They say that when the twin towers of the World Trade Center came crashing down and airliners plowed into the Pentagon and a Somerset field, all Americans were traumatized.

Illustration by Stacy Innerst, Post-Gazette Today we begin a five-part series on the mysteries and meanings of dreams. The series will feature dream excerpts from local residents, as well as stories on the history of dream research; nightmares and how to confront them, strange sleep disorders that include people who walk, fight and eat while dreaming; and the controversial topic of lucid dreaming -- being able to know you're in a dream and partially control it. Click illustration for larger image. Related stories Dreams: From falling to failing, it's the same old stories The 12 universal dreams Dreams and the arts: PG critics pick their dream theme team Insight from a decade-long journal of dreams A dream journal 'how-to'

The proof, if any was really needed, lay in their dreams.

For days after 9/11, people across the United States reported unusually intense dreams. They didn't necessarily dream about towers or airplanes, but the images were more highly charged and linked to emotions such as fear and terror than before the attacks.

In some cases, people dreamed of being swept away by whirlwinds or tidal waves.

"What they were dreaming," said researcher Dr. Ernest Hartmann, "is that they were overwhelmed."

It may seem like simple common sense that a traumatic event could have powerful effects on our dreams, but dreams by their nature are so ephemeral that Hartmann, a psychiatrist at Tufts University School of Medicine in Boston who has studied dreams for more than 30 years, believes he may be the first person who has systematically analyzed the link between a national trauma and the dreams it caused.

And so we have one more small clue to the mystery of why we dream.

People have always wondered about dreams. They have been enchanted by their fanciful and jumbled storylines, shaken by their eerie images and frustrated by the way they can dissipate upon awakening, like a whiff of perfume floating away on the breeze.

Scientists, whose endless quest for objective facts is often trumped by the subjective nature of dreams, have yet to develop a common theory on what purpose dreams serve or what they mean.

But dreams are slowly giving up some of their secrets.

For instance, scientists have now managed to gather evidence that helps explain why dreams, which have a happy connotation for most people, nevertheless are often infused with fear and anxiety.

In just the last six years, researchers at the University of Pittsburgh, the National Institutes of Health and the University of Liege, Belgium, have used advanced imaging techniques to peer deep within the brain of sleeping subjects to see which areas are most active during dream episodes.

They've found that when dreaming is at its peak, the sleeper's brain is highly aroused, almost as if it were on emotional overdrive. Not all parts of the brain are active, but those that are appear to be working as hard during dreams as during waking hours.

No device yet allows researchers to look over a dreamer's shoulder to evaluate the content of dreams, so scientists like Hartmann still must rely on the recollections of dreamers, which are necessarily incomplete, hazy and sometimes self-censored. In his case, he gathered dreams from 16 people around the country, culling dreams from 10 days before and 10 days after 9/11.

Sigmund Freud argued a century ago that dreams are highly symbolic and could be interpreted to reveal a person's hidden desires, but few scientists today agree with that. Still, intriguing evidence is providing hints about their possible purposes.

As suggested by the response to 9/11, these nocturnal experiences engage the emotional center of our brains and may play an important role in regulating our emotions. Dysfunctional dreaming, on the other hand, appears to play an important role in post-traumatic stress disorder and may be involved in depression as well.

Other evidence suggests that dreams may aid in learning new mental skills and in sorting and consolidating our memories.

Even if dreams ultimately turn out to be nothing but the brain's biochemical reset switch, it now seems clear that dreams do have a function and are not just hazy, confused meanderings within a groggy mind.

"What are dreams for? That's like asking, 'What is waking for?' " said Rosalind Cartwright, chairman of psychology and a sleep disorders specialist at Rush University Medical Center in Chicago.

Dreams are the mind's work.

What do they mean?

Just what that work consists of, though, is still open to considerable debate.

Graphic by Byron Spice and Steve Thomas/Post-Gazette
	Graphic: The dreaming brain (in pdf format)

One theory is that REM -- or rapid eye movement sleep, when the most intense dreaming occurs -- somehow resets special brain mechanisms and re-sensitizes the brain to certain neurotransmitters, the chemicals that brain cells use to communicate with each other. REM sleep, discovered just 50 years ago, appears to be the only time when the brain stops secreting serotonin, a neurotransmitter involved in regulating mood, and norepinephrine, which helps control heart rate, blood pressure and other bodily functions.

A once popular idea was that dreams were a mechanism for forgetting -- that the brain used REM sleep to dump information it no longer needed.

Certainly, dreams themselves are easily forgotten. Neuroimaging studies show that the part of the brain that is responsible for short-term, or working, memory -- the prefrontal cortex -- is largely inactive during REM sleep. It's as if the brain isn't supposed to remember dreams, or as if remembering dreams isn't essential to their function.

Unless people wake up during a dream -- or teach themselves to rouse themselves during a dream -- they seldom remember much about them. Late-morning dreams are the most likely to be remembered, because that's the part of the night when REM sleep periods are longest and sleep itself is less deep.

But the view that dreams are simply a means of clearing out old junk no longer has much support, said Dr. Daniel Buysse, medical director of the University of Pittsburgh Sleep Evaluation Center and past president of the American Academy of Sleep Medicine. To the contrary, quite a few researchers suspect that dreams and REM sleep may play a role in learning certain tasks and in consolidating memories.

Dreaming, said James McClelland, a Carnegie Mellon University psychologist, "is the opposite of forgetting."

During the day, he explained, a person's experiences are stored in an area of the brain called the hippocampus. The hippocampus, which is active during REM sleep, replays these episodic memories before they are translated into long-term memories stored in the brain's cortex.

Studies have shown that people who suffer damage to the hippocampus have trouble making new memories, he noted.

As the hippocampus replays snippets of the day's experiences, the brain can sort out the experiences, strengthening connections between brain cells in such a way that memories are made and new skills are incorporated.

In animal experiments, one of McClelland's collaborators, Bruce McNaughton, of the University of Arizona, has implanted electrodes in the brains of rats, recording the activity of more than 100 nerve cells during both waking and REM sleep. Distinctive patterns of neural activity that McNaughton recorded while the rats ran a maze or a track were repeated during REM sleep.

It's not possible for researchers to wire up humans in the same way, but presumably people also relive daily experiences during their dreams and perhaps rehearse new skills, McClelland said.

Residue of the day

Freud, the previous century's guru of dream theory, talked about dreams being day residue -- that daily events became the stuff of that night's dreams. This day-residue effect recently was explored by Canadian researchers, who studied dream logs kept for a week by 470 students at the University of Alberta.

They found that dreams that had a strong impact, such as changing the dreamer's mood or making the dreamer reconsider actions, were often related to an event from the previous day. Those dreams also were the most likely to display a dream-lag effect, where an event is again incorporated into a dream a week later.

Just why a seven-day lag should occur is unclear, said the lead author, Tore Nielsen, of the University of Montreal Sleep Research Center. The delay may be part of a sorting mechanism that the mind uses to evaluate potential memories, he suggested, or it may simply mean that it takes awhile to transfer information into long-term memory.

Several studies have shown that sleep is involved in procedural learning -- learning how to ride a bike, drive a car, play games -- but is less important for declarative memory, involved in remembering facts, said Pitt psychiatrist Dr. Eric Nofzinger. The fact that both the hippocampus and the brain's motor centers are active during REM sleep suggests that this might be a time when the brain can effectively practice such behaviors.

Nofzinger also thinks the brain may use REM to rehearse instinctual behaviors.

"We may all have genetically programmed behavior for responding to situations that we don't need on a daily basis," he said, such as reacting to danger or protecting a loved one from harm. REM sleep may be a time when the brain, prompted by some experience or an anticipated event, may riffle through these stored behaviors, select one that fits the situation, and then rehearse it.

This may be a particularly important function of dreams in youngsters, said Anne Germain, a dream researcher and postdoctoral scientist in Pitt's Sleep Evaluation Center.

Children often dream about monsters, for instance. When they dream about running away from the monster, Germain said, they are practicing a behavioral pattern that teaches them how to respond to threats.

Working out problems

One of the most intriguing theories about the purpose of dreams comes from one of the giants of the dream research field, Rush University's Cartwright.

		INTRIGUED? We'd like to hear about your dreams. The Pittsburgh Post-Gazette welcomes interested dreamers to e-mail samples that will be published in the newspaper or on the PG Web site. If you haven't been keeping track of your dreams, get started now. Send any dreams you feel comfortable in sharing by Dec. 19 to dreams@post-gazette.com. Type in "dreams" in the subject line. Please include your name, age and home town or neighborhood. Obviously, we can't publish them all, but we'll do our best to pull together as wide a selection as possible.

She notes that when sleep research subjects answer standard psychological questionnaires designed to elicit their mood before bedtime and after rising in the morning, they almost always feel better when they wake up.

It's not because dreams themselves are happy experiences. Rather, study after study has shown that negative emotions such as fear and anxiety are far more common in dreams than feelings of warmth and frivolity. Perhaps that's why the mind isn't wired to remember most dreams.

But when sleep researchers wake volunteers during each of their REM periods during the night and ask them to describe their dreams, they usually find that dreams feature the most negative emotions at the beginning of the night and become more positive as the night progresses.

What appears to be happening, Cartwright suggested, is that an upsetting experience or mood will trigger memory networks associated with that emotion and, during the course of a night, those thoughts and their related imagery will get a workout. The dream then evolves during the night; the plot line changes, and older memories are tapped.

"By the last dream of the night, you've worked it through to be something positive," she said.

Or, at least that's how it's supposed to work. People suffering from depression seem to feel just as bad or worse in the morning than they do before going to bed, Cartwright said. Their REM sleep is dysfunctional, and their dreams are often less vivid than those of their healthy counterparts.

It may be that depression makes a person's dreaming abnormal -- but it may also be that dream problems could result in depression, or at least make depression worse. "They're not working it through," Cartwright said, so sleep doesn't restore a balance in mood as it normally does.

Depressed patients who have the most REM sleep and "can give you a good dream story" seem to be the most likely to recover, she said. "It means that the mind is working productively at night."

Problems with dreaming also underlie post-traumatic stress disorder.

As Hartmann's study suggested, even a generalized trauma such as 9/11 can affect dreams. But people who suffer more immediate and intense traumas, such as someone directly involved in the 9/11 attacks or in combat, may have dreams so upsetting they disrupt sleep and make it impossible for the mind to get itself back in emotional balance, said Germain, who specializes in treating patients with post-traumatic stress.

Most traumatic dreams tend to subside within a month of the event, Germain said. But in post-traumatic stress disorder patients, the ugly dreams just keep on coming.

"It's like being re-traumatized every night," she said, with the sufferer feeling the same emotions as during the original experience.

That is when techniques such as visualizing dreams in advance and suggesting new endings to troubling dreams can be highly effective, Germain said.

Other researchers, such as Tufts' Hartmann, suggest dreams may play other roles, such as boosting creativity. In a dream, he explained, the mind is "hyperconnective," making unusual and sometimes unexpected links between various ideas and images that can lead to scientific discoveries or artistic accomplishments.

"It's not that you can't do it when you're awake, but it's easier when you're asleep," Hartmann said, citing lowered inhibitions and a lack of interruptions.

If there's any function that dreams don't seem to serve, it appears to be the one people assume is most important: to give the mind a rest.

To the contrary, said Cartwright: "The mind does not shut off. ... It just changes channels."

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Tomorrow: Dreams throughout history