7.4. Critical Periods of Visual Development.

Sensory experiences during early development are critical for perception. All sensory systems need to be stimulated early in life in order for them to develop normally (Cisneros-Franco et al., 2020). All of our senses are stimulated to some extent when we are still in the womb, with the exception of vision. Hence, when we are born, our visual system is less developed than our other sensory systems. The exact timing of critical periods varies across our senses. However, permanent deficits can arise if normal stimulation does not occur during a critical period. For example, if a newborn has visual problems that prevent them from seeing normally in the first year of their life they have permanent difficulties with face perception for the rest of their lives (Pascalis et al., 2020). Similarly, young children with eye problems often develop a condition called amblyopia (lazy eye), where they are unable to see fine details, such as small letters. Amblyopia is permanent if the eye problem is not corrected before the age of 8 years old. Therefore, it is important to detect and resolve any sensory issues, e.g., eye and ear problems as early as possible (Cisneros-Franco et al., 2020; Pascalis et al., 2020).

Each aspect of vision has its own critical period. For example:

• Face Perception: Disruption of normal vision in the first year of life can lead to lifelong difficulties with face recognition
• Stereovision: If the eyes are misaligned (strabismus) during early development, depth perception may never develop properly
• Visual Acuity: Problems with vision before age 8 can lead to amblyopia (lazy eye), a condition where fine detail vision never fully develops

Amblyopia

Amblyopia can develop in two main ways:

1. Strabismus (Misaligned Eyes)

• The eyes don’t point in the same direction due to muscle imbalance
• The brain receives conflicting images from the two eyes
• To avoid confusion, the brain suppresses input from the weaker eye
• Without treatment, vision in the suppressed eye remains permanently impaired

2. Anisometropia (Unequal Focus)

• One eye needs a much stronger prescription than the other
• The brain receives one clear and one blurry image
• Over time, the brain learns to ignore signals from the blurrier eye
• This results in monocular rather than binocular vision

Treatment of Amblyopia

The main goal of amblyopia treatment is to strengthen vision in the weaker eye and restore proper binocular vision. Treatment typically involves two main approaches:

1. Treating the Underlying Cause

• For strabismus: Surgery may be needed to realign the eyes
• For anisometropia: Corrective lenses (glasses or contacts) to equalize focus between the eyes
• Any other vision problems, like cataracts, must be corrected

2. Forcing Use of the Weaker Eye

• Patching (occlusion therapy): The stronger eye is covered with a patch, forcing the brain to process input from the weaker eye (Figure 7.12).
  • Patching may be required for several hours per day
  • Treatment duration varies from months to years depending on severity
• Atropine drops: An alternative to patching where drops blur vision in the good eye
  • More convenient than patching but not suitable for all cases

The earlier treatment begins, the better the chances of success. While the traditional view was that treatment after age 8 would be ineffective, recent research suggests some improvement may be possible even in older children and adults. However, outcomes are still best when treatment occurs during the critical period of visual development.