Science & Spirit: Seven Keys to Memory
New research in the last two decades offers us unprecedented glimpses into both healthy and damaged brains as they remember and forget. Here, a summary of the top seven insights we have into memory today.
VALERIE REISS
01/01/2004
1) The ABCs of Memory: What, Where, and How
A memory is formed when a nerve cell receives a signal in a dendrite (one of its tendrils) and sends it across a synapse to another nerve cell. That connection is flexible at first and can "vanish." But when the original signal is repeated or is especially strong because it is reinforced by emotion, these synapses strengthen and send signals to other cells, until there's a little solid system of neurons involved with the memory. Not all nerve cells will relate to each other in this way. "But with certain experiences, they might develop a relationship," says Dr. Pauline McHugh, an assistant professor at the Center for Brain Health at New York University's School of Medicine. "Repetition of that relationship might be a marriage rather than a quick flirtation."
There are many theories about where and how different kinds of memory are stored and retrieved: "How some memories get activated and others don't, that's like the holy grail," McHugh says. One thing is certain, though: The storage capacity is vast.
"There are millions and millions of brain cells," says McHugh. "And out of these, about a million possible synapses can be formed. If you think about it, it'll knock you backward. It's just so complex, it's unbelievable."
Some researchers compare memory to a computer, with files that can be retrieved or lost. But David Rubin, a psychology professor at Duke University, thinks this denies the holistic nature of memory. "Memory is a process," says Rubin, who works mostly with autobiographical and cultural memory. Memory is not a retrievable object; it's "an interaction between your environment now and things that have happened in the past," Rubin says.
Can we physically locate the memory that knows how to ride a bicycle? "That's the wrong way of thinking about it," Rubin says. "Memory is like hitting a ball in tennis: You never hit it exactly the same way twice."
2) The Top Three Tools: MRIs, fMRIs, and PET Scans
Technological advances in the last two decades have given researchers unprecedented glimpses into both healthy and damaged brains as they remember and forget. Researchers use three main kinds of technology to peek into brains. The type of research dictates the tool.
MRI: Essentially a large magnet that uses no X-rays, structural MRI (magnetic resonance imagery) gives detailed gray-scale pictures of the brain's shape and size. Radio waves bounce off the water and tissue in the brain to give a three-dimensional model that can be seen in "slices" on a computer screen and sorted through like cross-sections of brain in a lab. The great detail allows researchers to measure the volume of various brain parts and observe deterioration that may indicate disease.
The cutting edge of new research is being done with functional MRI, or fMRI. It allows us to watch which parts of the brain activate during specific activities. By measuring blood flow in the brain, researchers can map brain activity while subjects execute simple tasks, like tapping, visualizing, calculating numbers, and remembering. Interpretation of changes in brain activity is still considered controversial; just because an area "lights up"—increases blood flow and activity—does not mean a certain kind of memory is "stored" there. Still, it does suggest the area's involvement.
PET (positron emission tomography) scanning: This also measures blood flow in the brain. But instead of using magnets and radio waves, the device detects nuclear radiation. The subject is injected with radioactive glucose and the brain is studied for speed and efficiency of absorption over forty-five minutes. Areas working the hardest to absorb the glucose are assigned the color red; the second-hardest-working looks yellow, the next-hardest green, then blue. In its daily life, the brain relies on glucose to function. In a PET scan, "you're not seeing what your brain is doing, but you are seeing what your brain is like, in general, in terms of how much glucose you're using," says NYU's McHugh. "It's more trait than state—which is what particular state your brain is in at that moment in time. Whereas trait is how is your brain is working in general."
The ideal brain-scanning technology, says McHugh, would be "to have this kind of technology but to have it go quickly enough that you'd see it in real time. But we're not there yet." So, would she like to see video PET scans? "That would be my dream machine," she says. "I would totally want that for Christmas."
3) Imagination Makes it "True"
Memory is like a DVD, says Stephen M. Kosslyn, a Harvard University psychology professor who studies visual memory with fMRI. The disk surface is shiny, holding rainbows in the light, he says. "But you can't really see what the information is on it. Once you play it on the DVD player, you can see pictures and stuff on the screen, hear sounds," he adds. "The same is true of memory."
To draw out the mind's "encoded bits," Kosslyn created a series of questions designed to awaken visual memory. "What shape are a cat's ears?" he often asks study subjects. To answer the question, the volunteer usually must imagine a cat and inspect its velvety, triangular ears, making implicit (unconscious) memory explicit (conscious). "What mental imagery can do for you in part," Kosslyn says, "is help you discover things about what you know that you didn't know you know."
Kosslyn has found that ninety percent of the brain regions used for imagining visual images are the same ones used when actually seeing them. The potential and current therapeutic applications of this are broad—from "relearning" traumatic memories to "flooding" to heal phobias. Kosslyn guesses that at least a half a dozen psychological approaches have integrated visualization as a way of healing.
Kosslyn is also planning to study monks who meditate with images because they claim to possess a "holistic" visual memory. "When they visualize something complicated, like an image of a palace," Kosslyn says, "it will seem like a 'fish leaping out of water,' appearing all at once, fully formed. Whereas for us it's a laborious process of building it up one part at a time." Kosslyn thinks this is possible, but wants to see it in action. For now though, the project is on hold because, "We're waiting for an appropriate monk."
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