Critical Period in Vision Therapy

The human brain possesses an incredible capacity for adaptation and learning. Nowhere is this more evident than in the field of vision therapy, where the concept of the critical period plays a vital role. In this blog post, we will explore the significance of the critical period in vision therapy. Furthermore, we will find out how it influences the treatment outcomes.

What is Critical Period?

The critical period refers to a specific window of time in early childhood when the brain is most receptive to developing visual skills. In other words, critical period refers to how the brain shapes according to the environmental impact. During this period, the brain is exceptionally receptive to sensory input, allowing it to establish and refine neural connections necessary for visual processing.

In this phase, environment has such a strong impact on how the brain shapes like no other time. Our brain evaluates sensory data, and based on that, decides how our body will respond to signals from the environment.

The brain not only uses our senses to make judgements, but it can also change and learn from previous experiences, a phenomenon known as plasticity.

When does Critical Period in Visual Development Happen?

Studies show there are different critical periods for different visual functions during the development of the visual system. What is more, some time frames for certain visual functions are even unknown. For example, there are three periods in the development of visual acuity and in the development of ocular dominance. During the first 3 to 5 years of life, acuity develops from less than 20/200 to near 20/20.

Role in Vision Therapy

During the critical period, the brain is more receptive to changes and interventions aimed at correcting vision problems. The visual system’s plasticity allows for the reorganization of neural circuits, facilitating improved communication between the affected eye and the brain. By addressing amblyopia during this developmental window, there is a greater chance of achieving substantial visual improvement.

Critical Period and Amblyopia

 In the study from 2018, a group of children with amblyopia were all prescribed a patch to wear for a certain period of time. The weaker eye’s visual acuity was reassessed and compared across age groups after wearing the patch. Even youngsters who had their weaker eye patched at the same age had varying degrees of recovery.

Significantly, each child in this study wore their patch for the exact same amount of time. One would anticipate a similar degree of progress in people of the same age if everyone had the same important period. The conclusion that the critical period for vision is highly individualistic is largely supported, however, by the fact that people of the same age demonstrate varying levels of visual progress.

What does it Mean for the Treatment of Lazy Eye?

The critical period for vision typically lasts until about age seven. After this age, monocular deprivation becomes a significantly less effective treatment for amblyopia, although the exact time when the critical period closes differs between each person (Holmes & Levi, 2018). That’s why binocular treatment approach is so important! 

What if Amblyopia is untreated?

If amblyopia goes untreated or is not adequately managed during the critical period, the window of opportunity for optimal recovery gradually diminishes. As the brain matures and neural connections become more established, it becomes increasingly challenging to rewire the visual system and restore normal visual function. Research suggests that after the critical period, the visual system becomes less responsive to treatment, and the potential for significant improvements decreases.

Critical Period is not as Rigid as Believed

Vision therapy employs neuroplasticity, i.e. tasks that assist the brain develop new pathways or improve weak ones. With these exercises, visual skills improve, such as visual acuity, or stereopsis.

This can be achieved by practicing single-eye skills (monocular). Alternatively, it can be achieved by practicing abilities that require the use of both eyes simultaneously (binocular), which has been increasingly popular. Tools that make an individual use both eyes together include red/green or red/blue spectacles, polarized glasses, or virtual reality.

Hope for Adults: Brain Plasticity

While the critical period primarily emphasizes the benefits of early intervention, it doesn’t imply that adults cannot benefit from vision therapy. However, due to reduced neuroplasticity, achieving similar levels of improvement may require more time, effort, and patience. The age-related decline in plasticity necessitates the implementation of more intensive and targeted therapies tailored to individual needs.

Conclusion

The critical period in vision therapy highlights the remarkable plasticity of the human brain during early childhood. By taking advantage of this period, vision therapy interventions can have a profound impact on visual development and functional abilities. While the critical period serves as a reminder of the importance of early intervention, it doesn’t diminish the potential for improvement in adults.

Frequently Asked Questions

Source:
1. Brown, K., & Grimaud, J. (2021). Critical Periods in Vision: Why my Friend Was Wearing an Eyepatch.
2. Holmes JM, Levi DM. Treatment of amblyopia as a function of age. Vis Neurosci. 2018 Jan;35:E015. doi: 10.1017/S0952523817000220. PMID: 29905125.