The Myth of Critical Periods
by Paul Harris, OD, FCOVD, FACBO
copyright © 1994
Conventional wisdom in the fields of neurology and development has led to
general acceptance of the fact that "critical periods" exist in the
development of certain aspects of the human visual process. However, as more
understanding of the supporting physiology of the neural networks that comprise
the brain emerges, the length of time that makes up critical period must be
extended to coincide with the only true critical period there is -- death.
Originally, the concept of a critical period emerged from observations of
young chickens and ducks. Specific insights into critical periods of vision
development came from studies involving single cell recording from the visual
cortexes of cats and monkeys. Many of these studies involved altering the
distribution of the cells that reacted to monocular versus binocular stimulation
after varying periods of single lid suturing and various types of stimulation
programs after unsuturing the lid. The early studies, which were prior to 1976,
all supported the view that, in order for the underlying neurology to develop
the ability to handle binocular information, the eye->optic nerve->optic
radiation complex required binocular stimulation during a certain critical
period. In the late 1970s, a series of studies done by Hubel & Wiesel,
Blakemore & Pettigrew, and Hirsch & Spinelli began to find evidence of a
repression/de-repression model. This means that there are processes that
regulate periods of increased and decreased plasticity. Separate studies by
Cynader and Pettigrew state that attention systems, originating in the reticular
activating system (RAS), appear to be the regulator of the critical period
phenomena.
Separate reversal studies by vonNoorden, vanSluyters and Pettigrew all
showed that complete reversals of experience deprivation could be overcome well
after established critical periods had come and gone . Several of these studies
centered on the locus coeruleus, a center in the RAS which has as many synapses
in the visual cortex (V1, formerly called Broadmans area 17) as come from the
primary optic radiations.
Singer, Tretter, & Yinon and a separate study done by Kasamatsu showed
that stimulation of the locus coeruleus after the critical period opened up a
new period of increased or even hyper plasticity. According to Dr. Steven
Cool, the midbrain reticular activating system mechanism associated with
selective attention/arousal levels sets a "gate" on the primary
sensory information input system and helps to determine what and how much
information will get through. The primary visual information, the retino->geniculo->cortical
system, is modulated by the selective attention system.
The majority of this research was done specifically in the area of
binocularity. In my opinion, this was because these cells were more easily
accessible to the researchers than the cells directly associated with high
spatial frequency information in single channel (amblyopia). It goes without
saying that if the work was done with lid suturing techniques, that the cats and
monkeys were made amblyopic. It also goes without saying that, if totally
normal binocularity was restored, a pre-requisite to this level of binocularity
is normal information throughout the full spectrum of spatial frequencies, which
would mean that no amblyopia would be present.
© Copyright 1996/BABO
