The findings, by a British researcher, show that the disorder is linked to a subtle inability to process visual information about moving objects.
Scientists researching dyslexia, which affects an estimated two million people in the UK, now think it may be caused by a general inability to process fast-changing data from any of the senses.
The difference discovered in the latest work is so small that it makes no difference in other everyday activities. But the method used to find it could allow dyslexia to be diagnosed without reading tests, using Magnetic Resonance Imaging (MRI), which can look at activity within the brain.
In a three-year study, Guinevere Eden, of the National Institutes of Health in Bethesda, Maryland, used MRI to study the activity of a particular part of the brain's visual processing system, called V5/MT.
The MRI system spots changes in the flow of blood in the brain, so it highlights any area that is particularly active.
Six dyslexics and eight people with normal reading abilities were compared as they watched fast-moving dots on a screen. In the normal subjects, V5/MT showed heightened activity; in the dyslexics, it did not. When the same people were shown stationary dots, the differences disappeared. The results are published in the science journal Nature today.
Dr Eden pointed out that her work does not show that this difference is the cause of dyslexia. "This really indicates that dyslexia is a biological abnormality, not the result of upbringing or education. It also shows that there's some involvement of the visual system in dyslexia." She noted that the V5/MT area is fully formed before birth, showing that dyslexia must be innate.
The reduced activity in the visual cortex does not mean that dyslexics have problems following words on the printed page, said Chris Firth, of the Institute of Neurology in London.
"The problem described is very small. It wouldn't directly affect the ability to read. You could only detect it in the lab."
But there may be related problems with the auditory cortex, which processes signals from the ears, he suggests.
Dr Eden intends to study that area of brain function next.
Dr Firth believes that dyslexics might all have more deep-seated problems in processing data, either from eyes or ears.
Dyslexics have problems in detecting whether words or letters rhyme - a process which depends on recognising changes in the frequency of syllables.
The British Dyslexia Association welcomed the results.
"It should show the doubters, of whom there are too many, that it's not just the invention of frustrated middle-class parents who are disappointed at their children's performance in school," said a spokesman.
"It's very welcome."