Figure 1. Upper: Water tank apparatus used for testing the elemental and structural discriminations. Rats were placed in the water facing the near wall; discriminative stimuli were at the far end of the tank. An opaque partition separated the reinforced pattern (S+) from the nonreinforced pattern (S–). (The stimuli depicted are the first pair from the structural discrimination.) A submerged platform was always present underneath the reinforced pattern. Middle: The two stimuli used for the elemental discrimination, circle versus cross. The identity of the S+ stimulus was counterbalanced across the rats. Lower: The compound stimuli used for the structural discrimination task (Stages 1–6). Stimuli were formed from the elements black (B), white (W), and horizontal (H) presented in pairs with specific spatial relationships, for example, B|W, W|H, H|B. Reinforced stimuli are depicted in the left column (S+), and nonreinforced stimuli (S–) are depicted in the right column. Stimuli in the top row were used for Stage 1. Stimuli in the top two rows were used for Stages 2 and 3 of the structural discrimination, and all three rows of patterns were used in Stages 4–6. When all three discriminations were presented concurrently, every element was presented an equal number of times on the left or right of a compound stimulus as an S+ or S–. Thus, to solve the task the rat needed to learn the left and right positions of each specific pair of elements.