Studying patients who have electrodes inserted in their brains, scientists have been able to track the creation of memories in real time and in more detail than ever before. Their work, published in today's Science (p. 1582), shows how differences in brain activity make all the difference between remembering and forgetting a word.
The study focused on the medial temporal lobe of the brain, a region long known to be vital for memory formation. Imaging techniques such as PET and fMRI, which rely on changes in blood flow to the brain, are not quick enough to pinpoint the rapid neural events that occur when a memory is born. Realizing that a better yardstick of memory formation is electrical activity, a team at the University of Bonn in Germany led by cognitive neuroscientist Guillén Fernández bootstrapped a study onto surgeries performed on epileptic patients, some of whom get electrodes inserted in their brains before an operation to help surgeons decide where to cut. Twelve such patients were asked to memorize 12 nouns flashed successively on a screen, while scientists measured activity in different brain regions. After 30 seconds of distraction, the patients were asked to recall the words, which they could do for about 30% of them.
Initially viewing a word produced electrical activity in Wernicke's area, a part of the brain involved in verbal processing, as well as in the anterior parahippocampal gyrus and the hippocampus, two areas of the medial temporal lobe implicated in memory formation. But after about 310 milliseconds, the words later remembered started to spark a negative potential, or nerve impulse, in the anterior parahippocampal gyrus that was about 50% stronger than the peak produced by forgotten words. And after 500 milliseconds, words that were recalled caused a positively charged nerve impulse in the hippocampus that was four times greater than with words that slipped into oblivion. The results suggest that to be remembered, information has to elicit sharp responses in at least these two different parts of the brain. "Now we have more of an idea when this process may happen," says Fernández. By refining the experiments, he hopes to tease out further what kind of processing takes place in each brain location.
Researchers say the results confirm what they had suspected: "Anatomy tells us there is a sequential relationship from the cortex to the parahippocampal region to the hippocampus," says cognitive neuroscientist Howard Eichenbaum of Boston University. "This study shows that there are functional, not just anatomical, connections between these structures."