critical period

Meaning: A critical period is a limited window in development when a particular experience is needed for a brain system or behavior to develop normally. Plasticity is high during the window. Afterward, circuits become more fixed, so missing input often leaves a lasting deficit, with limited recovery. Some texts use sensitive period for a softer version, in which learning is easier during the window but still possible after it.

On the MCAT: Examples include language acquisition (children raised with little language exposure, such as Genie, show lasting grammar deficits), Lorenz's imprinting in geese, and Hubel and Wiesel's kittens, whose visual cortex developed abnormally after one eye was closed early in life. A passage may deprive groups at different ages; the group deprived during the window shows the largest deficit.

Common confusion: Students blur critical and sensitive periods, or read either as a simple deadline. In a critical period, recovery after the window is limited or absent, as in the kittens' lasting visual deficits. If the same experience simply matters more inside the window and learning stays possible later, that is a sensitive period. Also keep it separate from Piaget's stages, which describe general cognitive levels, not sensitivity to a specific input.