Selective visual responsiveness to variations in visibility, form, and novelty are measured during the first six months of life in normal and exceptional populations of infants in order to trace the early development of perceptual-attentional behaviors and to distinguish among the possible developmental influences. These influences include possible initial facilitating effects of the early experience of preterm infants, possible later-appearing retardation in early preterm or other high-risk populations and the retarding genetic effect from Down's Syndrome. In order to make visual preference testing more useful for both diagnostic and experimental purposes, we will especially try to distinguish between sensory or motor influences and more central perceptual or cognitive bases of the developments.