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The visual pathway from the eye to the brain

Trace vision from the retina to the visual cortex and learn about visual field loss in kids with CVI.

Abstract graphic of child and a brain with an orange line showing the visual pathway from the eye to the back of the brain

This is the third article in our five-part series about the visual system. 

Following the early visual pathway

Think of the visual pathway like a highway, with neurons as the cars and your vision as the driver. Ideally, the pathway is smooth and efficient, with predictable curves and directions leading from your optic nerve to your occipital lobe’s primary visual cortex. But sometimes that pathway is interrupted—and the driver has to adjust.

Graphic of visual pathway looking down onto the brain: eyes, optic nerves, optic tract, LGN, optic radiations, visual cortex
 
Here are the key points along the route:
Image of anatomy of eye, optic nerve, LGN, and visual cortex
If your child has interruption or damage to certain areas along this pathway, it could cause reduced visual acuity and contrast vision, and visual field loss. 

Optic nerve conditions 
Kids with CVI are often found to have abnormal optic nerves in one or both eyes. Some children with CVI have optic nerve hypoplasia (ONH), a congenital condition in which the optic nerve is underdeveloped, or optic nerve atrophy (ONA), mild to severe damage to the optic nerve. Some parents say that their child’s eye report notes small, pale optic nerves.  The impact on visual function is wide-ranging and unique to each child, but common issues include:

What is a visual field?
When our eyes are in a fixed position straight ahead, the area that we see is called a “visual field.” This visual field changes with age. Newborns usually have a visual field of just 30 degrees. But by two months old, it expands up to 90 degrees; by four months, it’s 180 degrees; and by toddlerhood, it’s similar to older kids and adults.

Visual field loss happens when a portion of the visual field is absent. Children with CVI can have a range of visual field deficits, depending on where the damage or interruption happens in the brain. For example, damage on the right side of the primary visual cortex in the occipital lobe may cause left homonymous hemianopia, or field loss in the left side of each eye (see image below). 

Visual field loss terms: Hemianopia, one half of visual field; quadrantanopia, one fourth of field; homonymous, same side of vertical midline in both eyes

Lower visual field loss is common in kids with CVI. Often, it occurs from damage or interruption to the upper part of the occipital lobe. Clinicians and researchers theorize that the upper occipital lobe can be more susceptible to damage; this could be why we see more lower visual field loss in children with brain-based visual impairments. CVI Scotland has great images that give us an idea of what lower visual field loss is like for kids with CVI. 

Image of visual pathway looking down on a middle cross section of the brain. Title: examples of visual field loss

In the image above: Examples of visual field loss when fibers of certain areas of the visual pathway are interrupted 

  1. Midline of optic chiasm→ hemianopia on the outer half of eye;
  2. Optic radiations in left side→uneven right homonymous hemianopia
  3. Full right optic tract, LGN→left homonymous hemianopia
  4. All left optic radiations→total right homonymous hemianopia
  5. Left visual cortex→ right homonymous hemianopia with central fields spared
  6. Right visual cortex→left homonymous hemianopia with central field and a sliver of peripheral field spared
  7. Right side beginning of visual cortex →left peripheral loss 

Adapted from Remington (2012) Clinical anatomy and physiology of the visual system

It’s important to remember that crowding, motion, clutter, and noise affect your child’s ability to visually attend in their visual fields. That’s why their visual field test, in a quiet doctor’s office, might be normal while their experiences in the real world might be quite different. Be sure to talk to your doctor and educational team about issues you observe at home and out in the community.

You may notice that your child tilts their head a certain direction when looking at something. And this might happen at a specific time of day, when presented with a new object, or in unfamiliar environments. Your child may seem to look in one direction and ignore the other or prefer to walk on the right or left side of a walkway. When navigating, your child may look down to avoid clutter and movement. 

Supports for visual field loss

If your child has a visual field loss, there are many ways to support them. Parents and teachers can:

To evaluate how your child uses their unique vision in a variety of settings, it’s important to obtain a comprehensive functional vision assessment by a teacher of the visually impaired (TVI). It should include a functional vision assessment (FVA), CVI-specific assessments, and a learning media assessment (LMA).

Since visual field loss is so affected by clutter, noise, crowding, and new environments, an orientation and mobility (O&M) specialist should also complete environmental assessments to evaluate the environment’s impact on your child’s learning and to determine which supports will help your child to move through their world safely. 

Go to the next article in this series: Higher-order visual pathways and the CVI brain. We will dive deeper into the higher-order processing visual pathways, the dorsal and ventral streams, to better understand how our brain recognizes and interprets our visual world and what happens if these visual pathways are damaged or interrupted.


References: 

Banich, M. T., & Compton, R. J. (2018). Cognitive Neuroscience. Cambridge, United Kingdom: Cambridge University Press.

Dutton, G.N. (n.d.). Cerebral Visual Impairment: Working Within and Around the Limitations of Vision. Publication for Royal Hospital for Sick Children, Glasgow, Scotland.

Merrit, Paul. (2017). Vision: From Retina to LGN [YouTube Video]. Retrieved from https://www.youtube.com/watch?v=zYa9TAePOi4&t=27s

Remington, L. A. (2012). Visual Pathway. In Clinical anatomy and physiology of the visual system (233-253). St. Louis: Elsevier/Butterworth-Heinemann.

Zihl, J., & Dutton, G. N. (2015). Cerebral Visual Impairment in Children: Visuoperceptive and visuocognitieve disorders. Wien: Springer.

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