Although initial stimulation of each sensory system is internal or endogenous, at a critical or sensitive point in development, external stimulation and experiences are needed for further development. Experiences that influence fetal, infant, and child development can come from their physical, sensory, chemical, nutritional, social, and/or emotional environments. Events and stimuli from any of these components of environment are capable of altering the course and outcome of developmental processes producing changes in brain development that can be either positive or negative.