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E/I Balance refers to the balance of excitatory (E) and inhibitory (I) activity in the brain.

When neurons communicate with each other, they can secrete excitatory or inhibitory neurotransmitters – chemicals that dial up or down, respectively, the electrical activity (aka “firing”) of other neurons. E/I balance is important for the brain’s plasticity, or capacity for change. Early in brain development, this balance is skewed towards excitation. Over time, neurons that make inhibitory neurotransmitter mature, turning on “critical periods” or “sensitive periods”—time windows when the brain is readily shaped by environmental experience.

Disruptions of E/I balance are thought to underlie several nervous system disorders, including mental disorders such as autism and schizophrenia.

Recently, optogenetic experiments demonstrated that altering E/I balance in the prefrontal cortex of mice interferes with cellular information processing and leads to impairments in social function and cognition reminiscent of those occurring in psychiatric disorders. (Visit Schizophrenia Research Forum for coverage of this work and researchers’ comments on it.)