In young infants, activation or inhibition of body movements on perception of environmental events is important to enable them to act on the world or understand the world. To reveal the development of this ability, we observed movement patterns in all four limbs under the two experimental conditions. Infants assigned to the interaction condition were provided a connection between their arm and a toy, which allowed movements of the critical arm to produce movements in the overhead mobile. Infants assigned to the stimulation condition were presented with similar movements of the toy emulated by the experimenter. In 3-month-old infants, the arm movements increased under the interaction condition, whereas they decreased under the stimulation condition. Such condition-dependent behavior dissociation was not observed in 2-month-old infants. These results suggest that the distinct behavior of an "observer" of environmental events was differentiated from that of a "player" interacting with the surroundings between 2 and 3 months of age. The emergence of differentiation of behaviors points toward the development of self-referential and inhibition mechanisms in the perceptual-motor and attention/evaluation system in the cortico-subcortical network. (Contains 9 figures.)