  The classic example of a critical period is due to the Nobel  prize-winning work of Hubel and Wiesel. By suturing shut one of a  kitten's eyes at various stages of development and for various  periods of time, Hubel and Wiesel (1970) showed that certain cortical and thalamic areas supporting binocular vision (specifically, ocular  dominance   columns[20]  and cells in the lateral geniculate body) will not develop normally  unless kittens receive patterned visual stimulation during the 4th to 12th weeks of life. They found that while the damage was sometimes  reversible to some extent, depending on the exact duration and timing of the occlusion, occlusion for the entire first three months of life produced irreversible blindness in the deprived   eye.[21] 