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Lessons

6b - Blindsight

Posted by Helen St Clair Tracy in Level 6 - CVIs Visual Field Impairments
Published: 28/01/2020, 12:44am | Updated: 24/02/2020, 7:41am

Video Link: https://vimeo.com/392468914

This lesson picks up from the information in the lessons on Reflex Vision (lesson 3h). Please review this lesson if necessary. We also refer to the different areas of the brain throughout this lesson, if needed please review Level 1 The Brain.

We all have a form of vision that we do not know about. It is automatic. It makes us blink before a fly hits our eye. Even before we have consciously seen it!

The word blindsight is used to describe being able to respond to a movement despite not being able to see due to brain injury. Blindsight is the result of an automatic movement detection systems which is still working.

Blindsight is a non-conscious rapid visual reflex ability to respond, triggered by movement

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Blindsight is processed in a part of the brain called the thalamus.

It starts its journey through the eyes.

People who are eye blind, cannot have blindsight.

Being a non-conscious reflex means that those who have blindsight have no control over it. It is something our brains have been pre-programmed to do automatically.

There are a number of patterns of blindsight.

  • Occipital lobe injury on one side causes loss of vision to the opposite side (hemianopia lesson 6a). Some people with hemianopia are able to react to movement on their blind side.
  • While those with occipital injury on both sides can develop complete loss of conscious vision. Yet some people with this type of loss of vision can respond to movement on both sides.

Movement is not just things moving around you. If you move, you create movement of everything you see too. So for some people with cerebral blindness, rocking in a rocking chair can create the movement needed to activate blindsight.

For most of us, when rocking in a chair, the image our occipital lobes creates, with the added movement (see lesson 3e) is enough for us to see clearly around us.

So what does blindsight look like if you can't see it?

Let us imagine you have a right hemianopia (lesson 6a), and are sat looking out across a lake with hills in the background.

Everything visually on the right side is absent, apart from where there is movement. Below is a suggested interpretation of the experience of blindsight, as described by affected adults. Can you work out what is on the right?

Gordon N DuttonVideo Link: https://vimeo.com/391954378

There are different forms of blindsight

  • The child with cerebral palsy and cerebral blindness meaning there is no conscious vision, may occasionally respond to a movement, sometimes more so on one side or the other. Such a child may also respond to a smile this is called affective blindsight (lesson 3h)
  • The child or adult with hemianopia may be able to correctly guess when a laser beam spot is moving, or detect fleeting movement on their blind side, more often than not.
  • Safe adult drivers have sometimes been found by their optometrist to have hemianopia they never knew about! They have the ability to rapidly detect movement and to look and see what it is due to.
  • Rare adults and young children we have seen, who have lost all conscious vision due to blocked blood vessels to both occipital lobes, have still been able to move around without collision and to accurately pick things up.

Key, is that the non-conscious reflex described above can in some, evolve into a form of useful conscious vision. This useful conscious blindsight vision is called the Riddoch Syndrome, as explained in lesson 3h. The vision may not be good enough to locate something small, but can be enough to navigate large obstacles. In those who have the higher visual processing difficulty, of reduced visual attention due to simultanagnostic vision (lesson 3g), but their occipital lobes are working typically, movement can often help to attract visual attention. This movement is not blindsight though. Here, the occipital lobes have done their job and created the image, and so blindsight is not needed.

And so we have the beginning of a sort of hierarchy of vision, where each element can be affected (or in the case of reflex vision - revealed as blindsight) by cerebral visual impairment

hierarchy of visionhierarchy of vision

At the base, is our primitive reflex vision. This is not blindsight as such, because it is present in all of us (the word blindsight relates to those with CVI). It is our protective reflex vision that manifests as blindsight in those who have lost occipital lobe function.

Next up, the picture created in our occipital lobes - this is when, for those with occipital lobe damage, the primitive reflex vision we all have can becomes evident as blindsight and in some can be used to good advantage.

In a child who has no conscious vision due to occipital lobe injury and requires spoon feeding, moving the spoon of food across from the side towards the mouth can create blindsight and some children learn to recognise the movement and open their mouth for the food at the appropriate time.

Moving the spoon of food across from the side towards the mouth can create blindsight.Moving the spoon of food across from the side towards the mouth can create blindsight.

A few people who cannot see because they have lost occipital lobe function on both sides, can move freely, and respond to moving things. In these people (including the safe adult drivers), it is the middle temporal lobes that are still present, which are working.

Blindsight has been explained to these able adults and children who have gone on to effectively use it, to help navigate obstacles, helping with their safe independent mobility.

If a person has combined occipital and higher processing visual difficulties, then the blindsight may evolve, to visually support where there is an absence of vision due to the occipital lobes. This can happen spontaneously, but if the affected person is able, teaching them about their own blindsight and how it can help them will make the most difference.

Checklist

Before moving on to the next lesson, please check you understand:

  • 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
  • Blindsight does not support higher visual processing deficiencies where the occipital lobes are performing typically.

Next lesson: Level 6c CVIs Visual Field Impairments - Hemi Inattention and Neglect

Further reading is not necessary to proceed, but if interested you may find the following enjoyable:

What is CVI? Blindsight

Blindsight Newspaper Feature

Homonymous Hemianopia (New Vision in Absent Visual Field) Paper. Where blindsight was developed to create conscious vision, called the Riddoch Phenomenon.

Child with 'More than Blindsight' Paper

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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.