Which city is farther north--Portland, Ore., or Portland, Me.? Unless for some reason you recently committed to memory the latitudes of all large U.S. cities, you probably have only a rough map of the country in your head and can call up at best an approximate mental image of their locations. Or perhaps your mental picture is so precise that you know the right answer (Portland, Ore.). This ability to conjure internal images may seem matter-of-fact to you, but from a scientific perspective it is anything but.