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.
How the brain generates and processes mental pictures has been a matter of much debate in the research community. The solution to the problem would illuminate an important facet of our conscious experience [see "The Movie in Your Head," by Christof Koch; Scientific American Mind, Vol. 16, No. 4, 2005]. The core of the issue is an even more fundamental question that has occupied philosophers for millennia: What are thoughts made of, and how are they represented in the brain? As with many other areas of neuroscience in recent years, brain-imaging technology is providing some insights.