Figure 1.

Schematic diagram of the Conceptual Set-Shifting Task (CSST). Each panel represents the touchscreen for a single trial, and contains 3 stimuli differing in shape (triangle, disc, star) and in color (red, blue, green). The first row of panels presents trials on the initial concept (“red”), during which the red stimuli (triangle, star, and disc, respectively) are scored as correct. The second row of panels show three trials for second concept presented (“triangle”). For this concept the triangle is always correct, regardless of color. Correct responses are indicated by “*” (which is not visible to the chimpanzees).