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.
Here are the key points along the route:
- Retina: This is your eye’s on-ramp. It’s made up of two types of photoreceptors, rods and cones, which are neurons that detect light. Rods are on the periphery of your retina. They process low levels of light and motion; cones, mainly in the center of your retina, distinguish color and detail.
- Optic nerve: This cranial nerve sends visual information from your retina to your brain. It consists of more than 1 million nerve fibers. It’s a busy highway!
- Optic chiasm: Think of this like an intersection. It’s where the nerves cross over, allowing your primary cortex to get information from both eyes. Like a busy intersection, this is where visual information is sorted and broken up into component parts for more processing. The left field of vision is processed by the right side (or cerebral hemisphere), and vice versa, moving along an optic tract toward the thalamus.
- LGN (lateral geniculate nucleus) in the thalamus: Think of this as a central tollbooth where all sensory information stops to enter. From here, visual information is organized from the retina and sent off to the primary cortex. Just like a highway, there’s one on either side of the thalamus.
- Optic radiations: We’re almost at the end of our journey! The optic radiations are axons—or nerve fibers—that convey information to your visual cortex.
- Visual cortex: This is where images received from your retina begin to get processed. The visual cortex has six layers and is the very beginning of your brain’s process of interpreting and recognizing what you see. Within these layers, depth perception is processed, and form, color, and motion are perceived.
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:
- Light sensitivity (photophobia)
- Reduced acuity and contrast vision
- Visual field loss
- Reduced depth perception
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).
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.
In the image above: Examples of visual field loss when fibers of certain areas of the visual pathway are interrupted
- Midline of optic chiasm→ hemianopia on the outer half of eye;
- Optic radiations in left side→uneven right homonymous hemianopia
- Full right optic tract, LGN→left homonymous hemianopia
- All left optic radiations→total right homonymous hemianopia
- Left visual cortex→ right homonymous hemianopia with central fields spared
- Right visual cortex→left homonymous hemianopia with central field and a sliver of peripheral field spared
- 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:
- Present materials in their most accessible visual field. For example, if their most accessible visual field is the upper right, use a slant board, All-in-One board, or some type of adjustable stand to place pictures, objects, or a tablet so they can better visually engage with the activity.
- Draw attention to objects using verbal directives or sound. For example, tapping the object on a table.
- Use CVI supports such as motion, light, and color to attract attention in a weaker visual field. For example, colored tape at the top of the railing can help alert your child to a drop-off, stairs, or ramp; slowly moving an object in a steady position in a certain visual field can support visual attention; and using task lighting can bring attention to a target.
- Many children with CVI use white canes due to visual field loss, especially lower visual field loss, to support safe navigation. Additionally, people with CVI continue to report that clutter, crowding, noise, busy environments, and visual fatigue reduce their visual field and/or cause blurred vision.
- Teach the child to scan the full environment using wider head movements.
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.
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.