Period: A developmental phase in which the brain requires certain environmental input or it will not develop normally. Scientists have documented critical periods for the presence of certain nutrients (e.g., folic acid, iron, protein and calories), for certain types of sensory stimuli (such as vision and speech sounds), and for certain emotional and cognitive experiences (attachment, language exposure). However, there are many mental skills, such as reading, vocabulary size, and the ability to see color, which do not appear to pass through tight critical periods in their development. See also: Windows of Opportunity.