Studies of children treated for dense cataract shed light on the extent to which pattern stimulation drives normal visual development and whether there are sensitive periods during which an abnormal visual environment is especially detrimental. Here, we summarize the findings to date into five general principles: (1) At least for low-level vision, aspects of vision that develop the earliest are the least likely to be adversely affected by abnormal visual input whereas those that develop later are affected more severely. (2) Early visual input is necessary to preserve the neural infrastructure for later visual learning, even for visual capabilities that will not appear until later in development. (3) The development of both the dorsal and ventral streams depends on normal visual input. (4) After monocular deprivation has been treated by surgical removal of the cataractous lens, the interactions between the aphakic and phakic eyes are competitive for low-level vision but are complementary for high-level vision. (5) There are multiple sensitive periods during which experience can influence visual development.
The studies described here have important implications for understanding normal development. They indicate that patterned visual input immediately after birth plays a vital role in the construction and preservation of the neural architecture that will later mediate sensitivity to both basic and higher level aspects of vision. The period during which patterned visual input is necessary for normal visual development varies widely across different aspects of vision and can range from only a few months after birth to more than the first 10 years of life. The results point to new research questions on why early visual deprivation can cause later deficits, what limits adult plasticity, and whether effective rehabilitation in other areas can provide new clues for the treatment of amblyopia.