Critical Period

What is a critical period?

A critical period is a developmental interval in which particular experience is required for a function to develop normally, with effects that later ordinary experience may not fully reverse.

Critical period versus sensitive period

Knudsen (2004) distinguishes this from the broader idea of a sensitive period, when experience has an unusually strong effect. His review contrasts early visual input shaping the visual cortex with systems where later experience can restore typical performance.

Examples: Vision and Imprinting

Knudsen describes two classic cases.

Vision

In studies of young mammals, depriving one eye of input changes its representation in the visual cortex, with changes that ordinary later visual input does not fully reverse.

Filial imprinting

In ducks and chickens, filial imprinting involves learning the sights and sounds that identify a parent shortly after hatching. The resulting preference can persist beyond that early learning window.

Critical periods in sensory development and learning

Timing and reversibility depend on the function and species. Evidence about a sensory circuit does not establish a single deadline for all language, attachment or learning. A period of heightened sensitivity also does not imply that later learning is impossible.