When we think of mental rotation, we usually think of an engineer visualizing some sort of machine--a car, say--and turning it to look at it from the front, back, up, down, left, right. I think people like me, who are bad at mental rotation, tend to think of it as transformation of one distinct thing (the object from one side) into something that looks very different (the object from another side). I think someone good at mental rotation sees it differently. Instead of having multiple representations that they transform into each other, they likely have a representation for the invariant structure of the object that might give them cues for what they'll see when they turn it. Their repesentation of the car is the car's invariant structure, and it calls up different views depending on how they want to turn it.
This system works great for 3D objects. But what happens when skilled mental rotators confront the wonderful world of print? While a spoon is a spoon no matter how you turn it, a b becomes a d or a p becomes a q. Yet these letters have the same basic structure: a half-circle with the open side connected to a line. b differs from d and p differs from q only in the rotation of the half-circle about the line. b and d differ from p and q only in whether the semi-circle is at the top or the bottom of the line. So it's not surprising that skilled mental rotators would mirror-reverse letters during reading or writing, or that their visual-conceptual problem with letters might cascade into problems matching graphemes to phonemes. In other words--it's easy to see how dyslexic kids' strengths might create their weaknesses.
Here's how the Eides illustrate this idea:
"I remember being surprised when our then young son brooded over how to write the letter "f"... he said, "f, f, f,... oh that's right, it's a flipped over 'j' with a line through it." Huh? I hadn't even thought about the relationship between the letter 'f' and 'j' before that. To this day both he and Brock are able to read words backwards more quickly than me."This, they suggest, is what kids with dyslexia do all the time.
So, have we explained dyslexia? Can all the dyslexia researchers pack their bags, go home, and congratulate themselves on a job well done? Probably not. This is almost certainly only one of several paths that lead to the behavioral symptoms known as dyslexia. Each article I've read on the subject takes it as a given that dyslexia is fundamentally a problem with poor grapheme (letter) to phoneme (sound) matching caused by poor "phonological awareness" and more basic auditory processing problems. A lot of studies seem to show that dyslexics don't process either speech or non-speech sounds exactly the way typically developing kids do.
So, do different causes of dyslexia imply mutually exclusive subgroups (i.e., different developmental disorders cause "dyslexia")? Or do a child's strengths (mental rotation) and weaknesses (auditory processing, phonological awareness) interact to produce a tradeoff between spatial and verbal ability? All I know is, we need more research.