A "critical period" in the sense of medicine refers to a period of time that the neural circuitry of the brain develops, strengthens, and then becomes relatively unchanged. This window of development is highly dependent on experience - the more a child uses a skill, the more fine-tuned the neuronal pathways become. At a cellular level, neurons are busy creating new synapses and undergoing a process called pruning to enhance the neural pathway associated with skills such as vision, language, and hearing. As a child ages, the ability to change these neural pathways is increasingly harder. The concept of the critical period is often applied to visual development - here the idea is that the brain only develops certain visual skill up to a certain age, and after reaching that age, the thought was that it is very difficult or impossible to change how the brain uses visual input from both eyes.
Early research on critical periods in vision development was performed by scientists Hubel and Wiesel. They were able to show that cats deprived of light (vision) in one eye significantly influenced the cellular development of the visual pathway (in the brain, not the eye itself). In young animals, neurons from the normal eye dominated visual input compared to neurons from the deprived eye (these are monocular neurons, as the input is from just one eye) and neurons dedicated to combined visual input from both eyes (these are called binocular neurons). Older animals were much less affected by a poor visual experience, suggesting that their visual system was already developed. Repeating experiments such as this and altering the amount of time and level of visual degradation, as well as performing the experiment on animals of various ages allowed researchers to better identify when periods of visual development occur, and when recovery was unlikely to occur. This became known as a critical period.
Experiments such as these have been repeated on multiple animals and have formed the basis for the amblyopia (lazy eye) treatment of patching. The idea of patching was to force the brain to primarily use the weaker eye with the hope of "equaling" the balance of input from the weak eye compared to the normal eye. Further, the concept of the critical period "closing" (neural development essentially stopping around adolescence) led to many older lazy eye patients being told that treatment for their lazy eyes was impossible.
So is this all bad news? In short, no. The eye care and scientific communities have spent an incredible amount of time researching how to best treat lazy eye, especially in older patients. So what have we learned thus far?
This may be due to an unequal glasses prescription, eye turn, or obstruction of light (deprivation). Regardless, amblyopia doesn't develop in older patients - it happens during brain and visual system development.
Eye examinations are now recommended by the time a child is only 6 months old
This is perhaps one of the most critical understandings as it helps us understand the importance of early detection. Eye examinations are now recommended by the time a child is only 6 months old. The American Optometric Association recommends "The child's first eye and vision examination should be scheduled at 6 months of age (or sooner if signs or symptoms warrant)." If this early exam is considered normal, the next exam should be scheduled by age 3. Early examination allows the detection of abnormalities that may affect visual development.
Evidence has shown that the critical period is not as firm as once believed. It is possible to learn or gain function at older ages, though it is a bit more difficult compared to a child that is still developing. This is better described as a period of recovery. During the period of recovery, the brain is still capable of making changes, though the changes require more intense and targeted visual experiences. Brain plasticity (also called neuroplasticity) is the ability of the brain to change and mold and prune neural pathways.
Vision therapy focuses on the concept of neuroplasticity by using perceptual learning - this is essentially targeted, practiced tasks that help the brain form new pathways (or improve on weak existing pathways) to improve a visual skill such as visual acuity, fusion, or stereopsis. This may be done by working single-eye skills (monocular) or recently (and now more popular) working on skills that require the use of both eyes together (binocular). Binocular training may use red/green or red/blue glasses, polarized glasses, or virtual reality. This is sometimes referred to as "dichoptic" or "anti-suppression" training if a patient's visual system ignores visual input from one eye (called cortical suppression or just suppression). Later phases of treatment focus on stereopsis (3D) skill training. Levi and colleagues suggest that stereopsis training (which is done with 3D glasses or virtual reality) may even be more beneficial that 2D training of one or both eyes, especially for patients with an eye turn (strabismus).
Neuroplasticity is great news for older patients that may not have been diagnosed until later in life or those that had limited intervention and may not have been fully treated for a binocular vision disorder when younger. A group called the Pediatric Eye Disease Investigator Group (PEDIG) was formed to further research on eye disorders that affect children - such as amblyopia and strabismus. This is a collaborative group of eye care providers (optometrists, ophthalmologists, and vision scientists) that study the effects of various forms of treatment for disorders of binocular vision. PEDIG's research continues to shape how vision disorders are treated in the US and abroad.
Early animal studies helped the visual community better understand how the visual system develops and the importance of early detection of problems such as lazy eye and strabismus. Studies that focus on visual development and more specifically rehabilitation of visual disorders will continue to shape how visual disorders such as lazy eye are treated.
Ribic & Biederer. Emerging Roles of Synapse Organizers in the Regulation of Critical Periods. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6745111/
AOA Clinical Practice Guidlines - Pediatric Eye and Vision Examination. https://www.aoa.org/documents/optometrists/CPG-2.pdf
Levi, Knill, and Bavelier - Stereopsis and Amblyopia: A Mini-Review. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4519435/