An interactional model of brain function is postulated, in which activity and reactivity are basic properties of the central nervous system from the very beginning. Variability is an essential aspect of normal functioning. Primary or indiscriminate variability consists of the development of many strategies for the execution of motor acts. It is predominant during fetal age and infancy. Toddler age is characterized by secondary or adaptive variability, which reflects the ability to select the adequate strategy at the right moment. This is important for the development of proper planning of movement patterns and is a prerequisite for skill development. Recognition of the types and quality of variability helps to detect developmental disorders at an early age.
