They kneel before the camera. They are rigid with fear. Or they are crying, clearly unhinged, begging for their lives. They are hostages, flanked by masked men who carry swords or AK-47s -- the grim reapers of this age of televised terror.

We watch. We watch, mainly, because these images are ubiquitous. Turn on the tube, and you will see the forlorn face of the late Margaret Hassan, the CARE International leader in Iraq. She had begged. She had wept, even fainted while forced to face the murderers' lens. And now British officials believe Hassan is the person executed with a bullet to the head, point-blank, as shown on yet another videotape. (Mercifully, al-Jazeera, the network that obtained the tape, has decided not to broadcast it, for now.)

Hostage-taking, beheadings, executions have become almost commonplace. With the Iraq war heating up recently, violent imagery has been the daily bread.

Mental health experts say it is unwise to watch, unhealthy even. But when news broke that a U.S. soldier in Fallujah shot and killed a wounded and unarmed Iraqi on tape, the video clip swept broadcast networks, played over and over.

You'd think all this imagery would have numbed us by now. But it hasn't, experts say. Instead, it has given us new cognitive scripts to follow: planes used as missiles, innocent people beheaded. These are things we once did not consider as ways of dying.

And it has embedded new threats, new fears, into our brains. Subtly, perniciously, our sense of well-being trembles with each new, jolting image. That is what the scientists say. We have become a society of witnesses. As surely as the witness to a horrible crime is shaken by what he has seen, so too are we haunted and stressed as secondary witnesses to the horrific acts of our time.

That was the lesson of Sept. 11, 2001. And the phenomenon continues.

"One of the concerns I have as a psychologist is that there are images that people are being exposed to in the media that many people are going to have difficulty ridding themselves of," says Jeffrey Johnson, who studies risk factors for mental disorders at the New York State Psychiatric Institute.

Take Shabnaz Yousefia. She's an ordinary young woman with a horrible memory she can't shake, something she saw that won't go away. Such imagery has a nasty way of lingering, as the 22-year-old now knows.

Talking about it, her dark eyes moisten. She tosses her head, side to side, her earrings swinging like pendulums.

She's a University of California at Santa Barbara student on a semester-long Amnesty International internship in the District. She's also taking a class on U.S.-Mideast policy at Georgetown, where she talks one evening about her traumatic episode:

It happened in May, back in Santa Barbara, she explains. A group of students were studying when one of them opened an e-mail attachment. It contained a video of a beheading. Students gathered around the computer screen.

Yousefia's foreign policy studies, plus her Iranian-Swedish-American upbringing, make her especially interested in what's happening in Iraq. So she, too, joined the small group, five in all, and bore witness to the murder in May of Nicholas Berg, a U.S. contractor in Iraq.

"It was horrifying," Yousefia says, getting upset at the memory.

"How could anyone do that?

"I was crying. And someone else was crying.

"It was so inhumane."

The students went their separate ways that day. They didn't chat about what they'd seen. But the image did what such images are known to do: It got stuck, submerged in Yousefia's brain, only to reemerge later.

She groans as she speaks about it at Georgetown. She takes a deep, exasperated breath. She fans her face. For a moment, it seems she might cry.

"I haven't thought of it for a while," she says, seeming to apologize for her involuntary reaction to a trauma replayed in her head.

Roxane Silver, a social scientist at the University of California at Irvine, adamantly advises against watching such imagery. She knows horror and trauma. She has extensively studied psychological responses to the 9/11 terror attacks.

"Watching those videos is not psychologically beneficial in any way," Silver says of beheadings.

She also cites the research of Harvard psychologist Dan Wegner, who says that when we try to suppress a thought or image, the more the thought intrudes, almost like an obsession.

In his well-known "white bear" experiments, he told his subjects not to think of a white bear, only to find that they could think of little else but a white bear even as they tried to forget the white bear.

Wegner says his experiment applies to memories of trauma as well.

"A lot of work we put into trying not to think about traumas can actually make us more horrified by them" because of the repetition of the image or memory in our brains, he says.

Another reason images of trauma can scarcely be forgotten is found in research pioneered by neuroscientist Joseph Ledoux of New York University. He has shown that an almond-shaped part of the brain called the amygdala stores the body's reactions to fear. Those reactions are triggered when the source of the fear is encountered.

That is why the mere sound of the theme music from "Jaws" can have such an impact on a previously frightened "Jaws" viewer, for instance. More seriously, our "fight or flight" responses to a threat reside there, in the amygdala.

This is the physiological side of the story. The body has physical reactions to what the brain perceives.

"When you see these images on-screen, your body reacts maybe not so intensely [as to a live event] but surprisingly intensely," says Joanne Cantor, professor emeritus at the University of Wisconsin and a leading authority on the impact of visual violence on children.

Our pulse races, our glands fire off hormones, our muscles tense, in response to stimuli that conjure an earlier fear. In other words, we get stressed all over again.

It is what Kaethe Weingarten, another Harvard psychologist, calls "common shock." It is the broad experience of shock shared by a culture, a society. Weingarten has studied responses to trauma in the United States as well as in South Africa and Kosovo.

She has chronicled the widespread numbness, anger, memory loss, sadness, helplessness, shame and aggression that are byproducts of the "common shock" induced by witnessing violence, even on television.

With up to 10 percent of the U.S. population suffering from post-traumatic stress disorder at any given time, it is those people who are most susceptible to being retraumatized, experts say.

One of the main triggers, for anyone, is believing that a horrific event can happen to you, too.

That is why so many people -- 17 percent of the U.S. population outside of New York City -- reported symptoms of Sept. 11-related post-traumatic stress two months after the attacks, according to research conducted by Silver. (Six months after the attacks, by the way, the level of such symptoms related to Sept. 11 had fallen to 5.8 percent and continued its decline.)

"The thing that creates the serious mental health problem is when it happens to you or you believe that it could happen to you or that you're at personal risk" of a traumatic event, says Randall Marshall, director of trauma studies at the New York State Psychiatric Institute.

Yousefia, for instance, was shaken to learn of the coordinated bombings aimed at Israeli tourists on the Sinai Peninsula, which killed 34 people last month.

"I was just there," she says. A month prior, she too had vacationed in that resort area of the Red Sea.

But as upsetting as the Berg video was, it did not feel personally threatening. She is here, not in Iraq, not at all likely to suffer such a terrible fate as Berg.

When not facing an immediate danger, people tend to "locate the fear, locate the threat outside of yourself and outside of your immediate environment," says Marshall.

It's not a matter of becoming numb. Rather, it's a protective mechanism, a way of putting horror in its proper, distant context, the experts say.

But how distant is it when we flip the channel and there in our living room is another hostage. Another mother, sister, father or brother -- kneeling, crying, begging for life.

Images that haunt us: Margaret Hassan, above, pleads for her life on camera. At left, the North Tower at the World Trade Center collapses on Sept. 11, 2001.