Sight Examinations and Assessments
Coloured Overlays Assessments and Treatments
(Duration 20 minutes)
What is a coloured overlay?
A Coloured Overlay is a coloured, but transparent, plastic sheet that is placed on top of reading or copy text. Its purpose is to minimise the discomfort of glare from a white page, and to mask the visual perceptual effects that can make reading unpleasant, or even impossible, for some individuals.
It is thought that the colour in some way alters the balance between central and peripheral retinal processing putting it in a more favourable equilibrium. This can improve clarity, eye movement control, ease of concentration and comfort.
This is not known for certain how the coloured overlays work but some interesting theories have evolved from neuro-physiological research. Click here for more information.
It should be remembered that the coloured overlay is simply a temporary tool to aid the progress of reading. The symptoms masked by the overlay are generally the result of an underlying deficiency in functional control over the visual system. Improvements are likely with lens and vision therapy and the overlay may then no longer be required. In the short term, however, while the underlying source is being treated, symptoms can be reduced to some extent by the use of Coloured Overlays.
In order to learn to read, write and spell it is essential that a child sees what is actually on the page in front of him. Each word sound must obviously always match up with the same sequence of letters in the same order. The strings of letters in a line must break in the correct place to form the individual words. Strange as it may seem, for many children who struggle to read, this is not the case.
Some children experience discomfort from the glare effect of black print on a stark white page. This can be so extreme as to distract attention from the content of the page, or to deter an individual from attempting to read. The glare can make it difficult to pick the print out from background or cause the spacing between the words to join together to create a river effect that stands out more vividly than the actual text. Letters may wobble, move, merge together and overlap, fade, and float.
Each time the child sees a particular word it may have a different letter arrangement making it extremely difficult to pick up a visual 'picture' of that word for instant recall when spelling or a fast sight-sound match on reading. Imagine how confusing and frustrating that must be. The source of these symptoms is generally a delayed development in functional control over the visual system and will be identified and treated through Behavioural Optometry Assessment, Lens Therapy and Vision Therapy.
How do coloured overlays work?
Central Visual Space
Light from the outside world falls onto the retina at the back of the eye. When we wish to visually inspect, or attend to, an object the eye is rotates until light from this object falls upon the relatively small macula area of the retina. The object now occupies central visual space . It is the centre of our attention.
Receptors in the central retina respond best to small, stationary, high contrast objects in good lighting conditions. Central space is, therefore, best suited to the discrimination of small, black on white, well-lit detail, such as letters and numbers. Once the eye has come to rest following an eye movement, these receptors will respond continuously to such detail for as long as the eye remains fixated upon the object.
Peripheral Visual Space
The retina does not have the same function all over. Receptors in the peripheral retina do not 'see' so clearly. They do not need to because they are not required for detailed analysis. Peripheral visual space forms a frame of reference, much like a three-dimensional map or picture, within which to centre or position the object of primary interest. From within peripheral space we can select another area of space for attention. Peripheral space gives us co-ordinates of how to get there, how to rotate the eye so that a new area of space can occupy central space. Receptors in the peripheral retina respond best to large, low contrast, moving objects in poorer lighting conditions and to changes in direction. Peripheral receptors respond only transiently, for a short spell, when changes to the overall 'picture' occur. This is why we are so distracted by an object that we see moving from the 'corner of our eye'.
Integration of Space
When the eye begins to move the receptors in peripheral retina send out signals that inhibit the response in the central receptors. This wipes out the signal from the previous fixation. When the eye comes to rest there will be a new response triggered by the new object of attention. This creates a definite time sequence of events, the contents of one fixation following on in time from the one before. This avoids jumbling or spatat avoids confusion. If the central and peripheral receptors are out of balance and do not integrate efficiently theinhibitory response from peripheral receptors will be too strong or too weak to extinguish information between fixations resulting in spatial confusion.
How do the coloured overlays help?
Altering the colour of the background will reduce the difference in tone between the figure (letters) and the ground (spaces). It will reduce the contrast of the target. This will reduce the response of central receptors which respond best to high contrast targets. The colour itself will also finely adjust the central response as only central receptors respond to colour. It is possible that the specific colour and the saturation (or concentration) of that colour makes it possible to control the level of adjustment to the central response so that central and peripheral responses can be placed in a better balance. This will, in turn, result in more efficient integration of central and peripheral space.