When infants are developing, there are specific periods of time when senses will begin to take shape. This is called a critical period. If infants are not able to develop their senses at this time, they will not develop correctly. Vision is one of the senses that has a critical period. It is essential that infants be able to develop their vision early on. In one experiment done by Hubel and Wiesel, a cat had one of its eyes sewn shut. When it was later allowed open, it was shown that the cat had only developed vision in the eye that was not sewn shut. Another experiment was done with humans. Some babies are born with cataracts. If these cataracts were not removed before the critical period, it was found that the child had not properly developed vision in the eye with the cataracts. It is difficult to perform experiments for this area of study because of ethical dilemmas. However, these two experiments were able to shed light on this subject. Both experiments prove that there is a critical period for vision.