Lessons

6g - Complete Absence of Vision

Posted by Helen St Clair Tracy in Level 6 - CVIs Visual Field Impairments
Published: 26/08/2021, 12:20am | Updated: 06/09/2021, 3:40pm

Video Link: https://vimeo.com/589918677

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...

Video Link: https://www.youtu.be/9ABQ-U6V0tY

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.

Affective Blindsight
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.
Checklist:

Before moving on please check that you have understood:

  • The part of the brain responsible for this complete absence of vision.
  • The difference between blindsight and Anton Syndrome.

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.

Affective Blindsight:

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

Level 6 Full Checklist

You should understand...

  • What the word hemianopia means
  • What part of the brain is responsible for hemianopia
  • That one side of the brain affects the opposite side of the image you see
  • That those affected by hemianopia may not always be aware that they have an absent visual field
  • Where in the brain blindsight is processed
  • That movement is needed to activate blindsight
  • That blindsight supports sight loss due to the occipital lobes
  • That blindsight uses visual information received through the eyes
  • The area of the brain affected by hemi inattention and visual neglect
  • That hemi inattention is different from frontal attention
  • That hemi inattention is different from hemianopia
  • Two important difference between hemianopia and hemi inattention
  • What body related / body centric means
  • That visual attention is body related
  • The difference between body related movement of vision and head / eye related movement of vision
  • Body centric movements and turns
  • The difference between things not being seen and things not being there
  • The part of the visual process responsible for causing lower visual field impairments
  • How lower visual field impairments affect the visual process
  • How visual attention is added, and how it can be altered when there is a lower visual field impairment.
  • The range of severity of lower visual field impairment, both in terms of how much of the lower visual field is affected, and whether the altered visual information is very mildly to severely different or absent.
  • How walking may be affected by lower visual field impairment
  • That there are considerable safety issues for some with lower visual field impairment
  • What the term quadrant means in relation the visual field
  • That the visual impairment in quadrantanopia can be a complete absence of vision or reduced vision in the affected quadrant
  • What superior and inferior mean in relation to quadrantanopia
  • Why the temporal and posterior parietal lobes can affect the visual field leading to quadrantanopia
  • Which parts of the occipital lobes correspond with which quadrants in the visual field
  • How a person can have vision in only one quadrant
  • The importance of identifying vision if present in only one quadrant
  • That a key reason for plotting the visual field is to find and best use the intact vision
  • What visual field constriction means
  • A cause of visual field constriction
  • Why visual field constriction is sometimes called 'central sparing'
  • Other conditions creating an 'Island of Vision'
  • The importance of the correct diagnosis (in relation to visual field constriction).
  • The part of the brain responsible for complete absence of vision
  • The difference between blindsight and Anton Syndrome

Congratulations on completing Level 6!Congratulations on completing Level 6!

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At CVI Scotland we are devoted to helping people understand cerebral visual impairments, and together working towards developing the understanding of this complex condition.