In this lesson, we are describing a form of blindness caused when the occipital lobes do not create a picture, usually due to damage, most commonly from a stroke.
The terms blindness and cortical blindness are used broadly in a way that can cover a range of visual disabilities, and so we have avoided using both these terms.
Please review our lesson 6a where we explain hemianopia. The complete absence of vision we are explaining in this lesson is essentially a double hemianopia, so both sides are affected, leaving no vision.
What does this look like compared to typical vision? That is a very difficult question to answer.
Describing the absence of anything is always going to be tricky.
Looking straight ahead, what do your surroundings behind you look like? This question is really to show you that you can't describe something that is not there.
Normally in our lessons we try to show images of what a visual impairment looks like compared to typical vision, but we can't show you what complete absence of vision looks like.
As we explained in lesson 3b, the visual information from the eyes travels first to an area in the occipital lobes called V1, where the detail of the picture is created. Then colour is added, and movement and the many other elements that create our visual world as described through our previous lessons.
If there is no picture at all, then there is nothing for all the other elements that create our visual world to be added to, so there is no vision...or is there?
For a long time it was though that damage to the V1 area of the brain on both sides (bilateral) meant no picture and no vision, until a man called George Riddoch made some fascinating discoveries.
These are explained in lesson 6b on blindsight. A number of people who have had this type of complete absence of vision somehow have been able to see movement, but without knowing. Hence the term 'blind-sight'. This is because a different sort of vision is created in another part of the brain when there is movement. George Riddoch has his name attached to the conscious perception of the movement. This is well described here...
There was a case of a child who suffered a brain injury as a baby that resulted in complete absence of vision... yet as he grew up, he seemed to gain pretty good sight, including seeing colours...even though there was no picture created in his mind to 'colour in'. His brain had incredibly created new pathways to connect different working parts of the brain, and made a different but very effective alternative system of vision. It was only when the boy was struggling with reading that tests showed V1 was completely dysfunctional.
Such cases show us how remarkable the human brain is.
Anosognosia / Anton Syndrome
Anosognosia means without knowledge of a disease.
There are cases where people have this complete absence of vision, but are unaware they can't see.
The first known case dates back over two thousand years when the Roman Philosopher Seneca describes a slave in his household, who on becoming suddenly blind 'denied her illness and argued irrationally about room darkness, constantly asking attendants to change her quarters'. https://pubmed.ncbi.nlm.nih.gov/29979335/
This unawareness of the absence of vision is called Anton Syndrome. It is extremely rare. Different definitions describe patients who are adamant they can see, when the evidence clearly shows they have a complete absence of vision. This is different from blindsight and Riddoch Syndrome (lesson 6b) where people can evidence conscious vision for the moving world - whether it is they or their surroundings that are moving, by correctly describing what they see or avoiding obstacles as they move.
Complete occipital absence of vision is most commonly due to a stroke or traumatic head injury. It is quite rare, and preservation of a small central island of vision probably due to a bit of surviving visual brain that may be supplied by a different blood vessel (visual field constriction lesson 6f), or absence of just once quarter of the visual field (quadrantanopia lesson 6e) or half the visual field (hemianopia lesson 6a) are much more common.
Some adults who are totally blind owing to profound injury to both occipital lobes remain able to use their 'blindsight' to detect the emotions of others through their facial expressions. This is likely because the automatic reflex brain that protects us kicks in at birth to allow infants to respond to their mothers, another fundamental protective reflex. We know of some children who sometimes return smiles despite not having any detectable vision due to occipital lobe injury, probably for the same reason. See links below if you are interested to learn more about affective blindsight.
Support for complete absence of vision is typically similar to the support for people who are completely eye blind.
Before moving on please check that you have understood:
Next Lesson: Level 7a CVIs Movement & Dorsal - Dyskinetopsia
Further reading is not necessary to proceed, but if interested you may find the following enjoyable:
Blindsight Newspaper Feature. Our introduction to a story on 'the blind woman who saw rain', explaining a little more about blindsight and Riddoch Syndrome.
Child with 'More than Blindsight' Paper. This is the child we described in the lesson, explaining his extraordinary story in more detail.
Lesson 3h Reflex Vision
What Is CVI? Blindsight Explaining what blindsight is and how it is experiences by people differently.
Newsletter 21 Working Together & Sharing
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