Hm, the obvious question there is: "How do you know you can sustain a full conversation, if you don't remember it at all later?" (..edit: With other people? Er, right. Somehow I was assuming it was an internal conversation.)

I've got some idea what you're talking about, though - focusing my consciousness entirely on sensory input. More useful outside of cities, and I don't have any kind of associated amnesia, but it seems similar to how I'd describe the state otherwise.

Neither your brother's nor your own thought processes otherwise seem to be any kind of match for mine. It's interesting that there's this much variation, really.

Otherwise.. see sibling post for more details.