This project will study the perceptual-cognitive development of human infants by investigating the acquisition, retention, and retrieval of a memory engram for a perceptual event and the infant's self-exposure to new stimuli as a function of comparing new stimuli with retained engrams. The basic procedure will be to present a standard stimulus to the infant over and over again until the infant habituates his attentional behavior, and then introduce a new or discrepant event. Current evidence indicates that it is reasonable to suppose that a) the rate of habituation to the repeatedly presented standard stimulus is a partial index of the acquisition of a memory engram, b) the differential response to a discrepant stimulus testifies to the retrieval of such a memory, and c) the response to discrepancy is likely to be an inverted-U function of the magnitude of discrepancy between the new stimulus and the retained engram. Furthermore, some data suggest the hypotheses that a) developmental changes occurring during infancy may have a greater effect upon the acquisition than the retrieval and use of a perceptual memory, b) the degree of maturity of a memory engram will determine whether discrepant stimuli will receive attention or be perceptually avoided, and c) older infants are more likely to avoid attending to extreme discrepancies than younger babies. These hypotheses and the general nature of the infant's perceptual-cognitive world will be studied by investigating the infant's habituation and response to different magnitudes of discrepancy as a function of age, habituation rate, amount of habituation, complexity of stimuli, retention interval, etc., for both visual and auditory stimuli. The proposed research has possible implications for the perceptual- cognitive development of human infants, the development of encoding and memory processes, early education, psychophysiology and brain function, and mental retardation.