Despite external changes such as those of magnitudes, the functional properties of the visual system also improve with increased age. According to Jean Piaget's centration/decentration theory, the process of perceptual development might continue until adulthood and even after. However, perceptual development should not be understood in all of its aspects as a linear improvement. For example, constancy is better developed in children approximately eight years of age than in either younger or older subjects. Therefore, in such cases, the perceptual development corresponds to a u-shaped curve. The same is true for depth perception, which is best developed in children approximately nine years old. These examples suggest an increased cognitive involvement in perceptual processing that leads to an overemphasized cognitive compensation. The experimental method used in research on perceptual development, however, might influence the determination of the so-called "critical age levels." Nevertheless, perceptual development in children does not correspond either to monotonously increasing nor decreasing improvement alone. Discontinuity seems to be a rule in many aspects of the perceptual development. (Author/FL)