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5c - Reduced Contrast Sensitivity

Posted by Helen St Clair Tracy in Level 5 - CVIs Basic (Occipital)
Published: 28/01/2020, 12:31am | Updated: 24/02/2020, 7:34am

Video Link: https://vimeo.com/391942696

This lesson follows on from Lesson 3c, please review this lesson if needed.

Contrast sensitivity is a measure, as a percentage, of how clearly shades of grey are seen.

Typically, most people can see a difference of about 1 or 2%

The measurement can be a useful guide as to how much contrast is needed for differences in contrast to be seen, but like everything with the brain, contrast sensitivity can vary, depending on many factors particularly the lighting of the environment.

There are tests to measure contrast sensitivity, although from people we meet we know that often actual measurements are not given. Instead advice like "requires high contrast' might be given.

Reduced contrast can affect how well things are recognised, including where there is high contrast, for example toys...

High-contrast owl toy, as it would be seen by someone with typical vision (left image) and mild to severe reduced contrast sensitivity.High-contrast owl toy, as it would be seen by someone with typical vision (left image) and mild to severe reduced contrast sensitivity.SIGHT-SIM www.sight-sim.co.uk

Reduced contrast sensitivity can also make recognising people by their faces more difficult.

How the same person would look to a person with different levels of reduced contrast sensitivity.How the same person would look to a person with different levels of reduced contrast sensitivity.SIGHT-SIM www.sight-sim.co.uk

When we meet people we know, they may look different, for example a different hairstyle, or in different light, so with reduced contrast sensitivity, it may be even more difficult to recognise the same person.

SIGHT-SIM www.sight-sim.co.uk

Targeted Support

Helping someone with reduced contrast sensitivity does not just mean picking something bright, bold, shiny or glowing.

Optimum contrast is black against white (or white against black).

Keeping images simple reduces the work load on the brain, so the image can be seen more clearly.

Visual Behaviours

Reduced contrast sensitivity can be extremely hidden, especially if from birth where the person will have no sense that they see things differently to everyone else. Often, things aren't missed altogether, just not completely seen or understood, so other reasons that are not related to vision are considered, like learning difficulties. Here's one reason why it is missed:

sight test cardsight test card

Sight tests are usually black on white, which is the highest level of contrast. If the person has typical visual acuity (Lesson 5b) then their reduced contrast sensitivity will not be picked up.

Reduced contrast sensitivity is commonly one of a combination of CVIs. We will look at the combined effects of different combinations of CVIs in the following level (6).

Reduced contrast sensitivity can cause or contribute towards some difficulties learning, for example, with diagrams.

In diagrams, there can be different coloured fonts on different coloured backgrounds, and only part of the text may be clearly accessible.

In the diagram above, the words 'greenhouse gasses' in black text on a dark blue background, would not be clearly visible to someone with only mildly reduced contrast sensitivity. This diagram is also an example of self-referencing (see below). The person who made this, assumes everyone who will be looking at it can see contrast as clearly as they can. So, this diagram for the person with reduced contrast sensitivity is incomplete, because the light text against the light background and the dark text against the dark background would be invisible. Sometimes the information missed might not be critical, at other times it might be, but the lack of available information makes learning more difficult.

This can lead to inconsistencies in learning for a person with reduced contrast sensitivity because some of the information is not available to see.

Things can also seem less interesting for a person with reduced contrast sensitivity, because they are not seen as clearly (in addition to the simulated images in this lesson, there are many more on this website, link at the bottom of the page). This can affect understanding and enjoyment of learning across all subjects and can affect all ages.

Reduced contrast sensitivity can also cause or contribute to difficulties affecting independence.

Imagine you are out and about, and approach a step or kerb in the pavement, like the one below:

Step in pavement kerb as seen by a person with typical vision (left) and reduced levels of contrast sensitivity.Step in pavement kerb as seen by a person with typical vision (left) and reduced levels of contrast sensitivity.SIGHT-SIM www.sight-sim.co.uk

Does this step go up or down?

How deep is it?

There is lighter paving to mark where the edge is of the pavement is, which is often not the case, but it is still difficult.

Have you ever accidentally missed a step and either tripped up or fallen down? It is something most of us have done occasionally. It is embarrassing but can also be dangerous, particularly to trip into a busy road. This may mean a person with reduced contrast sensitivity may not want to go to places they don't know very well, or may want to link arms with someone when out and about.

In social situations a person with reduced contrast sensitivity may not be confident because they struggle to both recognise faces and recognise facial expressions. This may mean they choose to stay with people they know and can become anxious when out and about.

Self Referencing

Believing that someone with CVI who has reduced contrast sensitivity sees the way you do can result in:

  • Assuming they can understand what you mean by your facial expressions (when this can be difficult because the face has low contrast).
  • Creating material, particularly for learning, with insufficient contrast.

Checklist:

Before you move onto the next lesson, please check you understand:

  • Luminosity, in terms of white and black.
  • What contrast sensitivity is a measure of.
  • How reduced contrast sensitivity can affect social confidence.
  • Why reduced contrast sensitivity may mean safety issues when out and about.
  • When reduced contrast sensitivity can affect learning and reading.
  • Why reduced contrast sensitivity can be missed in testing.

Next Lesson Level 5d CVIs Basic (Occipital) - Colours

Further reading is not necessary to move on to the next level, but if interested you may enjoy the following:

What Is CVI? Colour & Contrast Sensitivity

What Is CVI? The Technical Stuff, Contrast Sensitivity

Simulated Images

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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.