After imaging the brains of populations large enough to generate statistics, researchers have made sophisticated brain maps. There are maps to illustrate where we lose brain volume as we age, as AIDS progresses and as we use methamphetamines.
What would a complete map of the human brain look like? That depends on your interests. If you thirst to know the brain's structure, you might want to see that hypothetical Google Earth version that can begin with a picture of our cortex and zoom in to neuron number 888,898,432,857.
This complete, Google Earth type of map is stalled at many points. One such point is the imaging of all of the human brain's neurons and their connections. Even getting this data in the mouse is painstaking, says Harvard biologist Jeff Lichtman. At the rate the ATLUM and an electron microscope are now working, getting a map of all the interconnected neurons in the mouse brain would take 200,000 weeks, Lichtman estimates. The data would be "bigger than all the data on the Internet -- bigger than all the data in all the libraries in the world," he says. "At the moment, the kind of storage that's possible on computers is not quite up to task." The only "brain" for which we have a complete map of interconnecting neurons belongs to C. elegans, "a worm that's a millimeter long and has 300 nerve cells," says Lichtman.
Again, your definition of a complete brain map depends on your interests. If you're a neuropsychiatrist, for example, a complete map of the brain might be a time-lapse image showing how bipolar disorder unfolds in the brain from birth to the first symptom and what lithium does to stop the process.
That might not be enough for you. You might want to know the function of the brain's every last inch. Unfortunately, that's impossible. We can't capture functions that happen too quickly or too slowly, says neurobiologist Charles Wilson. Other processes take a lifetime. No imaging study has followed someone from birth to death. "No method that we know of handles every time of interest. No method we know of handles more than a tiny piece of it," says Wilson. At this point, Lichtman says there is no current effort underway to integrate all of these maps into one.
But there's no fundamental reason why we can't eventually have any - or all -- of these maps, says Wilson. "The problems are all practical technology problems that can be overcome. As with any map, if you start with a crude map, it's better than no map. And you don't need a new map. You just add information to the old map to make it more refined. We aren't going to wake up one day and have this. We are going to add a little today, a little tomorrow, and at some point, we are going to say, 'Wow, this is starting to look pretty good.'"
For more information on the process of brain mapping, take a look at the next page.