"The hippocampus is critical for learning," says Gluck, "and it's also one of the most volatile, unstable parts of the brain--one of the first parts damaged if oxygen is cut off. Think of it as a highly maneuverable kayak; it has to immediately capture a whole range of information about an event and needs the ability to go rapidly through many changes. We think the hippocampus serves as a filter, learning new associations and deciding what is important and what to ignore or compress. That's why it's critical for learning." The hippocampus is, in a sense, a collating machine, sorting and then sending various packets of information to other parts of the brain.
One of the most exciting advances in neuroscience may lie ahead as researchers begin to actually model the living brain on the computer--creating a new era of artificial intelligence called neural networks. Gluck and researchers at New York University have begun to model the hippocampus, creating "lesions" and watching what happens--in the hope that they can develop specialized tests that will identify Alzheimer's in its early stages, as well as develop machinery that can learn the way a brain does. Thus far their predictions about its role have been borne out--in fact, Gluck is developing applications for the military so that hippocampal-like computers can learn the early signals of engine mal-functions and sound the alarm long before a breakdown.
The hippocampus does not store memories permanently. It is a way station, though a supremely important one, Like a football player in the beat of the game, it passes the ball to other parts of the brain. This takes minutes, or maybe even hours, according to James McGaugh, Ph. D. of the University of California at Irvine. At that point, memories can still be lost. They need to be consolidated; the network of neurons responsible for a memory needs to be strengthened through repeated stimuli, until the memory exists independent of the hippocampus, a process known as long-term potentiation (LTP).
Once again, a word picture of this process is extremely crude. In actuality, Edelman points out, "the circuits of the brain look like no others we have seen before. The neurons have treelike arbors that overlap in myriad ways. Their signaling is like the vast aggregate of inter-active events in a jungle."
No one is certain how long it takes to fully consolidate a memory. Days? Weeks? Perhaps it even years until the linkages of networks are so deeply engraved that the memory becomes almost crystallized--easy to recall, detailed and clear. Individuals like M.P. seem to lose several years of memory just prior to hippocampal damage; so do Alzheimer's patients, who usually suffer hippocampal damage as their brains begin to malfunction, and who recall their childhood days with fine-etched clarity but find the present blurred.
A MAGIC RHYTHM OF MEMORY?
Just how and when do memories become permanent? Scientists now have direct evidence of what they have long suspected--that consolidation of memories, or LTP, takes place during sleep or during deeply relaxed states. It is then that brain waves slow to a rhythm known as "theta," and perhaps, according to McGaugh, the brain releases chemicals that enhance storage.
In an ingenious experiment reported in the journal Science last July, researchers planted electrodes in different cells in rats' hippocampi, the animals explored different parts of a box. After returning to their cages, the rats slept. And during sleep the very same cells fired.
There seems to be a specific brain rhythm dedicated to LTP. "It's the magic rhythm of theta! The theta rhythm is the natural, indigenous rhythm of the hippocampus," exclaims neuroscientist Gary Lynch, Ph.D., of the University of California at Irvine. Lynch is known for his inspiring, if slightly mad, brilliance. His laboratory found that LTP is strongest when stimulation is delivered to the hippocampus in a frequency that corresponds to the slow rhythms of theta, of deep relaxation. Research by James McGaugh seems to confirm this: the more theta waves that appear in an animal's EEG (electroencephalogram), the more it remembers.
No wonder, then, that recent experiments show sleep improves memory in humans--and specifically, the sleep associated with dreaming, REM (rapid eye movement) sleep. In Canada, students who slept after cramming for an exam retained more information than those who pulled an all-nighter. In Israel, researchers Avi Karni and Dov Sagi at the Weizmann Institute found that interrupting REM sleep 60 times in a night completely blocked learning; interrupting non-REM sleep just as often did not. These findings give scientific punch to "superlearning" methods like that of Bulgarian psychiatrist Georgi Lozanov, which utilizes deep relaxation through diaphragmatic breathing and music, combined with rhythmic bursts of information.
THE HAUNTED BRAIN
What happens when memory goes awry? It seems that some memories are so deeply engraved in the brain that they haunt an individual as if he were a character in an Edgar Allen Poe story. How, asks Roger Pittman, M.D., coordinator of research and development at the Manchester (New Hampshire) Veterans Administration Medical Center and associate professor at Harvard Medical School, does the traumatic event "carve its canyons and basins of memory into the living brain?"
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