In yesterday's Health section, an article about memory incompletely identified Alex Martin, who is chief of the section on cognitive neuropsychology at the National Institute of Mental Health. (Published 12/15/99)
December is a time of memories. Scents of cinnamon and fresh evergreen evoke holidays past. Dangling mistletoe conjures up long-ago kisses. With each refrain of Christmas carols, words and music not heard for a year come tumbling back.
How the human mind remembers has long intrigued scientists and philosophers. To Plato, recollections were gifts from "Memory, mother to the Muses." He imagined that the mind captured perceptions and thoughts, receiving impressions of them like the seal of a ring. Memory is the very essence of being human.
Without memory, people would be unable to read this sentence or find their way home tonight or grasp the jokes on their favorite television sitcom. Memory provides the foundation for learning throughout life, whether it's to explore emotions through psychotherapy, become fluent in a second language or master snowboarding.
All animals, from the lowly sea snail to humans, have some form of memory. But people possess the remarkable ability to make a nearly infinite number of memory associations. It's why the loss of memory--due to normal aging, illnesses such as Alzheimer's disease or accidents--is so profound. And it's why, as the Decade of the Brain draws to a close, neuroscientists are pressing to better understand this still mysterious process.
"Every thought we have, every word we speak, every action we engage in--indeed, our very sense of self and our sense of connectedness to others--we owe to our memory, to the ability of our brains to record and store our experiences," explain neuroscientists Larry R. Squire and Eric Kandel in their book, "Memory: From Mind to Molecules." "Memory is the glue that binds our mental life, the scaffolding that holds our personal history and that makes it possible to grow and change throughout life."
From the Mundane to the Magnificent
Researchers, using a variety of sophisticated new imaging devices, are beginning to understand some of neuroscience's most central questions: How are memories organized in the brain? Is there a particular location in the brain for the seemingly mindless daily habits, from climbing out of bed to brushing your teeth or making coffee? How does the brain recall vivid details from a movie, a book or a painting? Where does the memory for recognizing a face exist?
"It's a very difficult thing to find the anatomic parts of memory in the brain," said Daniel Alkon, director of the Laboratory of Adaptive Systems at the National Institute of Neurological Disorders and Stroke (NINDS). " . . . We see a picture of our father's face and hear his name and recall our relationship with him. But to find where that is stored [in the brain] is an incredibly difficult task."
Until recently, memory research was largely confined to animals and to those individuals in whom memory had begun to unravel. People suffering from amnesia, the aftermath of a stroke and various forms of dementia gave scientists rare glimpses into the mysteries of memory.
One of the most fascinating cases was of a 9-year-old boy who cracked his head on the sidewalk after being knocked down by a bicycle. His misfortune turned into a lengthy, classic study that provided the first riveting proof that memory is not one single process in the brain.
The child suffered debilitating seizures and frequent blackouts. By age 27, he was so incapacitated that neurosurgeons removed part of his brain--the hippocampus and two areas known as the medial temporal lobes.
The operation in the mid-1950s cured the man's seizures, but it left him unable to retain new information. Meals eaten, people met, even photos of himself as he aged held no meaning because he could not transfer them to his long-term memory. The man retained enough knowledge of language and life to hold a normal conversation. What he couldn't recall--even a few minutes afterward--was having the conversation.
Yet the man could vividly remember events that occurred before the accident and he could even learn some new skills. In a standard laboratory test, he was asked to trace the figure of a star. His tracings improved each day, just as they would in normal subjects. The difference was that the man had no memory of his previous drawings.
McGill University psychologist Brenda Milner, who has studied this man for 40 years, concluded that he couldn't retain new memories because he no longer had the medial temporal lobes and the hippocampus in his brain to store them. Yet since the man could recall events before his injury, Milner and her team determined that neither the temporal lobes nor the hippocampus could be the final storage sites in the brain for longer term memories.
The fact that the man could learn to trace a star and progressively improve at doing so--even if he didn't remember it--helped scientists to understand that memory is divided into two major forms, declarative and nondeclarative.
The surgery destroyed the man's declarative memory, or the "conscious recognition of facts and events," according to Squire, research career scientist at the Veterans Affairs Medical Center in San Diego.
Left intact was his nondeclarative memory, which includes the subconscious recall of motor skills and the ability to identify familiar objects more quickly with experience.
How Memory Works
In healthy individuals, the hippocampus and the medial temporal lobes are at the heart of declarative memory. But the process of capturing and recalling information spreads throughout the brain, often in milliseconds.
Each memory seems to be a compilation of tiny bits of information stored in a vast network of different cells. Just the simple ability to recall a phone number is believed to involve the activation of several thousand nerve cells, called neurons, distributed throughout the brain.
"Humans are in a class by themselves," said NINDS's Alkon. "The wiring inside the brain is designed to allow us at the flip of a coin or the snap of fingers to connect or associate any bits of information with any other bit. We can do it lightning fast. In terms of learning new associations, most of what we do can be formed in fractions of a second."
And the potential for storing long-term memories is thought to be nearly endless, given the multiple associations between cells. Each human brain contains an estimated 100 billion neurons, each capable of making up to 10,000 connections with other brain cells. The number of possible memories starts to approach the number of molecules in the universe, Alkon said.
There are, of course, highly skilled individuals who aptly demonstrate the vast capacity for declarative, long-term memory. The Olympic gymnast or gold medal ice skater who moves nonstop in intricate ways through a long routine relies on long-term memory. So does the concert pianist who plays an entire concerto by heart.
At Carnegie Mellon University in Pittsburgh, psychologists Herbert Simon and William Chase studied chess players and found that practice--more than exceptional memories--could account for differences between individuals.
In the study, each player was asked to spend five seconds examining a chessboard with 26 pieces arranged in game positions. The players were then told to reproduce what they had seen on an empty board. On average, skilled chess players--including some grandmasters--correctly replaced 16 of the 26 pieces, while beginners only positioned four pieces correctly. But the differences between skilled chess players and beginners disappeared when Simon and Chase randomly positioned chess pieces on the board.
Simon and Chase concluded that the grandmaster chess players had trained so much that they had stored thousands of game position variations in their brains but were no better than the beginner players in recalling random positions on the board.
A Look at Memory
Modern imaging techniques are enabling scientists to study the brain in ways once unimaginable and providing an understanding of the chemical processes that make memories.
With positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), for example, neuroscientists can pinpoint brain activity during different memory tasks. At NIMH, Martin and colleagues James Haxby, chief of the section on functional brain imaging, and Leslie Ungerleider, chief of the Laboratory of Brain and Cognition, use these techniques to explore how the brain stores information. They begin with such simple objects as animals, houses, chairs, pencils and tools. "We can now see this whole network, this whole cascade that is going on in milliseconds within the brain," Martin said.
The findings reveal a brain organized around the processes of learning, not the objects themselves, and by the way in which these objects are used. "A hammer, for instance, is stored in an area that involves motion, while the image of a cat is placed in a part of the brain that contains other visual shapes," Martin said.
To identify an object, he said, "we instantly retrieve information by the features that define it. What does it look like? What color is it? How does it move or how do we manipulate it, if it's a tool?"
For example, the studies found that verbs are stored in areas of the brain just in front of regions involved in the perception of motion. Colors of objects--the memory of a bright yellow hue of a pencil, for example--are stored next to the perception of color.
Thus a hammer is stored three ways in the brain: once for its form, once for its use or motion and once for the memory of the motor skill needed to use it. "We store these bits of information about objects near their features," Martin said. ". . . It's all very logical."
These findings also support clinical observations made by British researchers Elizabeth Warrington and Rosaleen McCarthy, who noticed a highly selective loss in the ability to name or retrieve objects in some brain-damaged patients. Depending on where the injury occurred in the brain, some patients lost the ability to name small inanimate objects, such as mops, forks and chairs, but retained the ability to recall living things and large objects, such as kittens, autos and clouds.
"It looks like how we store information is not randomly scattered, but follows a plan and the plan is organized the same way our sensory and motor systems are organized," Martin said. "Color goes with color, form goes with form, motor information with motor information."
Neuron by Neuron
But scientists are searching even deeper, seeking to unravel how individual brain cells store and share information. Last month, researchers at the University of Geneva in Switzerland announced that they had captured what appears to be the first electron micrograph image of the cellular changes involved in long-term memory. Reporting in the journal Nature, the team showed how the connections between two nerve cells in a rat's brain change significantly when long-term memory is established.
In an alteration that is believed to occur across nearly all species from rat to human, the team found significant changes in the spiny dendrites that form at gaps between nerve cells called synapses. Scientists believe that this change allows for a host of associations to be made linking a particular memory with experiences, thoughts, emotions, sights, sounds and smells.
At the chemical level, memory storage seems to activate the nerve cell to start a cascade of reactions that can last a fraction of a second or linger for years, depending on the type of memory being stored. While much of this work has been conducted in sea snails, there's evidence to suggest that the process applies to higher species, including people.
Alkon and his colleagues at NINDS have found that the first change is in calcium, which floods into the neuron within milliseconds of the start of a memory task. This is one of the things that scientists believe happens when you look up a phone number for a restaurant and hold it in memory for just the short time it takes to place the call.
This ability "is what people refer to as working memory," said Robert Desimone, director of NIMH's Intramural Research Program.
But if you need to remember that phone number permanently, rehearsal is necessary and it prompts an intricate series of reactions within the nerve cell.
"How long does it take to rehearse things so that they get stored in long-term memory?" asks Desimone. "There's no clear answer to that. People who have a good memory might do it in two seconds, while [others] . . . might take two days. It is also a matter of how novel the stimulus is, like meeting a person for the first time. . . . We know that long-term memory stores novel things much more easily than it does repeated instances of familiar things."
Why some memories are saved and others are simply discarded in healthy people appears to be determined in part by attention. Throughout the day, the brain is bombarded by thousands of pieces of information. Remembering every activity of every day would quickly overload neural circuits.
So the novelty of certain situations and the emotions and other sensory cues about them help dictate their importance and their storage in our memories. Vivid, emotional experiences appear to release chemicals in the brain that aid in the storage of information, said NIMH's Desimone. "So, if I tell you, 'I'm going to give you this Pizza Hut number and I want you to remember it,' you probably would for a few minutes. But if I said, 'Remember this number or I will kill you,' it might actually get stored right away in your long-term memory."
Attention, whether prompted by fear or genuine interest, helps to filter what is saved to memory, and for this reason, it has become one of the new frontiers in memory research.
As the population ages, understanding memory is vital for battling Alzheimer's disease and other age-related dementias. But it's not just the elderly who can benefit from memory research.
Only by understanding how memory works can researchers open the door to new treatments for psychiatric illnesses and learning disabilities.
"Memory is the most significant of all the mental processes," Kandel said. "Memory is involved in every aspect of our lives."
MYSTERIES OF THE BRAIN
This year marks the end of the "Decade of the Brain," proclaimed by President Bush in 1990. Powerful imaging devices and new genetic information are revealing the secrets of the organ that makes humans unique. This is the third of several articles in Health about mysteries of the brain. Join us today at 2 p.m. for a discussion of memory on The Washington Post's Internet edition at www.washingtonpost.com/liveonline. Send in your comments and questions.
A Tapestry of Memories
The hippocampus and the medial temporal lobes are critical for a healthy memory. They are involved in the function of the declarative memory, one of two major types of remembering. All declarative memory is conscious, meaning that a person is aware of its retrieval. It includes such tasks as the ability to briefly hold facts and then discard them or recall episodes from earlier years.
The other main type of memory is nondeclarative, or the unconscious retrieval of information. This is the ability to walk, chew, swallow, reach for a pencil, pour a glass of water, turn on a light or put a key in the door without thinking. These skills are centered in the amygdala and other portions of the brain.
Different types of memory are often interwoven like a mental tapestry. The ability to read is a nondeclarative memory, because once the skill of reading is mastered, it becomes an unconcious act, said Leslie Ungerleider of the National Institute of Mental Health. But remembering what has been read would be a declarative memory because it involves consciously retrieving the information.
Source: Society for Neuroscience
Fending Off Forgetfulness
Worried about losing your memory? You're not alone, says Cynthia R. Green, director of the Memory Enhancement Program at Mount Sinai School of Medicine in New York.
Green, author of "The Memory Workout: Eight Easy Steps to Maximum Memory Fitness" (Bantam; $23.95), uses three guidelines to determine whether someone with memory problems should see a specialist for a full memory work-up:
Has memory worsened over a period of time, such as the past six months?
Do memory problems interfere with work performance or cause difficulties at home in managing finances, hobbies or other activities?
Are family and friends concerned about your memory lapses?
If the answer to all of these questions is yes, Green advises seeing a health professional experienced in assessing memory, such as a neurologist, psychiatrist or psychologist.
Difficulties with memory can be caused by a wide variety of problems, including normal aging and stress. Here are some simple steps to help improve memory:
Get a good night's sleep. Evidence suggests that sleep deprivation can impair memory. Studies also point to sleep as an important way for the brain to consolidate what it has learned during the day.
Exercise. Physical activity helps the brain as well as the body by improving circulation, lowering blood pressure and increasing blood oxygen levels.
Eat a well balanced diet. Vitamin B12 is just one of the vitamins necessary for neurological function. Depending on your age and general condition, doctors sometimes advise taking a vitamin supplement to help assure adequate nutritional intake.
Don't smoke. Cigarettes lower oxygen levels circulating in the blood and chronic smoking decreases blood flow to the brain. Neither of these effects is good for the brain.
Go easy on the alcohol. Heavy, chronic drinking can cause significant neurological impairment, but even moderate drinking appears to interfere with working memory. And if working memory is damaged, information can't be stored in long-term memory.
Reduce stress. Being too busy can interfere with attentiveness, an important factor in retaining information.
Keep your mind active. The latest research shows that the brain continues to grow and change throughout life, a feature called plasticity. The more the brain is stretched to do mental activities, the more it stays vital and healthy.