Not long ago, when a child was born with strabismus, the defect was usually not corrected until after adolescence. Today, surgical correction is always attempted during early childhood. Why? How does strabismus affect the connections in the brain, and how does it affect vision?
Strabismus is a common visual disorder in humans in which the eyes are not perfectly aligned. As a result, the fovea in each eye is not focused on the same point in the visual field. Ocular misalignment must be corrected in early childhood, as soon as surgically feasible, to avoid permanent visual disability. This is because strabismus prevents corresponding binocular input to cortical neurons (input from both the right and left eye to cortical neurons representing the same point in space). This noncorrespondence prevents the development of binocular cortical neurons, which are essential for stereopsis, the ability to discriminate fine differences in three-dimensional spaces. In addition, people with strabismus often favor one eye over the other. The nonprefered eye is at a disadvantage during the critical period of visual cortical development when the process of binocular competition determines which eye wins synaptic space in visual cortex. The preferred eye establishes more than its share of synaptic contacts in visual cortex, and after the critical period ends, the unequal distribution becomes permanent. Corrective surgery for strabismus during adolescence may realign the eyes, but the cortical connections will not change. Establishing the correct cortical circuitry requires that corrective surgery be done early, before the critical period for cortical development ends.