One day fMRI might be used to diagnose dyslexia early, as every parent of a child at risk hopes. Even more tantalizing, the scans might be used to track what kind of intervention might restore brain activity to normal in children who are slow readers. Those studies are now underway. A more parsimonious explanation is available, which also explains various other oddities in dyslexia. The real villain here may well be a defective corpus callosum.
Foveal vision involves interhemispheric transfer of information.
Peripheral vision goes to one hemisphere only. Perhaps the left hemisphere can learn to deal with direct input from the peripheral vision more easily than it can make sense of double, non-matching input from both sides. The corpus callosum is suspect in dyslexia in other ways. In addition, persons with dyslexia are poor at tactile localization, which also requires accurate transfer of information across the corpus callosum. The corpus callosum helps control the arousal level of the hemispheres, enabling one to choose which side to use during cognitive tasks.