The twelve display configurations constructed from the three cartoon items (apple, duck, grapes) used in the MTS procedure. Pigeons were trained with either the top or bottom six displays. Notice that the role of each item in each of the two training sets is counterbalanced for sample frequency, correct comparison position, and incorrect comparison position. The six displays of the set not used in training were used to test for if-then rule learning. Note. From Figure 1, “Concept learning and learning strategies,” by A. A. Wright, 1997, Psychological Science, 8, p. 120. Copyright 1997 by Blackwell Publishing. Adapted with permission.