A minority of dyslexics have predominantly visual problems and most of these dyslexics surprise their parents and teachers with their difficulty learning to read because they have normal verbal development. When these ( a minority of dyslexics ) describe symptoms such as not being able to read because the letters are moving around to much or there is an amount of time before the next word comes into focus, it is easy to see their problem isn't phonological.
The prejudice against visual problems being involved with dyslexia partly is generated by the public response to those overselling the concept that there is a single color for a particular dyslexic that will help their dyslexia. When the evaluation is actually the product being sold there is no recourse when there is failure and no financial reason to limit those tested as the profits are the same for both success or failure.
This failure of high priced evaluations to determine a particular color for a particular dyslexic should not result in the conclusion that dyslexics have no visual problems. All the failure does is to make it a financially risky action to try those high priced evaluations.
Why then are we reading lately that fMRI studies have now proven that dyslexia is caused by phonological processing areas of the brain? Simply because that is the latest study reported and almost all fMRI studies conclude that they have found the cause.
My opinion is that there just doesn't seem to be any glory in concluding that a factor in dyslexia has been found. In fact every part of the brain that involves reading ,when studied by fMRI , shows the same type of results when a group of dyslexics is compared to a group of non dyslexics. The results show differences between groups but have so much overlap that individuals can not be identified as being dyslexic or not.
Now, I do not question any of the brain imaging results. I do note the fact that the differences observed ( when looking at any single brain location ) only show up when comparing groups and not individuals.
I suggest that the dyslexics in those studies whose results overlap non dyslexics actually have their dyslexia caused by different factors than what is being studied. I suspect that 1 group that skews the results have visual processing problems. This also works in reverse when studying the visual centers by the phonologically impaired.
Having a group of dyslexics made from dyslexics that actually have different factors causing their dyslexia would produce the fMRI results that are reported. I think that dyslexia having more than one cause the more reasonable conclusion rather than there is one cause which can not identify a individual as being dyslexic.
To consider dyslexia a syndrome and not be constrained by the single cause theory is much more likely to result in arriving at being able to identify a dyslexic by the fMRI method. This would require that several areas of the brain are evaluated for signs of dyslexia and at least might identify those individuals whose cause of dyslexia is able to be evaluated by fMRI data.