Scotopic sensitivity syndrome
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Scotopic sensitivity syndrome also known as Irlen Syndrome refers to visual perceptual disorder(s) affecting primarily reading and writing based activities. It can however have wider impact on memory and concentration. Its existence is not recognized as a homogenous condition by the American Academy of Pediatrics or the American Optometric Association, although it is recognised by an overwhelming body of expert opinion including the British Psychological Society, and the leading educational and cognitive psychological bodies in all industrialised countries. There is agreement that visual perceptual disorders exist and can be debilitating, but there is controversy over the pathology of the condition.
Similar symptoms were separately described by two people working individually, each unaware of the other's work. In the early 1980s New Zealand teacher Olive Meares described the visual distortions some individuals reported when reading from white paper, while American therapist Professor Helen Irlen wrote a paper about the use of coloured overlays aiding the reading abilities of some people. Irlen, who was the first to systematically define the condition, named her findings "scotopic sensitivity", though in the discussions and debates over the following years some often referred to it incorrectly as Meares-Irlen syndrome. There remains to this day, stark controversy over whether non Irlen certified Meares-Irlen Syndrome and the original and genuine Irlen Syndrome are the same conditions. Irlen Syndrome for example, seems to include a broader array of symptoms, including severe variants of the core condition. Basic testing for scotopic sensitivity was tried by optometrists, opticians, and orthoptists in UK hospitals, and by optometrists and opticians in private practice employing a technique that used the Intuitive Colorimeter, developed under Medical Research Council license. An alternative approach to correct Irlen Syndrome was also tried by Orthoscopics franchise in the UK, with wide color coverage and tints manufactured by Hoyato match. Other commercial organisations have produced sets of therapeutic tints, although most have not received scientific evaluation. 
A Field Requiring Study 
The disorders have been studied in a very few institutions, including the former Applied Psychology Unit, Cambridge University in England, and in the case of Meares Irlen Syndrome, Visual Unit at Glasgow Caledonian University in Scotland. As of 2012 the Visual Stress Unit offers non-commercial diagnostic and therapeutic services to individuals, and provides advice to the Scottish National Health Service. The small amount of peer-reviewed literature on the topic suggests that much is unknown about the cause of these disorders, ranging from the 2011 study in the journal of the American Academy of Pediatrics Irlen Colored Overlays Do not Alleviate Reading Difficulties  and the 2012 study in the journal Brain Topography A Functional Neuroimaging Case Study of Meares–Irlen Syndrome. The first, purely in relation to Meares Irlen Syndrome, finds that there is no evidence for one of the fundamental claims of therapeutic benefit, the second which focused primarily on Irlen Syndrome did however find compelling evidence of unique brain function linked to the syndrome.
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 (paper, for example) there might be only a limited colour set: 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.
Scotopic sensitivity syndrome Symptoms 
One or more of these symptoms may be related to the condition:
- Headaches (including migraine)
- Nausea, including visually related motion sickness
- Problems with depth perception (catching balls, judging distance, etc.)
- Restricted field of view and span of recognition
- Discomfort with busy patterns, particularly stripes ("visual stress" and "pattern glare")
- Discomfort with extreme conditions of bright/dark contrast (i.e. backlighting)
- Discomfort or difficulty reading (reading involves busy patterns, particularly stripes. People with strong symptoms of the syndrome find it very difficult to read black text on white paper, particularly when the paper is slightly shiny.)
- Text that appears to move (rise, fall, swirl, shake, etc.)
- Losing text content and only seeing rivers of white through the text
- Words moving together becoming one unrecognizable word
- Attention and concentration difficulties
- Seeing the part and losing the whole
- Epileptic seizure related to strobing or pattern glare
The use of tinted lenses in glasses and coloured overlay sheets has been prescribed by many doctors; however, the efficacy of such treatment is questionable. It has been felt to be efficient treatment by some, and inappropriate by others, because more conventional treatments are sometimes more appropriate.
The College of Optometry (UK) has specified guidelines for optometrists who use the colorimeter system. A society for colored lens prescribers  has been established to provide a list of eye-care practitioners with expertise in the provision of colored lenses for the treatment of visual stress.
The Promethean Trust, a Norwich-based charity for dyslexic children, has found that the use of a cursor has eliminated the need for colored overlays or lenses. The cursor is simply a piece of card or plastic, approximately the size of a business card, with a notch cut out of one corner. The reader (or the remedial teacher) uses this to track print from left to right, and at the same time the card prevents the eyes from wandering ahead.
Irlen Method 
The Irlen Method is the original and current system which uses Irlen coloured overlays and tinted lenses in the form of glass or contact lenses. The established method, practiced worldwide since the 1980s is intended to reduce or eliminate perceptual processing errors; the resultant retiming of visual signals in the brain improves the reading difficulties associated with scotopic sensitivity syndrome.
Use of the genuine Irlen Method requires intensive individual training, resulting in an issued certificate signed by founder Helen Irlen. Individuals should always ask to see proof of valid certification.
Irlen Screener 
"Irlen Screeners are certified to administer the first testing session and determine whether an individual will benefit from further evaluation for Irlen Spectral Filters." 
Irlen Diagnosticians 
"Irlen Diagnosticians are certified to administer both testing sessions. During the initial testing session, an individual is screened to determine whether wearing Irlen Spectral Filters will make a difference and the amount of improvement for reading and other academic activities. In addition, during this session 12 other areas will be evaluated and recommendations made. Only Irlen Diagnosticians are certified to test and determine your customized spectral filter worn as Irlen glasses or contact lenses and conduct yearly rechecks." 
Intuitive Colorimeter 
Developed by Arnold Wilkins, Ph.D., University of Essex, England, an alternative system for the identification of tint to reduce symptoms.
Skepticism relating to scotopic sensitivity syndrome's exact pathology, has evolved on several fronts:
- Whether it exists as a distinct, predictably identifiable disease with a reasonable pathophysiological mechanism, or whether instead a range of symptoms from other conditions are not being placed under this convenient heading;
- Whether it is causally or incidentally related to dyslexia, autism, or other conditions; and
- Whether existing methods of scotopic sensitivity syndrome treatment are appropriate and effective.
A 2009 report by The American Academy of Pediatrics (AAP) does not believe that there is conclusive scientific evidence for the use of colored lenses (one treatment used to relieve symptoms of Scotopic Sensitivity Syndrome) although it does acknowledge anecdotal evidence to the contrary. When discussing its scientific basis, the AAP mentions that "[t]he method used to select the lens or filter color has been highly variable,the color selection has also shown considerable variability,and the test-retest consistency has been poor" (p. 843)
The association of scotopic sensitivity syndrome and dyslexia has been challenged by many authors in both the optometric and ophthalmologic communities. Furthermore, many special education departments within universities also challenge the validity of coloured lenses as an effective treatment for the condition. As outlined by the Macquarie University Special Education Centre:
In a joint statement, The American Academy of Ophthalmology, American Academy of Pediatrics, American Association for Pediatric Ophthalmology and Strabismus and American Association of Certified Orthoptists firmly repudiated the use of lenses, stating that there was no scientific evidence supporting their use. The expense of such treatment is unwarranted and may divert resources from evidence-based treatment.
Critics claim that the symptoms of those with Scotopic Sensitivity Syndrome are related to already known visual perceptual and neurological disorders. According to a statement released by the American Optometric Association in 2004:
There is evidence that the underlying symptoms associated with specifically Meares Irlen Syndrome, are related to identifiable vision anomalies, e.g., accommodative, binocular, and ocular motor dysfunctions, in many patients seeking help from colored lenses. Furthermore, such conditions return to normal function when appropriately treated with lenses, prisms, or vision therapy. When patients exhibiting Meares Irlen Syndrome were treated with vision therapy, their symptoms were relieved. These patients were no longer classified as exhibiting this syndrome, and therefore did not demonstrate a need for the colored overlays or tinted lenses.
As outlined by Hyatt, Stephenson and Carter (2009):
In 1990, the Journal of Learning Disabilities published a special issue that provided intensive coverage of Irlen lenses. In the preface to the issue, the editor in chief, Wiederholt, noted that each of the studies had serious theoretical, medical/physical, and methodological ﬂaws, but were published along with articles critiquing the studies to provide readers with an overview of the Irlen procedures as well as guidance for conducting quality research studies. He, along with Hoyt (1990), Parker (1990), and Solan (1990), noted that these initial studies by Blaskey et al. (1990), O’Connor, Sofo, Kendall, and Olsen (1990), and Robinson and Conway (1990) failed to support the treatment validity of colored overlays.
Critics assert that the term "scotopic sensitivity" is a misnomer given that the symptoms of "Scotopic Sensitivity Syndrome" reportedly occur during photopic conditions. The term "Scotopic Sensitivity" seems dubious, given that scotopic vision is the vision of the eye under low light conditions and as such vision is provided by rod cells on the retina, which have little or any role in colour vision, it does not make sense that a coloured lens or coloured overlay would have any impact upon "Scotopic Sensitivity Syndrome".
Bestselling autistic author, Donna Williams, in her book Like Colour To The Blind wrote about her experience of tinted lenses (Irlen filters) after being diagnosed with scotopic sensitivity. In this book she described the lenses as enabling her to have cohesive, unfragmented vision, able to see faces, bodies and objects as a whole for the first time and reducing the extremity of experiences such as meaning-blindness, face blindness, inability to learn to read facial expression and body language and the social consequences of these impairments. This led to a worldwide raised awareness of scotopic sensitivity as a sensory perceptual problem common in many (but not all) people with autism and expanded awareness of the potential effects of Scotopic Sensitivity far beyond that of reading disability, also leading to awareness of the effects of fluorescent lighting on those with this perceptual disorder.
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- Musatcha.com Color Filter - Software color filter for use on computers
- Research in use of overlays