Visual Stress is said to affect 20% of the population – 13 million people in UK 40% of dyslexics.
Visual Stress is a visual processing disorder due to sensitivity to light and a believed hyper- excitability of visual cortex (visual brain)
- Words blur/go out of focus
- Letters move shimmer or shake
- Glare from page
- Coloured halos around words
Evolution: Sparse Code and Nature
Humans are adapted to view natural images, which share a particular characteristics which include spatial frequency and luminosity. Humans have a natural sparse code for this. Imagine a picture of a lake and mountains, beach, garden – is that comfortable to look at? Now imagine a stripy black n white shirt or a zebra – is that comfortable?
The brain is only 2% of body mass but requires 20% of ALL blood oxygen to fire 5-10% at any one time. Uncomfortable images cause a higher amount of neurons to start firing in the visual brain and this creates visual cortex excitability. Brain excitability leads to increased blood oxygen being diverted to the brain to cope with the additional neurones which are firing to try and make sense of what the visual system is seeing. This is inefficient and the body/ brain cannot sustain this and causes a response like pain – it makes the person to look away thus removing the stimuli and to calm the brain, to stop the additional blood oxygen – which is very valuable energy resource. Some people are genetically/naturally more sensitive and more susceptible to hyper excitability of the visual brain.
How does this link to reading text?
English text is made of striped patterns which are mainly written in black on white paper, to our brain look like stripes – not natural!
Filters reduce discomfort by reducing contrast and filtering out a particular wavelength of light that causes the neurons to fire unnecessarily. But it has to be correct colour, saturation and brightness. A person may be sensitive to a particular wavelength of light and this can explain why a tint stabilise the visual brain.
Binocular vision and reading
Lot of symptoms of visual stress are the same as if the eyes do not focus or/and work together properly. This could be due to eye muscle weakness so it’s VERY important to have full eye test and detailed binocular vision assessment. Unstable binocular vision can cause fixation to swap between eyes and print will appear to move so this can be fixed with prescription lenses/prisms to stabilise the vision at near. Then we can proceed to Colorimetry assessment which goes through 6000 colour combinations to find the correct tint (as shown in the picture above)
What is involved in a Full Assessment?
- Full eye examination to establish prescription – to check if short-sightedness or long-sightedness needs to be corrected (may be covered by NHS if applies).
- Eye dominance – to establish fixating eye the ‘locks’ onto the word when reading.
- Saccadic eye movement – ability to jump accurately from letter to letter or word to word.
- Binocular stability – to ensure that the eyes work together aligned at the near point of reading. This includes assessment of accommodation – focusing ability and convergence – ability to pull eyes together to make test single.
- Light sensitivity/Visual Stress – colourimetry assessment with Intuitive colorimeter to establish a tint to stabilise visual brain activity (hypersensitivity of neurones).
- Clinical eye tracker – to assess eye movements during reading and statistical analysis to monitor reading performance and treatment progress.