Visual stress (sometimes called 'Meares-Irlen Syndrome' or 'Scotopic Sensitivity Syndrome') is the experience of unpleasant visual symptoms when reading. Symptoms may include illusions of shape, movement and colour in the text, distortions of the print, loss of print clarity, and general visual irritation. Visual stress can also cause sore eyes, headaches, frequent loss of place when reading and impaired comprehension.
Figure 1: British Dyslexia Association [24th July 2012]
It is believed that about 15-20% of the population will show a degree of visual stress (Wilkins, 2003) and that the condition is caused by the brain and/or eye incorrectly processing/interpreting what the eye is seeing (i.e. it is neurological). These people will have increased difficulty reading and studying.
Visual stress is not the same as dyslexia but dyslexia seems to make the symptoms of visual stress much worse, and therefore it is particularly important that people with diagnosed dyslexia are screened for visual stress.
ssOverlay (the ss stands for Scotopic Sensitivity) places a coloured overlay on the screen. The colour and transparency levels are adjustable to suit the user.
Another program is T-Bar, a coloured bar which you can either drag around the screen or lock to your mouse. It can have ruled lines or not, depending on your preference. The colour can be chosen from pre-defined options, and then tweaked by altering the red, green or blue sliders to get the perfect colour; the transparency level can also be adjusted to suit the individual user.
A third program is RapidSet which allows quick and easy changing of the background and font colours, without having to go through the Screen Properties dialogs.
A fourth program which can also be useful is Vu-Bar which is an on-screen, slotted ruler which can help individuals who find it difficult to read large amounts of text by separating it into one or more lines at a time.
All these programs are free (open source) software which you can download and install on your home computer.
Wilkins, A. (2003) Reading through colour: how coloured filters can reduce reading difficulty, eye strain, and headaches. Chichester: Wiley.