Figure 2. The figure plots thresholds on density-varying trials against those on size-varying trials.

Data for Subjects 1 to 4 are indicated by symbol colors red, green, blue, and yellow, respectively. The shape of the symbols indicates the experimental condition. Circles: single stair size- and density-varying trials (size and density task). Squares: size- and density-varying trials are randomly interleaved, and the subject has to indicate not only in which direction the test stimulus is changed but also whether it differs in size or density (mixed task). Triangles: size- and density-varying trials are randomly interleaved, and the subject has to indicate whether the test has more or fewer dots than the standard (numerosity task). The error bars represent 95% confidence limits estimated by bootstrapping.