Paper: Neuroplasticity in cerebral visual impairment (CVI): Assessing functional vision and the neurophysiological correlates of dorsal stream dysfunction. Full details and link at bottom of page.
Dorsal stream dysfunction (DSD) is the name of a group of cerebral visual impairments that arise when there is atypical processing in a part of the brain called the posterior parietal lobes.
DSD can affect how accurately you can reach for things, and make finding and locating things difficult. For some, one side of their vision may be more affected than the other. This website has many pages dedicated to explaining DSD, including our Lessons, particularly lesson 6c (Hemi Inattention & Neglect) and Level 7.
DSD refers to a higher visual processing difficulty which is a complicated visual process, causing 'impairment of higher order visual spatial processing', as it is called in this paper.
"The ability of the brain to form and reorganise synaptic connections..." that sounds complicated!
Think of something you have learned to do well, like ride a bike or write or draw. When you started to learn this new skill, the brain started to make new connections. So when you pick up a pen, the way you hold it, the balance, the pressure you apply, and the movements to form the letters, are all guided by your many new memories and brain connections, different ones built up over time, working together to help you do the things you do.
At its most basic level, as we do something repeatedly, and get better, it becomes easier, that is because of ongoing brain development through neuroplasticity.
This paper looks at the neuroplastic adaptive changes in the brain relating to those with DSD. The first point to note is how little research there has been in this area, even though...
CVI is now the leading individual cause of congenital visual impairment in developed countries, including the United States and the United Kingdom, with prevalence estimates ranging between 20-40% of reported visual impairments
A key word in this quote is reported. We think there may be a considerable volume of unreported cases where DSD is not picked up in children, possibly because:
Because there is so much that can be done to help each affected person if their visual impairments are identified, and catered for.
Where one part of a brain processes atypically, it can have a knock-on effect upon many other areas. Problems that start off being small during younger years can grow and become more difficult if not identified and appropriately accommodated - and that means
We explain this in a little more detail in our page The Lists (for Profiling Individuals with CVIs).
So, how do we learn what a brain is doing differently?
One of the ways is through imaging, and the images used in this paper show how powerful they are not just to learn, but to convey how incredibly debilitating CVI can be. For a fuller explanation of the images and link to work by the same team, please see the 3D Rotating CVI Brain Pathways film, where you can see how thin the pathway between the posterior parietal lobes and the frontal lobes can be.
The paper begins with a very clear but brief history of the world of CVI. It starts with the brain's adaptations to ocular blindness through neuroplastic changes, and how much we have learnt from these changes, to help affected people.
The paper notes in relation to these findings:
Arguably, the most striking finding has been the observation that the occipital visual cortex (normally associated with visual processing) is functionally recruited for the processing of nonvisual sensory information and cognitive tasks such as memory and language
This shows how amazing the human brain is. With typical vision, the visual cortex makes the picture we can see, but when eye blind, this significant part of the brain is re-programmed to do something different. Section 3 Investigating brain structural connectivity in ocular blindness and CVI, goes into a lot more detail about these findings.
So, what about CVI?
Well, one of the challenges, beyond how complex and unique to each individual CVI is, is that it seems to vary not only from person to person, but also for each individual person, sometimes from moment to moment.
The paper features images of some tests such as spotting the school principal in a hall, and how having lots of people crowding the scene makes it much harder. You can look at an example of the test here:
Another test involves finding a toy in a box of toys, and finds out how difficult this becomes when there are many toys as compared with only a few. You can look at an example of the test here:
What these tests clearly and consistently show is that for people affected by CVI, finding things is more difficult where there are more different people or objects to process at once.
So that tells us something is difficult, but why?
This is where imaging of the bundles of nerves, and connections in the brain, that serve vision can help explain some of the challenges due to CVI.
Looking at the paper, scroll down to Figure 2. This shows the images of the brains of three different people. The left one is from someone with no visual impairments at all (called a control). The right image is from someone with CVI, due to dorsal stream dysfunction. This view is of the left side of the head and the upper streams of colour (mostly green) follow one of the super highways of the brain called the superior longitudinal fasciculus (pronounced fa-sick-you-lus). Or, in simple language, a bundle of fibres travelling lengthwise through the brain starting at the occipital lobe, going through the parietal lobe and on to the frontal lobe.
Sometimes it is referred to as the SLF, and the first part of it likely contains the dorsal stream from the occipital lobes to the posterior (back of) parietal lobes.
Look at the front end of the trail of colour on the SLF of the right image and compare it with the control image. That is the frontal lobe, your control centre where you understand your surroundings to make informed decisions.
The comparison between the two is staggering. See how few coloured lines (fibres) there are, and come to realise that so much less visual information about the scene can be made available to the frontal lobes to process.
CVI is linked with learning delays and behavioural difficulties, and we need to learn more about it. Neuroimaging helps us to understand why the brain of someone with CVI is not working typically as compared to someone who does not have CVI.
Ok, so how do these very complex atypically processing brains compensate for these difficulties?
We don't know...yet!
Which is why more research and understanding is needed into how the brain might find ways of doing things differently due to the challenges caused by CVI.
Then, we will be able to use this information to learn what we need to do to help people with CVI.
What do we need to do improve outcomes for this increasing population?
This is a conversation, a big, growing international conversation.
This paper shows what neuroimaging brings to this conversation and how important it is to help us learn more.
For a complicated subject, the paper is an enjoyable read with clear explanations.
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