Scotopic sensitivity syndrome is based on the theory that some individuals have hypersensitive photoreceptors, visual pathways, and/or brain systems that react inappropriately to some wavelengths of light. Vision occurs when photons are detected by the retina, initiating a biochemical process affecting the visual pathways and deep structures of the brain. A growing number of researchers are taking an interest in the view that inappropriate biochemical processing has the potential to cause physiological and/or visual perceptual problems. Many of these problems are grouped together under the label "scotopic sensitivity syndrome".
In simple terms, the theory is that some signals from the eye are not getting to the brain intact and / or on time. Although the eye might be functioning correctly, the brain receives what is like a double exposed picture where the location of items is confused. The brain tries to filter out the bad information and so the conscious mind receives a reconstructed image. That image may be of the items moving (the brain constantly changing its best guess of what is there), blurred outcomes (inability to form a view of what is there), gaps in wrong spots, and a variety of other minor errors. There may also be exhaustion (from the mental effort to unscramble) and sore eyes (from the eyes constantly seeking extra data to aid the process) The problem is worst where different colours do not all give a similar outcome. In nature you get a lot of consistent data but on a man made item (e.g. paper) there might only be limited colour sets. i.e. The condition does not generate practical problems where there is lots of redundant data for the brain to use. The pragmatic response by Irlen was not to try to fix the problem but to avoid it. By filtering out the light most likely to generate problem signals to the brain, she was able to improve the likelihood that the brain will correctly distinguish between good and bad information. It also seems likely that in some individuals, over time the brain learns which colours are the problem items and improves its ability to reconstruct an accurate image.