E/I Balance refers to the balance of excitatory (E) and inhibitory (I) activity in the brain.
When neurons communicate with each other, they release excitatory or inhibitory neurotransmitters – chemicals that intensify or dampen, respectively, the electrical activity of other neurons. The balance between these excitatory and inhibitory signals is important for the brain’s plasticity, or capacity for change.
Early in brain development, the balance is skewed towards excitation. As the brain matures, inhibition increases. Neurons that secrete inhibitory neurotransmitter are thought to turn on “critical periods” or "sensitive periods"—windows of heightened plasticity in brain development. These are the times, different for different brain circuits, when environmental experience most potently shapes neural architecture.