How do their brains compare to a standard brain?
I believe the answer lies in minicolumnar morphology in the neocortex. It's been shown that the minicolumns of autsitics and gifted individuals have narrower minicolumns, with greater spacing between each minicolumn. It's speculated that this creates an increased ability to distinguish percepts. Here is a paper on the topic:
Casanova MF, Switala AE, Trippe J, & Fitzgerald M. (2007) Comparative minicolumnar morphometry of three distinguished scientists. Autism 11(6):557-69.
I recommend reading through more of Casanova's work on the topic if you're interested.