I have noticed this behavior myself, and I used the same phrase, that my brain essentially shuts off when the computerized directions are being given.
What's weird though, is that the same thing doesn't seem to happen if I have an actual person giving me directions. If I listen to the computer, I can't remember shit. If a passenger looks at the map and does essentially the same function, I can remember everything fine and well. I wonder what the difference is between the two that results in such a different neurological outcome.