We know that the perceptual abilities of those with autistic spectrum disorders is different. For example, as I blogged, one study showed that those with ASD tend to have eagle eye vision. Individuals with ASD report other differences, including differences in the ability to perceive sight, sound, touch and taste.
I have no idea exactly how Brad takes in the world. However, I do have a theory: his foreground/background are the reverse of typical. Not the exact reverse. But different. For example, I often sleep in an oversized shirt emblazoned with a large "Old Navy" logo. When I wake my typical son, he greets me - sometimes with a smile, sometimes with a frown, but I believe he sees me in the foreground, and my shirt in the background.
With Brad, I think it's the reverse - it's like he sees my shirt in the foreground, and me in the background. Drawn to the visual stimulus, he recites the letters. He may be interested in affirmation for getting the letters correct, which is great, because that's joint attention. He's not detached from me. He just doesn't take me in or experience me the way my typical son does.
Similarly, noises that are in my background, like a plane flying overhead, appear to be in Brad's foreground. And when we go to a restaurant, he has an uncanny ability to pick out a baby crying, even if it's just a dull wimper from across the room.