Critical
Periods
Appropriate visual experience is essential to the development
of most visual functions. For some functions, it is critical
that this experience occur during a particular phase or stage
of an infant's development. If the infant does not receive
the appropriate stimulation during this "critical
period", it may be difficult, even impossible, to
develop that function later. Due to ongoing development of
the retina and visual brain, children remain susceptible to
the adverse effects of visual deprivation until about 7 to
8 years of age.
Critical
periods are often studied using selective rearing.
In these studies, animals receive or are deprived of particular
types of stimulation at different times in their development.
This allows vision scientists to determine, experimentally,
the length of critical periods and the effects of stimulus
deprivation or enrichment during them.
Considerable
knowledge about critical periods has also been gained from
the study of the effects of disorders on visual development.
For example, infants are often born with astigmatism that
is usually corrected naturally during the first six months.
If this does not occur within the first 2 years, the child
may suffer a lifelong impairment in the ability to resolve
targets of certain orientations (i.e. meridional amblyopia).
Similarly, a baby with infantile cataracts will be
deprived of well-defined spatial stimuli essential for developing
the cortical "feature detectors" needed for good
spatial vision. If left untreated for the first five or six
months, the infant could be impaired for life.
The critical period for the development of binocular function
begins at 6 months and peaks from 1 to 2 years. While the
most important part of the critical period for acuity is from
9 months to 2 years, the effects of deprivation can extend
to about 8 years of age. The critical period for the binocular
information needed for by ocular dominance begins at birth,
but is particularly crucial between 3 and 5 months. The development
of binocular depth perception (i.e. steroposis) is
most dependent on the availability of retinal disparity information
from about 3 1/2 and 6 months of age.
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