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Infants of rats and other mammals respond to a novel environment by becoming immobile, and then showing a process of motorial expansion called "warm-up." Starting from immobility, new types of movement are incorporated into the stream of behavior according to rather strict rules of order. Once a new type of movement has been performed, the infant reverts to it repeatedly. As a result, the earlier portion of the behavior appears stereotyped, giving the impression of an automatism. Later, as new types of movement are added to the infant's repertoire, the movement becomes increasingly rich and unpredictable, giving the impression of "free" behavior. The same rules of order operate within "warm-up" sequences of movement, and across such sequences, day by day. Concurrently, there is an increase in the amplitude of movements, resulting in a gradual expansion of the portion of the environment explored by the infant. The same rules of order seem to operate in the development of locomotion in more primitive vertebrates. In rats under the action of psychoactive drugs, the "warm-up" sequence is performed in reverse.