The development of the brain regions responsible for vision is not yet complete at birth. These regions mature during the first years of life. However, this requires a sharp retinal image in both eyes, and both eyes must actually be used. In cases of severe or unilateral refractive error, different diopter values between the two eyes, or when one eye is not used at all due to strabismus, the brain does not learn to see properly. This learning process cannot be made up for later in life, and the otherwise healthy eye remains visually impaired (amblyopic) for the rest of the person’s life.