Inactive areas, say those "absorbed" in watching a sitcom rerun, need less blood. Because the MRI is harmless, it can be used with the mammal that most fascinates neuroscientists -- the human being.
Using a highly accurate MRI, Susan Bookheimer, of the department of psychiatry and biobehavioral sciences at the University of California-Los Angeles, and colleagues watched while 10 people learned to associate names with faces. That's a classic "declarative memory" task that's should call for heavy lifting in the hippocampus.
Instead of looking at just the hippocampus, Bookheimer also measured some nearby brain structures (they're called the medial temporal lobe, or MTL, if you insist), that also "do memory."
Together, she found that specific parts of the MTL were devoted to either input (learning) or output (memory retrieval).
Cerebral cinnamon roll
Ditto for the hippocampus.
Instead of doing the obvious -- getting her hands full of cinnamon and butter -- Bookheimer digitally flattened the medial temporal lobe in a computer, and then found that three areas, the cornus ammonis 2 and 3 (CA2 and CA3), and the dentate gyrus, play a role in encoding, or establishing, memories.
Tellingly, these three bits of brain reside on the input circuits of the MTL -- where nerve impulses enter the hippocampus.
The binding of memory
To strengthen the link, some circuits in the medial temporal lobe simply echo the linked information back at themselves: "His name is Fred," "His name is Fred."...
The upshot of all this linkage, Bookheimer says, is "like a reference in a card catalog. You look up information for that face, and it sends you to the name."
After the association is nailed down, especially after it has been successfully retrieved even once, the input regions slack off, Bookheimer says, since their activity level is determined by "how much information is being bound together at that moment."
Like a student who's just passed finals, this bit of brain seems to figure, "We've learned this stuff already. Time to party." (We hate to be killjoys, but Bookheimer mentions that, "Hippocampal neurons are especially sensitive to drugs and alcohol, which can kill off all that hard work of binding connections in a single shot.")
A second lesson is that more associations make stronger memories. If you try to link the French noun "cycliste" with "bike rider," you have only one association to remember, and "when the hippocampus goes to rebuild it, it will have only what you initially encoded," Bookheimer says.
Instead, try adding some associations: "I watched cyclist Lance Armstrong win the big race," "I cycled on a black Schwinn one-speed as a kid," and "I love to cycle down a long hill on two wheels."
This technique provides plenty of memorable associations, and it's especially good since it includes verbal, visual, muscular, and spatial memories, giving multiple ways to retrieve the pesky definition.
Those tactics, Bookheimer says, will help the little cinnamon roll in your noggin decipher the world around you. "One role of the hippocampus is to reform memories given partial clues," she says. "The more associations, the more clues you have, the better you will be at finding a clue that is effective."
What happens to individual neurons when we learn?
©2003, University of Wisconsin, Board of Regents.