Slowness in processing affects all the senses in Specific Learning Difficulties. Perception refers to the interpretation the brain makes of information from the senses. If the senses can’t convey a rapid feedback due to lack of neurons devoted to the function then information is mistimed and misequenced.
In fact most Dyslexic children have been taken to have their hearing and eyes checked early on, only to be told by the optician and audiometric Ian that there is nothing wrong with their eyes or ears. This is sometimes bad news for them — their parents or teachers may make a negative judgment — laziness or stupidity or any number of reasons for their failure in learning to read. The accounts children give of blurring print, losing lines in reading text or music, headaches on reading, not being able to see the blackboard, copy fast enough or listen to the teacher are all hard for a parent to interpret after being told there is nothing wrong at the physical level. But these complaints are common and are due to inadequate processing of visual, auditory and kinesthetic information.
In fact up to 25/30% of children may have light sensitivity and colour based visual perceptual problems. The current opinion is that at least some of the observed problems (glare off the page, moving and blurring of text, sore, watery eyes on reading, losing lines, needing to reread constantly to get the sense,) are due to an analogous deficit (to the auditory problems) in the visual pathways. The lateral Geniculate nucleus has been identified as smaller in cellular content in Dyslexics than in normal readers by Margaret Livingstone at Harvard University “Physiological and anatomical evidence for a magnocellular defect in developmental Dyslexia” Proc. Natl.Acad. Sci. 1991:88.
Most visual information moving from the retina via the lateral geniculate nucleus of the thalamus travels through one of 3 visual pathways. One of these, the magnocellular is thought to carry visual information about space — such as movement, depth and the relationships between them. The magnocellular is thickly myelinated (for rapid transmission) and ends up in the parietal cortex. The other important visual pathway, the parvocellular, the “what” pathway, which ends in the temporal cortex, must synchronize for efficient reading, so the theory is that when the magnocellular is not able to keep up with the parvocellular, visual tracking and fixation is unstable when the eyes sweep across a page. Visual information carried through the magnocellular in Dyslexic brains, has been shown by brain imaging studies (Eden, G. in Nature, 1995) to be poor in identifying movement in comparison to normal readers. By the time the information gets to the visual cortex the signal is quite faint in Dyslexics compared to normal readers. Thus the magno can’t control eye movements or guide them to the object to be looked at. Some researchers believe this means the magno or “What” pathway acts as an attentional spotlight. (Vidyasagar T. (1999) “Impaired Visual Search in Dyslexia related to the role of the Magnocellular pathway in attention” Neuroreport; 10) Studies showed that Dyslexic children are poorer at a visual search task than normal readers and the more distracters there were in the background the worse they did because reading places great demands on the attentional spotlight, far more than a complex visual scene. In other words a slow visual processing analogous to the slow auditory processing mentioned above. This has led some researchers to propose an auditory magnocellular system, analogous to the visual
For some children the interaction of these problems causes a light sensitivity with headaches, pattern glare off white pages (copying their work on pastel colours helps) — this is known as scotopic sensitivity, first identified by Helen Irlen — see her help for parents book “Reading by the colours”. Her solution is to screen the child or adult using coloured lenses until the right portion of the spectrum is inhibited or enhanced. This detected by reading rate, clarity of depth perception and subjective feeling of comfort.
Behavioral optometrists give vision exercises to help strengthen the convergence and accommodation.
Other techniques are monocular occlusion (covering the left eye with opaque lenses while reading).