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FIGURE 12-2 Brain Maintenance Monkeys quickly learn to solve puzzles or perform other tricks to gain access to a door that looks out from their dimly lit quarters into an adjacent room. A toy train is a strong visual incentive for the monkey peeking through the door; a bowl of fruit is less rewarding.
From R. A. Butler and H. F. Harlow (1954). Persistence of Visual Exploration in Monkeys. Journal of Comparative and Physiological Psychology, 47, p. 260.
University of Wisconsin