Article
Higher-order visual pathways and the CVI brain
Exploring the dorsal and ventral streams, how they work, and what happens if there’s an interruption anywhere along the way
This is the fourth article in our five-part series about the visual system.
What are the higher-order visual pathways?
Now it’s time to dive deeper into higher-order visual pathways, which consist of the dorsal and ventral streams. This is how your child processes what objects are and where they exist in space. We’ll discuss what each pathway does, how it works, and what happens if there’s an interruption anywhere along the way.
Our visual cortex relies on two processing streams that originate in the back of the brain. The occipital-parietal visual area refers to the beginning of the dorsal or “where” pathway. Here, your child’s brain assesses where things are in space, like a ball zooming across a field or a puppy running toward a lap.
Dorsal damage can cause:
- Trouble with spatial perception and perception of complex movement
- Trouble with spatial orientation and navigation
- Impaired spatial guidance of motor activities (saccadic and pursuit eye movements; reaching, grasping and pointing; walking over steps; navigating crowds and obstacles)
- Trouble with seeing and distinguishing between multiple objects at the same time
The occipital-temporal visual area refers to the ventral or “what” pathway, where your child’s brain recognizes objects and shapes. The temporal areas provide your child’s long-term visual memories, helping them to recall what they’ve seen before and attach meaning to it. (For example, once you’ve seen a baseball, you know what it is next time you come across one!)
Ventral damage can cause:
- Impairments in contrast sensitivity, form and color vision, and depth perception
- Impairments in object and face perception and route-finding
Picture a crisp, juicy pear. Two things happen when you look at it: The ventral stream helps you see its color, shape, size, and texture. The dorsal stream lets you know where the pear actually is—high on a counter or tempting you on the plate.
New research on dorsal and ventral streams
For a long time, researchers thought that the dorsal and ventral streams operated separately. Recently, neuroscientists have begun to view them holistically based on new research.
Lotfi Merabet, a clinician-scientist at the Laboratory for Visual Neuroplasticity at Massachusetts Eye and Ear and Harvard Medical School, frames it this way: “Think of the streams as two highways that have multiple connectors between the two, or as two rivers that separate. There are many tributaries that exist between the two.”
As the brain develops, more “tributaries” form as information gets exchanged between the two streams. As more information gets exchanged, the system becomes increasingly efficient at identifying and interacting with objects in the visual world. The dorsal stream never acts alone, nor does the ventral stream. Your child needs both streams to work together and in an integrated way in order to fully leverage their visual system.
Understanding the dorsal stream
The dorsal stream helps us size up a visual scene in conjunction with other senses, like hearing. Our brain is always taking in fresh information about our surroundings, helping us plan our movements and contextualizing our environment. When we have an accurate picture of the world around us, we can maintain a rapid, accurate gaze to hone in on our chosen targets in the visual scene.
The dorsal stream does more than situate us and place objects in context, though. Researchers consider the dorsal visual pathway the “action” pathway—once we know what we see, we know what to do next (for instance, reach/grab that pear!).
Research shows dorsal stream vulnerability is common in people with brain-based visual impairments like CVI. In children with CVI, it’s possible that a full image reaches their primary visual cortex in the occipital lobe, but when it reaches the parietal lobe, their brain has difficulty dealing with the spatial information related to the image.
People who have interruption to the occipital or parietal lobe could have:
- Difficulty with visuospatial attention, which is how we focus and filter the visual information world around us
- Lack of or loss of being able to process simultaneous visual information; this is what researchers call the “totality of a scene”
- Difficulty shifting attention from one element of a scene to another because of difficulty processing too many pieces of visual information at once
- Impaired visual search; difficulty figuring out what to, and what not to, look at when there is competing visual stimuli
- Difficulty with sustained attention
- Difficulty with selective attention—in other words, taking in important stimuli but filtering out the rest (such as focusing on that pear, not the glass of milk next to it)
- Difficulty with spatial awareness and spatial relationships
Want to learn more? Check out this CVI Scotland simulation of dorsal stream dysfunction. You’ll notice that when there’s more visual information to take in and process, the field of vision becomes overwhelmed and constricted.
Why? Think about that pear. If it’s on a plate with peanut butter and a fork, a child with CVI might become overstimulated. Their dorsal stream can’t process all of the information. Your child’s window of vision could get narrower and narrower. When there’s less to process—just a pear on an uncluttered plate—it’s easier to see the big picture.
Understanding the ventral stream
Here’s where your child’s brain recognizes objects, patterns, textures, and shapes. It’s what helps them choose a pear in a bowl of fruit; pick out a circle block in a line of squares; distinguish between a puppy and a cat curled up on the sofa; or pick you out of a crowd at the playground.
Think of the ventral stream as your child’s visual library, aiding in recognition and memory. Much of this processing is handled by the inferior-temporal cortex.
After images get sorted in the first layer of the primary visual cortex (V1), they move on to more specialized areas:
- V2: Here’s where your child begins to process an object’s edges and contours.
- V3: This plays a role in color sensitivity.
- V4: Shape, depth, color, and lightness are processed here.
- Inferior-temporal cortex: Here’s where complex objects such as body parts, patterns, places, objects, words, and faces are processed, even when their position, angle, or size changes (for example, this is how your child would be able to recognize you, even with a haircut!). This ability is called perceptual or form constancy. For example, we might have an abstract visual memory of a pear, which we can match with the many ways pears might appear in reality: on its side, with different colors, in dark lighting, in photographs, or abstract drawings with shading or lines.
Kids with CVI can have trouble recognizing shapes and objects when either the left or right temporal lobe is damaged. They might have trouble distinguishing objects and different types of animals, or recognizing faces or perceiving facial expressions.
With temporal or occipital-temporal damage, several disorders can occur, including:
- Visual object agnosia, or the inability to recognize common objects.
- Prosopagnosia, or the inability to recognize faces. This happens when the right temporal lobe is damaged.
- Topographic agnosia, which affects your child’s ability to orient themselves in space or follow a route. This also happens with right temporal lobe damage.
Interestingly, ventral stream dysfunction usually happens in conjunction with dorsal stream dysfunction—even though dorsal stream dysfunction can happen on its own.
Check out this presentation on Ventral Stream Functions in CVI: Object and Face Perception by Dr. Corrina Bauer.
Remember, the visual system is complex and highly interconnected. Damage to the dorsal and ventral streams results in difficulty with visual attention and recognition, making it difficult to fully access the visual world. Children with CVI have unique visual behaviors and require CVI-specific assessments to evaluate their functional vision and access to the environment.
Want to learn more about CVI and the brain? Check out CVI and the brain: Q&A with Dr. Lotfi Merabet and The science behind visual fatigue and CVI.
Go to the final article in this series: The importance of neuroplasticity. Our wondrous brains adapt and rewire to respond to our environment, and that’s great news for all of us, including kids with CVI.
References:
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