A part of the brain called the medial temporal lobe (MTL) is critical for memory, but neuroscientists know little about which parts of the MTL do what. A new study, the first to tackle the question in humans, pinpoints one region of the MTL that lights up while new memories are being formed and another that blinks on when previously encoded memories are retrieved.
It's difficult to study the MTL in living people because much of it is folded up like a jellyroll. That makes it hard to distinguish key components in functional magnetic resonance imaging (fMRI) scans. To overcome this obstacle, Susan Bookheimer and colleagues at the University of California, Los Angeles, employed mathematical tricks similar to those mapmakers use to turn globes into wall maps. Their flattened maps of the MTL show the borders between different regions.
As subjects were learning to associate a set of names with faces, the researchers saw increased activity in three neighboring regions of the hippocampus, a key component of the MTL. This activity was highest the first time subjects saw a name-face pair and dropped off gradually as the pairs were repeated and became more familiar, suggesting that this activity reflected the encoding of new memories. Later in the test, people had to retrieve names from memory to match faces. This triggered activity in another region of the hippocampus, the subiculum, the team reports in the 24 January issue of Science.
The study provides some of the strongest evidence yet for a division of labor within the MTL when it comes to encoding and retrieving memories, says neuroscientist Larry Squire of the University of California, San Diego. Still, he and others say it's unlikely to be the last word on the issue. "There's a debate and the data are all over the place in terms of what areas of the MTL are involved in what aspects of memory," says John Gabrieli, a cognitive neuroscientist at Stanford University.