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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]

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]

Monocular deprivation is an experimental reduction of normal patterned vision through one eye while the other remains available. It is used to investigate how sensory experience shapes a developing nervous system. In classic cat experiments, researchers closed the eyelids of one eye for a stated interval, then reopened it and compared what the animal could do through each eye.^[ref]

P. B. Dews and colleagues varied both the starting age and duration of deprivation, testing the cats when they were at least a year old. Closure during the first four to six weeks left a lasting reduction in visual acuity. Longer early closure could also impair visually guided paw placement and pattern discrimination. Deprivation beginning at four months or later produced no lasting deficit detectable by their behavioural tests. The comparison identifies a critical period of heightened susceptibility in this species and experimental setting; it does not mean that all forms of adult neuroplasticity disappear.^[ref]

An illuminating feature of the design is its separation of when experience is lost from how long it is lost. A weak response immediately after opening an eye would not alone demonstrate a lasting developmental change; delayed testing and comparison with the other eye make that inference stronger.

P. B. Dews coauthored a 1970 experiment measuring how early visual experience affects later visually guided behavior. Kittens had one eye closed for different ages and durations; after they were at least a year old, behavioral tests compared the previously deprived eye with the normally exposed eye. The tasks distinguished visual acuity, guidance of paw placement and discrimination of patterns.^[ref]

The age and duration of monocular deprivation mattered. Longer early deprivation produced more severe impairments, while deprivation beginning at four months or later caused no detectable lasting effect in these tests. This is evidence for age-dependent neuroplasticity under the specified experimental conditions, rather than a universal timetable for every form of learning.

An especially revealing comparison concerned recovery: closing the normally exposed eye improved some behavioral use of the deprived eye, without a corresponding increase in its ability to drive recorded visual cortex cells. Behavioral performance and one physiological readout therefore did not change in lockstep. The experiment shows why measurements at different levels of organization should be related through evidence rather than assumed equivalent.^[ref]

A sensitive period is an interval of development during which a particular kind of experience has an unusually strong effect on subsequent behaviour or nervous-system organization. It is specific to a function and experimental outcome: susceptibility of visual acuity and susceptibility of a recorded neuron's response need not have identical schedules. Such timing is one aspect of neuroplasticity.

A revealing experiment varied both the age at which a kitten's eyelids were sutured shut on one side and the duration of closure. The investigators later compared vision through the two eyes using graded behavioural tasks. Starting deprivation at different ages produced different lasting effects, and longer early deprivation affected more demanding tasks. This separates the timing of experience from merely its total absence.^[ref]

The same paper reported some improvement when subsequent experience forced use of the previously deprived eye, including in one older animal. A sensitive period can therefore coexist with later improvement. Its experimental meaning depends on the measured behaviour, the timing and duration of deprivation, and the opportunity for recovery.^[ref]