During development, the eyes compete to connect with the brain's binocular zone, and one eye sometimes prevails. This can lead to amblyopia, the most common cause of childhood visual impairment, in which the brain's preference for the strong eye increases at the expense of the weaker eye. Patching the strong eye can help to correct amblyopia, but only during the critical period, which ends around the age of 7 years. After this, the connections are difficult to change, and visual impairment is likely to persist. New research led by Joshua Trachtenberg (University of California, Los Angeles) has identified a circuit in the brain that can be manipulated to allow remodeling of visual connections outside the critical period in mice (Nature published online 25 August 2013; doi:10.1038/nature12485).
Remodeling of brain connections proceeds only when cell activity is above a certain threshold. When vision through one eye is impaired, the activity of binocular zone cells falls below the threshold, and remodeling cannot occur. But over time, the cells' activity increases back to normal levels. Trachtenberg's team found that this increase is due to weakening of an inhibitory circuit. When vision through one eye is impaired, the inhibition from that circuit lessens, restoring the cells' activity and allowing remodeling.
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