, nearby stimuli are perceptually similar. (B) We hypothesized that participants make “SAME” or “DIFFERENT” responses to an image pair based on the dissimilarity between the two images. In the global block, when two images have the same global shape, we predict that response times are longer when the two images are more dissimilar. Thus, two diamonds made using Xs and Zs evoke a faster response than two diamonds made of circles or Xs, because the latter pair is more dissimilar than the former. By contrast, when two images differ in global shape, responses are faster when they are more dissimilar. Thus, dissimilarity can either speed up or slow down responses. (C) We also hypothesized that shapes that are more distinct (i.e. far away from other shapes) will elicit faster responses because there are no surrounding distractors. Thus, the diamond made of circles, which is far away from all other stimuli in the schematic space of panel A, will elicit a faster response than a diamond made of Zs.