Final answer:
Sensitive periods represent times in development when the brain is particularly open to learning new skills, often seen in early childhood. Critical periods involve heightened plasticity, like in language acquisition and sensory development, which necessitates proper stimuli for optimal growth. These periods underline the importance of suitable learning environments in early stages of life.
Step-by-step explanation:
Sensitive periods are referred to as 'windows of opportunity for learning' because they signify times during development when an organism is particularly receptive to certain types of environmental stimuli. During these critical periods, the brain's plasticity allows for rapid learning and development. For example, language acquisition is profoundly affected by the sensitive periods; children are more adept at learning new languages before puberty, after which the ability plateaus.
During the developmental milestone stages in early childhood, children master various skills easily due to their brain's adaptability. The brain and sensory systems, like vision as demonstrated in the kitten eye-suturing experiments, require specific stimuli for proper development. If these stimuli are absent or insufficient during the critical periods, it can result in long-term deficits in abilities such as language, vision, and even form attachments, as seen in studies of disorganized attachment.