Maybe I'm wrong but I don't think it's as much about attention span as it is about simple interest. They are interested = they pay attention, they are not interested = they couldn't care less.
IME attention also has a social component. Often an NT little kid, shown something with enthusiasm by an adult or a peer, will become interested not because of the intrinsic interest of the item, but because of a social impulse to share the interests of others.
Obviously, some people are more other-directed and some are more inner-directed-- but for a person with autism, the impulse to be interested *in order to share a social experience* is often just not there. So intrinsic interest can become the only kind that's operating.
DeeDee
