The potency of the environment to shape brain function changes dramatically across the
lifespan. Neural circuits exhibit profound plasticity during early life and are later stabilized. A
focus on the cellular and molecular bases of these developmental trajectories has begun to
unravel mechanisms, which control the onset and closure of such critical periods. Two
important concepts have emerged from the study of critical periods in the visual cortex:(1)
excitatory–inhibitory circuit balance is a trigger; and (2) molecular “brakes” limit adult …