This paper describes a study of six people who all have lower visual field impairments, but as tests show, they are all very different. Below is our brief explanation of some of the key points and processes, and at the end a link to the full paper.
In the vast majority of cases, a lower visual field impairment is caused by damage to the optic radiations (one on each side) as they pass through the posterior parietal lobes on their journey taking the visual information to the occipital lobes.
Think about what you are looking at, right now, these words and what else you can see around them, maybe a computer screen, or a window or wall with pictures. Now think about what is in the lower area of your visual field, maybe a table or desk you are working on. That visual information has passed through your eyes and is being processed...
...it travels along the optic nerve to a junction point called the optic chiasm, where it splits into two optic tracts. At the end of each optic tract is a relay station called the lateral geniculate body, and here the visual information is relayed into the optic radiations, which are bundles of fibres within each side of the brain. The optic radiations take the visual information to the occipital lobes.
You can follow that journey of visual information looking at both the diagram above, and the image of the visual pathways in the brain.
In the diagram above, the two upper curved white arrows (marked with the red arrows) show the route of the optic radiations that take the information from the lower visual field into the upper part of the occipital lobes. When there is a lack of the lower visual field, the cause is likely to be (but not always) due to injury or lack of function in both posterior parietal lobes (where the adjacent dorsal streams can also be affected), because the optic radiations transmitting the information from the lower visual field pass through the posterior parietal lobes, and can be altered where there is damage.
The lower curved white arrow (just above the word 'Temporal Lobe') shows the route of the right optic radiation carrying the information of the upper left visual field as it passes through the right temporal lobe, on its way to the lower part of the right occipital lobe (there would be a matching opposite arrow on the left, if it were visible from the other side).
This paper is fascinating, because it covers six patients who all were born prematurely and all have white matter damage of immaturity (abbreviated to WMDI in the paper) in both posterior parietal lobes, which is the result of conditions including periventricular leukomalacia and hydrocephalus.
You may have sat for a visual field test when having a routine eye examination at the optician's. Typically one eye will be covered, and you will be asked to look straight ahead into a machine looking at a target light, while little dots of light flash out to the side, and you pressed a button when you saw the light flash. The information provided a map of your visual field.
In this paper, where the small dot out to the side, was not seen, rather than mark the area as without vision, the researchers increased the size of the visual target to the side, to see if the patient may see something if it was bigger.
Figures 1 to 6 are the different patients results.
One image of each patient shows the visual fields recorded using the Goldmann visual field record sheet, like the one below, which maps out the visual field for each eye using a pinpoint of light moving in from outside until it is seen.
Goldmann Visual Field Record Sheet
The central red ring plots the positions in which a dim light moving in from the outside was seen, while the blue ring plots the positions for the brightest light (with green and orange in between). These lines are similar to contours. A contour is a plot of points of equal height. These lines are plots of equal sensitivity to light. The black circles and radial lines, are just like the lines of latitude and longitude on a map if you look down from the north pole. They provide the locations of where the isoptres are located.
Note the range of lower visual field impairments across the patients, they are all different, some slight, some considerable.
The images in the top right corner of Figures 1-6 in the paper show another way of plotting the visual field, but this time how clearly the person sees is marked by smaller and larger circles. The bigger the circle, the bigger the stimulus needed to be, to be seen. Where there is a solid black circle, this marks an area with no visual response.
So, if you look at figure 5, that patient has larger circles in the bottom right corner of his visual field, that means the he sees less clearly than is typical in this area - but this is not due to a problem in the occipital lobes where we associate visual acuity issues. Before the visual information has even reached the occipital lobes it has already been blocked to varying degrees by lack of functioning nerve fibres, akin to very fine wires (as explained above).
This paper shows what we have observed in many children and adults with lower visual field impairments... that there is a two level spectrum when it comes to lower visual field impairments, with ranges affecting:
Let us imagine we are all on the banks of the River Thames looking at the Tower of London (image below)
If mildly affected, the detail of this view may be slightly reduced (below):
Some of the detail in the lower visual field has been lost, but the picture still makes sense.
With a more severe lower visual field impairment, more of the detail is reduced, making it harder to understand the image (below):
Here (above), if you did not know where you were, would you know there was a river if you didn't know to look down to find it?
And at the most severe level, nothing is seen (below)
For some the visual impairment may just affect the very bottom part of their lower visual field and be unnoticed, even by the person themselves.
The peripheral lower visual field is not tested in standard visual field tests, and an affected person may not have many difficulties, but occasionally trip up or down pavements, and possibly find certain sports with low balls like hockey and football difficult.
When walking, when looking straight ahead, you are likely to be aware of the tip of your toe of the foot you are putting forward in your lower visual field - if not, you may possibly have a lower visual field impairment (a simple test is demonstrated on this video).
A lower visual field impairment can cover as much as the entire lower visual field, from the midline or eyeline down, or it may be only very peripheral, or there may be blurring of the peripheral lower vision.
This can lead to...
We have referred to CVI as a spectrum of spectrums, and it seems that this paper shows that the lower visual field impairment, is in itself a spectrum of spectrums, ranging in extent and severity for each affected person, with consequential visual and frontal attentional issues. Does that make CVI a spectrum of spectrums of spectrums? Increasingly, we think so!
Paper: Visual field defects in prematurely born patients with white matter damage of immaturity: a multipleu2010case study, Jacobson L, Flodmark O & Martin L, Acta Ophthalmologica Scandinavica, May 2006
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