A critical period in neural development is a bounded interval in which particular experiences have a strong role in establishing a specified function or circuit organization. Its timing must be stated for a species and a measured outcome. A sensitive period emphasizes heightened susceptibility without necessarily asserting that every later change is impossible.
A classic example comes from monocular deprivation in cats. P. B. Dews and colleagues closed one eye at different ages and for different durations, then compared vision through the two eyes when the animals were at least a year old. Starting deprivation at one month offered little protection against lasting impairment; starting at two months reduced the impairment, and starting at four months or later produced no lasting deficit detectable in their behavioural tests. The age comparison revealed a developmental window rather than merely a short-lived effect of having an eye closed.[ref]
Consequences of monocular deprivation on visual behaviour in kittens by P. B. Dews and others. Journal of Physiology 206, pp. 437–455; abstract points 1–3, starting age, duration and delayed behavioural testing
The design separates age at disruption, duration of disruption, and the function tested. A change in visual acuity and a change in the responses of visual cortex neurons need not have identical recovery profiles. The existence of a critical period for one visual measure does not establish a single deadline for the entire brain, or the absence of adult neuroplasticity.[ref]
Consequences of monocular deprivation on visual behaviour in kittens by P. B. Dews and others. Abstract points 4–5; behavioural improvement and cortical responses after reversal of eye closure
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